Philippe Askenazy
email: philippe.askenazy@ens.fr

Bruno Palier
email: bruno.palier@sciencespo.fr

France: Rising Precariousness Supported by the Welfare state

Keywords: Inequality, precarious employment, social contributions

This paper describes that France is apparently one of the few rich countries to have avoided a significant increase in income inequality in recent decades. However, stable average inequalities mask an asymmetric trends of income between age groups, the elderly improving their situation while the young see theirs worsening. Furthermore, it shows that behind this relatively still surface, a general trend of precarization of more and more ordinary workers is occurring. The importance of wage-setting processes and of regulation of the labour market is bought out, together with the way the tax and transfer systems have operated, in restraining the forces driving inequality upwards. Wage growth, while limited, has thus been reasonably uniform across the distribution and together with the redistributive system have kept household income inequality within bounds. However, in response to high unemployment both regulatory and tax-transfer systems have served to underpin the very rapid growth in precarious working over the last decade, representing a very serious challenge for policy.

La France est l'un des rares pays riches à avoir évité une augmentation significative des inégalités de revenus au cours des dernières décennies. Cependant, des inégalités moyennes stables masquent une tendance asymétrique dans l'évolution des revenus entre les groupes d'âge, les personnes âgées améliorant leur situation tandis que les jeunes voient la leur s'aggraver. En outre, sous cette surface relativement calme se développe une tendance générale à la précarisation de travailleurs de plus en plus nombreux. La croissance des salaires, bien que limitée, a été raisonnablement uniforme dans l'ensemble de la distribution et, avec le système de redistribution, elle a relativement circonscrit les inégalités des revenus des ménages. Cependant, en réponse à un taux de chômage élevé, les systèmes de réglementation et de transferts fiscaux ont soutenu la croissance très rapide du travail précaire au cours de la dernière décennie, ce qui représente un défi très sérieux.

Mots clefs: Inégalités, précarité professionnelle, contributions sociales

Introduction

In all OECD studies on inequality, France stands out as one of the very rare country where inequality has for long not increased, and then increased less than in other cases (such as Sweden or Germany for instance -OECD, 2008). Even if income inequalities have slightly risen since the late 2000s, there should thus be fewer worries about the fate of inequality and income in France, since comparatively it seems that the picture is quite reassuring. As we will show, most of this is due to social legislation and transfer and true as long as transfer to the elderly is taken into account. Stable average inequalities masks an asymmetric fate of income between age groups, the elderly improving their situation while youngster see theirs worsening. Furthermore, we show that behind this relatively quiet surface, a general trend of precarisation of more and more ordinary workers is occurring. And this is not only due to changes in the labour market but is partly supported by changes in legislation, employment policies and welfare state reforms.

In the first section, we show that inequalities have slowly increased in France, this general trend masking increasing gaps, especially between age groups. The flaw in redistribution is due to a pro-old welfare system that family policies or unemployment benefits cannot redress.

In section 2, we analyse the trend of increasing precariousness in the labour market and thus analyse the causes of increasing inequalities among the working population in France.

The Conditions that Lead to the Increasing Role of Intermediaries

A comparatively limited increase in inequalities in France

In the past three decades, the French business cycle was globally similar to others large European countries; however, the Great-Recession was less harsh in France -a drop of "only" 3% of GDPand the recovery was unsettled. The cumulative GDP increase was significant roughly 70% in 30 years (Fig 1a). Sources: Insee, National Accounts (provisional value for 2016); Insee Première "Les niveaux de vie en 2015 », N°1665. Note: The INSEE concept is the ratio of the disposable income by the number of units of consumptions (1 for the first adult + 0.5 for each member of the household aged 14 or more + 0.3 for each child).

Thanks to this growth, from the mid-1980s to 2007, on average, individuals and households enjoyed erratic but non-marginal gains in their disposable income (respectively, +30% and +23%). In the wake of the Great Recession, the growth of disposable income plummeted. It even turned negative in 2012 and 2013 because of tax increases but it now stands to its pre-recession level.

Consequently, over the last decade, the total evolution of the mean disposable income was near 0.

An important driver of the steady growth of disposable income of household is the impressive improvement of the activity and eventually employment rate of women while the employment rate of men is globally flat since the early 1990's (Fig. 2). Even the Great-Recession did not reverse this trend. This dynamics is not exceptional amongst OECD countries but, in the French case, it has not been associated with a dramatic rise in overall inequality.

Globally the median disposable income followed the same pattern as the mean (Fig. 1b).

One way to show that the entire income distribution has not changed much in France over the 1980s to the late 2000s, and much less than in most other OECD countries, is to refer to Louis Chauvel's strobiloids (see figure 3 below). His graphs compare the distribution of income in the late 1970s and in 2010 for various countries; France is appearing as the country where the distribution has changed the least [START_REF] Chauvel | The intensity and shape of inequality: the ABG method of distributional analysis[END_REF]. Source: Chauvel (2016) using microdata samples in the LIS datacenter Note: The strobiloid shows the income hierarchy (on the vertical axis, 1 = median). The curve is larger (horizontal axis) when the density at this level of income is higher: Many individuals are at the intermediate level near to the median and their number diminishes at the top and at the bottom. Thus, in strobiloids with a larger belly, the intermediate middle class is larger with a more equal distribution.

As far as inequalities are concerned, Gini coefficients for total population and of the working age population (market prices) are U-shaped but they are basically flat since the 1980's (table 1). Alternative indexes of aggregated inequality such as the Decile ratios do not exhibit major trends. This situation has been praised by OECD as an extraordinary case of almost no increase in inequalities until 2008, contrasting starkly with the rest of OECD countries (OECD, 2008). However, these global trends masks drifts in inequalities and emerging new divide within the French societies.

Behind a quiet surface, changing inequalities, between gender and age groups

Indeed, as soon as one decomposes the evolution of income by different groups, one sees new divides and inequalities that average masks. Of special importance are inequalities between gender and between age groups.

As far as gender inequalities are concerned, women are experiencing worse situation than men in France. Despite their increasing employment rate and their rising educational attainment, the median income for women is lower. These inequalities are partly due to the situation of women within the labour 1970 1975 1979 1984 1990 1996 1997 1998 1999 2000 2001 2002 2003 All market (we come back to this in the next section), and partly to the design of the French welfare state, which is highly influenced by a 'nuanced' male breadwinner model [START_REF] Lewis | Gender and the development of welfare regimes[END_REF]. In 2015, the net wage for a full-time equivalent worker was on average €2438 Euros per month for men, and €1986 Euros for women in the private and public firms. When eliminating disparities in responsibilities and lower working time, there is still a gender pay gap around 10% that reveals gender discrimination. That should be seen as a lower bound since women are also discriminated for the access of a full time job and higher paid tasks.

The risk of poverty is also biased against women in France. In 2015, the poverty rate (50% of median income) was 8.1% for women, and 7.9% for men (INSEE data). This is only partially due to the situation of women into the labour market that we just described. The concentration of single parent families amongst women is crucial at active ages. And the pension system magnifies income inequalities between men and women since they face lower annual wages and they cannot get a full pension in case of significant period of inactivity (for example to take care young children) or part time.

On average, women receive an old age pension of near 40% lower than men (before specific credits for years of children's education and survivors pensions).

If we turn to age groups, one sees that the average situation of the "young" elderly (65-76 years old) has improved much more than the one of the younger age groups over the last thirty years. The situation of the 76+ is less favourable because of the prevalence of women in this age group. This is also shown through discrepancies between individual poverty rates by age group. The situation of the elderly has much improved in term of poverty, while it has deteriorated for the youngest. These gaps can be explained by the main mechanisms of redistribution organised by the French welfare system, which is particularly pro-old. And the impact of reforms which will significantly reduce the generosity of the pension systems, is still limited for the current retirees. The French welfare system is a Bismarckian one, which is based on social insurances [START_REF] Palier | A long goodbye to Bismarck? The politics of welfare reform in continental Europe[END_REF]. It mainly organises a socialization of solidarity between active generations and older generations [START_REF] Chevalier | Mutations des âges de la vie et mutations du contrat de solidarité entre les générations[END_REF]. In 2013 (source: DREES, the social protection accounts), health risks (€ 232.8 billion in 2013, half of which goes to over 60) and old-age pension (€ 307.5 billion in 2013) represent respectively 34.6% and 45.8% of total social welfare benefits, ie more than 80% of social protection expenditure, or about 25% of GDP. Maternal-family and employment risks account for 8.4% and 6.1% respectively of social protection benefits. Finally, housing and povertysocial exclusion account for 5.1% of benefits.

Thus, almost half of social expenditure is spent on pensions. Adding the half of the expenditure on sickness that is benefiting people aged over 60, and the benefits for frail elderly, more than two thirds of social expenditure is devoted to the older than 60. Even if we add education spending, we can see that socialized solidarity (assumed by the community, through compulsory collective contribution and guaranteed benefits) is mainly made up of benefits for the elderly, while solidarity with the youngest (children and young adults) passes through family solidarity (d [START_REF] Albis | Les jeunes sont ils sacrifiés par la protection sociale ?[END_REF]. In Bismarckian systems, which are mainly based on social insurance and contributory benefits, there is a structural tendency towards a more senior orientation, at the expense of spending on younger people [START_REF] Lynch | Age in the Welfare State: The Origins of Social Spending on Pensioners, Workers, and Children[END_REF].

The French welfare state thus assumes that it is for families to take care of their children and young adults rather than the welfare state, as can be seen from the logic of family benefits (always provided to parents, in support of families), and not directly aimed at children, or can also be seen through the fact that before 25 years, young people are still considered as "social minors" [START_REF] Chevalier | Varieties of youth welfare citizenship. Towards a two-dimension typology[END_REF]. The age group which is specifically neglected by the French welfare system is youth. Young adults between the ages of 18 and 25 in difficulty do not have the same rights to benefit as other adults (the minimum income, 'Revenu de Solidarité Active' is only accessible from the age of 25, many social benefits are accessible to young people only through their parents -see the scholarship grant system in particular). Thus, many studies have shown that it is indeed the families who take charge of solidarity with their children (including when they have grown older, but are still in difficulty). This familialism in solidarity with young people leads to a reproduction of inequalities (an affluent family has more means to help its young people than a family in difficulty, where young people are most likely to encounter difficulties) [START_REF] Papuchon | Les transferts familiaux vers les jeunes adultes en temps de crise: Le charme discret de l'injustice distributive[END_REF].

