Harriette Gc

MD Van Spall

Lala Mdd

Thomas F Deering

M D Mbae

Barbara Casadei

M D Dphilf

Faiez Zannad

M D Phdg

Padma Kaul Phdh

Roxana Mehran Mdi

MD Gail D Pearson

ScDj Monica R Shah

M D Mhsk

Martha Gulati

M D Msl

Cindy Grines

M D Mscaim

Annabelle Santos

Volgman Mdn

Revkin Mdo

MD Ileana L Piña

M D Mphp

Carolyn Sp

Lam Mbbs Phdq

Hochman Mdr

MD Tabassome Simon

Walsh Mdt

Biykem Bozkurt

M D Phdu

Bozkurt : Bristol

Myers Squibb

Dr Harriette Gillian
email: harriette.vanspall@phri.ca@hvanspall

Christine Van Spall

Copeland

Ending Gender Inequality in Cardiovascular Clinical Trial Leadership: JACC Review Topic of the Week Brief title: Gender Inequality in Cardiovascular Clinical Trial Leadership WRITING COMMITTEE

Keywords: Gender, disparities, clinical trial, leadership, steering committee, principal investigator, equity

Women are under-represented as leaders of cardiovascular randomized controlled trials (RCTs), representing one in ten lead authors of cardiovascular trials published in high-impact journals. While the proportion of cardiovascular specialists who are women has increased in recent years, the proportion of cardiovascular clinical trialists who are women has not. This gap, underpinned by systemic sexism, has not been adequately addressed. The benefits of diverse RCT leadership extend to patients and professionals, and equality in the workplace is a human right that we must all uphold.

In this position statement, we present strategies adopted by some organizations to end gender inequality in research leadership. We offer an actionable roadmap for early career researchers, scientists, academic institutions, professional societies, trial sponsors, and journals to follow, with the goal of harnessing the strength of women and under-represented groups as research leaders and facilitating a just culture in the cardiovascular clinical trial enterprise.

Introduction

Despite gender parity in medical school admissions, women remain under-represented as cardiovascular specialists and as leaders of cardiovascular randomized controlled trials (RCTs). The gender chasm in cardiovascular medicineunder-representation of women in clinical subspecialties, along with lower likelihood of promotion, less recognition for scientific work, fewer opportunities at scientific meetings, and under-representation in research publications and journal editorial boards relative to men [START_REF] Blumenthal | Sex differences in faculty rank among academic cardiologists in the United States[END_REF][START_REF] Mehta | Current demographic status of cardiologists in the United States[END_REF][START_REF] Stone | Assessment of subspecialty choices of men and women in internal medicine From 1991 to 2016[END_REF][START_REF] Whitelaw | Flattening the hierarchies in academic medicine: the importance of diversity in leadership, contribution, and thought[END_REF][START_REF] Ouyang | Association between female corresponding authors and female co-authors in top contemporary cardiovascular medicine journals[END_REF][START_REF] Balasubramanian | Women representation among cardiology journal editorial boards[END_REF][START_REF] Silver | Where are the women? The underrepresentation of women physicians among recognition award recipients from medical specialty societies[END_REF][START_REF] Burgos | Gender equity imbalance in cardiology scientific sessions in the Americas[END_REF][START_REF] Denby | Representation of women in cardiovascular clinical trial leadership[END_REF][START_REF] Asghar | Sex differences in authorship of academic cardiology literature over the last 2 decades[END_REF] manifests on a larger scale in clinical trial leadership. [START_REF] Vitale | Under-representation of elderly and women in clinical trials[END_REF][START_REF] Whitelaw | Trial characteristics associated with under-enrolment of females in randomized controlled trials of heart failure with reduced ejection fraction: a systematic review[END_REF][START_REF] Wei | Trial leadership by women is associated with racial and ethnic diversity among heart failure clinical trial participants: a systematic review[END_REF] This gap, underpinned by systemic sexism, has not been adequately addressed in the research enterprise. We are called to end the gender inequality in clinical trial leadership not only to benefit the cardiovascular community, but also because equality is a human right that must be protected. [START_REF] Silver | Where are the women? The underrepresentation of women physicians among recognition award recipients from medical specialty societies[END_REF] We provide an actionable roadmap for early career researchers, scientists, academic institutions, professional societies, industry sponsors, granting agencies, and journals to close the gender gap and facilitate a just culture in the cardiovascular clinical trial enterprise.

Rationale for Ending Gender Inequality

Benefits of Diversity in Clinical Trial Leadership

A majority of cardiovascular clinical trials enroll patients who are predominantly male, racially homogenous, and younger than those afflicted with the disease, limiting the generalizability of results. [START_REF] Vitale | Under-representation of elderly and women in clinical trials[END_REF][START_REF] Whitelaw | Trial characteristics associated with under-enrolment of females in randomized controlled trials of heart failure with reduced ejection fraction: a systematic review[END_REF] Trials led by female investigators enroll a greater proportion of participants who are female, Black, Indigenous, and people of colour, [START_REF] Ouyang | Association between female corresponding authors and female co-authors in top contemporary cardiovascular medicine journals[END_REF][START_REF] Wei | Trial leadership by women is associated with racial and ethnic diversity among heart failure clinical trial participants: a systematic review[END_REF] generating better evidence to assess for sex and race or ethnicity as effect modifiers of intervention. [START_REF] Whitelaw | Trial characteristics associated with under-enrolment of females in randomized controlled trials of heart failure with reduced ejection fraction: a systematic review[END_REF] RCTs with geographic diversity among their leaders are associated with greater geographic representation among trial participants. [START_REF] Zhu | Geographic representation of leaders and collaborators in heart failure randomized controlled trials: A systematic bibliometric review[END_REF] Diversity in clinical trial leadership can increase innovation; decrease 'group think'; answer research questions relevant to diverse populations; tailor recruitment and retention strategies to patients' cultural norms, resources, and health literacy; and increase the external validity of studies. [START_REF] Ouyang | Association between female corresponding authors and female co-authors in top contemporary cardiovascular medicine journals[END_REF][START_REF] Whitelaw | Trial characteristics associated with under-enrolment of females in randomized controlled trials of heart failure with reduced ejection fraction: a systematic review[END_REF][START_REF] Wei | Trial leadership by women is associated with racial and ethnic diversity among heart failure clinical trial participants: a systematic review[END_REF][START_REF] Tshetshema | A systematic literature review of the relationship between demographic diversity and innovation performance at team-level[END_REF][START_REF] Campbell | Gender-heterogeneous working groups produce higher quality ccience[END_REF] Diversity begets diversity among cardiovascular professionals. Those who lead well-designed clinical trials often experience career growth, recognition, and opportunities to inform regulatory or policy decisions. When diverse scientists are represented in high-profile positions, diverse trainees and junior faculty are encouraged to broaden their aspirations and see a path ahead in their own research careers (Figure 1). Research collaboration networks are diverse when research leaders are diverse; clinical trial publications are more likely to include women lead and co-authors when senior authors are women. [START_REF] Whitelaw | Flattening the hierarchies in academic medicine: the importance of diversity in leadership, contribution, and thought[END_REF][START_REF] Ouyang | Association between female corresponding authors and female co-authors in top contemporary cardiovascular medicine journals[END_REF][START_REF] Zhu | Geographic representation of leaders and collaborators in heart failure randomized controlled trials: A systematic bibliometric review[END_REF][START_REF] Whitelaw | Characteristics of heart failure trials associated with under-representation of women as lead authors[END_REF]

Under-Representation of Women As Clinical Trial Leaders

Nowhere is the underrepresentation of women in cardiovascular medicine as conspicuous as in clinical trial leadership. More than half of cardiovascular trials published in three high impact factor journals between 2014 and 2018 lacked women investigators on their executive committees. [START_REF] Denby | Representation of women in cardiovascular clinical trial leadership[END_REF] No more than 10% of publications from these trials were led by a woman investigator. [START_REF] Denby | Representation of women in cardiovascular clinical trial leadership[END_REF] The underrepresentation of women is more pronounced in cardiovascular clinical trial leadership than in US academic cardiology (17%) or cardiology fellowship programs (25%); [START_REF] Blumenthal | Sex differences in faculty rank among academic cardiologists in the United States[END_REF][START_REF] Mehta | Current demographic status of cardiologists in the United States[END_REF][START_REF] Stone | Assessment of subspecialty choices of men and women in internal medicine From 1991 to 2016[END_REF][START_REF]ERAS Statistics[END_REF] and is more evident in procedural than nonprocedural CV specialties, reflectingalbeit, to a larger extent -the marked gender gap in procedural sub-specialties. [START_REF] Denby | Representation of women in cardiovascular clinical trial leadership[END_REF] The leaky pipeline from clinician to trialist manifests itself even in the subspecialty of heart failure, where the proportion of women clinicians is higher than in other cardiovascular subspecialties. [START_REF] Moayedi | Perceived generational, geographic, and sex-based differences in choosing a career in advanced heart failure[END_REF][START_REF] Defilippis | Representation of Women Physicians in Heart Failure Clinical Practice[END_REF] In a review of heart failure (HF) RCTs published in high-impact journals between 2000 and 2019, women represented less than 16% of first authors, 13% of senior authors, and 20% of all authors. [START_REF] Whitelaw | Characteristics of heart failure trials associated with under-representation of women as lead authors[END_REF] Despite an almost 2-fold increase in the median number of authors per trial over time, there was no temporal change in the proportion of women in any authorship position. Similarly, nearly half of all HF trials published in the last 2 decades did not have any women in their steering committees. [START_REF] Eliya | Temporal trends and clinical trial characteristics associated with the inclusion of women in heart failure trial steering committees: a systematic review[END_REF] Women comprised only 11% of steering committee members, with no change in gender composition over time. [START_REF] Eliya | Temporal trends and clinical trial characteristics associated with the inclusion of women in heart failure trial steering committees: a systematic review[END_REF]

Factors Associated with Under-representation of Women as Clinical Trial Leaders

Several reasons have been proposed to explain the gender chasm in cardiovascular clinical trial leadership, some not borne by evidence. These include a lack of qualified women in the clinical trial pipeline; systemic sexism; and an assumption that women are too burdened with familial responsibilities to lead trials. [START_REF] Denby | Representation of women in cardiovascular clinical trial leadership[END_REF][START_REF] Whitelaw | Characteristics of heart failure trials associated with under-representation of women as lead authors[END_REF][START_REF] Eliya | Temporal trends and clinical trial characteristics associated with the inclusion of women in heart failure trial steering committees: a systematic review[END_REF][START_REF] Yu | Minorities struggle to advance in academic medicine: A 12-y review of diversity at the highest levels of America's teaching institutions[END_REF] While the under-representation of women in cardiovascular medicine is a factor in the gender gap in cardiovascular clinical trial leadership, there is a larger pool of women candidates to draw upon as trial principal investigators (PIs) than clinical cardiovascular specialists. [START_REF] Denby | Representation of women in cardiovascular clinical trial leadership[END_REF][START_REF] Whitelaw | Characteristics of heart failure trials associated with under-representation of women as lead authors[END_REF] The pool includes non-physician clinicians and non-clinician cardiovascular researchers. Seven of ten recipients of the Master of Public Health degree are women, [START_REF] Leider | Trends in the conferral of graduate public health degrees: A triangulated approach[END_REF] suggesting a high level of training in research methodology among women. In fact, the numerator of women in trial leadership has remained static over time whilst the denominator of available women (including women cardiovascular specialists) is increasing. [START_REF] Whitelaw | Characteristics of heart failure trials associated with under-representation of women as lead authors[END_REF][START_REF]ERAS Statistics[END_REF][START_REF] Eliya | Temporal trends and clinical trial characteristics associated with the inclusion of women in heart failure trial steering committees: a systematic review[END_REF] Research training programs alone do not appear to address the broken pipeline that leaves a majority of women researchers out of leadership roles.

