Sami Fawaz

Olivier Mansier

Yann Pucheu

Séverine Marti

Harmony Leroy

Astrid Gaufroy

Jean Broitman

Chloe James

Thierry Couffinhal
email: thierry.couffinhal@inserm.fr

Clonal haematopoiesis and cardiovascular diseases: A growing relationship

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Summary

Cardiovascular diseases, particularly atherothrombosis, are the leading cause of death worldwide, but their mechanisms are not yet fully understood. Traditional cardiovascular risk factors have been known for many years, but are not enough to predict individual risk. Clonal haematopoiesis of indeterminate potential (CHIP) has been described recently; it corresponds to the clonal expansion of a population of haematopoietic cells in response to the acquisition of a somatic mutation, without any clinical or biological sign of haematological malignancy. The prevalence of this condition increases with age, reaching 10-20% of the general population aged > 70 years. Recent observational studies have shown a link between CHIP and cardiovascular diseases in humans, revealing that CHIP carriers have a higher risk of myocardial infarction, heart failure and severe aortic valve stenosis. The prognosis of these conditions also seems to be altered by the presence of CHIP. Experimental studies have identified that the immune system and inflammation -particularly interleukin-1β-secreting macrophages -play a critical role in enhancing the cardiovascular consequences of CHIP, through their action on the atherosclerotic plaque and myocardial tissues. We aimed to write an extensive review of what is currently known about CHIP and its cardiovascular consequences, the pathophysiological mechanisms leading to the increased cardiovascular risk and, finally, the expected influence on our daily practice and how we care for patients with CHIP.

Résumé

Les maladies cardiovasculaires, en particulier l'athérosclérose, sont la principale cause de décès dans le monde, mais leur pathogénie n'est pas encore complètement élucidée. En effet, les facteurs de risque cardiovasculaires traditionnels sont bien connus depuis de nombreuses années mais ne

Background

Atherothrombosis is the main cause of death worldwide. Although our knowledge and ability to treat patients has improved in recent years, some aspects of the disease remain unclear. We know from our daily practice that people without any traditional cardiovascular risk factors can experience cardiovascular events. Furthermore, within a group of people of the same age bearing the same cardiovascular risk factors, some will experience myocardial infarction and others will not. Thus, traditional risk factors account for only 75-80% of events associated with cardiovascular disease (CVD) [START_REF] Berry | Lifetime risks of cardiovascular disease[END_REF]. This underscores the need to identify other contributing factors to enhance our understanding of the disease and improve strategies for both treatment and prevention.

A new concept has emerged recently, which places the role of the haematopoietic system at the centre of this investigation. Here we provide a review of the current state of knowledge about clonal haematopoiesis of indeterminate potential (CHIP), its link with CVD, and the potential implications for our daily practice, both current and future.

CHIP

A somatic mutation is a genetic alteration acquired by a cell that can be passed to the progeny of the mutated cell during the course of cell division. Somatic mutations differ from germ-line mutations, which are inherited genetic alterations that occur in the germ cell. Somatic mutations accumulate in normal tissues as a function of time. The majority of these mutations have no effect on cell behaviour, so selection does not act upon them. Rarely, a mutation will arise that confers a growth advantage to the cell in which it occurs. Such a mutation would allow that cell and its progeny, referred to as a "clone," to expand progressively over time. This is now known to occur in a number of tissues, particularly in elderly individuals. When this happens in a haematopoietic stem cell (HSC), a condition known as "clonal haematopoiesis" appears as the result of the contribution of the mutated clone to the production of a substantial proportion of mature blood cells (Fig. 1). Clonal haematopoiesis is well known in the scope of haematological malignancies as being a step towards overt neoplasms, such as acute myeloid leukaemias, when the somatic mutation involves a leukaemia driver gene.

CHIP was described several years ago, and is defined by the clonal expansion of a haematopoietic cell population in response to the acquisition of a leukaemia driver gene mutation, at a variant allelic fraction (VAF) ≥ 2%, without any biological or clinical sign of haematological malignancy [START_REF] Steensma | Clonal hematopoiesis of indeterminate potential and its distinction from myelodysplastic syndromes[END_REF]. Mutations observed in CHIP frequently involve epigenetic regulators, the main ones being DNMT3A, TET2 and ASXL1 (Table 1) [START_REF] Jaiswal | Clonal hematopoiesis in human aging and disease[END_REF]. Because these somatic mutations do not significantly disturb cell differentiation, mutated mature cells are produced as normal cells, and circulate in the peripheral blood. Thus, during this premalignant state, patients have totally normal blood cell counts.

The involvement in CHIP of the JAK2 V617F mutation, which is known for being involved in myeloproliferative neoplasms, is a matter of debate [START_REF] Cassinat | Clonal Hematopoiesis and Atherosclerosis[END_REF][START_REF] Jaiswal | Clonal Hematopoiesis and Atherosclerosis[END_REF][START_REF] Phillips | Clonal Hematopoiesis and Atherosclerosis[END_REF].

How to detect CHIP

Because the acquired mutation is present in mature blood cells, detecting CHIP is feasible in deoxyribonucleic acid (DNA) from circulating leukocytes from a blood sample. A bone marrow test is not necessary. Next-generation sequencing has been used as the reference technique for genomic DNA sequencing for almost a decade; its cost has decreased rapidly over the last few years, currently allowing the entire genome to be sequenced in a few days for approximately €1000. However, wholegenome sequencing is a suboptimal technique to search for CHIP because of its lack of sensitivity. As CHIP is defined as the presence of a mutation with a VAF ≥ 2%, more than 500 DNA molecules would need to be analysed to obtain reliable results for a given gene. To date, such a sequencing depth cannot be achieved easily using whole-genome or even whole-exome sequencing.

Many genes have been identified as being involved in CHIP. However, most of the time, a patient with CHIP has only one mutation. Moreover, only some of these mutations have been identified as mutated recurrently in CHIP, and are detected in > 90% of CHIP carriers (Table 1). The three most frequently mutated genes (TET2, DNMT3A and ASXL1) represent > 70% of CHIP cases.

Thus, restricting sequencing analysis to a panel of genes is a worthwhile strategy for detecting CHIP. Indeed, by limiting the genomic regions analysed, targeted sequencing allows the depth of sequencing to be increased, thus improving the ability to detect CHIP with low-VAF mutations. Such a strategy also reduces the cost and time of analysis. In this way, CHIP could be detected in routine practice in a week for less than €100.