As a consequence, today in France, the poor are the "young". Whatever the way of calculating it, poverty rates are much higher today among the young population (and also among children) than among the labour force and even more so than among the elderly population. In France, poverty among the elderly has been divided by 7 between 1965 and today. As a result of the rise of wage-related pension schemes, fewer and fewer individuals are required to resort to minimum means-tested pensions. Thus, while 2.5 million people were receiving the minimum old-age pension at its creation (more than half of the elderly at the time) in France in 1956 (nearly two-thirds of those over 65 at the time), they are 557,800 at the end of 2013 (3.5% of the population aged over 60 years) [START_REF] Lelièvre | Minima sociaux et prestations sociales -édition 2015. Ménages aux revenus modestes et redistribution[END_REF]. Even since the crisis of 2008, the standard of living of the pensioners continued to increase to be 2,049 euros per month and per unit of consumption in 2013, being almost equal to that of the working age population (2 062 euros) and higher that the standard of living of the whole population (1,946 euros) and especially young people of 18-24 years (1,671 euros) (COR, 2015).

Why did France manage to keep inequalities relatively limited amongst the active population?

If inequalities have increased between age groups, they have been relatively limited within the working age population, where inequalities have increased in many other countries. It is not because France has been preserved from globalization or technological changes, but because it has settled conditions guaranteeing a relative compression of wage dispersion.

Two periods can be highlighted. At the bottom of the wage distribution, the rapid increase of the minimum wage in real terms and compared to the median from the 1970's to the early 1980's was made sustainable thanks to a hyper-productive model of production, especially in the service activities. The intensification of work was pronounced from the 1980's to the end of the 1990's1 . Large food retailers that concentrate low-paid female jobs are a good illustration. Productivity in the French outlets is even larger than US ones [START_REF] Ph | Working in large food retailers in France and the USA: the key role of institutions[END_REF]. Many factors including regulations of the products/services markets and social organisation explain this performance.

At the upper half of the distribution, wages were moderated by the massive effort to democratize education in France from the mid 1980's. In just a generation, France doubled her stock of workers with a tertiary degree.

However, the 1990's acted the limits of this strategy and the emergence of a new logic: diminishing the cost of labour for firms while keeping net income relatively high for low skill workers. The apparent job polarization between so-called "lousy" and "lovely" jobs has reached France as much as other OECD countries [START_REF] Catherine | Marché du travail: la grande fracture[END_REF][START_REF] Fernández-Macías | Job polarization in Europe? Changes in the employment structure and job quality, 1995-2007[END_REF]. As in many other countries, this evolution should lead to a jump in inequalities. However, France has put in place a system that allows -and actually favours-the development of low paid jobs while keeping net hourly wages of their tenants close to the rest of the income groups. Three main elements allow this: a general minimum wage applicable to all sectors (with only few exceptions), salary grids stated in industry agreements which cover a vast majority of workers, and a system of social contribution exemptions on low wage. The first significant social contribution exemptions were introduced in 1993. And both conservative and socialist governments went deeper in this strategy [START_REF] Ph | Blind Decades. Employment and Growth in France 1974-2014[END_REF][START_REF] Carbonnier | Tax cuts or social investment? Evaluating the opportunity cost of French employment strategy[END_REF]. It aims at compensating for the labour cost that wage minima are representing for firms employing low skill jobs. Basically, this system generates a disconnection between labour costs and net wages (before income taxes and transfers). [START_REF] Bozio | Taxes and Technological Determinants of Wage Inequalities: France 1976-2010[END_REF] provide a very clear illustration of this phenomenon. They use the DADS, the exhaustive logs of the social security for workers on private labour contracts. These data allow measuring the gross wage, the net wage (gross minus workers' social contributions before income taxes and transfers) and the wage bill for employers (gross + employers' contributions -exemptions). All compensations are included, except specific bonuses e.g. stock options (so gains of very top earners are probably under-evaluated). Figure 4 shows, by percentile, the log changes from 1976 to 2010 of the real net and gross wages of full time male workers; and of the wage bill for employer. Two facts are striking. First, on average, the gap between the net wage and the wage bill widened for financing the expansion of the welfare state, especially pensions. Second, across the bottom 98% of the distribution, net wages of full time salaried men experienced a clear compression. By contrast, as in many other OECD countries, the dispersion of wage bills significantly enlarged.

Figure 4 also shows that the median net wage (gross wage minus social contributions) of full time full year working men increased in real terms by about 15% from 1976 to 2010 in private and public firms. Half of this gain occurred before the de-indexation of wages in the early 1980's. The median net wage continued to increase but at a very low pace; it was poorly cyclical, even the Great Recession didn't drive a significant drop of median wages. Provisional statistics suggest that median net wage of men came to a standstill since 2010. Figures are rosier for full time full year women: their median net wage gained around 30% from 1976 thanks to, again, their rising educational attainment. The French strategy can appear as optimal, limiting inequality and preserving the firm competitiveness. However, as will be developed in the next section, the basis is crumbling.

A crumbling lower-middle working-class: from the "ordinary worker" to the "ordinary precariousness"

We show in the previous section that under the apparent stability of aggregated indicators, a multidimensional income inequality raised in France. The same story is true for the labour market. The unemployment rate is high but in the same range from decades, and the proportions of employees, fixedterm and open-ended contracts have slightly moved until recently. However, when we explore each of these dimensions and the dynamic of carrier paths, the precariousness has deepened and expended concerning now a large part of the working-class. The next subsection develops the most striking stylized facts that drive inequality.

Employment and future pensions: a growing concern for low-middle educated men

Apparently, the French economy exhibited certain resilience during the Great Recession. The unemployment rate remained within its long-run range -between 7 to 11%-, consistently lower than its level after the recession in the first part of the 1990's. The adjustment of employment was far below what macroeconomic models of the French economy predicted. Contrary to numerous OECD countries including the US, France exhibits an employment rate that is comparable to the one just before the crises. More precisely, the employment rate of men aged 20-64 slipped by about 2 points while women enjoyed a slight improvement (see Fig. 2). However, this observation is biased by the consequence of the reforms of the pension system: end of pre-retirement schemes, increase of the normal retirement age, longer period of employment for getting a full pension etc. (Pa-lier, 2014). They push workers aged 55 or more to remain active. Their employment rate thus dramatically improves. By contrast, the employment rate of the 25-64, especially of men, declines. And within men, a gap has deepened between the tertiary educated workers who still enjoy a 90% employment rate and the less educated who have faced a drop of 10 point (Figure 5). In 2015, less than 7 out of 10 low-educated men aged 25-64 were jobholders. Rather than marking a step, the aftermaths of the Great Recession have initiated declining trend in their employment opportunities. Upper secondary educated are also facing increasing difficulties.

One corollary is that more and more low and secondary educated workers will experience incomplete professional carrier. This will translate in massive cuts in their pensions. Consequently, in terms of permanent income, inequality has certainly more widened than current income figures, in the past decade.

Women: more opportunities but headwinds

Increasing female participation is not only the result of their rising education but also of active policies. For decades, nursery school starts at the age of 2 or 3; children have regular schedules 4-days a week from around 8.30am to 4.30am. In addition, in the past 3 decades, public but also private crèches have flourished. Firms that create crèches for their staff enjoy tax cuts. Parents who are working (or actively looking for jobs) have the priority in the public crèche. The cost for the parents is related to their income and even the wealthiest benefit an income tax credit. The social security also provides a financial support for employing child caregiver.

Increasing employment rate of women was positive for couple. According to the Labour Force Survey, the proportion of bi-active working age couples was around 50% in the early 1990's and is now about 60%. Women provide an additional income (actually the main income in one case out of three), and bi-activity offers a significant security against the individual risk of being sacked.

However, women, especially lone mothers, still face significant headwinds [START_REF] Meurs | Growing Unequal?: Income Distribution and Poverty in OECD Countries[END_REF]. Women are paid around 20% less than men. The convergence rate is so low, that the gap would be still around 10% by 2100. Disparity in working time is one of the main causes of this gap.

In the 1990's, the development of part-time work, which was low in France compared to successful European countries, was considered as one of the most promising avenues. In 1992, the government introduced a massive reduction in employer social security contributions for hiring (or transforming a worker's job to) part-time; it corresponded to a drop of around of 10% in labour costs. France experienced in the 1990's the largest growth of part-time work within Europe. However, the general social security reduction for parttime jobs was eventually removed in 2000.

Actually, this strategy was in contradiction with the education policy that provides to mothers practical conditions for working full-time (or long parttime). In addition, most of the created jobs were the transformation of fulltime ones in multiple short-times ones. The proportion of under-employed dramatically rose; the transition rate from part-time to full-time work has also drastically slowed. Despite the end of public support to part-time, the employers continued to develop these jobs that give them a significant flexibility, especially in services -e.g. retail trade-where the weekly number of rush hours is lower than 35. According to the Labour Force Survey, the proportion of under-employed women topped 10% in 2010 and was still about 9.5% in 2016.

This evolution fuelled the phenomenon of (working) poor among sole mothers. In 2012, 20.5% of people living in a single-parent family -1.188 million people -live on less than 50% of the median income, compared to 8.1% of the total population. The number of recipients of the Lone Parent Benefit (Allocation Parent Isolé) in metropolitan France rose from 129,500 to 201,000 between 1990 and 2013, an increase of about 64%. The difficulties are all the greater when the number of dependent children is high and mothers work part-time or low-paid (30.2% of single mothers in employment work part-time in 2014 and, of the 3.7 million people in employment with an income of less than 964 euros a month, 70% are women). With the introduction of means-tested allowances for families and the deployment of a new minimum income: the so-called "active solidarity income" (Revenu de Solidarité Active), social transfers contribute to the fight against the poverty of single-parent families. But after social and fiscal transfers, according to the latest statistics from INSEE, nearly 40% of sole mothers were still poor (60% of the median income) in 2014. These women also face an additional dramatic alteration of the French labour market: the epidemic of very short-term contracts.

The epidemic of very short-term contracts

The use of temporary workers by firms can follow two basic strategies. First, short-term work can improve the production and matching processes. It is a tool for adapting the staffing to the activity and thus to increase productivity while the core of the workforce is stable and motivated. Short-term contracts can also be an efficient step for selecting workers on open-ended jobs. Second, the strategy can be to minimize labour costs and thus maximize current profits since workers on these jobs are less costly: no tenure bonus (cash or vacations), no additional benefits, no performance pay schemes, no dismissal cost, etc. Indeed, most of the industry agreements offer such advantages to workers but only with a minimal tenure; profit sharing is mandatory in firms with 50 or more staff, but only for workers employed at least 3 months during the year.