An important barrier to women leading clinical trials is the systemic sexism that under-recognizes their academic accomplishments. Women receive less research funding than men, [START_REF] Carnes | Women are less likely than men to be full professors in cardiology[END_REF] and research submitted under a woman's name is likely to receive a more critical review than the same research under a man's name. [START_REF] Asghar | Sex differences in authorship of academic cardiology literature over the last 2 decades[END_REF][START_REF] Budden | Double-blind review favours increased representation of female authors[END_REF] The systematic biases that favor men via research funding and salary support, speaking engagements, and academic promotion also make them favorable candidates for downstream opportunities, including leadership of high-profile trials. The process is self-fulfilling, creating an exclusive circle where individuals who meet leadership criteria set by industry are rewarded with incremental success. [START_REF] Denby | Representation of women in cardiovascular clinical trial leadership[END_REF][START_REF] Chopra | Industry funding of clinical trials: Benefit or bias?[END_REF][START_REF] Fabbri | The influence of industry sponsorship on the research agenda: A scoping review[END_REF][START_REF] Marušić | Five-step authorship framework to improve transparency in disclosing contributors to industry-sponsored clinical trial publications[END_REF][START_REF] Flanagin | Prevalence of articles with honorary authors and ghost authors in peer-reviewed medical journals[END_REF][START_REF] Lundine | Gender bias in academia[END_REF] The sexism in the clinical trial enterprise is evident in the fact that after adjusting for relevant factors, women are less likely to be first authors of trials in which the senior authors are men, and that steering committees are less likely to include a woman when the trial leaders are men. [START_REF] Whitelaw | Characteristics of heart failure trials associated with under-representation of women as lead authors[END_REF][START_REF] Eliya | Temporal trends and clinical trial characteristics associated with the inclusion of women in heart failure trial steering committees: a systematic review[END_REF] Men dominate leadership roles in high-profile, multi-center pharmaceutical trials, [START_REF] Whitelaw | Characteristics of heart failure trials associated with under-representation of women as lead authors[END_REF][START_REF] Moayedi | Perceived generational, geographic, and sex-based differences in choosing a career in advanced heart failure[END_REF] which garner publicity, career advancement, and remuneration for the PI. [START_REF] Chopra | Industry funding of clinical trials: Benefit or bias?[END_REF][START_REF] Rose | Gender fifferences in physicians' financial ties to industry: A study of national disclosure data[END_REF] Industry-initiated trials appear to favor leadership selection from a small pool of individuals who have previously served in leadership, marketing, and consultant roles; the collaborations that ensue are restricted to a group that rarely include women. [START_REF] Denby | Representation of women in cardiovascular clinical trial leadership[END_REF][START_REF] Whitelaw | Characteristics of heart failure trials associated with under-representation of women as lead authors[END_REF][START_REF] Eliya | Temporal trends and clinical trial characteristics associated with the inclusion of women in heart failure trial steering committees: a systematic review[END_REF][START_REF] Rose | Gender fifferences in physicians' financial ties to industry: A study of national disclosure data[END_REF][START_REF] Wang | Time to end "Manels" in clinical trial leadership[END_REF] Industry sponsors may be swayed by an investigator who previously led a product from testing to approval, and may choose that individual again to garner product success. Name recognition of the investigator may also be deemed desirable for regulatory approval of a product.

Benevolent sexismoverlooking women for leadership and collaboration opportunities with the assumption that they have competing familial responsibilitiesis unjustified. While the leadership of investigator-initiated multi-center trials poses challenges, women are under-represented as leaders of pharmaceutical trials [START_REF] Whitelaw | Characteristics of heart failure trials associated with under-representation of women as lead authors[END_REF] for which there is ample industry infrastructure, funding, and logistical support. Benevolent sexism perpetuates false assumptions of women's' leadership aspirations, commitment to advancement, and engagement in their work, when in fact, it is systemic sexism that holds women back. [START_REF]Introduction to the Principles and Practice of Clinical Research (IPPCR)[END_REF]

Roadmap to Closing Gender Inequality in Clinical Trial Leadership

Trainees and Early Career Investigators

Leading a successful investigator-initiated clinical trial requires competencies in trial design, grant writing, leadership, and communication (Table 1). These research competencies can be facilitated by advanced research degrees and research fellowships. Certificate programs and online training options [START_REF]Introduction to the Principles and Practice of Clinical Research (IPPCR)[END_REF] can be useful adjuncts, and leadership training can strengthen one's capacity to execute trials, negotiate, and manage interactions with multiple stakeholders. [START_REF] Chang | Increasing promotion of women faculty in academic medicine: Impact of national career development programs[END_REF] While the competencies of the PI in industry-initiated trials are similar to investigatorinitiated trials, leadership is not open to all, and the degree of the PI's involvement in trial planning and logistics varies. Funding, scientific expertise, analysis, and writing services are often offered, and trial oversight is typically provided by the industry sponsor. The PI is typically an opinion leader in the field who lends professional credentials, works as the trial advisor, disseminates trial results, and helps market the product.

Beyond obtaining competencies through training programs, early career researchers are called to:

1. Register in research directories, attend networking events, and seek mentorship / sponsorship from trialists.

2. Prioritize peer-reviewed grants and publications over other academic activities to build academic currency.

Engage in knowledge dissemination of their work on digital platforms to overcome institutional

barriers that may keep talent hidden.

Hold leaders accountable when diversity and inclusion principles are not manifest.

Clinical Trialists and Scientists

The progression of novel therapies from bench to bedside relies on diverse translational research teams. [START_REF] Tachibana | From the lab to the clinic and back: Translational research training and careers[END_REF] In order to promote diversity and inclusion, scientists are called to: [START_REF] Bates | Moving the needle on gender equity: A call for personal and organizational action[END_REF] 1. Avoid gender stereotyping and linguistic asymmetries that diminish the professional accomplishments of women in evaluations, reference letters, and conferences.

2. Participate in recruitment, retention, and mentorship programs for women and other underrepresented groups.

3. Build diverse research teams. [START_REF] Whitelaw | Flattening the hierarchies in academic medicine: the importance of diversity in leadership, contribution, and thought[END_REF]. Make participation on panels, research collaborations, and committees contingent on diversity.

5. Hold institutional leaders accountable in reporting gender-disaggregated metrics in recruitment, research compensation, and promotion.

6.

Step up, speak up, and be an agent of change when they observe discrimination or gender harassment.

Academic Institutions

In order to harness the strength of women as cardiovascular researchers, academic institutions must be purposeful in recruiting and retaining women as cardiovascular trainees and faculty members (Table 2). Supporting the career growth of existing women cardiovascular researchers may be a strategic way to build capacity for clinical trial leadership among women; trials with women senior authors are more likely to have women first authors after adjusting for other factors. [START_REF] Whitelaw | Characteristics of heart failure trials associated with under-representation of women as lead authors[END_REF] Academic institutions must implement bias-free processes for research awards and grants, public recognition, academic promotion, and leadership; and ensure a respectful, harassment-free work environment (Table 2). The gender gap in academic advancement is more pronounced among women of colour. [START_REF] Whitelaw | Flattening the hierarchies in academic medicine: the importance of diversity in leadership, contribution, and thought[END_REF][START_REF] Bates | Moving the needle on gender equity: A call for personal and organizational action[END_REF][START_REF] Alshebli | The preeminence of ethnic diversity in scientific collaboration[END_REF] Academic institutions should report the demographic breakdown of faculty who are recruited, promoted, and awarded with institutional grants and research chairs. It is often these internal funding awards that drive the success of external funding applications, so equal opportunity is of utmost importance.

Incorporating diversity and inclusion into institutional research strategic plans is a step towards acknowledging women and under-represented groups as equal partners in the advancement of science. [START_REF] Alshebli | The preeminence of ethnic diversity in scientific collaboration[END_REF] Meaningful change will require institutional commitment, frequent measurements, and targeted improvements to achieve the desired goals.

Catalyzer Organizations

Catalyzer organizations are uniquely positioned to advance women and other underrepresented groups as cardiovascular clinical trial leaders. [START_REF] As | Women As One[END_REF][START_REF]Global CVCT Forum[END_REF] Given their connections with academia, government agencies, and industry partners, catalyzer organizations are uniquely positioned to create national platforms that convene key stakeholders and tackle disparities. For example, Women as One advocates for the professional advancement of women in clinical research through talent directories, networking opportunities, speaking engagements, and competitive research awards. In partnership with industry, the Global CardioVascular Clinical Trialists (CVCT) Forum builds research capacity in under-represented groups through the Future Trialists Fellowship, which enables early-career investigators to join clinical trial steering committees; learn about trial conduct; experience data analysis and interpretation; and co-author trial sub-studies.

Catalyzer organizations could work synergistically to facilitate research leadership in underrepresented groups. Strategic partnerships between catalyzer organizations that focus on distinct but related goals (e.g., gender equity and racial equity) and between catalyzer organizations and industry could increase the return on investment made by individual organizations.