Prevalence

In 2014, two studies evaluated the prevalence of individuals presenting with CHIP (Fig. 2) [START_REF] Genovese | Clonal hematopoiesis and blood-cancer risk inferred from blood DNA sequence[END_REF][START_REF] Jaiswal | Age-related clonal hematopoiesis associated with adverse outcomes[END_REF]. When analysing blood samples, CHIP seems to be rare before the age of 40 years; its prevalence then increases with age, with an estimated frequency of 5% at 60 years, and reaching 10-20% of those aged > 70 years. Recently, Busque et al. studied a population with a mean age of 75 years, and found that 23% of the cohort had CHIP [START_REF] Busque | High-sensitivity C-reactive protein is associated with clonal hematopoiesis of indeterminate potential[END_REF]. Some authors reported a higher prevalence of CHIP in healthy individuals (40% after 50 years of age) when detecting mutations in the bone marrow with a sensitivity threshold of 1% [START_REF] Guermouche | High prevalence of clonal hematopoiesis in the blood and bone marrow of healthy volunteers[END_REF].

CHIP and haematological malignancy

Although Jaiswal et al. demonstrated that CHIP carriers had an 11-fold increased risk of developing haematological malignancies (myelodysplastic syndromes, acute leukaemias or even lymphomas), the absolute risk of evolving to a haematological cancer remains low (about 1% per year).

CHIP links to CVD (Table 2) Atherothrombosis

Surprisingly, when Jaiswal et al. looked at the prognosis of CHIP carriers in 2014, they found that in addition to an increased risk of haematological malignancies, they had an increased risk of overall mortality [START_REF] Jaiswal | Age-related clonal hematopoiesis associated with adverse outcomes[END_REF]. This increased mortality was driven primarily by increased cardiovascular-related mortality, and -interestingly -not deaths resulting from cancer. In this study, the risk of cardiovascular ischaemic events was increased by a factor of 2, and the risk of ischaemic stroke by a factor of 2.6. Because of these surprising results, the same team tested the hypothesis that CHIP is correlated to CVD. They chose two cohorts of people with mean ages of 60 and 70 years, bearing cardiovascular risk factors, and followed them for several years; they found a twofold increase in the risk of coronary artery disease in CHIP carriers compared with non-CHIP carriers [START_REF] Jaiswal | Clonal Hematopoiesis and Risk of Atherosclerotic Cardiovascular Disease[END_REF]. Interestingly, in two additional cohorts of people aged < 50 years, they found a fourfold increase in early onset myocardial infarction in those who presented with CHIP [START_REF] Jaiswal | Clonal Hematopoiesis and Risk of Atherosclerotic Cardiovascular Disease[END_REF]. Additionally, there was a significant increase in the coronary artery calcium score evaluated by computed tomography in CHIP carriers: compared with non-carriers, CHIP carriers had a threefold increased risk of having a coronary artery calcium score > 615 Agatston units, which identifies those at highest risk of coronary artery disease [START_REF] Genovese | Clonal hematopoiesis and blood-cancer risk inferred from blood DNA sequence[END_REF][START_REF] Jaiswal | Clonal Hematopoiesis and Risk of Atherosclerotic Cardiovascular Disease[END_REF].

These observations raise the question of how much influence CHIP has in the development of CVD, when compared with traditional risk factors, such as smoking, hypertension, dyslipidaemia, diabetes or family history of CVD. Jaiswal and Libby thus studied the weight of each factor in the development of CVD, based on previous cohorts, and found that the presence of CHIP could be one of the strongest factors associated with CVD, conferring a higher relative risk than smoking, hypertension and dyslipidaemia [START_REF] Jaiswal | Clonal haematopoiesis: connecting ageing and inflammation in cardiovascular disease[END_REF]. However, some questions remain. It is still not known whether CHIP is an independent risk factor or if its presence potentiates existing ones, increasing the risk of CVD developing in people harbouring traditional risk factors.

Heart failure and valvular heart disease

CHIP made an entrance into the world of CVD through the study of atherosclerotic CVD, but the association is likely to be much deeper. Dorsheimer et al. studied the prevalence and prognosis of CHIP in a population of 200 patients with chronic heart failure, with a mean age of 65 years [START_REF] Dorsheimer | Association of Mutations Contributing to Clonal Hematopoiesis With Prognosis in Chronic Ischemic Heart Failure[END_REF]. The team found an increased prevalence of CHIP in this population compared with what is described in the general population, with more than 18% of the total cohort harbouring CHIP [START_REF] Genovese | Clonal hematopoiesis and blood-cancer risk inferred from blood DNA sequence[END_REF][START_REF] Jaiswal | Age-related clonal hematopoiesis associated with adverse outcomes[END_REF][START_REF] Zink | Clonal hematopoiesis, with and without candidate driver mutations, is common in the elderly[END_REF]. This increased prevalence was constant in each age category, with a more than twofold increase in all groups.

Consistent with previous data in the general population, DNMT3A and TET2 were the two genes most affected by somatic mutations. CHIP carriers appeared to have a significantly worse prognosis than non-carriers regarding both death and hospitalization for heart failure. Indeed, 37% of patients harbouring CHIP died during a 4-year follow-up period, versus 24% of those who did not have CHIP.

The risk of death combined with rehospitalization for heart failure was multiplied by a factor of 3.

Interestingly, there seemed to be a dose-response effect between a poor prognosis and the clone size estimated by the VAF.

More recently, Mas-Peiro et al. studied the prevalence and prognosis of patients harbouring

CHIP among a population with a mean age of 83 years undergoing transcatheter aortic valve replacement (TAVI) for the treatment of degenerative aortic valve stenoses [START_REF] Mas-Peiro | Clonal haematopoiesis in patients with degenerative aortic valve stenosis undergoing transcatheter aortic valve implantation[END_REF]. They found a higher prevalence of CHIP in this population than in healthy controls, with approximately 33% of the TAVI group harbouring a mutation in DNMT3A or TET2 (the only two genes studied) at a VAF ≥ 2%. Similar to the general population, the prevalence seemed to increase with age, from 25% in patients between the ages of 55 and 69 years, to more than 52% in those aged > 90 years. Again, as in atherosclerotic CVD and heart failure, the prognosis seemed to be worse in the presence of a candidate driver gene mutation. Over an 8-month follow-up period, it was found that CHIP carriers had a fourfold increase in their risk of death.

Taken together, these observations strongly suggest that clonal haematopoiesis may play a role in the development of different CVDs.

How does CHIP increase cardiovascular risk?

Experimental approaches

In 2017, Fuster et al. evaluated the role of TET2 mutations in the development of atherosclerosis in mice [START_REF] Fuster | Clonal hematopoiesis associated with TET2 deficiency accelerates atherosclerosis development in mice[END_REF]. Using a mouse model in which 10% of HSCs were inactivated for TET2, they observed an amplification of blood cells bearing the TET2 mutation, as well as a marked increase in atherosclerotic burden, with a 60% increased thickness of the atherosclerotic plaques in the aortic root compared with control mice. This finding is consistent with the results of a study conducted by Jaiswal et al. in the same year, which reported that mice bearing a homozygous mutation of TET2 showed an increase in atherosclerotic lesions of the aorta, from 1.4-fold in the aortic root to 2.7-fold in the descending aorta compared with control mice [START_REF] Jaiswal | Clonal Hematopoiesis and Risk of Atherosclerotic Cardiovascular Disease[END_REF].