There are consistent figures that suggest French Firms moved from the first strategy to the second in the past fifteen year [START_REF] Ph | Blind Decades. Employment and Growth in France 1974-2014[END_REF]Erhel, 2016, 2017), hampering labour productivity. The most striking one is the dramatic rise in very short-term contracts, whereas, the flow of open-ended contracts and longer fixed-term contracts remained flat, just affected by the business cycle. According to the exhaustive social security logs2 , in 2003, the quarterly number of recruitments on fixed contract of less than one-month was about 1.7 million. On the eve of the great recession, it reached 2.9 million.

In the wake of the recession, it declined a bit but it rapidly recovered. Since 2015, each quarter at least 4 million people are hired on fixed-term contract of less than one month. About half are on contracts of less than one week. This upsurge has been concentrated in the tertiary sector, with spectacular increases in e.g. advertising agencies or entertainment. In the hotels and restaurants, quarterly job flows are higher than the static stock of jobs.

While the global stock of fixed-term contracts did not increase up to 2015, this phenomenon resulted from the conversion of "long" short-term contracts (6-month up to 18-month) in very short contracts.

These evolutions challenge the standard interpretation of the cause of the dualism of the French labour market: the rigidity of open ended contracts should push firms to hire for short-term jobs. Indeed, probatory periods for open-ended jobs are at least 2 months for clerks and blue collars, 3 months for technicians and first line managers, 4 months for engineers, professionals and managers (and can in some circumstances even be doubled). The employers are free to sack workers during this period at no cost. Therefore, the protection of open ended contracts following such a quite long probatory period can hardly explain why employers do hire for less than one week. Other mechanisms seem more relevant. They converge for improving the attractiveness of precarious jobs. All the parties-unions, employers and governments-supported reforms to simplify hiring on very short-term jobs, with the goal to reduce overall unemployment. The hiring is now fully digitalized and can be completed in just a few minutes on a dedicated web site. New employee had to be examined by an occupational practitioner. But this condition had been first dramatically alleviated: there was no need of such exam if the workers have passed one in the past two years. From 2017, there is no more medical examination.

In addition, in a series of decisions, on November 2003, the Court de of Cassation clarified the regulation of temporary contracts 'd'usage constant' (CDD d'usage). In industries where short-term contracts are historically used (20 industries: sports, entertainment, education, restaurants etc.), an employer has no quantitative limit to hiring on the bases of such contracts, even for the same worker and the same job, except if this job can be considered as permanent3 . In the other sectors, the employer cannot hire the same workers on two consecutive short-term contracts. But this condition is impinged upon by the possibility for that the employer to re-hire the worker after a transition period. For example, in a supermarket open every day, you can hire a worker 4 days from Monday to Thursday; then you apply the transition period (at least half of the length of the contract), from Friday to Sunday, and then you can hire again from Monday to Thursday etc.

This liberty of rehiring the same workers is massively used. According to the social security logs, in 2015, about 80% of the "new" recruited are former workers of the same firm. Note that since July 2013, an extra social contribution has been introduced for very short-term contracts, but it is very low and does not concern most of the configurations including the CDD d'usage. In this framework, for both employers and employees, the individual working income is not wages but net wages plus unemployment benefits. Employment periods are chosen to minimize labour costs and maximize this working income. The great recession has shaken this model (fig. 7). Employment is more sliced. The yearly working income at the bottom of the distribution plummeted in real tem and inequality rose: the median/D1 ratio jumped from 3.5 in 2008 to 3.8 in 2014. But it would be twice in the absence of unemployment benefits.

We observed a massive mobility from one year to another at the bottom of the distribution (around 40% of workers exit the first decile). But it is partially an artefact: the income is so low at the bottom of the distribution that the chance to get more jobs one given year can dramatically boost in your income rank, and you can lose ground also quickly the next year. Therefore, it is worthwhile to describe carrier paths for a multi-year period.

Precarious carrier paths

Older surveys -health and professional itineraries 2006 and 2010 (Ministries of Labour and of Health)-provide a picture of dynamics for all workers who started to work (after exited school) at least one decade before the survey; by construction, this picture does not include young workers.

Using analyses based on these data, we propose in Askenazy et al. ( 2013) a classification of workers in 4 groups before the great recession. Findings by [START_REF] Mette | Emploi, conditions de travail et santé des 30-50 ans : des trajectoires contrastées entre 2006 et 2010[END_REF] from the 2010 wave, are consistent with our classification; they suggest that the two first years of the great recession did not massively alter workers' situation.

One-fifth workers had precarious trajectories, typical of working poor. Over a decade, they -mainly women-experienced employment, then long-term unemployment, inactivity or activity. In addition, they faced hard working conditions and their health deteriorated, especially for men.

Around one-fifth workers had fallen or were falling in precariousness. Employees initially stable for years, they experienced a professional breakdown marked by a phase of unemployment most often of long duration, or they entered a declining professional trajectory.

A same proportion of workers were threatened. Their professional trajectories were still unbroken but lastingly low-middle paid, with no prospect of an upward career. With poor training during their career and occupying the same type of job than workers in in the previous category, they could at any time join a precarious trajectory, in the event of their position being suppressed, or of closure of their establishment. These workers also experienced work intensification, participating in their professional exhaustion. Since long-term unemployment among men aged 25-49 or 50-64 has doubled since 2009, we can expect that some of these threatened have indeed fallen in the previous category.

A last group -around big third of workers-, enjoyed paths either continuously ascending or stable and highly qualified. Most of them are managers and professionals (private and public salaried or independent) at time of the survey.

This classification moves the frontier of the dualism of the labour market in France. Rather than focusing on the static observation of around 20% "outsiders" (unemployed plus non-permanent jobholders) versus 80% "insiders" (permanent jobholders), we can argue that the French working population is split in two classes: 40% enjoy job security and opportunities, while a large low-middle class struggles in an insecure environment.

The inequality in opportunities is magnified by the fact that temporary contracts act less and less as stepping stones [START_REF] Andrews | Discussion of the French and British Conundrums[END_REF]. According to OECD estimates based on EU Statistics on Income and Living Conditions EU-SILC data), France has one of lowest three-year transition rates from temporary to permanent employment (2008)(2009)(2010)(2011): only 36%, compared to e.g. 80% in the UK. On-the-job training is significantly lower for temporary workers in France.

Youths are primarily concerned. And here, we can observe a long-run stigma of the great recession. The non-tertiary educated, young workers have more and more difficulties to get a permanent job. According to the generation surveys, in April 2015 (at the date of the last survey), 27.4 percent of youth who exited school in 2010 with a secondary diploma are unemployed (table 4). The deterioration is massive for men. In just a dozen of years, their chances to be 5 years after exited school on full-time permanent job (including civil servants), dropped from nearly 60% to 37%. The situation of young secondary-educated women was also far from improving. Facing the trends towards high-frequency job turnover, the unemployment insurance system (managed by the employers' and workers' organisations) has been adapted in order to cover workers with shorter contribution periods (4 four months instead of 6 six since 2009); and in July 2014, better coverage for the recurrently unemployed has been acted. In the wake of these reforms, we observed a new phenomenon (Askenazy and Erhel, 2017): While the share of non-permanent jobs was during two decades increasing very slowly, since late 2014, according to the labour force survey, the employment rate on fixed-term contracts reaches its highest historical records (figure 8). At the same time, the employment rate on open-ended salaried jobs plummets.

New low-income entrepreneurs

While the focus is in general on the salaried workers, it is worthwhile to stress the case of non-salaried ones in France. In past decade, a new selfemployment status massively altered their portrait: the "auto-entrepreneurs" (called now "micro-entrepreneurs").

In 2009, this new social contribution and fiscal regime was created for selfemployed individuals as unincorporated enterprise. Becoming an auto-entrepreneur requires only a quick registration on the Internet. Then, these freelance entrepreneurs declare their quarterly revenue online. Social contributions and income tax are immediately charged and for most of the case, it is just the application of flat rates; these rates are low compared to taxes paid by classic independent contractors or wage workers. The status may be cumulated with a salaried job (with strong restriction for public employees); around one third of the micro-entrepreneurs are in this situation.

The regime immediately met with great success representing about half of all newly created enterprises. According to the exhaustive social security statistics (Acoss-Urssaf), in the second quarter 2016, 640.000 micro-entrepreneurs were "economically active" i.e. had a positive turnover. The turnover of these micro-entrepreneurs in the second quarter 2016, is particularly low: 1150 Euros monthly on average. Even if we restrict to the micro-entrepreneurs who do not cumulate with a salaried job, the average gross income remain below 1600 Euros.

At the same time, the mixed income of the classic self-employed (artisans, retailers, etc...) fell in 2009, but recovered rapidly, while their numbers dropped. This decline of classic self-employed was driven by the recession but also by the competition of the auto-entrepreneurs who charge lower prices. The net impact of this substitution in terms of jobs or the total number of worked hours by non-salaried workers is then just slightly positive according to Insee National Accounts.

Because of this dramatic composition change, the mixed income per worked hour of non-salaried workers was cut in real term by about 20% in the past decade [START_REF] Ph | Exploring the French Productivity Puzzle[END_REF]. And income inequality among non-salaried workers boomed: according to the INSEE non salariés database, the interquartile gross income ratio is about 4 for "classic" non-salaried workers but tops 7 when micro-entrepreneurs are included.

In addition of the creation of the status of micro-entrepreneurs, the emergence of platforms of the on-demand economy e.g. Uber is fuelling the development of this category of low income independent contractors. Low or secondary educated youths are primarily concerned (see also table 4). This trend along with the growing share of fixed-term salaried should translate in a current increase of inequality among the working-age population. In addition, standard tools to address labour earning inequality -the minimum wage or unemployment benefits-prove their limits in face of these mutations of the labour market.