Professional Societies

Cardiovascular professional societies can play a pivotal role in increasing diversity in clinical trial leadership (Figure 2). These societies promote the interests of their members, and advocate for research funding at the local, state, and federal levels. Because of their reach, professional societies are uniquely positioned to:

1. Create awareness regarding the importance of diversity via their communication platforms.

Trial Leadership Selection Committees

An optimal process for clinical trial executive or steering committee selection should be transparent, competence-based, and bias-free. It should offer equal opportunity, reflect commitment to diversity and excellence, facilitate justice, and leave no room for tokenism. It should include reporting the names, institutional affiliations, and demographics of steering committee members in the associated publication.

Research teams and organizations that are in a position to select clinical trial leaders and executive committee members must change the selection process from "gender unequal" to "gender transformative" [START_REF] Whitelaw | Flattening the hierarchies in academic medicine: the importance of diversity in leadership, contribution, and thought[END_REF] (Table 3). We propose an approach adopted from strategies that achieve gender equity in the workplace. [START_REF] Who | WHO Gender responsive assessment scale: Criteria for assessing programmes and policies[END_REF] (Figure 3).

Step 1. Responsible organizations must understand why it is necessary to address the gender inequality in RCT leadership and commit to making changes to the culture that normalizes sexism.

Step 2. Organizations should perform gender analysis of a) clinical trial leadership; b) application to filled ratios for trial leadership; and c) the composition of the selection committee.

Trial sponsors should set measurable targets and track progress towards gender equality.

Step 3. Standardized selection processes for trial executive committees should include transparent selection criteria based on competence and expertise; open calls for applications; selection committees trained in implicit bias and committed to diversity; blinded reviews of candidates; safeguards against exclusivity; and management of conflicts of interest. These processes can be included in trial design publications. [START_REF] Williams | National hiring experiments reveal 2:1 faculty preference for women on STEM tenure track[END_REF][START_REF]Convention on the elimination of all forms of discrimination against women[END_REF] Step 4. Equity-based organizational policies allocate resources to match need and acknowledge the historic preferential treatment of some demographic groups over others. [START_REF] Who | WHO Gender responsive assessment scale: Criteria for assessing programmes and policies[END_REF] These policies should purposefully aim for gender as well as racial and ethnic diversity. Sponsors should uphold the principle of equal pay for equal work across demographic groups and report remuneration schemes in a transparent manner. [START_REF] Rose | Gender fifferences in physicians' financial ties to industry: A study of national disclosure data[END_REF] Step 5. Intentional strategies to increase the representation of women and underrepresented groups in trial leadership should be structured, not symbolic, and include a wide outreach to universities, academic societies, journals, funders, and research networks. Sponsors along with societies and organizations should collaborate to create databases of women who have methodologic expertise, experience, and interest in clinical trial leadership.

Step 6. A clear pathway to trial leadership should be outlined so that diverse researchers have the opportunity to meet prerequisite steps such as methodologic or regulatory training and grant funding applications. The same prerequisites should be required from all candidates.

Industry Sponsors

A majority of cardiovascular trials are funded by the pharmaceutical industry and led by men. [START_REF] Whitelaw | Characteristics of heart failure trials associated with under-representation of women as lead authors[END_REF][START_REF] Chopra | Industry funding of clinical trials: Benefit or bias?[END_REF] Among trials of heart failure published in high impact journals, pharmacotherapeutic interventions are independently associated with under-representation of women as first authors; and there is a numeric association between industry funding and under-representation of women as first authors. [START_REF] Whitelaw | Characteristics of heart failure trials associated with under-representation of women as lead authors[END_REF] After controlling for key factors, women physicians receive significantly lower compensation for similar activities compared to men, with 75% of industry payments made to men physicians. [START_REF]Catalyst. Why diversity and inclusion matter[END_REF] These suggest that industry sponsors are "gender-unequal", reinforcing the systemic sexism that hold women back. [START_REF] Whitelaw | Flattening the hierarchies in academic medicine: the importance of diversity in leadership, contribution, and thought[END_REF] The processes in industry-initiated trials that may indirectly favour men as PIs over women must also be questioned. The financial viability of companies relies on trial leaders and their message to the scientific community. Clinician-scientists are paid as consultants to speak on behalf of industry, are offered shares or paid positions on advisory boards, and are often offered unrestricted funds that can be applied toward educational or research initiatives. [START_REF] Chopra | Industry funding of clinical trials: Benefit or bias?[END_REF] These incentives may undermine objectivity, introduce bias to the study design and inferences, and indirectly disfavour women, who may be less likely to "toe the line" and therefore seen as undesirable forces to lead the trial. Research in this area is lacking but warranted, particularly given the associations between lack of diversity on industry executive boards and suboptimal corporate governance. [START_REF] Rose | Gender fifferences in physicians' financial ties to industry: A study of national disclosure data[END_REF] Industry research sponsors should implement solution in a timely manner to end gender, racial, and ethnic inequalities (Table 4). Industry-academia collaborations should be contingent on academic medical centers supporting workplace diversity, and on academic investigators assuming independent responsibility for the design, conduct, analysis, and reporting of clinical trials. Industry A new model of industry collaboration based on competence, critical thinking, innovative trial designs, and merit-based leadership will make for better science and greater inclusivity.

Effective initiatives to encourage diversity will require the establishment of metrics that ensure continuous and meaningful progress (Tables 4,5). Sponsors should publicly announce their own policies, systemic changes, and goals to address and eliminate inequity in clinical trial leadership. They should suggest remedial processes and surrogate measures (e.g., proportion of trial leadership positions filled after open calls) to close gaps when the desired diversity goals are not achieved.

Beyond actions to close the gender gap in research environments, industry sponsors may do well to close the gender gaps on their own boards. In addition to the established benefits of gender diversity on company boardsgreater financial performance, fraud prevention, higher-quality audits, greater environmental and social stewardship [START_REF] Rose | Gender fifferences in physicians' financial ties to industry: A study of national disclosure data[END_REF] -gender diversity among industry leaders and board members may increase gender diversity among those granted the opportunity to lead industrysponsored research.

Grant Funding and Federal Agencies

National funding agencies such as the United States National Institutes of Health (NIH), the Canadian Institutes of Health Research (CIHR), the United Kingdom National Institute for Health Research (NIHR), the German Research Foundation (DFG), and others have adopted equity, diversity, and inclusion (EDI) statements to advance the cause of women and members of underrepresented communities in clinical trial leadership. The NIH recognizes that its mission depends upon scientists from diverse backgrounds. [START_REF]Notice of NIH's interest in diversity[END_REF] Similarly, CIHR aims to increase the diversity of grant review teams and governance committees, and to increase equitable access to funding opportunities among researchers. [START_REF]Tri-Agency statement on equity, diversity and inclusion[END_REF] NIHR supports diversity and inclusion in the people who lead the research they fund, [START_REF]Promoting equality, diversity and inclusion in research[END_REF] and the DFG requires that reviewers of research funding applications take into consideration researcher diversity. [START_REF] Dfg | DFG Emphasises importance of sex, gender and diversity to research projects[END_REF] The Food and Drug Administration (FDA) considers increasing diversity of patients enrolled in clinical trials as a priority, but trial leadership choices remain under the purview of the study sponsors. However, since the FDA is highly interested in sex-specific data, it behooves study sponsors to pursue approaches that increase gender diversity in trials, including enlisting more women PIs. [START_REF] Whitelaw | Trial characteristics associated with under-enrolment of females in randomized controlled trials of heart failure with reduced ejection fraction: a systematic review[END_REF] Grant agencies should diversify their selection and leadership committees, and report the demographic composition of their study sections, grant applicants and awardees. They should establish charters with academic institutions to drive the cultural change required to remove systemic barriers that are placed in front of women and underrepresented groups in the research environment.

The major Canadian federal grant agencies have adopted such a charter program, through which academic institutions report on and address systemic roadblocks associated with multiple dimensions of diversity, including age, education, parental status/responsibility, immigration status, religion, language, ethnicity, and socio-economic status. [START_REF]Dimensions: equity, diversity and inclusion Canada[END_REF] Journal Editorial Teams Scientific journals are gate keepers for the publication of clinical trials and can prioritize diversity through policies that influence all aspects of journal content. [START_REF] Van Spall | Medical publishing under review[END_REF] Journals can give a voice to stakeholders otherwise underrepresented through the stipulation of diversity among research teams and invitations for themed issues, viewpoints, and editorials. In so doing, journal editorial teams have the unique opportunity to provide a platform to the next generation of clinical trialists.

Editorial leadership teams should comprise individuals who represent the journal's readership, but from 1998 -2018, there was only 1 woman editor-in-chief of an American or European general cardiology journal. [START_REF] Balasubramanian | Women representation among cardiology journal editorial boards[END_REF] While there has been an increase in the representation of women, editorial boards should also aim for diversity in age, race, ethnicity, and geography.

Journals should rely on open calls and competence-based criteria rather than informal networks to select editorial board members and reviewers, an intervention that may facilitate diversity and excellence instead of tokenism. [START_REF] Van Spall | Medical publishing under review[END_REF] Women represent a minority of first and last authors in cardiovascular research publications, [START_REF] Ouyang | Association between female corresponding authors and female co-authors in top contemporary cardiovascular medicine journals[END_REF][START_REF] Whitelaw | Characteristics of heart failure trials associated with under-representation of women as lead authors[END_REF] and blinded reviews are associated with selection of manuscripts based on quality rather than name recognition. [START_REF] Van Spall | Medical publishing under review[END_REF] Reviewers are more likely to select submissions of the same gender and country as themselves, especially when the reviewers are men. [START_REF] Van Spall | Medical publishing under review[END_REF] Journals are called to institute blinded and more diverse reviewers to increase diversity among published authors. Journals must constantly monitor their peer review processes to eliminate gender bias.

A Plan for Change

We must adopt transformative strategies to harness the strengths of women as cardiovascular clinical trial leaders and deconstruct the sexism that has been normalized in the clinical trial enterprise. [START_REF] Lundine | Gender bias in academia[END_REF][START_REF] Who | WHO Gender responsive assessment scale: Criteria for assessing programmes and policies[END_REF][START_REF] Williams | National hiring experiments reveal 2:1 faculty preference for women on STEM tenure track[END_REF][START_REF]Convention on the elimination of all forms of discrimination against women[END_REF] For far too long, recruitment, advancement, mentorship, and sponsorship have followed gender lines, with decisions generally made by men for men. The research enterprise, which propels men into positions of leadership, must be transformed by multi-level actions (Central Illustration) into one that provides equal opportunity to women who have the skillset, funding, and capacity to lead clinical trials but don't fit the prevailing face of leadership.