Both studies showed that macrophages, which populated the plaques and harboured the mutation, had enhanced inflammatory activity, producing more cytokines, particularly interleukin (IL)-1β. Of note, Fuster et al. showed that this overproduction could be explained by an increase in activity of the nucleotide-binding oligomerization domain-like receptor family pyrin domain containing 3 (NLRP3) inflammasome, an intracellular oligomeric protein complex that cleaves the pro-IL-1β into IL-1β (Fig. 3) [START_REF] Fuster | Clonal hematopoiesis associated with TET2 deficiency accelerates atherosclerosis development in mice[END_REF].

Other studies have focused on mechanisms underlying the relationship between CHIP and heart failure. Sano et al. demonstrated in both ischaemic and non-ischaemic heart failure models that TET2 mutations were associated with adverse remodelling, a decrease in left ventricular ejection fraction and an increase in left ventricular systolic and diastolic volumes, as well as larger areas of fibrosis in the marginal zone [START_REF] Sano | Tet2-Mediated Clonal Hematopoiesis Accelerates Heart Failure Through a Mechanism Involving the IL-1beta/NLRP3 Inflammasome[END_REF]. As for atherosclerosis, this phenomenon was linked with greater macrophage infiltration in the marginal zone, and the expression of various proinflammatory cytokines and chemokines, including IL-1β. In the same year, Sano et al. also demonstrated in another heart failure model that DNMT3A mutations were responsible for a similar proinflammatory phenotype of mutated myeloid cells [START_REF] Sano | CRISPR-Mediated Gene Editing to Assess the Roles of Tet2 and Dnmt3a in Clonal Hematopoiesis and Cardiovascular Disease[END_REF]. In summary, these studies support the hypothesis that TET2 and DNMT3A mutations confer an advantage to mutated cells, as well as a proinflammatory phenotype, responsible for enhanced recruitment of leukocytes to cardiovascular tissues, and leading to the development of atherosclerosis or heart failure (Central illustration). This deleterious phenotype involves the NLRP3 inflammasome, which could represent a therapeutic target in various CVDs linked to CHIP. By comparison, little is known about the pathophysiology linking other CHIP-associated mutations to cardiovascular complications.

Human data

Although studies conducted in a mouse model have suggested that CHIP triggers an inflammatory state, few studies have been conducted in humans to verify this hypothesis. High-sensitivity CRP (hsCRP) has been used as a predictor of cardiovascular events for many years, as it reflects the patient's inflammatory state and their susceptibility to plaque rupture [START_REF] Rifai | High-sensitivity C-reactive protein: a novel and promising marker of coronary heart disease[END_REF]. In a recent study, Busque et al. suggested that CHIP carriers had slightly higher concentrations of hsCRP than non-carriers [START_REF] Busque | High-sensitivity C-reactive protein is associated with clonal hematopoiesis of indeterminate potential[END_REF].

Mas-Peiro et al. also speculated that CHIP was associated with a proinflammatory state in patients [START_REF] Mas-Peiro | Clonal haematopoiesis in patients with degenerative aortic valve stenosis undergoing transcatheter aortic valve implantation[END_REF]. Thus, TET2 mutations would be linked to an increase in the frequency of inflammatory monocytes known to be involved in atherosclerosis development, whereas DNMT3A mutations would favour a proinflammatory polarization of T lymphocytes.

Implications for clinical practice

What is the place for CHIP detection in the management of CVD?

Despite accumulating data, clonal haematopoiesis still needs to be validated as a full-fledged cardiovascular risk factor. To do this, observational studies are needed to reassess the prevalence of CHIP in the general population, as well as in patient populations seen in cardiology clinics worldwide.

These studies should seek to confirm the link between the presence of CHIP, a higher inflammation level and a more pronounced atheroma volume.

The medical community will need to assess the best way to include CHIP in daily practice.

The first question is how to select patients for CHIP screening. We could focus on patients who have a cardiovascular event without a high level of risk, based on traditional risk factors. This would allow us to better understand why the event happened, and give us tools to improve prevention of a recurrence alongside standard care. Another interesting population to screen could be patients with high concentrations of hsCRP, because they may represent a population with a high prevalence of CHIP.

In order to let inflammation decrease and avoid an overestimation as a result of the acute cardiovascular event, hsCRP can be measured a few months after the event, when inflammation comes back to its basal level [START_REF] Lafitte | High sensitivity C reactive protein, fibrinogen levels and the onset of major depressive disorder in post-acute coronary syndrome[END_REF]. Finally, we could propose screening of patients presenting with a severe atheroma load, such as cases of multitroncular coronary artery disease or extensive peripheral artery disease. Screening for CHIP in the scope of primary prevention will be central in the future, if CHIP is verified as a risk factor with as much weight as smoking or dyslipidaemia in the development of CVD. Given that CHIP could have a synergistic action with other traditional cardiovascular risk factors, it will be important to adjust current guidelines to account for this risk.

We have seen that screening for CHIP can be quite easy to perform in practice. It can fit to standard care requirements, in that the material is simple to get via a standard blood sample, the results are available within a few days and the costs are decreasing to the point where they will likely become affordable for routine practice.

The identification of CHIP in our daily practice will then raise the question of how to treat these patients.

Anti-inflammatory therapies

Recent studies have tested the hypothesis that anti-inflammatory molecules could improve the prognosis of people with CVD. In 2017, Ridker et al. studied the effect of canakinumab, a human monoclonal antibody targeting IL-1β, in a population of patients with a history of myocardial infarction who had an hsCRP blood concentration of ≥ 2 mg/L [START_REF] Ridker | Antiinflammatory Therapy with Canakinumab for Atherosclerotic Disease[END_REF]. The team showed a decrease in the risk of cardiovascular deaths, non-fatal myocardial infarctions and non-fatal strokes in patients treated with the highest doses of canakinumab after nearly 4 years of follow-up. Canakinumab significantly decreased hsCRP blood concentrations. Interestingly, a subgroup analysis showed a trend towards a reduction in the risk of hospitalization for heart failure and heart failure-related mortality in patients who were receiving canakinumab [START_REF] Everett | Anti-Inflammatory Therapy With Canakinumab for the Prevention of Hospitalization for Heart Failure[END_REF]. These data suggest that targeting IL-1β and reducing the degree of inflammation may be an efficient way to reduce the risk of adverse cardiac events in secondary prevention.