Conclusion

Despite an apparent resistance to increasing inequalities, France is actually going into deep socio-economic transformations. First of all, inequalities have changed since poverty has shifted from the elderly to the young, touching especially young women with children. Second, the labour market is currently drifting towards the emergence of more and more precarious situations, here again touching first young workers (especially unskilled ones). If the policies aimed at reducing labour costs while maintaining minimum wage at the best OECD standards, prevent a deep increase in inequalities, the welfare state is less and less able to mask inequalities. On the contrary, the process of dualization of welfare benefits itself contributes to stabilize new inequalities.

Two distinct worlds of welfare have come to coexist in the public welfare system. One is the remaining realm of social insurance, comprised mainly of old age pensions and unemployment insurance, where what is called in France, 'professional solidarity', is central and benefits are still acquired through work, albeit linked more closely than before to the amount of contribution paid. This world of social insurance is no longer offer comprehensive coverage i.e. cover the whole population and provide the insured with sufficient benefits to sustain their standard of living. For those whose inclusion within the labour market is either absent or too intermittent (such as in the case of very short term contracts), the main social insurances are irrelevant, and they need to rely either on negative income tax and or on social assistance.

The French population itself seems to be increasingly divided into, on the one hand, those who can rely on a rather generous social insurance program and continue to have access (thanks to their employers or their own wealth) to private complements, and on the other hand, those who have fallen out of that system and are dependent on minimum benefits. To the latter group, one should probably add those being 'activated' into atypical contracts under which they benefit from second rank labour and social protection. Between those on minimum incomes (10 per cent of the French and those of the working population with an atypical working contract (short term, part time, with lower wage than normal, and other subsidized jobs), it seems that almost a majority of the French population does not participate in the 'normal' labour market and social protection arrangements, despite the rise of the participation rate. French social protection reforms have thus contributed to increase inequalities and divide society [START_REF] Palier | Institutionalizing dualism: Complementarities and change in France and Germany[END_REF].

The objectives of social policy have undergone fundamental changes. Before, social protection was mainly conceptualized as a way of guaranteeing a substitute income for people who could no longer work, temporarily or permanently. Benefits were conceived as an entitlement, earned over years of social contributions. More and more, social policies are conceived as instruments for modifying individual behaviour, in particular with regards to employment, and as a tool for spurring the economy -not by supporting household consumption, but by encouraging citizens to work as much as possible and by developing private social protections. The French social protection system is gradually supposed to become an instrument of competitiveness: in business (decreases in employer social contributions, development of private social protection activities), of the state (decrease in taxation, control over the rise in public social expenditure), and of individuals (activation policies).

Fig

  Fig. 1a: GDP growth and evolution of the mean disposable income per capita.1986-2016 In %

Fig. 2 :

 2 Fig. 2: Employment rates of women and men. aged 20-64, 1983-2015. In %

Fig. 3 :

 3 Fig. 3: Distribution of income in the late 1970s (or 1980s) and in 2010.Selected countries.

Fig. 4 :

 4 Fig. 4: Change in log real wage by percentile, full-time, full-year private male workers. 1976-2010

Fig. 5 :

 5 Fig. 5: Employment rate of men 25-54 by education. In %

Fig. 6 :

 6 Fig. 6: Number of private contracts signed each quarter by duration (excluding temp agencies). 2000-2017

Fig. 7 :

 7 Fig. 7: The evolution of real annual employees' working income (wages + unemployment benefits -social contributions) by decile: D1, D2, median, D8. 2002-2014. Base 100 = 2008

Fig. 8 :

 8 Fig. 8: The employment rate of the 15-64 on permanent (CDI) and temporary jobs (fixed-term and temps) 2003Q1-2017Q2. In %

Table 1 . Gini (after taxes and transfers) by age groups. 1970-2015. INSEE or OECD concepts

 1

Source: INSEE (corrected for breaks), and OECD stats, break in 2012 (consulted 02/01/2018).

Table 2 : Annual mean equivalised disposable income (OECD definition: income of the household/square root size), by selected age groups. 1984-2015. In current Euros.

 2

		1984	2000	2015
	26-40	10,760	17,620	23,800
	41-50	12,070	20,460	26,530
	66-75	10,330	18,620	27,640
	76+	9,620	17,650	24,720
	Source: OECD stat (consulted 02/01/18).

Table 3 . Individual poverty rate (at 50%) after taxes and transfers by age groups. 1984-2015 1984

 3

			2000	2015
	0-17	7.7	9.2	11.3
	18-25	8.8	9.8	13.9
	26-40	5.9	6.8	8.0
	41-50	7.3	6.7	7.4
	51-65	11.3	6.0	6.7
	66-75	3.3	3.6	2.4
	76+	5.6	6.0	3.9
	Source: see table 2.

Note that the reduction of annual working time in 1982 was associated with a proportional jump of hourly productivity. Optimisation of work schedules and thus productivity was fuelled by the industry and firm agreements on the 35-hour workweek in the late 1990's.

Updated stats are available on: http://www.acoss.fr/home/observatoire-economique/publications/acoss-stat.html

E.g., the Cour de Cassation (supreme civil court) judged in 2008 that a teacher hired during 14 consecutive years on an annual contract should be qualified as a permanent worker.

Statistics for all the 15-29 ages confirm their difficulties (Dares Résultats, 2017-16, based on the French Labour Survey). On the one hand unemployment particularly affects young people, since the 15-29 age unemployment rate was 18.9% in 2015 (against 10.4% for all groups) and their employment rate 43,2% (against 63,8% for all 15-64 years). On the other hand, among young employees, 37.3% are on temporary jobs, compared with 14.6% for all employees. By contrast, the shares of part-time and under-employment (ILO concept) are similar amongst youth and all ages.


Kévin Zimny

Benoit Mascaro

Thomas Brunet

Olivier Poncelet

Christophe Aristégui

Jacques Leng

Olivier Sandre

Olivier Mondain-Monval

Design of a fluorinated magneto-responsive material with tuneable ultrasound scattering properties

Introduction

Possessing numerous original properties, fluorinated materials are used in a wide variety of applications such as coating agents for cooking devices or fabrics, ion exchange membranes or as biomaterials for cardiovascular implants. 1 They present low thermal conductivity, high mass density, chemical inertia, and unflammability. Thus, several companies developed low viscosity perfluoro-or semi-fluorinated oils with good biocompatibility for applications such as blood substitutes, medical devices or ultrasound contrast agents, together with low environmental cost i.e. zero ozone-depletion and low greenhouse effect of their vapour in the atmosphere. 2 The spreading in environment by industry of perfluoroalkanes motivated toxicological studies. If perfluorooctanoic acid (PFOA) was shown to exhibit very slow clearance from living organisms, 3,4 as ascribed to the outstanding stability of carbon-fluorine bonds, other fluorinated chemicals are considered as "bio-safe". Among them, fluorinated ultrasound contrast agents (UCA) commonly referred to as "microbubbles" are authorized for intravenous injection in humans under brand names as SonoVue  , Optison  , Imavis  , Definity  , Imagent  ... Due to their very low solubility in water (less than 10 ppm) and, on the contrary, to their high compressibility and ability to dissolve large quantities of gases (O 2 , N 2 , CO 2 ...), 5 volatile fluorocarbons can indeed be used to fill the inner components of microbubbles. The UCA echogenicity relies on the impedance contrast between tissues mainly made of water (Z water ≈ 1.5×10 6 Rayl) and the materials to be injected, which is often a gas phase (Z air ≈ 340 Rayl). However gas microbubbles also have a short lifetime due to their physicochemical instability in a fluid and 50 tendency to rapidly burst or to coalesce. One way to increase the lifetime of UCA in blood is to mix air with a perfluorocarbon gas that presents very low solubility in water, thus acting as "osmotic agent" and slowing down the Ostwald ripening process. 5 Several routes were developed to obtain longer-lasting UCA, on the one 55 hand by coating the bubbles with a stabilising shell of lipids or polymers, 6, 7 on the other hand by adding to air or nitrogen a partial pressure of fluorinated gas, in that case wrapped by a shell of hydrogenated 8 or F-alkylated double-tailed phospholipids. 9 Typical volatile fluoroalkanes incorporated in microbubbles are 60 octafluoropropane, 10 decafluorobutane, 11 tetradecafluorohexane (commercialised by 3M as the Fluorinert FC-72 reference), 8, 9 or perfluorooctylbromide (PFOB). 7 Recently, several teams reported the decoration of the surface of microbubbles by iron oxide nanoparticles, both for pure air 12 and for mixed 65 air/fluorocarbon gas bubbles. 10, 13 The idea was to be able to guide such magnetic microbubbles against the strong flow-rate of blood circulation by the use of a magnetic field gradient.

Thus, the US imaging community is still in search for alternatives to gas bubbles, which present a high echogenicity but 70 poor long term stability. We propose here a new type of materials made of fluorinated ferrofluid oil droplets exhibiting both a large sound-speed contrast (∼1/3) with aqueous media (500 m•s -1 / 1500 m•s -1 ) and sensitivity to an external magnetic fluid. When dispersed in an aqueous yield-stress matrix, magnetic fluorinated oil droplets are not only magnetically guidable but also exhibit strong Mie resonances at specific frequencies, 14 varying with the intensity of an applied magnetic field and its orientation with respect to the wave propagation vector. Such magneto-responsive attenuation properties are described precisely in a companion article. 15 The present manuscript deals with chemistry and softmatter methods necessary to build this tuneable acoustic material. First, we describe the process used to synthesise the magnetic nanoparticles and to coat them with a layer of perfluorosilane. This fluorinated ferrofluid is then fully characterised by different spectroscopic, microscopic and magnetic methods. At last, we present the fabrication of a monodisperse emulsion in an aqueous yield-stress fluid and an example of tuneable acoustic properties.