The clinical trial enterprise must be restructured so that it includes a broader range of ideas, voices, and interests. The recalibration efforts must address the structure and processes that lead to the gender gap. Meaningful gender-disaggregated measurements (Table 5) at the level of academic institutions, trial sponsors, national societies, and journals will allow us to track progress as we move to close the gender gap in cardiovascular clinical trial leadership. This goal can be realized through a plan (Figure 4) that includes intentional actions, repeated measurements, and cycles of change until equality is achieved.

Main Messages of the Position Statement:

 Women are under-represented in the leadership of high profile, multicenter cardiovascular clinical trials.

 Diversity in clinical trial leadership is associated with more diverse trial participants, which improves the generalizability of results and analysis of treatment interactions.

 Systemic sexism must be overcome to enhance the representation of women in cardiovascular practice and trial leadership.

 Deliberate action and monitoring of key metrics by individual investigators, academic institutions, professional societies, industry sponsors, funding agencies, and scientific journals are needed to overcome gender inequalities in cardiovascular clinical trial leadership.

FIGURES Central Illustration. A Roadmap to Close the Gender Gap in Clinical Trial Leadership

Multi-level actions can transform the research enterprise to provide equal opportunity to women and men who have the capacity to lead clinical trials.

Figure 2. How Professional Societies Can Diversify Clinical Trial Leadership

Professional societies can build research capacity in diverse groups, make grant funding contingent on diverse trial leadership, and advocate for funders and journals to make diversity a priority. A plan comprising intentional actions, gender-disaggregated measurements, and cycles of change can be adopted by academic institutions, industry sponsors, grant funding agencies, national societies, and journals.

TABLES Table 1. Clinical Trial Leadership Competencies Table 2. Strategies for Academic Institutions to End the Gender Inequality in Research Leadership Table 3. Gender Responsive Assessment Scale*: Criteria for Assessing Programs and Policies

*Adapted from WHO Gender Responsive Assessment Scale46 . This approach can also be adapted for other kinds of diversity, including age, ethnicity, race, culture, immigration status, parental status.

Table 4. Strategies for Industry Sponsors to End the Gender Inequality in Clinical Trial Leadership Table 5. Metrics to Access Gender Equality in Research Leadership

2 .

 2 Provide education that increases research knowledge among diverse learners. 3. Make grant funding contingent on diverse trial leadership and advocate for similar contingencies by other funders. 4. Encourage society-associated journals to advocate for diversity in trial leadership and publications. 5. Provide a resource database of potential trial leaders from under-represented groups. 6. Ensure diversity in societal leadership. The American College of Cardiology (ACC) has launched the Clinical Trials Research: Upping Your Game for its members [40] and the European Society of Cardiology (ESC) has launched a Master of Science program in clinical trials. [41] Participants in the ACC program learn clinical trial methodology; network with clinical trialists, regulatory stakeholders, and industry leaders; and develop personal career plans. The program is intended to increase the proportion of individuals historically underrepresented in cardiology and in trial leadership. The ESC program has similar goals. The American Heart Association has launched a "Health Equity for All" initiative, withresearch programs focused on solving health inequities and increasing diversity among cardiovascular leaders.[START_REF]Diversity & Inclusion[END_REF]

  sponsors can facilitate greater diversity in research leadership by engaging women and those from under-represented groups in mentorship and sponsorship programs. 'Trialist' career opportunities can be marketed broadly so that qualified individuals with new voices can serve on steering, data safety/monitoring, and adjudication committees. Fellowship programs in clinical trials can facilitate merit-based progression to positions of research leadership. Collaborations with catalyst organizations that focus on diversity can help discover new voices, ideas, and leaders with the potential to improve industry-initiated research.

Figure 1 .

 1 Figure 1. Benefits of Diverse Clinical Trial Leadership

Figure 3 .

 3 Figure 3. Proposed Steps for Selection of Trial Executive and Steering Committees

Figure 4 .

 4 Figure 4. A Five-Year Plan to Close the Gender Gap in Clinical Trial Leadership

Figure 1 .

 1 Figure 1. Benefits of Diverse Clinical Trial Leadership. Diverse clinical trial leaders are associated with diverse clinical trial participants and research mentees.

Figure 2 .

 2 Figure 2. How Professional Societies Can Diversify Clinical Trial Leadership. Professional societies can build research capacity in diverse groups, make grant funding contingent on diverse trial leadership, and advocate for funders and journals to make diversity a priority.

Figure 3 .

 3 Figure 3. Proposed Steps for Selection of Trial Executive and Steering Committees. An optimal process for selection of trial executive committee should be transparent, competence-based, and biasfree. It should reflect commitment to both diversity and excellence.

Figure 4 .

 4 Figure 4. A Five-Year Plan to Close the Gender Gap in Clinical Trial Leadership. A plan comprising intentional actions, gender-disaggregated measurements, and cycles of change can be adopted by academic institutions, industry sponsors, grant funding agencies, national societies, and journals.

Volgman: Novartis, MSD/Bayer Global Advisory Board Member, Bristol Myers Squibb Foundation Diverse Clinical Investigator Career Development Program National Advisory Committee Walsh: None Zannad: Steering committee personal fees from Applied Therapeutics, Amgen, Bayer, Boehringer, Novartis, Janssen, Cellprothera and CVRx, advisory board personal fees from, AstraZeneca, ACC -American College of Cardiology CIHR -Canadian Institutes of Health Research DFG -German Research Foundation EDI -Equity, Diversity and Inclusion ESC-European Society of Cardiology FDA -Food and Drug Administration NIH -National Institutes of Health NIHR -National Institute for Health Research PI -Principal Investigator


Rodrigo Osuna-Orozco

Nathanaël Machicoane

Peter D Huck

Alberto Aliseda

Nathanael Machicoane

Effect of electrostatic forcing on coaxial two-fluid atomization

Keywords: two-fluid coaxial atomization, electrostatic atomization, spray, primary breakup

come

I. INTRODUCTION

Multiphysics control of atomization is a promising new area of research that could result in more robust and versatile atomizers that can produce the desired range of droplets under very diverse operating conditions, enabling their use in applications that require the atomization process to operate within tight bounds under the influence of a changing environment and external perturbations (e.g., as it may occur in the combustion chamber of a jet engine onboard an aircraft or in a coating process with heat and mass transfer and phase change). Although a very wide range of atomizers has been designed for various applications, the spray characteristics produced are typically determined by the design itself and offer limited controllability in the presence of perturbations. These could be environment changes, variability of the liquid properties, of the injected flow rate, etc. leading to fluctuations in the produced droplet size and number density that could drift outside acceptable spray properties. The development of robust and versatile closed-loop control a) Electronic mail: roo3@uw.edu strategies for atomization remains a fundamental challenge, and including multiphysics actuation (e.g. electrostatics, acoustics, swirl, etc.) may offer a possible solution for this fundamental problem or for specific applications [START_REF] Arai | The Possibility of Active Attitude Control for Fuel Spray[END_REF][START_REF] Murugappan | Optimal control of a swirl-stabilized spray combustor using system identification approach[END_REF][START_REF] Osuna-Orozco | Feedback control of coaxial atomization based on the spray liquid distribution[END_REF][START_REF] Osuna-Orozco | Feedback control of the spray liquid distribution of electrostatically assisted coaxial atomization[END_REF] .

Although both electrostatic and coaxial atomization have each been studied extensively [START_REF] Eggers | Physics of liquid jets[END_REF][START_REF] Grace | A review of liquid atomization by electrical means[END_REF][START_REF] Aliseda | Atomization of viscous and non-newtonian liquids by a coaxial, high-speed gas jet. Experiments and droplet size modeling[END_REF][START_REF] Kumar | Liquid jet breakup unsteadiness in a coaxial air-blast atomizer[END_REF][START_REF] Lasheras | Break-up and atomization of a round water jet by a high-speed annular air jet[END_REF][START_REF] Lasheras | Liquid Jet Instability and Atomization in a Coaxial Gas Stream[END_REF][START_REF] Dumouchel | On the experimental investigation on primary atomization of liquid streams[END_REF] , atomization that combines both a high-speed coaxial gas jet and electrostatic forces remains relatively unexplored, especially at high gas-to-liquid momentum ratios [START_REF] Li | Temporal instability of coflowing liquid-gas jets under an electric field[END_REF][START_REF] Si | Modes in flow focusing and instability of coaxial liquid-gas jets[END_REF] . Most of the work exploring the combination of electrostatic atomization and a coflowing gas has been done from the perspective of the electro flow-focusing (EFF) technique introduced by Gañán-Calvo et al. [START_REF] Gañán-Calvo | The combination of electrospray and flow focusing[END_REF][START_REF] Gañán-Calvo | Electro-Flow Focusing: The High-Conductivity Low-Viscosity Limit[END_REF] . This technique has seen very fruitful applications for mass spectrometry [START_REF] Forbes | Primary and secondary droplet and charge transmission characteristics of desorption electro-flow focusing ionization[END_REF][START_REF] Forbes | Mass spectrometry detection and imaging of inorganic and organic explosive device signatures using desorption electro-flow focusing ionization[END_REF] and in the context of microfluidics involving two liquid phases [START_REF] Castro-Hernández | AC electrified jets in a flow-focusing device: Jet length scaling[END_REF][START_REF] He | Low-frequency ac electro-flow-focusing microfluidic emulsification[END_REF][START_REF] Vilanova | Drop size control in electro-coflow[END_REF] . Allaf-Akbari and Ashgriz 21 explored atomization regimes of electrospray in gaseous crossflow, involving either stable or unstable Taylor cone jets where the main atomization mechanism is electrostatic. Notably, research has focused mostly on situations where the mass flow rate and Reynolds number of the liquid jet are very small (amenable to the aforementioned applications). The current work explores a substantially different regime where atomization is mostly achieved by the aerodynamic stresses at the interface. This regime of electrostatically-assisted coaxial atomization is useful to introduce additional actuation capabilities for control of two-fluid atomization.