Although canakinumab seems interesting in the prevention of CVD in patients at risk, it is not yet an approved treatment or used routinely in cardiology. Another study therefore evaluated the benefit of a widely used anti-inflammatory therapy -colchicine [START_REF] Tardif | Efficacy and Safety of Low-Dose Colchicine after Myocardial Infarction[END_REF]. This drug is known to reduce inflammation through various mechanisms, in particular interference with NLRP3 inflammasome, resulting in reduction of IL-1β release, which in turn reduces the production of various cytokines and proinflammatory molecules, such as IL-6, tumour necrosis factor α and C-reactive protein [START_REF] Slobodnick | Colchicine: old and new[END_REF]. In the COLCOT trial, Tardif et al. compared the effect of colchicine or placebo on prognosis in patients who had recently had a myocardial infarction. The authors showed a significant reduction in major adverse cardiovascular events, mainly driven by a decrease in the risk of stroke and the need for urgent coronary revascularization. These data demonstrate the beneficial effect of targeting the immune system and reducing inflammation to prevent CVD. CHIP would be responsible for a low-grade inflammation that could favour CVD. Therefore, we can speculate that the efficacy of anti-inflammatory drugs could be mainly driven by an effect on CHIP carriers, not yet identified in these studies, who could be more receptive because of their proinflammatory state [START_REF] Svensson | TET2-Driven Clonal Hematopoiesis Predicts Enhanced Response to Canakinumab in the CANTOS Trial: An Exploratory Analysis[END_REF]. While we wait for more specific treatments, antiinflammatory therapies may be one way to properly treat patients with CHIP.

Conclusions

CHIP is a common condition, particularly in the elderly, and is associated with an increased risk of CVD. Recent studies have suggested its implication in the pathogenicity of CVD, and models have shown that inflammation and dysregulation of the immune system are at the core of this involvement.

As our knowledge grows, development of specific treatments will be a goal in order to move towards more personalized care and improve prevention of CVD.

Sources of funding

None.

  Abbreviations: CHIP, clonal haematopoiesis of indeterminate potential; CVD, cardiovascular disease; DNA, deoxyribonucleic acid; HSC, haematopoietic stem cell; hsCRP, high-sensitivity Creactive protein; IL, interleukin; NLRP3, nucleotide-binding oligomerization domain-like receptor family pyrin domain containing 3; TAVI, transcatheter aortic valve replacement; VAF, variant allelic fraction.

Central illustration. Clonal haematopoiesis of indeterminate potential (CHIP) occurs particularly in the elderly; it is the result of an acquired mutation occurring in a population of haematopoietic stem cells, conferring a growth advantage to their progeny. Because of the proinflammatory phenotype, it favours atherothrombosis, leading to coronary artery diseases such as myocardial infarction, and is also associated with heart failure (ischaemic and non-ischaemic) as well as aortic stenosis, altering the prognosis of CHIP carriers.

Disclosure of interest 12

The authors declare that they have no conflicts of interest concerning this article.


M C S Ferreira

B Furieri
email: bruno.furieri@ufes.br

J M Santos

A Ould

El Moctar

J.-L Harion

A Valance

P Dupont

N C Reis

An experimental and numerical study of the aeolian erosion of isolated and successive piles

Keywords: Wind Erosion, Pavement, Fugitive Emissions, Turbulent flow, Wind Tunnel, Emission Measurements

Open storage yards at industrial sites usually comprise several piles of granular materials, representing a particulate matter source that may significantly deteriorate air quality. The aeolian erosion of stockpiles is affected by changes in airflow patterns due to the pile shape and the presence of nearby piles or buildings. The aim of this study was to analyse the impact of wind erosion of successive parallel stockpiles on flow behaviour and particle emissions. A wind tunnel experiment was conducted in six configurations: one isolated pile and two successive piles separated by gaps of 0.9h and 1.8h (h is the pile height) oriented to 60 • and 90 • with respect to the main wind flow direction. The particles in the piles had a bimodal particle size distribution consisting of sand that was erodible (white) and non-erodible (black) in the investigated velocity range. The contrasting colours enabled the visualisation of the non-erodible sand accumulation. The mean field of the wall shear stress distribution and flow pathlines predicted by numerical simulation were associated with the experimental erosion patterns. The emitted mass was experimentally quantified as the difference between the initial and final

Introduction

Diffuse emission from the aeolian erosion of granular materials from stockpiles or exposed granular beds may be difficult to quantify due to the large extension and shapes of the sources and the factors that affect the process such as atmospheric conditions (e.g., wind velocity and direction, precipitation, humidity and temperature), particle size distribution, modification of the wind flow due to the presence of obstacles and topography (e.g., flat or inclined surfaces). Granular materials typically have a wide particle size distribution, including larger grains that may not be lifted by wind. As erosion occurs, the concentration of the coarser particles on the pile surface increases. It has been verified that the accumulation of these grains plays a protective role in particle emissions, whether on a flat or an inclined surface [START_REF] Furieri | Effects of non-erodible particles on aeolian erosion: Wind-tunnel simulations of a sand oblong storage pile[END_REF][START_REF] Li | Aerodynamic dislodgement of multiple-size sand grains over time[END_REF][START_REF] Smith | Aeolian processes as drivers of landform evolution at the South Pole of Mars[END_REF][START_REF] Swet | Reduction in soil aggregation 16[END_REF][START_REF] Yang | A continuously weighing, high frequency sand trap: Wind tunnel and field evaluations[END_REF]. Non-erodible particles create wake zones, reducing the drag on pile zones that would otherwise be erodible. A temporal decrease thereby occurs for the emitted mass flux, and the total amount of particles emitted due to wind erosion is strongly reduced, which is known as the pavement phenomenon.