Since decades, sol-gel methods have opened a golden gate towards the so-called "soft chemistry" which enables to obtain organic-inorganic nano-sized or nano-structured hybrid materials based on silica exhibiting a wide variety of structures and morphologies, and more recently on metal-oxide frameworks. 16 Organosilanes are commonly used to functionalise silica surfaces by a hydrolysis-condensation mechanism initiated at the inorganic surface, that is rich in silanol moieties (Si-OH) arising from the hydrated state of the solid surface. Monolayers of semifluorinated alkylsilanes are chosen for instance to render glass surfaces highly hydrophobic. 17,18 The same reactions exist for the hydrated surface of iron oxide (Fe-OH groups), but they have been much less developed. The chemical grafting of molecules onto iron oxides often starts by the coating with a molecular monolayer of (3-aminopropyl)triethoxysilane (APTS). [19][20][21][22][23][24][25][26][27] This short molecule is too small to bring enough electrosteric repulsions between particles to confer any colloidal stability to the MNPs in water, but they provide amino groups enabling 35 further functionalisation steps. Several studies involve a ligandexchange process starting from an initial oleic acid monolayer that is replaced by the organosilane. 32 or 2-bromo-2-methyl-N-(3-(triethoxysilyl)propyl) propanamide to initiate polymer chain growth by a "grafting-from" controlled polymerisation method. 33 In almost all the previous works, silanization was an intermediate step needed to anchor a molecular shell around the magnetic 45 cores by covalent bonds, which is preferable than most common adsorption routes with chelating ligands such as carboxylic acids, phosphates, sulfonates or catechols. Only macromolecular silanes such as triethoxy(methoxypolyethyleneoxy)silane 29 or trifluoroethylester-poly(ethyleneglycol) 30 were shown to achieve 50 direct colloidal stabilisation of MNPs in water before any further coupling reaction.

Approach developed here

In this study, we propose to accomplish the chemical grafting of 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTS) leading to 55 stable colloidal suspensions of iron oxide MNPs in fluorinated oils. The goal for this magnetic fluid is to remain monophasic whatever the strength of an applied magnetic field, which is the definition of a fluorinated "true ferrofluid" (as opposed to a magneto-rheological fluid, where the MNPs make dipolar chains charge reversal in nitric acid and oxidation with iron nitrate according to Massart procedure. [START_REF] Massart | Magnetics[END_REF] At first, 180 g of FeCl 2 •4H 2 O were mixed with 100 mL of HCl (37%) and 500 mL of deionised water. The mixture was stirred until complete dissolution of FeCl 2 . After addition of 365 mL of FeCl 3 (45%) and 2 L of 10 deionized water, 1 L of ammonia solution (28%) was added rapidly under vigorous stirring (800 rpm). The obtained magnetite particles were then collected after sedimentation on a strong magnet. After precipitate washing with water, 360 mL of nitric acid and 2 L of water were added. Then, 323 g of Fe(NO 3 ) 3 •9H 2 O was added with 800 mL of water in order to oxidise the magnetite particles (Fe 3 O 4 ) into maghemite (γ-Fe 2 O 3 ). After sedimentation and washing, the addition of water leads to the formation of a stable aqueous ferrofluid. The obtained magnetic nanoparticles are dispersed in water due to their 20 positive surface charge in acidic conditions. The synthesised nanoparticles were separated into two fractions of hydrodynamic sizes, respectively D H =11 nm (PDI=0.013) and D H =36 nm (PDI=0.024) following a size-sorting procedure based on the phase-separation by addition of electrolyte in excess to screen electrostatic repulsions. 35 The smaller size fraction (SP aqueous ferrofluid) was used mainly for the study of the fluorinated coating, while the larger size fraction (LP aqueous ferrofluid) enabled preparing magnetic emulsions in water-based gels with larger magnetic susceptibility and thus higher deformations of the 30 drops for a given magnetic field.

Fluorinated surfactant coating

At first, the MNPs were coated with a high molecular weight (7500 g•mol -1 nominal, ∼5000-6000 g•mol -1 measured 36 ) perfluoropoly(ether) end-functionalised with a carboxylic acid group (PFPE-COOH, Krytox 157-FSH, Dupont), which enables transferring them from water to perfluoro-2-butyltetrahydrofuran (Fluorinert FC-75, 3M) (see step 1 on Scheme 1). Krytox 157-FSH was first mixed with ammonium hydroxide to form a carboxylate salt. The aqueous ferrofluid was afterwards directly 40 added to this mixture (molar ratio n(γ-Fe 2 O 3 )/n(PFPE)=30). In such conditions, the deprotonated carboxylic head-groups of Krytox molecules bear a negative charge and adsorb onto the positively charged iron oxide surface. The obtained system was then washed several times with acetone in order to separate the Krytox-covered MNPs from the polar mixture. Then, ethanol was added and the precipitate was collected after sedimentation on a strong permanent magnet. After three steps of washing with ethanol and acetone, fluorinated oil FC-75 was added to achieve a suspension around 4 wt% and the residual solvents were eliminated by heating 1 hour at 353 K in an oven. After this drying step, the ferrofluid is referred to as "surfacted ferrofluid". These surfacted-ferrofluids present a high viscosity (> 0.1 Pa•s) incompatible with deposition through capillaries. This high viscosity is due to the relatively high viscosity of the 55 FC75/Krytox continuous phase. In addition, the interfacial tension γ between the ferrofluid and a dilute Carbopol solution (γ = 25 mN•m -1 as measured using a pendant drop experiment on a pendant and sessile drop tensiometer Krüss DSA100) seems too high to observe a deformation of the droplets under a 60 magnetic field of reasonable intensity (i.e. of induction B less than 60 mT).

Ligand exchange and silanization

A second step of ligand exchange was performed in order to replace Krytox 157-FSH surfactants by organofluorosilanes, 65 with the purpose to graft them covalently onto the MNPs (step 2 on Scheme 1). The chosen fluoroalkylsilane, 1H,1H,2H,2Hperfluorodecyltriethoxy-silane (PFDTS, Sigma-Aldrich) was added in large excess (n(PFDTS)/n(PFPE)=12) directly to the surfactant-based ferrofluid in FC-75 (or another oil from the 70 Fluorinert series such as the less volatile FC-40). It was first hydrolysed and then condensed around the MNPs via a sol-gel process. The reaction occurred under basic conditions with tetramethylammonium hydroxide (TMAOH, Sigma, 0.1 M) as catalyst. The mixture was placed under stirring (300 rpm) for 3 75 days at 343 K. The sample was washed several times with water, ethanol and acetone then isolated on a magnet in order to remove the un-reacted silane. MNPs were finally dried in a vacuum oven during 1 hour at 353 K and afterward dispersed in fluorinated oil FC-75 at a concentration of 300 g•L -1 , representing volume and 80 weight fractions of respectively 6 vol% (15 wt%) of iron oxide according to the mass densities (respectively 5 g•cm -3 for γ-Fe 2 O 3 and 1.77 g•cm -3 for the fluorinated oil FC-75). After this second step, the ferrofluid referred to as "silanized ferrofluid" presents low values of viscosity and interfacial tension with a dilute 85 solution of Carbopol in water (γ = 11 mN•m -1 as measured by a pendant drop experiment).

Preparation of the monodisperse magnetic emulsion

Aqueous gel

A yield-stress (Bingham) fluid was prepared by mixing 0.5 g of 90 ramified sodium poly(acrylate) (Carbopol 2050 ETG) with 100 mL of deionised water at 323 K. After 30 minutes of stirring at 170 rpm, the mixture was aged at room temperature during 40 min. Finally, 0.5 mL of NaOH at 10 M was added under vigorous stirring until pH reached 7±0.5. The gel was then diluted with 95 deionised water to obtain a range of concentration comprised between 0.05 % and 0.5 % w/w then centrifuged at 6000 rpm in order to remove bubbles. Rheological measurements were performed on an AR2000 TA Instruments rheometer to determine the yield stress at different concentrations. The magnetic 100 emulsions described in this paper were prepared in Carbopol gels at 0.05 % w/w. In this case, the rheological curve exhibits a yield stress of 1.5 Pa.

Injection of ferrofluid in gel to prepare the emulsion

Using robotics, the fluorinated ferrofluids were injected through silica capillaries (coated by polyimide) directly into the aqueous gel. Silica capillaries with respective internal diameter (ID)/outer diameter (OD) of 40/105 µm and 75/150 µm purchased from Polymicro Technologies were inserted in corresponding sleeves of OD=1/16″ (respectively F-237 and F-238, Upchurch). The sleeve was inserted in a 10-32 PEEK nut with a 1/16″ OD ferrule (Upchurch) connected through a P-659 Luer-lock to a syringe. As previously done for bubbly media 37 or fluorinated oil 10 droplets, 14, 38 periodic rows of regularly spaced fluorinated ferrofluid droplets were deposited by a motorised motion of the capillary relatively to the gel. The magnetic fluorinated oil was continuously pushed into the aqueous gel by a 0-1.8 bar pressure generator (Elveflow AF1, Elvesys  , Paris, France). Motorised displacements of the plate led to the formation of aligned droplets. Samples made of series of drop lines suspended in the hydrogel were then transferred into the measurement cell (4×4×3 cm 3 ) placed inside an electromagnet made of two solenoids (1000 turns each) connected in serial with a U-shape soft iron polar 20 piece and an air gap of 4 cm. The magnetic induction B was measured by a LakeShore 425 Hall-probe gaussmeter.

Analytical methods

Transmission electron microscopy (TEM)

Transmission electron microscopy micrographs were recorded on 25 a Hitachi H7650 microscope working at 80 kV. Samples were diluted at ∼ 0.2 g•L -1 in the volatile perfluorohexane (Fluorinert FC-72) and sprayed onto the grids. The images were analysed by automated particle counting after threshold and watershed filters using the ImageJ software (http://rsbweb.nih.gov/ij/).

Infrared spectroscopy (ATR IR)

Infrared measurements were performed on a Bruker Tensor 27 FT-IR spectrometer using the Diamond attenuated total reflection method. Samples were analysed after drying during 1 h in a vacuum oven at 353 K. The IR spectra were recorded in the wave number range 800-4000 cm -1 .

X-ray photoelectron spectroscopy (XPS)

A VG Scientific 220 i-XL ESCALAB spectrometer was used for the MNPs surface analysis (at a maximal depth of 10 nm) with a non-monochromatised MgK α source (hν = 1253.6 eV) at 130 W 40 (13 kV and 10 mA). A pressure of 10 -7 Pa was maintained in the chamber during analysis. The analysed area was about 150 μm in diameter. The full spectra (0-1150 eV) were obtained with constant pass energy of 150 eV and high resolution spectra at constant pass energy of 40 eV. Charge neutralisation was required for all insulating samples. The peaks were referenced to Si 2p maximum shifted at 103.5 (± 0.1) eV. High resolution spectra were fitted and quantified using the AVANTAGE software provided by ThermoFisher Scientific. Dynamic Light Scattering (DLS) 50 DLS measurements were performed using a nanoparticle size analyser Vasco DL135 (Cordouan Technologies, Pessac, France) equipped with a diode laser operating at a 650 nm wavelength at 30% of full power, and a photodiode detector collecting backscattered light at an angle of 135°. For each 55 sample, intensity measurements were carried out at 298 K in a multi-acquisition mode implying 20 correlograms. Nanoparticles Z-average diameters and polydispersity indexes (PDI) were obtained by fitting each correlogram with the 2 nd order Cumulants algorithm. The time interval and number of channels 60 were fixed for each measurement at 11 and 1000 µs respectively. Size distribution histograms were generated after each acquisition using the Pade-Laplace inversion algorithm. After accumulation of 20 curves, the obtained histograms were fitted with a lognormal function. Hydrodynamic size histograms obtained by the 65 Cumulants and Pade-Laplce analyses were compared.