In this paper, we present experimental observations of the primary breakup of an electrically-charged laminar liquid jet by a turbulent coaxial gas flow and an external electric field. Section II describes the experimental setup and experimental methods, section III summarizes the results and section IV presents the discussion of the results and conclusions.

II. METHODS

A. Atomizer

We studied the atomization of an electrically charged laminar liquid jet exposed to a turbulent coaxial gas stream for gas-to-liquid momentum ratios ranging from 1 to 25. We define the momentum ratio, M , as the ratio of the dynamic pressure of the gas to that of the liquid, M = ρ g U 2 g /ρ l U 2 l , where ρ is the density and U the mean velocity computed as the volume flow rate divided by the cross-sectional area of the nozzle, and the subscripts indicate the gas and liquid phases. The working liquid was reverse-osmosis purified water with a conductivity of 3µS/cm and the gas was standard air. The canonical coaxial atomizer (see Fig. 1) used for this study is available in open-source [START_REF]The atomizer design is made[END_REF] . The design, fabrication and operational aspects have been described in detail in previous work [START_REF] Machicoane | Synchrotron radiography characterization of the liquid core dynamics in a canonical two-fluid coaxial atomizer[END_REF] . Figure 1 shows a schematic for the atomization setup.

B. Electric Field Geometry

A strong electric field approaching the dielectric strength of air was established by connecting the metal liquid nozzle to a high voltage power supply at an electric potential of -5kV and grounding a copper ring at the perimeter of the gas nozzle (see Fig. 1). Thus, the liquid droplets issuing from the atomizer were negatively charged. To shape the geometry of the electric field in the atomization region, a pair of metallic plates running along the spray axis were either grounded or set to a potential of -5 kV (equal to that of the liquid nozzle). Electrical arcing was observed to occur for all conditions when the voltage reached -6 kV.

Figure 2 illustrates the geometry of the electric field in the configurations explored, as obtained from finite element simulations. When the plates are grounded, the electric field is predominantly radial (Fig. 2, left), whereas when the plates are at the same potential as the liquid nozzle the electric field has a large axial component (Fig. 2, right). For these simulations we assumed a straight liquid jet of a length that's relevant to the range of M studied. The non-dimensional group describing the ratio of electrostatic to inertial forces, the electrical Euler number [START_REF] Li | Temporal instability of coflowing liquid-gas jets under an electric field[END_REF] , was estimated as

Eu = ϵ g E 2 0 /ρ l U 2 l ≈ 10 -2
, where ϵ g is the permittivity of air and the electric field strength E 0 is given by the voltage drop and the distance between the nozzle and the grounded ring. For all the explored conditions, the momentum ratio was at least two orders of magnitude larger than the electric Euler number, indicating a dominance of the aerodynamic shear forces over the electrostatic forces.

C. High-Speed Imaging

High-speed shadowgraphy imaging with a temporal resolution of 100 µs (10,000 frames per second) was used to identify the liquid core and the resulting droplets. The spatial resolution of the imaging was 91.3 or 27.3 microns per pixel, where the lower resolution was utilized for the lower momentum ratio conditions. Image postprocessing identified the presence of liquid along the line of sight of the camera following methods described previously [START_REF] Machicoane | Influence of steady and oscillating swirl on the near-field spray characteristics in a two-fluid coaxial atomizer[END_REF] . Figure 3 illustrates the liquid core as delineated digitally from the images, with the intact length L b defined as the farthest distance downstream from the nozzle where there is liquid continuously connected to the issuing liquid jet. Similarly, the resulting droplet population was characterized based on identification of approximately circular objects in the image. Only detected objects whose perimeter was less than 2π times their equivalent diameter (where the equivalent diameter is computed from the objects' projected area on the image) are used in the analysis as valid droplets.

The spray angle and virtual origin (the point where linear projections of the points defining the width of the jet converge, see Fig. 3) were obtained following the method proposed by the authors in previous work [START_REF] Machicoane | Influence of steady and oscillating swirl on the near-field spray characteristics in a two-fluid coaxial atomizer[END_REF] . The liquid core length was obtained from the mean image intensity map using a threshold to identify the region always occupied by liquid.

The entrainment speed, u e , and wavelength, λ, of the initial interfacial instabilities were computed by tracking of local maxima of the gas-liquid interface radial position. The growth rate of the instabilities, ξ/ξ (where ξ is the radial position of the interface relative to the unperturbed state), was approximated based on the relation proposed by Villermaux 25,[START_REF] Villermaux | Mixing in coaxial jets[END_REF] (equation 1). The velocities were estimated using tracks of local maxima (similar as done in previous work [START_REF] Ricard | Role of convective acceleration in the interfacial instability of liquid-gas coaxial jets[END_REF] ) that were at least four frames long and that could be fitted with a quadratic expression (assuming nearly constant acceleration) with R 2 > 0.99.

u e ≈ λ ξ ξ (1)
Proper orthogonal decomposition (POD) from the images was performed by an implementation of the singular value decomposition analysis. For the purpose of identifying the dynamics of the liquid core, POD was applied to the images after applying a mask that removed drops and ligaments. To derive the empirical dispersion relation, a 2D Fourier transform on all POD modes, and a 1D Fourier transform on their corresponding time vectors, was performed. For robustness, a PSD-weighted mean of the temporal frequency and spatial wavelength was computed. Thus, we obtained unique frequency-wavelength pairs for each POD mode (Fig. 4).

III. RESULTS

A. Liquid core lengths

Figure 5 shows snapshots of atomization at three momentum ratios with (right) and without (left) electric field. The snapshots show the electrostatic effect for the electric field with a stronger axial component attained for the charged plates configuration. The influence of the electric field is readily apparent in the increased curvature and decreased intact length of the liquid core.

The radial electric field results in significantly shorter intact lengths at all momentum ratios explored, as opposed to the axial electric field that leaves the intact length approximately unchanged (Fig. 6). The mean liquid core length reveals the same trend as the intact length indicating a clear reduction due to the axial electric field across all momentum ratios explored 6. For the case of the radial electric field, there is a smaller reduction in the liquid core length for the two smallest momentum ratios. The observed liquid core lengths (especially at low M) are in good agreement with the expression derived by Lasheras and Hopfinger 10 :

L c d l ≈ 6 √ M 1 - U l U g -1 (2)
Interestingly, Figure 7 shows that the standard deviation of the liquid length remains relatively unaffected by the electric field, whereas the skewness is strongly affected by the axial electric field. All the values of the kurtosis are close to 3 (the kurtosis of a normal distribution) indicating that the distributions do not have very broad tails (Fig. 7).

In addition to the intact length, we characterized the mean liquid cone geometry, in terms of the spreading angle and virtual origin (Fig. 8). For low momentum ratios, enhanced atomization, due to the axial electric field, leads to a much larger spreading angle. In contrast, for larger momentum ratios, the spreading angle is noticeably smaller whenever the electrostatic forces tend to repel the droplets away from the plates (i.e. for the case when the plates are held at the same potential as the liquid nozzle).

B. Liquid core frequencies

We measured two key aspects (and their corresponding frequencies) of the liquid motion that are critical to understanding the break-up dynamics: the flapping frequency and the shedding frequency. The flapping frequency can be detected by the transversal variations of the light intensity in the liquid core region, whereas the shedding frequency can be identified from the time series of the intact length. Determination of these frequencies from the power spectra of the aforementioned time series has been described in detail in previous work [START_REF] Machicoane | Influence of steady and oscillating swirl on the near-field spray characteristics in a two-fluid coaxial atomizer[END_REF][START_REF] Kaczmarek | Spatial characterization of the flapping instability of a laminar liquid jet fragmented by a swirled gas co-flow[END_REF] . As shown in figure 9, no significant impact of the electrostatic field or the droplets electric charge on the flapping frequency was observed. In contrast to the flapping frequency, the shedding frequency does get modified by the electrostatic forcing (Fig. 9). The shedding frequency was observed to increase for all momentum ratios due to electrostatic forcing in the grounded plates configuration (predominantly radial electric field).

The ratio of the shedding to the flapping frequency decreases as a function of momentum ratio and it appears to have an asymptotic value of one half. This reveals a differential effect of the hydrodynamic instabilities of the liquid-gas interface on the shedding dynamics and consequently on the intermittency of the droplet generation. Furthermore, the characteristic frequencies computed from the time series of the number of drops crossing the edge of the image, shown in figure 9 c, matched well with the liquid core shedding frequency.

C. Empirical Dispersion Relations

In order to investigate whether the electrostatic forcing modified the dynamics of smaller scale features of the primary breakup, we computed empirical dispersion relations (Fig. 10). As expected for all the conditions, longer wavelengths (smaller wave numbers) corresponded to lower frequencies. More interestingly, the empirical dispersion relations clearly captured the faster dynamics prevalent at all length scales as the momentum ratio increased. Moreover, for the momentum ratio of 1, the empirical dispersion relation did reveal that the axial electric field resulted in faster dynamics at smaller scales. However, at higher momentum ratios, no configuration of the electric field resulted in noticeable changes in the dispersion

relations.

D. Initial Interfacial Instabilities

Along with the global characterizations of the spray, we explored the dynamics of the initial interfacial disturbances by tracking their position to evaluate their velocities, as well as their characteristic wavelengths. The entrainment velocity clearly increases with momentum ratio but appears unaffected by the electric field (Fig. 11). The entrainment velocity was observed to be proportional to the square root of the momentum ratio, consistent with the work of Villermaux and coauthors [START_REF] Villermaux | Mixing and spray formation in coaxial jets[END_REF][START_REF] Villermaux | Mixing in coaxial jets[END_REF] , whereby u e ≈ λ ξ/ξ ∝ U g .

In contrast, the wavelength of these interfacial disturbances follows the same trend as the core length, with the charged plates configurations exhibiting systematically shorter wavelengths than the other two configurations. These two trends result in estimated growth rates that are always faster in the presence of the axial electric field.

E. Droplet Population Characterization

Finally, we performed optical identification of drop sizes in order to compute drop size distributions. As seen in figure 12, there are small differences in the tails of the mean diameter distributions, indicating a smaller fraction of large drops in the presence of the axial electric field. The differences in the volume-weighted distributions are more noticeable and clearly show that smaller drops are a larger proportion of the total spray volume flux for the axial electric field case.