The most widely used methodology to estimate fugitive dust emissions has been developed by the USEPA (United States Environmental Protection Agency) [START_REF]2.5 Industrial Wind Erosion[END_REF], based on data from wind tunnel experiments for two geometries of isolated stockpiles and three wind flow directions, and does not consider the pavement phenomenon. The rapid decrease in aeolian emissions linked to a wide size distribution is mentioned in USEPA [START_REF]2.5 Industrial Wind Erosion[END_REF], but it is not explicitly incorporated in the proposed method. The particle size distribution is also not explicitly incorporated into the method. So, if the model was applied to two granular materials with very different particle size distributions but the same maximum particle diameter, it would result in similar emission values. Experimental results show that wind erosion emissions can be widely different with the same maximum particle diameter but a greater proportion of larger particles which are non-erodible. Furthermore, the various configurations of different piles shapes that can be found at industrial sites are not covered by the elements provided by USEPA [START_REF]2.5 Industrial Wind Erosion[END_REF]. Industrial facilities usually contain more than one storage pile, which greatly affects erosion patterns since piles act as obstacles to the free path of atmospheric flow, modifying the flow field dynamics. Therefore, it is important to broaden the models application to more realistic situations. Several researchers have already performed this using both numerical (Computation Fluid Dynamics -CFD) and experimental (wind tunnel) approaches to investigate the physical phenomenon of wind erosion of granular materials beds and storage piles [START_REF] Ferreira | Numerical and wind tunnel modeling on the windbreak effectiveness to control the aeolian erosion of conical stockpiles[END_REF][START_REF] Huang | Part I : a priori study of erosion and deposition with large eddy simulation of turbulent flow over multiple 2D sandy Gaussian hills[END_REF][START_REF] Kheirabadi | Sediment flux, wind erosion and net erosion influenced by soil bed length, wind velocity and aggregate size distribution[END_REF][START_REF] Lopes | Numerical simulation of sand dune erosion[END_REF][START_REF] Novak | Numerical Modeling of Dust Lifting from a Complex-Geometry Industrial Stockpile[END_REF][START_REF] San | Numerical simulation and optimization study of the wind flow through a porous fence[END_REF][START_REF] Turpin | Numerical modeling of flow structures over various flat-topped stockpiles height: Implications on dust emissions[END_REF][START_REF] Webb | Threshold wind velocity dynamics as a driver of aeolian sediment mass flux[END_REF][START_REF] Yeh | An investigation into the sheltering performance of porous windbreaks under various wind directions[END_REF]. Badr and Harion [START_REF] Badr | Effect of aggregate storage piles configuration on dust emissions[END_REF] and Torano et al. [START_REF] Torano | Influence of the pile shape on wind erosion CFD emission simulation[END_REF] numerically predicted the wind flow over isolated piles in different scenarios with various geometries and wind orientations and used the USEPA formulation for the emission factor to quantify particle emissions using the friction velocity previously calculated by CFD.

Turpin and Harion [START_REF] Turpin | Effect of the topography of an industrial site on dust emissions from open storage yards[END_REF] investigated the flow structures over coal stockpiles in an actual power plant configuration. The authors performed numerical simulations in a configuration that included three coal stockpiles and different buildings in the surrounding area, and found that upstream buildings significantly increased emissions from the piles. Nevertheless, if the stockpile was located upstream of the buildings in the configurations tested by Turpin and Harion [START_REF] Turpin | Effect of the topography of an industrial site on dust emissions from open storage yards[END_REF], then the erosion processes was slightly attenuated by a flow stagnation zone upstream of the buildings. The authors concluded that all wind perturbations including surrounding buildings and stockpiles have an impact, and must be accounted for in dust emissions estimation. Diego et al. [START_REF] Diego | Simultaneous CFD evaluation of wind flow and dust emission in open storage piles[END_REF] and Cong et al. [START_REF] Cong | Effect of aggregate stockpile configuration and layout on dust emissions in an open yard[END_REF] employed similar techniques to estimate dust emissions by integrating CFD data into the USEPA methodology. The former work studied the wind flow around an arrangement of two parallel flat-crested stockpiles separated by a gap of 0.44h (where h is the height of the piles) and highlighted the different contribution from each pile to the total eroded dust. The latter work evaluated dust emissions in an open yard with a complex geometry (16 stockpiles arranged in 4 columns) varying the pile shape (flat-topped oval and conical), the gap between the piles (0.6h and 1.2h) and the orientation of the wind direction (from 0 to 90 • ). In both studies, a sheltering effect created by the neighbouring piles was noticed, which resulted in lower dust emissions. It is found that the front pile created a downstream shelter, then the wind velocity over the back pile was reduced. However, Cong et al. [START_REF] Cong | Effect of aggregate stockpile configuration and layout on dust emissions in an open yard[END_REF] verified that the total dust emissions are greater when the gaps between piles are larger. The authors implied that a gap larger than the shelter area completely exposes the downstream piles to wind erosion, similar to an isolated pile. In addition, it has been suggested that more studies with arbitrary space variation between the piles are necessary to better understand the role of the stockpile layout in limiting wind erosion in an actual yard. Furieri et al. [START_REF] Furieri | Comparative analysis of dust emissions: Isolated stockpile VS two nearby stockpiles[END_REF] compared emissions (using CFD and emission factors) from an isolated and two successive oblong stockpiles separated by 0.9h and 1.8h gaps oriented by 60 • to the main wind flow direction. Numerical results showed that an isolated stockpile emitted less dust than each pile in the successive pile configuration. Therefore, a protective role of the upstream pile was not observed. Ferreira and Fino [START_REF] Ferreira | A wind tunnel study of wind erosion and profile reshaping of transverse sand piles in tandem[END_REF] performed wind tunnel experiments to study the erosion of an isolated sinusoidal pile and the interference effect resulting from two closely spaced piles (gaps of 0 and h). The authors found that the aeolian erosion of the downstream pile was considerably larger due to flow disturbances induced by the first pile. Therefore, the literature is contradictory, and the wind erosion of two parallel piles still requires additional investigation.

This study aimed to investigate, through numerical and experimental approaches, the impact of several configurations of parallel piles on wind erosion and the effect on emissions of the proportion of non-erodible particles. The parameters analysed were (i) wind velocity, (ii) the orientation of the piles to the main flow direction and (iii) the gap distance between the piles. A wind tunnel experiment was performed to estimate and compare emissions from an isolated sand pile and from two successive piles. The particles that constituted the piles had a bimodal particle size distribution encompassing erodible and non-erodible particles in two different proportions. The accumulation of non-erodible particles enabled an analysis of the erosion patterns of the piles. Numerical simulations with identical configurations were carried out to obtain the wall shear stress distribution on the pile surface and the flow pathlines, which support our understanding of the physical phenomena.

Experimental study

The experiment was conducted in the wind tunnel facilities at the Industrial Energy Department (IMT Lille Douai, France). Figure 1 shows the experimental set-up and the principal dimensions of the wind tunnel. Multiple roughness obstacles were placed in rows close to the tunnel entrance to enable the formation of a turbulent boundary layer. The validation of the experimental profiles upstream the test section is based on the comparison between measured profiles of velocity and turbulent kinetic energy (carried out in the same wind tunnel by Turpin [START_REF] Turpin | Amélioration des modèles de quantification des émissions particulaires diffuses liées à l'érosion éolienne de tas de stockage de matières granulaires sur sites industriels[END_REF] and Furieri et al. [START_REF] Furieri | Experimental surface flow visualization and numerical investigation of flow structure around an oblong stockpile[END_REF]) and the profiles of fully developed turbulent boundary layer from the literature [START_REF] Schlichting | Boundary Layer Theory[END_REF]. In the vicinity of the wall, the velocity profile follows the linear law and then, moving away from the wall, follows the logarithmic law. The wind-tunnel generates a fully developed turbulent boundary layer at the level of the measurement zone with a thickness greater than the stockpile height (δ = 16 cm > h = 7.7 cm). More details of the wind-tunnel characteristics are given by Furieri et al. [START_REF] Furieri | Experimental surface flow visualization and numerical investigation of flow structure around an oblong stockpile[END_REF] and Turpin [START_REF] Turpin | Amélioration des modèles de quantification des émissions particulaires diffuses liées à l'érosion éolienne de tas de stockage de matières granulaires sur sites industriels[END_REF]. The Reynolds number of the fluid flow inside the wind-tunnel is approximately between 36000 and 48000 based on the free stream velocity (from 6 to 8 m/s) and stockpile height.