Volume fraction measurements

At first, MNPs were diluted, either in dilute HNO 3 (at pH 2) for aqueous ferrofluids or fluorinated oils for the fluorinated ferrofluids (FC-40 or FC-75). They were then analysed by UV-70 Vis. UV-Vis spectra were recorded in quartz cuvettes on a Molecular devices spectrophotometer SpectraMax between 240 and 800 nm by step of 4 nm. Molar concentrations were determined by fitting a calibration curve 39, 40 and converted into volume fractions using the tabulated mass densities. Another 75 method based on direct mass density measurement was employed for highly concentrated suspensions for which the volume was impossible to control with a micropipette. In that case, a plastic tubing of given length and diameter was simply weighted using a precision balance in order to determine the mass density. Then, 80 the MNPs volume fraction φ was computed from the apparent mass density ρ = φ • ρ iron oxide + (1-φ) • ρ FC-75 where ρ iron oxide = 5 g•cm -3 and ρ FC-75 = 1.77 g•cm -3 are the mass densities of iron oxide and of the FC-75 oil, respectively.

Small angle neutron scattering (SANS)

85 SANS curves were recorded on the PACE spectrometer at the LLB-CEA Saclay neutron facility, France. The scattering wave vector q ranged from 0.00243 Å -1 to 0.37 Å -1 thanks to the use of 3 different configurations: "small-q range" (sample-to detector distance D = 4.7 m, neutron wavelength λ = 17 Å), "medium-q 90 range" (D = 3 m and λ = 6 Å), and "large-q range" (D = 1 m and λ = 6 Å). The samples analysed in 1 mm-thick quartz cuvettes exhibited low incoherent background do to their low hydrogen content. The curves were converted into absolute intensity unit (cm -1 ) using calibration by standards (subtraction by empty cell 95 and normalisation by signal of H 2 O to take into account the efficiency of the detector). 41 The SANS curves were fitted with a polydisperse core-shell form factor using the SASView program available at http://www.sasview.org/. The values of neutron scattering length density (SLD) were calculated using chemical 100 formulas and tabulated values of the atomic neutron scattering lengths. These SLD values were 6.98, 2.1, and 3.84×10 10 cm -2 respectively for γ-Fe 2 O 3 , PFDTS, and FC-75.

Superconducting quantum interference device (SQUID)

Magnetisation curves were recorded at room temperature using a 105 Quantum Design MPMSXL SQUID magnetometer from 0 to 7 T. MNPs were diluted at 1 % v/v in HNO 3 pH 2 for aqueous ferrofluids or in FC-75 for fluorinated ferrofluids. A wet mass around 1 mg of each sample (including solvent) was weighted precisely in an impermeable bag sealed at the end of a plastic 110 straw inserted inside the magnet hole. The diamagnetic contribution of the solvent was fitted by a negative slope line (dominant at high field) subtracted from the total curve to yield the contribution of the MNPs only.

Results and discussion

Sizes and colloidal stability of MNPs in fluorinated oils

The size and colloidal stability in fluorinated oils was assessed by DLS (Table 1). Two kinds of fluorinated nanoparticles were prepared in this work. They were noted 'surfacted' after coating with the Krytox surfactant (step 1 on Scheme 1) and 'silanized' after grafting with the PFDTS silane (step 2 on Scheme 1). Sample 1 was prepared from an aqueous ferrofluid of larger nanoparticles noted 'aqueous LP' and used to produce magnetic emulsions in gels whereas Sample 2 was prepared from 'aqueous SP' and used for the SANS experiment described further. The different size measurements performed on the two samples at the various steps of the coating procedure are given in Table 1. The two methods used to analyse the DLS data lead to roughly the same average particles diameters and broadness of the distribution (the values of PDI and CV being clearly correlated). After step 1 (coating with Krytox), the 'surfacted' nanoparticles remain dispersed and stable but exhibit a significant size and sizedispersity increase (Z-average hydrodynamic diameters respectively of 68 and 92 nm for the two batches LP and SP, PDI respectively 0.07 and 0.09). This result indicates that the MNPs tend to aggregate when coated with the fluorinated surfactant Krytox 157-FSH. After step 2 (chemical grafting of perfluorinated organosilane at the particles surface), one observes a decrease of the sizes of MNPs and of the polydispersity index that reveals an improved colloidal stability and a narrower size distribution. The mean diameter of the MNPs and its coefficient of variation (CV), both calculated by fitting the Pade-Laplace histogram with a log-normal distribution law, show the same evolution. The results exhibit a large CV difference between surfacted and grafted ferrofluids. Figure 1 shows the evolution of the hydrodynamic diameter for three different samples before and after grafting with PFDTS. In all cases, each 'silanized' ferrofluid presents a lower hydrodynamic diameter and a sharper distribution than the corresponding 'surfacted' ferrofluid.

Transmission electron microscopy (TEM)

The MNPs were also directly imaged by TEM (Fig. 2). The sizes 50 of the MNPs deduced by image analysis of these pictures are given in Table 1.

The sizes by TEM are much smaller than those deduced by DLS, which indicates that the magnetic cores observed individually on the TEM pictures form clusters of several 55 nanoparticles when suspended in a liquid. To represent the sizedispersity, the histograms of the diameters d obtained by DLS (Fig. 1) or TEM (Fig. 2) were all fitted by a log-normal distribution law of median value d 0 and characteristic broadness σ, as written below. 60 ( )

2 2 0 1 1 exp ln 2 2 d P d d d σ pσ       = -         (1)
On a logarithmic scale such as the horizontal axis of Fig. 1, this distribution looks like a Normal law, σ being the standard deviation of ln(d). Then, the moments of the distribution can be easily calculated. In particular the number-averaged (d n ) and volume-averaged (d w ) diameters are obtained respectively by following formulas.

( )

2 0 exp 2 n d d d σ = < > = ⋅ (2)
( ) For example, 'silanized' sample 1 (LP) has d n TEM =10.2 nm and d w TEM =19.1 nm. At larger magnification (Fig. 3), the TEM micrographs evidence the presence of a shell around the nanoparticles, which has a lower electron density than iron oxide. The size of this shell can be estimated at approximately 3 nm for sample 1 (Fig. 3). These observations are consistent with the formation of multi-layers of fluorinated silica around the MNPs. With sample 2 (from the aqueous SP batch), the size of the MNPs goes from around 5.8 nm before grafting to 7.3 nm after grafting. The 1.5 nm difference can be assigned to the formation of a silica 15 shell around the MNPs.

4 3 2 0 / exp 7 2 w d d d d σ = < > < > = ⋅ (3)

Small angle neutron scattering (SANS)

The effect of the different coatings on the colloidal stability of the MNP dispersion in FC-75 oil was further studied by SANS with the nanoparticles of smaller sizes (sample 2). The intensity curve of the PFDTS-silanized MNPs (Fig. 4) was well fitted by the 25 form factor of a polydisperse suspension of core-shell particles modelled by an inorganic core radius R 0 =2.34 nm (Log-normal dispersion of standard width σ=0.33, SLD of γ-Fe 2 O 3 ) wrapped by a silicon-rich shell of thickness t 0 =1.45 nm (polydispersity σ=0.64, SLD of PFDTS) suspended in a medium with the 30 theoretical SLD of the FC-75 oil. Near the lowest scattering vectors q, the Guinier approximation enables to measure a radius of gyration R G =5.1 nm compatible with individually dispersed MNPs. On the contrary, the curve for the Krytox-surfacted ferrofluid could not be fitted simply by a core-shell form factor, 35 presumably due to the contribution of cross-particle correlations (structure factor due to attractions). In that case, the Guinier plot leads to a gyration radius R G =29 nm, attesting the presence of finite sized aggregates of MNPs. From these SANS experiments, we deduce that the grafting of a fluoroalkylsilane onto the MNPs 40 greatly improved the quality of the dispersion in the fluorinated oil compared to mere adsorption of a fluorinated surfactant. Fig. 4 SANS curves of the Krytox-surfacted ferrofluid (blue markers), PFTDS-silanized MNPs (red markers) and fit by a polydisperse core-shell 45 form factor (solid line). The volume fractions φ are respectively 0.5% and 3.4%. Inset: Guinier plots of ln(I(q)) as a function of q 2 enabling to measure the gyration radii RG from the slope -RG 2 /3.

Spectroscopic study of the chemical grafting onto iron oxide ATR IR spectroscopy 50

The efficiency of the grafting reaction was assessed by FT-IR spectroscopy. We display the spectra in three different regions (800-1400 cm -1 , 1500-2000 cm -1 , and 2500-4000 cm -1 ) in order to ease the discussion on all the information that can be deduced.

Evidence for the surfactant adsorption at the surface of the The band at 1780 cm -1 (d) is assigned to carbonyl stretching of carboxylic acid. 43 For Krytox-surfacted ferrofluid, band (d) is replaced after step 1 by three new bands at 1680 (e), 1630 (f) and 1540 cm -1 (g) respectively attributed to ammonium carboxylate salt of Krytox, asymmetric and symmetric COO -stretching.