The small difference in the arithmetic drop distribution seen above was observed through most experimental conditions (except at the lowest momentum ratio) and can be expressed in terms of the arithmetic mean diameter (Fig. 13). Higher moments of the drop size distribution, however, were smaller in the presence of the axial electric field as compared with the other two configurations. In particular, the d 43 values for the radial and axial electric field configurations were about 2% and 14% smaller than the values without electrostatic forcing, respectively, over the whole range of M explored here.

IV. DISCUSSION

Observations of the liquid core dynamics of coaxial atomization under electric field revealed that electric fields can significantly alter the wavelength and growth rate of interfacial instabilities, as well as notably shorten the liquid core lengths and reduce the mean drop diameters. This effect was observed to persist up to much higher momentum ratios than had been explored before in electrostatic atomization, where the ratio of electrostatic to aerodynamic forces (as estimated from the electrical Euler number and the momentum ratio) was well below unity (about 10 -3 ). These observations are consistent with linear stability analysis in the literature [START_REF] Li | Temporal instability of coflowing liquid-gas jets under an electric field[END_REF] , whereby an electric field in the presence of surface charge increases the growth rate and shortens the wavelength of the interfacial instabilities. The entrainment velocity of the interfacial corrugations was observed to be proportional to the gas velocity in agreement to the theory developed by Villermaux and coauthors [START_REF] Villermaux | Mixing and spray formation in coaxial jets[END_REF][START_REF] Villermaux | Mixing in coaxial jets[END_REF] . Interestingly, the entrainment velocity appeared to be unaffected by the electrostatic forcing. This suggests that the axial acceleration of interfacial corrugations was dominated by the aerodynamic stresses, a result that is consistent with the small ratio of Euler number to momentum ratio.

Moreover, the enhanced growth rate and shorter characteristic wavelengths of the interfacial instabilities due to electrostatic forcing showed a strong dependence on the electric field geometry for similar field strength. Notably, an electric field with a stronger axial component (and resulting electrostatic stresses in a direction opposite the flow) resulted in a greater enhancement of the interfacial instability growth rate and reduction of their wavelength. A substantial shortening of the liquid core length consistent with the enhanced growth rate of the instabilities was also observed.

Pertaining to the initial growth of interfacial instabilities, a shortcoming of the present work is that helical modes cannot be adequately characterized by two-dimensional shadowgraphy data. As indicated by earlier work [START_REF] Li | Temporal instability of coflowing liquid-gas jets under an electric field[END_REF][START_REF] Si | Modes in flow focusing and instability of coaxial liquid-gas jets[END_REF] , electrostatically-assisted coaxial atomization presents strong helical modes in addition to the axisymmetric ones.

In contrast with what was observed for the initial interfacial instabilities and liquid core lengths, some of the more global metrics reveal little impact of the electrostatic forcing on the primary breakup dynamics. In particular, there was no discernible effect of the electrostatic forcing on the flapping frequencies, or on the characteristic frequency of the drop crossing times. A slight increase in the shedding frequency was observed for all momentum ratios under the effect of the predominantly radial electric field. This is hypothesized to be caused by enhanced radial acceleration of the liquid core, resulting in faster ligament shedding.. Moreover, while the empirical dispersion relations clearly separate the dynamics for different momentum ratios, they only show noticeable change under the axial electric field at very low momentum ratios (of order unity). These observations suggest that the overall breakup dynamics are still dominated by aerodynamic forces.

A sizeable reduction in the mean drop diameters was observed under the axial electric field configuration. In particular, the reduction of the mean droplet diameter was largest for the higher moments of the distribution (volume-weighted or volumetric flux-weighted) indicating a difference caused by the electric field in the reduction of the largest drops formed by inefficient aerodynamic atomization. The histograms of the droplet diameter clearly confirm this effect: a deficit in the number of the largest drops in the distributions. As shown by earlier work [START_REF] Aliseda | Atomization of viscous and non-newtonian liquids by a coaxial, high-speed gas jet. Experiments and droplet size modeling[END_REF] , the mean diameter of the resulting droplet population is proportional to the wavelength of the interfacial instabilities, a conclusion consistent with the observations presented in this work. The increase in the growth rate of the surface instabilities due to Maxwell stresses along with the unaffected entrainment velocity results in smaller structures that detach from the liquid core at a higher frequency; this, in turn results in a reduction of the droplet size in the ensuing spray.

V. CONCLUSIONS

Electrostatic forcing is shown to noticeably impact the primary atomization in a coaxial atomizer, especially if the electric field is predominantly radial at the liquid nozzle exit. Our observations are consistent with the presence of Maxwell stresses that accelerate the liquid jet towards the turbulent gas stream, which would lead to an increased efficiency in the kinetic energy transfer from the gas and the liquid. Furthermore, these effects persist at higher momentum ratios than had been studied in the past. The Maxwell stresses, resulting from an electric field with strong axial component, were observed to shorten the wavelength of the interfacial instabilities, leading to shorter liquid core lengths and smaller droplet diameters. In contrast, an electric field with a radial component of similar strength, but smaller axial component, did not have such an impact on the length scale of the interfacial instabilities. More research is necessary to optimize the electric field characteristics that maximize the mechanical coupling between the phases, for improved atomization.

12 . 7 FIG. 1 .

 1271 FIG.1. Side cut and front view of atomizer showing relevant dimensions; d l is the liquid nozzle inner diameter, D l is the liquid nozzle outer diameter and dg is the gas nozzle inner diameter. The metallic plates are 8"x8" and 5 inches apart. The gas nozzle is made of plastic whereas the liquid nozzle is made of aluminum.

FIG. 2 .

 2 FIG.2. : Finite Element Method (FEM) simulation illustrating the voltage and electric field lines for the two cases considered: grounded plates (left) and charged plates (right). Notice that when the plates are grounded the electric field is predominantly radial, whereas when the plates are charged the electric field has a strong axial component over a broader region.

FIG. 3 .

 3 FIG.3. Metrics used to characterize the spray morphology. Wavelength, λ, of interfacial instabilities and instantaneous liquid intact length shown for a single snapshot(left). Metrics derived from the mean intensity field (right) included the virtual origin (VO), spreading angle (SA), and the liquid core length.

8 .

 8 FIG.4. POD modes (left), shown along with their 2D FFT (middle) and FFT of the corresponding time vector (right) for modes #1 (top) and #22 (bottom). Higher order modes contain shorter wavelengths in space and higher frequencies in time.

FIG. 5 .

 5 FIG.5. Snapshots of the liquid core at momentum ratios of 1, 2 and 5 (Ug = 15, 23, 35 m/s), with no electric field, Eu = 0, (left) and with the axially-oriented electric field, Eu = 10 -2 , (right) attained in the charged plate configuration. Notice the higher curvature and shorter intact length for the case with the electric field.

L&H 2000 FIG. 6 .

 20006 FIG. 6. (left) Probability density function (PDF) of intact length for M=11. Only the radial electric field has a substantial effect in reducing the intact length. (middle) Mean intact length as a function of momentum ratio. (dashed lines are power laws fits, displaced by a constant). (right) Liquid core length as a function of M . The black line is the expression proposed by Lasheras and Hopfinger 10 . The fits are all exponentials plus a constant with R 2 > 0.95 .

FIG. 7 .

 7 FIG.7. First four statistical moments of the liquid intact length.

FIG. 9 .

 9 FIG. 9. : Flapping (top left), shedding (top right) and drop crossing (bottom left) frequencies as a function of momentum ratio. Error bars indicate the spectral resolution of the Fourier transform used to compute the Welch's power spectral density estimate. The ratio of the shedding to the flapping frequency (bottom right) decreases as a function of momentum ratio to an asymptotic value of 0.5 (error bars in the ratios are derived from the error bars in the frequencies).

FIG. 10 .

 10 FIG. 10. Empirical dispersion relation. As a function of M (top) and electric field for M=1 (middle) and M=5 (bottom).

FIG. 11 .FIG. 12 .

 1112 FIG.11. Entrainment velocity normalized by the mean liquid injection velocity, U l , along with a linear fit to the square root of the momentum ratio (left), wavelength of interfacial disturbances (middle) and growth rate of disturbances (right). The error bars are the standard error of the mean.

FIG. 13 .

 13 FIG. 13. Drop mean diameters, (top-left) d10 arithmetic mean diameter;(top-right) d10 volume mean diameter;(bottom-left) d32 Sauter (area-weighted) mean diameter;(bottom-right) d43 De Brouckere (volume-weighted) mean diameter.

ACKNOWLEDGMENTS

This work was sponsored by the Office of Naval Research (ONR) as part of the Multidisciplinary University Research Initiatives (MURI) Program, under grant number N00014-16-1-2617. The views and conclusions contained herein are those of the authors only and should not be interpreted as representing those of ONR, the U.S. Navy or the U.S. Government.


Vincent Klapczynski

Dylan Le Maux

Mickael Courtois

Emmanuel Bertrand

Pascal Paillard

Surface tension measurements of liquid pure iron and 304L stainless steel under different gas mixtures

Keywords: surface tension, aerodynamic levitation, oscillating droplet method, stainless steel, Marangoni effect

In this paper, the surface tension of molten pure iron and 304L stainless steel is measured under different gas mixtures (pure argon and Ar-2.5 %vol. H 2 ). Measurements are realized by aerodynamic levitation with an acoustic excitation. The surface tension is measured between 1756 K and 2227 K. The addition of H 2 in the surrounding gas is supposed to create a desorption of oxygen and modify surface tension. The effect of oxygen on surface tension is then analyzed. Measurements show a linear variation of surface tension with temperature for pure iron under Ar-2.5 %vol. H 2 and a nonlinear relationship for the 304L stainless steel even when an active gas is used. This phenomenon can be explained by the short time at high temperature which does not allow oxygen desorption. Increasing the time at high temperature may change the surface tension but at the risk of evaporation of some alloy elements. The intensification of the evaporation would also disturb the surface tension measurements. In this study, the time spent at the liquid state is equivalent to that of welding and additive manufacturing processes, allowing to determine the thermocapillary coefficient (Marangoni effect) for both metals considered in these conditions. The comparison between our measurements and different data are consistent.