A bimodal granulometry of sand with density equal to 2650 kg/m 3 was used to represent erodible and non-erodible particles: fine white sand and coarse black sand with ranges of diameters from 56.0 to 194.2 µm and from 700.0 to 1300.0 µm, respectively. The sand colours allowed the visualisation of high shear stress zones, which were marked by the accumulation of non-erodible particles.

The threshold friction velocities u * t min and u * t max at which the largest erodible particles (194.2 µm) and the smallest non-erodible particles (700.0 µm), respectively, are lifted from a horizontal surface were calculated using the take-off criterion obtained by Shao and Lu [START_REF] Shao | A simple expression for wind erosion threshold friction velocity[END_REF]:

u * t s = 0.11 ρ P -ρ ρ gD + γ ρD , (1)
where u * t s is the threshold friction velocity, ρ is the fluid density, ρ P is the particle density, g is the gravity, D is the particle diameter and γ is a surface energy that characterises the cohesion. Shao and Lu [START_REF] Shao | A simple expression for wind erosion threshold friction velocity[END_REF] recommended values of γ ranging between 1.65 × 10 -4 and 5.00 × 10 -4 kg/s 2 . In the present work, γ = 2.86 × 10 -4 kg/s 2 was adopted. This value was calculated by fitting Equation 1 to the experimental threshold required to lift loose particles as proposed by Kok and

Renno [START_REF] Kok | Enhancement of the emission of mineral dust aerosols by electric forces[END_REF].

Figure 2 shows the take off-criterion, the fine and coarse sand size ranges used for the experiments and the friction velocities u * t min = 0.23 m/s and u * t max = 0.42 m/s. The corresponding freestream flow velocities U min and U max , determined based on u * min and u * max , were calculated using the expression proposed by Kurose and Komori [START_REF] Kurose | Turbulence structure over a particle roughness[END_REF]:

u * = u * smooth (1 + 0.00431h + ) , (2)
where h + is the dimensionless mean diameter of the non-erodible particles (h + = h N EP u * s /ν, where h N EP is the mean diameter of the non-erodible particles, and ν is the kinematic viscosity), and u * smooth is the friction velocity in a smooth wall.

It is calculated by the relation proposed by Mollinger and Nieuwstadt [START_REF] Mollinger | Measurement of the lift force on a particle fixed to the wall in the sublayer of a fully developed turbulent boundary layer[END_REF]:

u * smooth = 0.036U ∞ + 0.033 . (3)
Equation 2 represents the effects of roughness elements on the vertical velocity profiles. Specifically, in the present experiments the roughness length is assumed to be the diameter of the coarse (non-erodible) particles. Therefore, Equations 2 and 3 yielded U min = 5.5 m/s and U max = 9.5 m/s. Three different velocities were experimentally tested: 6, 7 and 8 m/s. These velocities were between U min and U max , the minimum and maximum velocities at which the finer and the coarse particles remained erodible and non-erodible, respectively, for a horizontal surface.

However, the threshold friction velocity u * t s (θ) on a surface with a slope tan θ is different from that found in a flat bed due to the distinct relative contributions of the forces acting on the particle. If the slope is positive (flow upwards), gravity is a resistive force, and thus, the friction velocity must be stronger to lift the grains.

In contrast, if the slope is negative (flow downwards), the friction velocity required to lift the particles is lower than the threshold friction velocity for the beds.

The sand stockpile models were formed inside the wind-tunnel using a device similar to an industrial hopper, shown in Figure 3(a). Furieri et al. [START_REF] Furieri | Effects of non-erodible particles on aeolian erosion: Wind-tunnel simulations of a sand oblong storage pile[END_REF] have performed several tests to ensure that the pile shape and dimensions given by this device were reproducible. The dimensions of the sand stockpile had a scale ratio of approximately 1:200 to an actual stockpile, and they are shown in Figures 3(b) and 3(c): 7.7 cm (height), 23.6 cm (length), 57.9 cm (width), and 34.5 • of angle of repose. It must be mentioned that the experiments did not accurately simulate full-scale conditions because the pile dimensions were smaller than the saturation length, the length that transport requires for saturation (i.e., to reach the saturated flux), which is approximately 1 to 2 m [START_REF] Andreotti | Measurements of the aeolian sand transport saturation length[END_REF]. Therefore, the experimental sand piles did not behave in a similar way to piles of scale several metres from the erosion point of view: the first would be primarily eroded by aerodynamic entrainment, while the second would be subject to impact entrainment.

Configurations of one isolated stockpile and two successive stockpiles were used in the wind tunnel experiments. For the two pile configuration, the edge-to-edge separations tested were 0.9h and 1.8h, where h is the height of the pile. The effect of the stockpile orientation to the main wind flow direction was analysed for 60 • and 90 • . The piles were built with a mixture of the erodible and non-erodible sand with two different mass fractions of non-erodible particles (α N E ): 10 and 20%.

The six pile configurations used in the 36 tests are shown in Table 1.

After determining the initial stockpile mass and setting it on the wind tunnel floor, the free stream velocity was set by a frequency controller, and the fan was turned on. The duration of the flow was 15 minutes in all cases. No additional emissions occurred after this period, and a progressive pavement process that finally suppressed erosion at the eroded areas was observed. A camera installed over the wind-tunnel ceiling (transparent glass wall) registered the erosion evolution by analysing the contrasting colours (black and white sand). The photographs were taken at the beginning of the experiment and every 30 seconds for 5 minutes. The last picture was taken after 15 minutes. These pictures allowed for a qualitative assessment of the temporal evolution of different wind erosion exposure zones on the pile.

For the two parallel pile configuration, each pile was weighed separately after the experiment. The mass of particles emitted was calculated as the difference between the initial and final stockpile weights.