These bands indicate that the Krytox carboxylate moiety is chemisorbed onto the metal oxide through bidentate chelating interaction. Now comparing spectra of PFDTS before and after hydrolysis and condensation (Fig. 6: spectra C and D), the spectrum of unhydrolysed PFDTS silane (D) presents a group of three peaks at 2980, 2933 and 2892 cm -1 (h) that can be attributed to the C-H stretching from the CH 2 and O-CH 2 -CH 3 groups of PFDTS. The broad peak observed at 959 cm -1 (i) is characteristic of the Si-OEt bonds. After hydrolysis-condensation, (spectrum C), the intensities of bands (h) and (i) decrease strongly, thus confirming almost total hydrolysis of PFDTS. Indeed, upon hydrolysis, the O-CH 2 -CH 3 groups initially present on the PFDTS molecules are transformed into CH 3 -CH 2 OH and no longer contribute to the signal of band (h). A new broad band appears at 3340 cm -1 (j) corresponding to the O-H stretching mode of H-bonded silanol groups, which shows that condensation is not complete in the bulk system. Spectrum (C) presents three peaks at 1020, 1070 and 1112 cm -1 (k) that can be attributed to the formation of siloxane bonds Si-O-Si. 47 The other bands observed at 1143, 1200 and 1230 cm -1 (l) are characteristic of the -CF 2 stretching modes. Finally, on the spectrum of 'silanized' MNPs (Fig. 7: spectrum E), the almost total disappearance of band (j) observed after grafting onto the MNPs indicates a good condensation of the silica species around the MNPs. 49 Besides, the bands of CH 2 groups of PFDTS are still detected (h), which indicates the presence of fluorosilane at the MNPs surface. Adsorption due to fluorinated chains (l) partially overlap the band at 1070 cm -1 (k) associated with the formation of the Si-O-Si network. The presence of the bands at 1070 and 1112 cm -1 (k) indicates a high degree of condensation of silica species. 47 By comparing spectra (A) and (E) (pure Krytox vs. 'silanized' MNPs), a strong 45 decrease of the intense band (c) at 1230 cm -1 is observed compared to the spectrum of pure Krytox. This is due to the global decrease of the number of CF x groups as the perfluoropoly(ether) Krytox is replaced by the much shorter PFDTS molecule. However, the lower but still present band (a) 50 observed in spectrum (E) indicates that Krytox is still not completely removed even after several washing steps.

XPS spectroscopy

In order to prove the successful grafting of PFTDS onto MNPs, samples were also analysed by XPS. Fig. 8 and Fig. 9 present the evolution of XPS spectra of C 1s and O 1s sub-regions of 60 respectively the Krytox-surfacted and the PFDTS-silanized ferrofluids. The corresponding surface elemental concentrations are presented on Table 2. In the C 1s region (Fig. 8a), the de-convoluted XPS spectrum of the Krytox-surfacted ferrofluid presents three peaks (1, 2, 3) at In the O 1s region (Fig. 9), the XPS spectrum of Krytoxsurfacted ferrofluid exhibits two peaks at 530.2 eV and 535.1 eV attributed respectively to oxygen in γ-Fe 2 O 3 environment and in perfluoropropylene oxide groups of Krytox. The deconvoluted XPS-spectrum of the O 1s region of 'silanized' ferrofluid shows that after grafting, the O 1s peak corresponding to oxygen from maghemite practically disappears in favour of a new peak at 532 eV. This peak at +2 eV is generally attributed to the formation of Si-O bonds and the peak at 531.2 eV, obtained after deconvolution, is assigned to the formation of Fe-O-Si bonds. 51 The strong intensity decrease of the peak at 535 eV, assigned to the oxygen in the perfluoropoly(ether) chains, indicates that Krytox is almost entirely eliminated during the washing steps.

The disappearance of the peak at 530 eV after silanization is correlated to a silica coating around the MNPs. Moreover, the concentration of iron atoms on the surface of MNPs (Table 2) decreases strongly from 1.7 to 0.4 after grafting whereas the concentration of Si reaches 3%. The elemental concentrations measured for the silanized-ferrofluid are extremely closed to those calculated for the pure PFDTS powder. These observations are consistent with the formation of a dense silica shell, which attenuates the photoelectrons and almost totally hides the iron atoms at the surface of the MNPs. The XPS and FT-IR spectroscopy results are thus in total agreement to evidence the success of the chemical grafting reaction of the fluoroalkylsilane onto the iron oxide surface.

Magnetic properties of the suspensions in fluorinated oils

The magnetisation curves of PFDTS-silanized MNPs (sample 1, LP) were obtained from superconducting quantum interference device (SQUID) magnetometry measurements at 300K (Fig. 10). After subtracting the diamagnetic contribution of the solvent, the magnetisation curve M(H) follows a Langevin law describing the progressive orientation of the magnetic moments along the magnetic field followed by their saturation at a plateau value M s . 40 This saturation magnetisation reads M s =φ •m spe , where m spe is the specific magnetisation approximately equal to 3×10 5 A.m -1 for colloidal maghemite nanoparticles and φ is the MNPs volume fraction (the magnetisation being defined as the volume density of magnetic moments). 35 This superparamagnetic behaviour in 45 the liquid state direct evidences that the silanization reaction at the surface of the MNPs enabled their stabilisation against aggregation up to a magnetic field induction as high as 7 T. The magnetic susceptibility defined as the slope of the 60 magnetisation curve at low field was χ = 0.05 for this sample diluted at φ = 1%. Therefore, the susceptibility of samples at any volume fraction can be calculated by χ = 5 •φ. The exact shape of the measured magnetisation curve M(H) could be fitted by the convolution of the Langevin law of superparamagnetism with a 65 size distribution of the MNPs for which we assume a log-normal law 35,52 as written on eqn (1). By this method, we obtained a median diameter d 0 SQUID = 6.1 nm and a logarithmic size width ). This difference is ascribed to the reported presence of a nonmagnetic dead layer around ferrite MNPs 53-55 and to the silicon-rich shell that is visible on the TEM images but does not contribute to the magnetic properties.

Preparation of monodisperse magnetic emulsions as medium for acoustic measurements

To evidence great potential applications of silanized fluorinated ferrofluids to control the propagation of ultrasounds, we prepared narrowly size-dispersed emulsions in a water-based Carbopol yield-stress matrix, whose interest is to oppose to the settlement of droplets by gravity (fluorinated oils being much denser than aqueous media). We show here results obtained with an emulsion prepared with the LP 'silanized' ferrofluid (sample 1) composed of MNPs with a hydrodynamic diameter of 51 nm (PDI = 0.05). The deformation of an isolated ferrofluid droplet by a magnetic 55 field was studied several decades ago both theoretically 56 and experimentally. 57 The model is very robust and has been verified by experimental studies on aqueous biphasic ferrofluids 58, 59 and on non aqueous, e.g. silicone, 60 ferrofluid droplets. Figure 11b shows another representation of the axial aspect ratio a/b of the 60 ferrofluid droplet as a function of the magnetic Bond number B m . This adimensional number compares the magnetic pressure P m acting on the interface between the ferrofluid droplet and the non magnetic gel to the Laplace pressure ΔP L arising from the curvature of the initial droplet: deduced previously. Therefore, even if the applied field is at its lowest value corresponding to B m = 0.04, the pressure acting on the magnetic poles of the droplet is estimated at P m ≈ 0.01×230 = 2.3 Pa. This value is slightly above the yield stress of the gel (measured value of 1.5 Pa). We can thus conclude that the applied magnetic pressure overcomes the yield stress of the water-based gel, which allows fluid motion around the droplet accompanying its deformation. In the opposite situation when the droplet is elongated and the magnetic field is switched off, the slow shape relaxation kinetics can be ascribed to a pressure gap between the poles and the equator of the ellipsoid lower than P m acting when the field was on, but still sufficient to cause the flow of the gel (i.e. above the yield stress).

Monodisperse magnetic emulsions in water-based gel

The silanized fluorinated ferrofluid was injected through a silica capillary (with internal diameter ID of 40 or 75 µm) into the gel (Carbopol 0.05 % w/w) under a controlled pressure. 14, 38 The deposition was obtained by motorised translation of the plate at a speed of 1 mm•s -1 (Fig. 12) for the smaller capillary or 10 mm•s -1 for the larger one. The yield stress of the Carbopol gel was measured at 1.5 Pa for a concentration of 0.05 % w/w by standard rheological measurement (cone-plan AR2000 rheometer). We calculated that fluorinated ferrofluid droplets (mass density d = 1.9 g•cm -3 ) can stay suspended as long as their diameter remains lower than 250 µm (balancing the hydrostatic pressure P h = 2/3r 0 •g•ρ with the yield stress). The sizes of the emulsion droplets were measured on optical images. The mean diameter and coefficient of variation CV (ratio of the standard width to the mean value, assuming Gaussian distribution) were deduced from these measurements on a large statistics. The conclusion is that nearly monodisperse magnetic emulsions can be produced by this method, an example being presented on Fig. 12.

Acoustic properties

These emulsions were analysed by multiple-echo ultrasound spectroscopy acquired with a piezoelectric transducer of 5 MHz central frequency stuck directly on the measurement cell. Fig. 13 65 shows the evolution of the attenuation coefficient vs. frequency curves for magnetic inductions of 0, 20, and 40 mT. Under a zero magnetic field, the attenuation spectrum exhibits several peaks at specific frequencies, in accordance with previous results obtained for pure (non magnetic) fluorocarbon emulsions of CV ≈ 5 %. When a magnetic field was applied perpendicularly to the direction of acoustic wave propagation, these attenuation peaks were shifted towards higher frequencies. This evolution is 80 explained by the change of the shape of the ferrofluid droplets from spheres to ellipsoids, in that case elongated perpendicularly to the US wave propagation vector. Qualitatively, the dimension 0 e → of the droplets probed by the ultrasounds becomes smaller, thus the resonance frequencies become higher. A quantitative description of this effect with a complete fitting of the spectra under varying magnetic field strengths and directions is the subject of another article. 15

Conclusions

In summary, we described a chemical route based on aqueous coprecipitation synthesis, perfluoropoly(ether) surfactant coating, ligand exchange and sol-gel reaction to achieve a stable fluorinated ferrofluid. After silanization, the MNPs coated by a thin fluorinated silica shell can be dispersed individually in any fluorinated oil. The obtained dispersions of silanized-MNPs exhibiting a high colloidal stability and low viscosity were used to prepare monodisperse emulsions in a water-based Bingham gel with a microfluidic injection device. The lower viscosity 15 compared to a mere 'surfacted' ferrofluid is ascribed to the replacement of the macromolecular surfactant with a thin layer of much shorter perfluorosilane (since in the denomination Krytox 157-FSH, the H letter means "high viscosity"). Concerning the decrease of interfacial tension γ with the aqueous phase, this 20 might be a consequence of the ligand-exchange that causes desorption of the surfactant from the iron oxide surface and adsorption of a small fraction of Krytox still remaining after the washing steps (as seen by FT-IR and XPS spectroscopy) at the fluorinated oil/water interface. Such a reducing effect of γ was 25 indeed also noticed with another fluorinated surfactant, Dupont Zonyl FSO (data not shown).