Introduction

Surface tension plays a key role in high temperature processes such as GMAW (Gas Metal Arc Welding) or WAAM (Wire Arc Additive Manufacturing). A variation of surface tension and its dependence to temperature may affect the geometry of a welding pool, which is known as Marangoni effect [START_REF] Lu | Marangoni convection and weld shape variations in He-CO2 shielded gas tungsten arc welding on SUS304 stainless steel[END_REF][START_REF] Rajesh Kannan | On the effect of temperature coefficient of surface tension on shape and geometry of weld beads in hot wire gas tungsten arc welding process[END_REF][START_REF] Berthier | Analyse des phénomènes de transfert et couplages physico-chimiques lors de la transformation des métaux sous arc électrique[END_REF][START_REF] Lu | Effects of Oxygen Additions to Argon Shielding Gas on GTA Weld Shape[END_REF]. The surface tension of pure metals decreases with temperature, leading to a negative surface tension gradient (dγ⁄dT < 0) [START_REF] Egry | Surface tension of liquid metals and alloys -Recent developments[END_REF][START_REF] Ozawa | Influence of oxygen partial pressure on surface tension and its temperature coefficient of molten iron[END_REF][START_REF] Mills | Review of surface tension data for metallic elements and alloys: Part 1 -Pure metals[END_REF]. However, the presence of impurities such as oxygen or sulfur can inverse this temperature dependence and lead to a temperature domain with a positive gradient (dγ⁄dT > 0). This temperature range is limited and a transition to a negative gradient, such as in pure materials, is observed at high temperature.

Thus, the variation of surface tension as a function of temperature leads to a "boomerang shape curve" [START_REF] Ozawa | Surface Tension of Molten Silver in Consideration of Oxygen Adsorption Measured by Electromagnetic Levitation[END_REF]. It is shown in literature that oxygen modifies the surface tension gradient and can inverse the convection flows in liquid metals. Several models have been developed to evaluate the dependence of surface tension of liquid metals to temperature and impurities concentration such as oxygen and sulfur [START_REF] Sahoo | Surface Tension of Binary Metal -Surface Active Solute Systems under Conditions Relevant to Welding Metallurgy[END_REF][START_REF] Brillo | Surface tension of liquid Ti with adsorbed oxygen and its prediction[END_REF][START_REF] Gheribi | Modelling the surface tension of liquid metals as a function of oxygen content[END_REF][START_REF] Gheribi | Temperature and oxygen adsorption coupling effects upon the surface tension of liquid metals[END_REF]. These models were developed to estimate the surface tension of a metal in presence of impurity (mainly oxygen or sulfur). Although they were developped for pure metals, they are used for couples alloy/surfactant in numerical simulation to predict liquid alloy motion in welding pool [START_REF] Dong | Modeling of the Weld Shape Development During the Autogenous Welding Process by Coupling Welding Arc with Weld Pool[END_REF][START_REF] Wang | Effects of surface active elements on weld pool fluid flow and weld penetration in gas metal arc welding[END_REF]. A good knowledge of surface tension of metals and alloys is then necessary.

A large amount of surface tension data can be found for pure metals like iron [START_REF] Morohoshi | Surface Tension of Liquid Iron as Functions of Oxygen Activity and Temperature[END_REF], nickel [START_REF] Brillo | Surface tension of nickel, copper, iron and their binary alloys[END_REF] or titanium [START_REF] Brillo | Surface tension of liquid Ti with adsorbed oxygen and its prediction[END_REF]. First measurements were performed by sessile drop method (SD). More recently, containerless techniques such as Electromagnetic Levitation (EML) were developed to avoid substrate contamination [START_REF] Egry | Surface tension of liquid metals and alloys -Recent developments[END_REF]. Even if this technique is accurate, the surfactant concentration cannot be easily controlled. That is why values of surface tension found in litterature are scattered for a given metal.

Only few experiments have been performed on alloys. Recently, Ozawa et al. [START_REF] Ozawa | Influence of Oxygen Partial Pressure on Surface Tension of Molten Type 304 and 316 Stainless Steels Measured by Oscillating Droplet Method Using Electromagnetic Levitation[END_REF], Fukuyama et al. [START_REF] Fukuyama | Thermophysical Properties of Molten Stainless Steel Containing 5 mass % B 4 C[END_REF] and Dubberstein et al. [START_REF] Dubberstein | Surface tension and density data for Fe-Cr-Mo, Fe-Cr-Ni, and Fe-Cr-Mn-Ni steels[END_REF] studied the surface tension of stainless steel by EML. All of them reported that evaporation occurs during an experiment. According to Dubberstein [START_REF] Dubberstein | Surface tension and density data for Fe-Cr-Mo, Fe-Cr-Ni, and Fe-Cr-Mn-Ni steels[END_REF], manganese evaporation in 304L steel during surface tension measurement leads to an overestimation the surface tension. As a pure metal, this element has a lower surface tension than other alloying elements and its presence as alloying elements should lower the surface tension value. A short time at high temperature should be imposed to perform an accurate measurement.

The aim of this work is to measure the surface tension of pure iron and 304L stainless steel by aerodynamic levitation under two gas mixtures: pure argon and argon with 2.5 %vol. H 2 . Dihydrogen is mixed with argon in order to remove traces of oxygen and sulfur at the surface of the liquid metal. Firstly, pure iron is characterized under Ar-2.5 %vol. H 2 in order to confirm the validity of the measurement technique. Then, 304L is characterized under pure argon and Ar-2.5 %vol. H 2 in order to highlight the effect of oxygen under these conditions. The effect of oxygen on surface tension is estimated, discussed and compared with data of literature. The role of the experimental parameters (measurement technique and parameters) is also discussed.

Materials and methods

99.95 % pure iron (supplier: Neyco) and 304L stainless steel (supplier: QUADA V+F) are characterized in this experience.

The composition of 304L stainless steel studied is given in Table 1 45 mg samples are cut from a bulk material. They are melted under controlled atmosphere in the levitation device in order to obtain a sphere shaped sample. Then, a flat area is grinded with SiC papers in order to stabilize the levitation and avoid the rotation of the sample, as shown on Fig. 1.

Surface tension measurements were performed by aerodynamic levitation on a device developed at IRDL -CNRS

Laboratory. Details about the levitation technique and the experimental procedure of this study are available in a previously published work [START_REF] Le Maux | Density measurement of liquid 22MnB5 by aerodynamic levitation[END_REF]. The experiment consists in putting a sample in levitation by using a numerically controlled gas flow (Brooks SLA5800) at 2.4 L min -1 . The chamber is purged twice before the levitation with a primary vacuum pump and filled with the required gas. This procedure ensured a low residual oxygen content (below 5 ppm when using argon). Once in levitation, the sample is heated until melting with a 300 W diode laser (IPG YLR 300/3000 QCW λ = 1070 nm). The laser sequence is constituted of a very short time with a high power in order to melt the sample quickly (1.5 s at 140 W) and a power stabilization stage (for example 5.5 s at 30 W) in order to reach several temperature range, see Fig. 2. Temperature of the sample is measured using a bichromatic pyrometer (Optris CT Ratio 2MH1). The emissivity ratio is calibrated by measuring the solidification temperature of pure iron which occurs at 1811 K. Fourier Transform (FFT) is calculated, see Fig. 4. The spectrum allows to estimate the Rayleigh frequency ν r , for the n=2 mode and then to calculate the surface tension, using Eq. 1 [START_REF] Brooks | Observation of the Effects of Oxide Skins on the Oscillations of Electromagnetically Levited Metal Droplets[END_REF][START_REF] Sansoucie | Effects of Environmental Oxygen Content and Dissolved Oxygen on the Surface Tension and Viscosity of Liquid Nickel[END_REF][START_REF] Seyhan | The Surface Tension of Undercolled Binary Iron and Nickel Alloys and the Effect of Oxygen on the Surface Tension of Fe and Ni[END_REF].

Eq. ( 1) However, the spectrum reveals three frequencies peaks in most of the experiences, or five peaks in some cases see Fig. 4.

These peaks correspond to the ± 2 and ± 1 degenerated modes [START_REF] Cummings | Oscillations of magnetically levitated aspherical droplets[END_REF]. Actual experimental setup does not allow to index these peaks. Because there is a small error and it's easier to measure, only three peaks are used to calculate the Rayleigh frequency when they are observed, as suggested by Millot et al. [START_REF] Millot | High-Temperature Properties of Liquid Boron from Contactless Techniques[END_REF].

Results and discussion

Surface tension of pure iron under Ar-2.5 %vol. H 2

Firstly, the surface tension of pure iron is measured in a range of 368 K (1859 K -2227 K). These results are compared to experiments of literature on Fig. 5 and summarized on Table 2. Measures are in agreement with literature and a linear dependence to temperature is observed. The surface tension gradient is estimated to -1.54 × 10 -4 N m -1 K -1 and the temperature dependence can be approximated by: As shown in table 2, the absolute value of the surface tension gradient is lower than the data from literature. However, the only measure of surface tension of pure iron by ADL [START_REF] Wille | Thermophysical Properties of Containerless Liquid Iron up to 2500 K[END_REF] also highlights a higher surface tension gradient. Wille et al. only estimate a value lower than 4 × 10 -4 N m -1 K -1 , see Table 2. They performed a measurement up to 2500 K, which is the highest temperature found in literature. This comparison indicates a good agreement even if no mathematical correction is employed in ADL.

Evaporation during a surface tension measurement

Several alloying elements, such as manganese and copper are known to evaporate in molten 304L [START_REF] Bellot | Evaporation of Fe and Cr from Induction-stirred Austenitic Stainless Steel. Influence of the Inert Gas Pressure[END_REF][START_REF] Collur | Mechanism of alloying element vaporization during laser welding[END_REF]. Preliminary tests were performed to study the impact of manganese evaporation. This experiment consists in heating several samples with different laser sequences. After the test, the loss of mass is controlled and the variation of samples composition is analyzed by Energy-Dispersive X-ray Spectroscopy (EDS). An exponential dependence between the loss of mass and temperature is evidenced, see Fig. 6. At 1997 K, the loss mass is of 0.33 %. This value increases up to 7.37 % at 2194 K. An EDS analysis, is performed to identify and quantify the evaporation of our alloy elements, as visible on table 3. Measurements show that manganese evaporation is mainly responsible of the loss of mass, followed by copper and chromium.

The impact of this phenomenon can be observed during the recording of the oscillating droplet, see Fig 7 . Metallic vapor can be observed above the droplet. In order to reduce at maximum this phenomena, the dwell time at high temperature is limited to 6 seconds, the loss of mass is limited to a maximum of 3 % with these conditions.