The repeatability of the experimental measurements was tested using the configuration with α N E = 20%, orientation 90 • , 1.8h gap and U ∞ = 7 m/s. The emitted mass for the three different tests of repetitions had a reasonable coefficient of variation equal to 4.5% (the coefficient of variation is defined as the ratio of the standard deviation to the mean and is given by 23.3 and 524.3 grams, respectively).

Numerical simulations set up

Numerical simulations were performed to solve the flow structure over several pile configurations representing the wind tunnel experiment (see Table 1). The threedimensional Reynolds Averaged Navier-Stokes equations of mass and momentum were solved using the commercial software Fluent 15.0 [START_REF]ANSYS Fluent User's Guide[END_REF], providing the mean fields of the flow properties. The k -ω Shear Stress Transport (SST) model was used to incorporate turbulence effects. The governing equations of transport are described below.

-Continuity equation: -Momentum conservation:

∂(ρu i ) ∂x i = 0 (4)
∂(ρu i u j ) ∂x j = ∂τ ij ∂x j -ρδ 3i g (5)
τ ij = 2µS ij -p + 2 3 µ ∂u k ∂x k δ ij (6)
S ij = 1 2 ∂u i ∂x j + ∂u j ∂x i (7)
Where, δ ij represents the Kronecker Delta, g is the acceleration of gravity The model uses the following two transport equations for k and ω:

[m/s 2 ], p is the mechanical pressure [N/m 2 ], τ ij is the stress tensor [N/m 2 ],
∂k ∂t + ∂k ∂x j ( ūj ) = P k -β * kωY k + ∂ ∂x j (ν + σ k ν t ) ∂k ∂x j (8) ∂ω ∂t + ∂ω ∂x j ( ūj ) = αS 2 -βω 2 + (ν + σ ω ν t ) ∂ω ∂x j + 2(1 -F 1 )σ ω2 1 ω ∂k ∂x i ∂ω ∂x i (9)
Where the coefficients and auxiliary relations are given by:

ν t = a 1 k max(a 1 ω, SF 2 ) ( 10
)
F 1 = tanh min max 2 √ k β * ωy , 500ν y 2 ω , 4σ ω2 k CD kω y 2 4 ( 11
)
CD kω = max 2ρσ ω2 ∂k ∂x i ∂ω ∂x i , 10 -10 (12)
F 2 = tanh max( 2 √ k β * ωy , 500ν y 2 ω ) 2 (13)
P k = min min τ ij ∂u i ∂x j , 10β * ky , C lim ρ (14)
The tensor of the turbulent stresses is treated as:

τ i,j = µ t ∂u i ∂x j + ∂u j ∂x i - 2 3
∂u k ∂x k δ i,j - 2 3 ρkδ i,j ] (15)
The constants of k-ω SST model are summarized as follows:

σ k = 0.85; σ k,1 = 1.0; σ ω,1 = 0.5; σ ω,2 = 0.856; β 1 = 0.075; β 2 = 0.0828; β * = 0.09; α ∞,1 = 5 9 ; α ∞,2 = 0.44; a 1 = 0.31; C lim = 10.
This turbulence model was chosen based on a study performed by Badr and

Harion [START_REF] Badr | Numerical modelling of flow over stockpiles: Implications on dust emissions[END_REF], who simulated analogous configurations under similar flow conditions and compared the results using different closure models. Figure 4 presents the computational domain dimensions and boundary conditions. The numerical model of the stockpile presents a not evolving surface during erosion. The pile retained its shape during erosion in the proposed model. A no-slip condition was set at this boundary. The inlet boundary conditions for the velocity, the turbulent kinetic energy and the specific dissipation rate were retrieved from a converged field obtained by previous numerical simulations of a flat plane surface with the same dimensions, for which a periodic streamwise flow was set. In these precursors numerical simulations, we imposed a mass flow rate correspondent to the desired free stream velocity. The converged vertical velocity profile is logarithmic. The inlet profiles of wind velocity and turbulence of the numerical simulations were validated using the experimental data carried out by Turpin [START_REF] Turpin | Amélioration des modèles de quantification des émissions particulaires diffuses liées à l'érosion éolienne de tas de stockage de matières granulaires sur sites industriels[END_REF] in the same wind tunnel described in Section 2. A fully developed flow was assumed for the outflow conditions, that is, all flow variables except pressure were assumed to have a zero normal gradient. The SIMPLEC algorithm was used for pressure-velocity coupling [START_REF] Van Doormaal | Enhancements of the SIMPLE method for predicting incompressible fluid flows[END_REF]. Symmetry conditions were applied to the upper domain boundary (normal gradients of all variables were set to zero). Finally, no-slip conditions were imposed for the lateral boundaries, the ground and the pile surface.

The geometries and meshing were generated with Gambit software (Geometry and Mesh Building Intelligent Toolkit) [START_REF]ANSYS Gambit User's guide[END_REF]. The mesh was built by an extrusion of triangular face cells from the pile and ground walls towards the upper wall of the computational domain creating triangular-based prism cells (see Figure 5).

The grid is irregular following the shape and orientation of the geometries. A mesh refinement near the walls (y+ < 5) was required due to the expected intense gradients close to these areas and due to turbulence modelling requirements.

Mesh sensitivity tests were previously carried out for similar configurations [START_REF] Badr | Numerical modelling of flow over stockpiles: Implications on dust emissions[END_REF].

Turbulence model choice was based on previously validated numerical calculations performed by Badr and Harion [START_REF] Badr | Numerical modelling of flow over stockpiles: Implications on dust emissions[END_REF] and Turpin et al. [START_REF] Turpin | Numerical modelling of aeolian erosion over rough surfaces[END_REF]. For the different configurations, the computational grids ranged from 3800000 to 5300000 cells. The simulations with two successive piles arrangement required the larger number of cells in order to allow sufficient refinement between the piles.

Results

The results are presented in three sections. In the first section, the erosion patterns are interpreted in the light of the numerical data of the basal shear stress map.

Then, a general overview of the pavement process for a tested case is presented.

Finally, the mass loss measurements for the different configurations are discussed in the third section. The isolated stockpile will subsequently be referred to as pile p 1 , and the upstream and downstream piles in the successive arrangements will be referred to as piles p 2 and p 2 , respectively. Distinctive wind erosion regions are highlighted according to the degree of wind exposure. To facilitate the analysis, 4 zones were highlighted on the isolated stockpile and are designated as (region B 2 , with high levels of shear). Nevertheless, especially for the gap of 0.9h, the dimensionless values of shear stress for the piles oriented 90 • (region B 2 ) were higher than for the pile with no interference (region B 1 ), with maximum values of 8.6 and 6.9, respectively. These findings are consistent with the erosion patterns observed in Figure 7(a).