The droplet shape elongates along the direction of the applied magnetic field. These reversible deformations can be perfectly controlled by modulating the magnetic field induction. Acoustic measurements have shown that these emulsions behave as a "resonant material" exhibiting attenuation peaks at specific frequencies in the Mie scattering regime. By applying a magnetic field, we can tune the resonant acoustic response of such a scattering medium in a reversible manner. By varying the magnetic field from to 0 to 40 mT, we can precisely control the attenuation frequency and the attenuation coefficient of the material. The droplets described here have a radius around 100 µm, while typical diameters of "microbubbles" used as contrast agents in ultra-sonography are rather in the 1-10 µm range, i.e. 40 not larger than red blood cells, to avoid clogging of blood vessels or kidney. Nevertheless, magnetic emulsions allow convenient size selection using a method based on their sensitivity to an external magnetic field, by forming dipolar chains preferably made of droplets of analogous diameter, and that can be separated

  Scheme 1 Fluorinated ferrofluid prepared in two steps. The proper dispersion of iron oxide nanoparticles in a fluorinated oil involves: Step 1: adsorption of a perfluoropoly(ether) surfactant (Krytox 157-FSH) onto the nanoparticles and phase transfer into a fluorinated oil such as perfluoro-2butyltetrahydrofuran (Fluorinert FC-75); Step 2: ligand exchange, and then chemical grafting (sol-gel reaction) of 1H,1H,2H,2Hperfluorodecyltriethoxysilane (PFDTS) directly onto the hydroxyl groups of the iron oxide surface. The two steps were performed in alkaline media (respectively ammonium hydroxide and tetramethylammonium hydroxide).

a

  2 nd order cumulant fit (Z-average diameter and polydispersity index) and b Pade-Laplace fit (mean diameter, coefficient of variation) of the DLS correlograms for aqueous, Krytox-'surfacted' and PFDTS-'silanized' ferrofluids. Sample 1 was prepared from the large fraction (aqueous LP). Sample 2 was prepared from the smaller size fraction (aqueous SP). c NM: not measured.

Fig. 1

 1 Fig. 1 Evolution of the hydrodynamic diameter at different steps of the 45

Fig. 2

 2 Fig. 2 TEM micrographs and corresponding size histograms deduced from image analysis and fitted with a log-normal function. a) 'Silanized' sample 1 (LP, 9.3 ± 3.4 nm); b) 'Surfacted' sample 2 (SP, 5.8 ± 2.6 nm); c) 'Silanized' sample 2 (SP, 7.3 ± 2.3 nm). Scale bars represent 100 nm.

5

 5

Fig. 3

 3 Fig. 3 High resolution TEM micrographs of the 'silanized' sample 1 (LP). The scale bar represents 50 nm. The thickness of the silica-rich shell is measured at several locations on this image.20

55 MNPs (step 1 )

 551 can be deduced from the comparison of the spectra of pure Krytox 157-FSH and of the Krytox-surfacted ferrofluid (Fig.5: Spectra A and B). The IR spectrum of pure Krytox 157-FSH contains a large band at 980 cm -1 (band (a) on Fig.5) characteristic of the stretching of CF 3 bonds in the 60 fluoropropylene oxide groups of the perfluoro-poly(ether) backbone.42 The broad band at 1117 cm -1 is ascribed to C-O stretching mode (b). Absorption bands at 1180, 1200 and 1230 cm -1 correspond to different stretching modes of CF x groups (c).

44 - 46 Fig. 5

 44465 Fig. 5 ATR-IR spectra of Krytox 157-FSH (spectrum A) and of Krytox-surfacted MNPs (spectrum B) in two wave number ranges. The letters indicate absorption peaks discussed in text.

48

 48

Fig. 6

 6 Fig. 6 ATR-IR spectra of hydrolysed and condensed PFDTS (spectrum C) and of un-hydrolysed PFDTS (spectrum D) in two wave-number ranges. The letters indicate absorption peaks discussed in text.

Fig. 7

 7 Fig. 7 ATR-IR spectra of Krytox 157-FSH (spectrum A), PFDTS powder (spectrum C), and PFDTS-coated MNPs (spectrum E). The letters 55

Fig. 8

 8 Fig. 8 De-convoluted XPS spectra in the C1s region for a) the Krytox-65

  70 294, 292 and 291 eV attributed respectively to carbon in CF 3 , CF 2 and CF groups of Krytox. The peak (4) at 288 eV is assigned to carbon of carboxylate groups. After grafting of PFDTS onto the MNPs (Fig. 8b), a new peak at 285 eV reveals the presence of carbon in alkyl chains of PFDTS, 50 such CH 2 groups being absent 75 in the Krytox perfluoropoly(ether). The strong decrease of the intensity of the peak at 294 eV indicates that Krytox is almost entirely eliminated.

Fig. 9

 9 Fig. 9 XPS spectra in the O1s region for Krytox-surfacted ferrofluid (black line), PFDTS-silanized ferrofluid (red line) and deconvolution of XPS spectrum of PFDTS-silanized ferrofluid (dotted line).

50 Fig. 10

 5010 Fig. 10 a) Magnetisation curve of the PFDTS-silanized ferrofluid sample 1 (LP) in FC-75 (φ = 1% v/v) by SQUID normalised by the saturation value at high field; inset: Log-normal distribution of magnetic inorganic core diameters: by number (black line) and by volume (red line); b) Magnetisation curve on a log-log plot to highlight the fit quality at low 55

σ

  SQUID = 0.42. The inset of Fig. 10 a) shows the particle size distribution by number and by volume. The number-averaged diameter d n SQUID = 6.7 nm is determined by eqn (2), whereas the volume-averaged diameter d w SQUID = 11.4 nm is calculated by eqn (3). These two values are both smaller than those obtained by TEM images analysis (d n TEM = 10.2 nm and d w TEM = 19.1 nm

20 TEM 35 Fig. 11 a

 203511 Fig. 11 a) Evolution of the axial aspect ratio a/b of a droplet of the PFDTS-silanized ferrofluid suspended in a water-based gel, of initial radius r 0 = 104 µm, under increasing values of magnetic induction B. The apparent hysteresis when the field is decreased back to zero is ascribed to slow relaxation of the gel medium 40

=

  µ 0 = 4π×10 -7 kg•m•A -2 •s -2 is the magnetic permeability of vacuum, γ is the interfacial tension between the ferrofluid and the surrounding fluid (here the aqueous yield-stress gel), and the values of magnetisation M were 70 obtained from the SQUID measurement curve M/M s vs. B at low field (inset of Fig. 11b) with M s = φ •m spe where φ = 6 % and m spe = 3×10 5 A•m -1 . The droplet radius ( ) is measured at 104 µm from the volume V that remained constant within 1 % when 75 plotting V = 4π a•b 2 /3 whatever the applied field strength (data not shown). The values of B m were calculated using M values corresponding to each magnetic field applied to elongate the droplet of PFDTS-silanized ferrofluid. We observed that the axial aspect ratio a/b varies perfectly linearly with B m . Such linearity is generally observed at low B m values, typically for aspect ratios below 3. 57 Introducing a theory of the deformation of magnetic droplets at any B m values, Tsebers 56 showed that at low deformation, when the eccentricity of the ellipsoidal shapeexperimental plot of a/b vs. B m with a line of slope π/2 and intercept 1, we get the value γ = 11.8 mN•m -1 for the interfacial tension, in excellent agreement with the interfacial tension γ = 11 mN•m -1 measured by the pendant drop method. The Laplace pressure difference L the interior of the initially spherical droplet and the gel is estimated at ΔP L = 230 Pa with the interfacial tension

Fig. 12

 12 Fig. 12 Optical micrograph and corresponding size histogram deduced 55

38

 38

70

  These attenuation peaks result from Mie resonances created by the sound-speed contrast existing between the host matrix and the fluorinated ferrofluid droplets.

Fig. 13

 13 Fig. 13 Acoustic spectrum (attenuation coefficient vs. wave frequency) 75

26, 28-30

  Apart from APTS, other organosilanes were grafted onto the surface of iron oxide MNPs, such as (3-glycidyloxypropyl)trimethoxysilane,31

40 cyanoethyltrimethoxysilane,

Table 1

 1 Hydrodynamic (DLS) and physical (TEM) diameters of the nanoparticles for several prepared ferrofluids LP (Sample 1)

		Cumulants a	Pade-Laplace b	TEM analysis
		Z-ave	PDI	<D>	CV (%)	<D>±ΔD
	Aqueous	(nm) 36.5	0.024	(nm) 35.9	6.9	(nm) NM c
	Surfacted	68	0.07	79.3	135	NM c
	Silanized	51	0.05	51.5	6.4	9.3±3.4
			SP (Sample 2)
		Cumulants a	Pade-Laplace b	TEM
						analysis
		Z-ave	PDI	<D>	CV (%)	<D>±ΔD
	Aqueous	(nm) 11	0.013	(nm) 10.1	11.2	(nm) NM c
	Surfacted	91.5	0.09	99.1	79.6	5.8±2.6
	Silanized	56.2	0.065	57.3	8.6	7.3±2.3

Table 2

 2 Surface elemental concentration obtained by XPS

		Surface elemental concentration (%)
		Si2p	O1s	C1s	F1s	Fe3p
	γ-Fe2O3 control a	2.06	51	27.19	4.34	15.4
	Surfacted-ferrofluid	0.31	11.16	27.57	59.19	1.77
	Silanized-ferrofluid PFDTS powder b	3.2 4.19	6.22 6.35	29.01 30.24	61.24 59.22	0.33 0

a possibly contaminated by PFDTS; b after hydrolysis-condensation.

Synthesis of magnetic iron oxide nanoparticles

Magnetic nanoparticles (MNPs) were prepared by coprecipitation of iron +II and iron +III salts in alkaline media followed by surface by a field strength gradient. 61, 62 Starting from a polydisperse magnetic emulsion, this size-grading method can lead to monodisperse emulsions with target sizes ranging from the submicrometric up to the millimetric range, thus opening the way to acoustic applications from the audible (20 Hz -20 kHz) to the 50 US regime (100 kHz -10 MHz). The locally resonant emulsions described in this article appear to be the constitutive brick elements for the preparation of tuneable acoustic materials. 63

Notes and references