Surface tension of 304L stainless steel under 100 % Ar

The surface tension is measured in a range of 325 K (1815 K -2140 K). These results are visible on Fig. 8. The behavior of surface tension with temperature can be split into two parts: the first domain shows an increase of surface tension with temperature and then a decrease of the surface tension with temperature, which is characteristic of a metal with impurities. In this case, samples have naturally few ppm of sulfur and oxygen. This presence, in very low quantity, is sufficient to modify the surface tension gradient. The inflection point is located around 1980 K. It is difficult to measure at high temperature the surface tension. This is due to the higher evaporation rate which disturbs flows and levitation stability. This may lead to disrupt the calculation of ν r and then the calculation of surface tension.

Laser power (W) ∆m (mg) Loss (%) Temperature (K) Mn (W %) Cu (W %) Cr (W %) 0* / / / Fig. 8. Surface tension of molten 304L under 100 % Ar, comparison with Ozawa [START_REF] Ozawa | Influence of Oxygen Partial Pressure on Surface Tension of Molten Type 304 and 316 Stainless Steels Measured by Oscillating Droplet Method Using Electromagnetic Levitation[END_REF] and Dubberstein [START_REF] Dubberstein | Surface tension and density data for Fe-Cr-Mo, Fe-Cr-Ni, and Fe-Cr-Mn-Ni steels[END_REF].

The surface tension temperature dependence can be approximately expressed by two equations:

The surface tension gradient is first positive with a value of 7.95 × 10 -4 N m -1 K -1 and at around 1975 K the sign changes with a value of -3.61 × 10 -4 N m -1 K -1 . These results are in good agreement with Ozawa's and Dubberstein's measurements [START_REF] Ozawa | Influence of Oxygen Partial Pressure on Surface Tension of Molten Type 304 and 316 Stainless Steels Measured by Oscillating Droplet Method Using Electromagnetic Levitation[END_REF][START_REF] Dubberstein | Surface tension and density data for Fe-Cr-Mo, Fe-Cr-Ni, and Fe-Cr-Mn-Ni steels[END_REF]. Moreover, no clear variation can be observed by comparing all data. Despite the presence of few ppm of surfactants (oxygen and sulfur) and the variation of manganese concentration, see Table 4. The manganese evaporation has no obsverbable effect on the surface tension value between these expirements. Nevertheless, the comparison seems pertinent and it allows to confirm a tendency.

Reference

Mn (W%) S (ppm) O (ppm) Ozawa [START_REF] Ozawa | Influence of Oxygen Partial Pressure on Surface Tension of Molten Type 304 and 316 Stainless Steels Measured by Oscillating Droplet Method Using Electromagnetic Levitation[END_REF] < 0.01 < 10 / Dubberstein [START_REF] Dubberstein | Surface tension and density data for Fe-Cr-Mo, Fe-Cr-Ni, and Fe-Cr-Mn-Ni steels[END_REF] 1.38 125 55 This paper 1.84 80 83 Table 4. Manganese, sulfur and oxygen content before trials.

Surface tension of a 304L stainless steel under Ar-2.5 %vol. H 2

During the experiment, the oxygen content in the gas cannot be measured because the sensor can be damaged in presence of hydrogen. Yet, with this gas mixture, the probable oxygen concentration is maintained to a very low value due to the hydrogen/oxygen reaction. The surface tension is measured in a range of 324 K (1756 K -2080 K). Measurements are shown in Fig. 9.

Fig. 9. Surface tension of molten 304L under Ar-2.5 %vol. H 2 , comparison with Ozawa [START_REF] Ozawa | Influence of Oxygen Partial Pressure on Surface Tension of Molten Type 304 and 316 Stainless Steels Measured by Oscillating Droplet Method Using Electromagnetic Levitation[END_REF].

The surface tension can be approximated by using these two equations:

No clear difference can be observed with the measurements done under argon. A boomerang shape curve is also observed.

The surface tension increases up to 1975 K with a coefficient of 3.65 × 10 -4 N m -1 K -1 and then decreases slightly with a value of -3.43 × 10 -4 N m -1 K -1 .

This unexpected result can be explained by experimental conditions. In order to limit the loss of mass and the modification of chemical composition, the duration above the liquidus is limited at ≈ 6 s. This duration is too short to let the desoxidation of 304L by hydrogen. Thus, experimental conditions should limit the oxygen desorption.

With these measurements, a longer dwell time at high temperature under Ar-2,5 %vol. H 2 must lead to the results of Ozawa et al. [START_REF] Ozawa | Influence of Oxygen Partial Pressure on Surface Tension of Molten Type 304 and 316 Stainless Steels Measured by Oscillating Droplet Method Using Electromagnetic Levitation[END_REF]. However, carrying a longer experiment would lead to the evaporation of manganese and other elements of the alloy giving results not transposable to welding and additive manufacturing processes. This can be a problem when an alloy with a high content of manganese is characterized.

Comparison between argon and Ar-2.5 %vol. H 2 measurements

The comparison of these two experiments highlights small differences. Higher values are observed in a range of 1723 K - This difference can be caused by a small oxygen desorption even if the melting time is short. The oxygen concentration at the surface may be lower in presence of Ar-2.5 %vol. H 2 gas. Then, the surface tension values slightly increase below 1975 K under Ar-2.5 %vol. H 2 . Beyond this temperature, no difference can be observed between these two experiments. In this temperature range, the values are equivalent to the surface tension of 304L without surfactant. The pure state value can be approximated by this equation:

This equation is in agreement with the value given by Ozawa et al. [START_REF] Ozawa | Influence of Oxygen Partial Pressure on Surface Tension of Molten Type 304 and 316 Stainless Steels Measured by Oscillating Droplet Method Using Electromagnetic Levitation[END_REF] with a gradient of -3.29 × 10 -4 N m -1 K -1 . Also, it is interesting to notice that the surface tension gradient for pure liquid iron is close from this value, see Table 2. The value given in this paper is the highest. This may be the result of the iron purity used (99.95 %).

Conclusion

An aerodynamic experimental set up is used to measure the surface tension of pure liquid iron and 304L stainless steel.

The temperature range is of 368 K (1859 K -2227 K) for pure iron.

For pure iron, measurements were performed under a mixture of Ar-2.5 %vol. H 2 . The variation of surface tension is in agreement with literature and validates the procedure.

For the stainless steel, surface tension was measured under two gas mixtures. The first plotted measurements under argon, presents a "boomerang shape". The curve is in good agreement with Ozawa's data with the same variation. Dubberstein does not observe a "boomerang shape". This can be explained by the shorter temperature range in his work. However, the surface tension in this temperature range is consistent.

The second curve under Ar-2.5 %vol. H 2 shows a "boomerang shape". This variation can be explained by the short time above the melting point. This dwell time does not allow long time reaction between oxygen and hydrogen. Thus oxygen desorption does not clearly appear. Longer dwell time at high temperature may affect the chemical composition of the alloy and thus modify the value. Yet, the surface tension measured may not be representative in the case of a 304L stainless steel with a higher manganese content. Results proposed here are the closest to welding and additive manufacting processes.

All these results indicate that this step up allows to measure precisely the surface tension by aerodynamic levitation up to

Fig. 1 .

 1 Fig. 1. Scheme of a sample before a measurement. The mass of the sample is measured with a precision scale (Mettler Toledo XS205DU). The loss of mass due to evaporation is obtained by comparison of the mass of the sample before and after the levitation experience. A scanning electron microscope (Zeiss MERLIN) associated with an Energy Dispersive Spectrometer (Oxford instruments plc) is used to perform chemical analysis.

Fig. 2 .

 2 Fig. 2. Standard temperature measurement on a sample.

3 .Fig. 3 .

 33 Fig. 3. Scheme of levitation device. A Matlab program is developped to analyze the record. The amplitude of the sample oscillations is analyzed and a Fast

Fig. 4 .

 4 Fig. 4. Example of a Fast Fourrier Transform (amplitude normalized to 1).

Fig. 5 .

 5 Fig. 5. Surface tension of pure 99.95 % iron under Ar-2.5 %vol. H 2 and data from literature. γ 1811 (N m -1 ) dγ/dT (10 -4 N m -1 K -1 ) Temperature (K) Purity (%) Method Reference 1.862 -1.54 1 859 -2 227 99.95 ADL Current work 1.969 -4.65 2 047 -2 360 / EML (Ozawa et al., 2011) [6] 1.87 -4.30 1 750 -1 925 / EML (K.C Mills and Brooks, 1994) [27] 1.92 -3.97 1 758 -2 064 / EML (J. Brillo and Egry, 2005) [16] 1.925 -4.55 1 664 -2 140 99.997 EML (Morohoshi et al., 2011) [15] 1.888 -2.85 1 735 -2 500 99.99 ADL (Wille, Millot, and Rifflet, 2002) [28] 1.918 -4.30 1 781 -2 015 99.99 EML (Kasama et al., 1983) [29]

Fig. 6 .

 6 Fig. 6. Sample loss of mass as a function of temperature -dwell time of 7.5 s.

Fig. 7 .

 7 Fig. 7. Photography of a sample without metallic vapor (left) and surrounded by metallic vapor (right).

Fig. 10 .

 10 Fig. 10. Surface tension of molten 304L under Ar and Ar-2.5 %vol. H 2 .

Table 1 .

 1 . Chemical composition (mass %) of a SUS 304L.

	Cr	Ni	Mn	Si	Cu	Mo	S	O
	19.24 7.93 1.83 0.39 1.21 0.22 0.008 0.0083

Table 2 .

 2 Surface tension data for pure iron measured by Aerodynamic Levitation and by Electromagnetic Levitation.

			1 ) Temperature (K)	Purity (%) Method	Reference
	1.862	-1.54	1 859 -2 227	99.95	ADL	Current work
	1.969	-4.65	2 047 -2 360	/	EML	(Ozawa et al., 2011) [6]
	1.87	-4.30	1 750 -1 925	/	EML	(K.C Mills and Brooks, 1994) [27]
	1.92	-3.97	1 758 -2 064	/	EML	(J. Brillo and Egry, 2005) [16]
	1.925	-4.55	1 664 -2 140	99.997	EML	(Morohoshi et al., 2011) [15]
	1.888	-2.85	1 735 -2 500	99.99	ADL	(Wille, Millot, and Rifflet, 2002) [28]
	1.918	-4.30	1 781 -2 015	99.99	EML	(Kasama et al., 1983) [29]

Table 3 .

 3 Estimation of the loss of mass for manganese, copper and chromium by EDS. * Initial composition as reference.

	1.83	1.21	19.24