An asymmetrical pattern was detected in the experimental and numerical results, despite the symmetrical geometry. The literature reported similar behaviour of the flow around geometrical symmetric obstacles [START_REF] Gopalan | Numerical study of the flow interference between tandem cylinders employing non-linear hybrid URANS-LES meth-ods[END_REF][START_REF] Grandemange | Turbulent wake past a threedimensional blunt body. Part 1. Global modes and bi-stability[END_REF][START_REF] Kim | Measurements of the flow fields around two square cylinders in a tandem arrangement[END_REF][START_REF] Zdravkovich | Review of flow interference between two circular cylinders in various arrangements[END_REF] as a consequence of the bi-stable intermittent nature of the flow in which the wake switches randomly at irregular intervals from the sides of the pile between the two stable states. The numerical simulation of bi-stability is a difficult task, due to its very long timescale. The average timescale of the RANS numerical simulations may not be sufficiently large to comprise the switch of the wake timescale.

Figure 7(a) revealed an accumulation of sand on the ground between the piles for the closely spaced configuration (0.9h gap). In addition, the interference of pile p 2 in the leeward wall of pile p 2 was more important in this case. When comparing the 0.9h gap to the 1.8h gap, the zone of ineffective erosion C 2 was smaller, and the black zone of high erosion D 2 was larger. For the 1.8h gap, the wake region of pile p 2 resembled the wake region of pile p 1 and the extent of areas C 2 were comparable, except for the narrow region of high friction that was impacted by the vortex (zone D 2 ) that was transposed to the lateral side due to the bi-stability.

A more severe erosive impact was observed for the second stockpile on the windward wall of pile p 2 . Figure 7 In the wake zone behind pile p 2 , the regions C 2 and D 2 were also highlighted.

The similarity between these zones can be noticed for the 0.9h and 1.8h gaps; however, in both cases, D 2 is larger than the corresponding zone in pile p 1 (zone D 1 ), which emphasises the impact on the downstream pile. The impact of this vortex can be clearly observed in zones D 1 and D 2 , a region with higher friction and significant agglomeration of black particles, although it had a weaker effect on pile p 1 . Moreover, the vortex had a greater effect on the closely spaced piles. Indeed, it impinged on the windward p 2 pile wall increasing the friction on the upper part of region B 2 .

The unexpected situation in zone A 2 with a low shear stress level and a high erosion rate was also seen for the 60 • orientation. This situation can be again attributed to the impact of the emitted particles from pile p 2 . Further evidence supporting the effect of particles in saltation from pile p 2 on the erosion of pile p 2 can be seen in Figure 10, which shows the final photographs and the dimensional contours of the shear stress for the velocities 6 and 8 m/s with the 1.8h gap and

α N E = 20%. In this figure the results are presented in the dimensional form in order to compare the absolute values of the shear stress for both velocities. The highlighted area in zone B 2 (red line) was highly eroded for U ∞ = 8 m/s due to the high levels of shear and the windward wall of pile p 2 was also highly eroded, as shown in Figure 10(a). Accordingly, as erosion is weaker for U ∞ = 6 m/s, the red region in zone B 2 and its corresponding directly frontal region in pile p 2 remained uneroded (see Figure 10(b)). Similarly, the upper part of region A 2

(highlighted with blue line) was highly eroded, suggesting emissions due to the impact of saltating particles from region D 2 of pile p 2 .

A significant concentration of coarse black particles in the windward wall of pile p 2 , as shown in Figure 9(a), reinforces the strong impact of a second parallel stockpile on erosion, especially for the smaller gap (0.9h).

The patterns on the leeward walls of pile p 2 behaved in a similar manner as the leeward walls of piles p 2 although with much less erosion intensity. In Figure 9(a), zone D 2 is larger than zone D 2 , especially for the 1.8h gap.

Visual analysis of the erosion patterns

Figure 11 illustrates the experimental temporal evolution of wind erosion for both granulometries (with α N E = 10 and 20%) and for the lowest and highest wind velocities (U ∞ = 6 and 8 m/s). Considering that similar pavement phenomenon behaviour was observed for all arrangements, a configuration with an orientation 60 • and a 0.9h gap was chosen to represent a typical case.

As mentioned in Section 2, the pavement phenomenon was observed after 15 minutes of wind exposure for all configurations, which means that the final erosion patterns are well represented in the fifth column of photographs shown in Figure 11. The extent of the final eroded areas varied little with the increase of α N E , although the final concentration of non-erodible particles in these areas was higher for α N E = 20% than for α N E = 10%, especially for U ∞ = 8 m/s. On the other hand, a significant increase of the eroded areas occurred as flow velocity increased.

For instance, modifications of the windward surface of the upstream pile are almost imperceptible for U ∞ = 6 m/s whilst the same region was highly eroded for

U ∞ = 8 m/s.
The temporal evolution of the pavement shown in the Figure 11 indicates that the phenomenon occurred faster in tests with a larger amount of non-erodible particles and higher velocities. Indeed, for α N E = 20% and U ∞ = 8 m/s, after 30 seconds a larger quantity of black particles accumulated than with α N E = 10% and U ∞ = 6 m/s, and after 1 minute and 30 seconds, the observed pattern was found to be very close to the final pattern. This behaviour was related to a temporal decrease in the emitted mass flux. The highest pavement rate for α N E = 20% supports previous findings in the literature [START_REF] Furieri | Effects of non-erodible particles on aeolian erosion: Wind-tunnel simulations of a sand oblong storage pile[END_REF].

Emission estimates

Table 2 shows the emitted mass measurements from an isolated stockpile (p 1 ) and from each stockpile in the successive arrangement (p 2 and p 2 ). The first result worth noting is the remarkable amount of sand emitted from the downstream pile p 2 . In all test cases, the emissions were higher for the pile p 2 , compared to either pile p 2 or p 1 , which is consistent with the experimental erosion patterns discussed in Section 4.1. The reason for this result is the emissions from the upward wall of pile p 2 due to the impact of saltating particles from pile p 2 (see Figure 10).

Pile p 2 had higher emissions in the 60 • arrangement if the stockpiles were separated by a 0.9h gap because of higher shear stress values, a greater effect of the main vortex shed from pile p 2 and interactions between the piles (more energetic saltation particles). These facts do not apply to pile p 2 (wall shear stress distribution on pile p 2 is quite similar for both gaps for the orientation of 60 • ), and the critical importance of the gap on the emitted mass for this pile was not observed. Nevertheless, the shear levels of pile p 2 were higher than those of pile p 1 , and the amount of emitted mass was also higher.

On the other hand, with the piles oriented 90 • to the main flow, pile p 2 and pile p 2 had higher emissions for stockpiles separated by a gap distance of 1.8h. However, this result is ambiguous because, as mentioned in Section 4.1.1, an accumulation of sand in the floor between the piles was noticed for the 0.9h gap. This mass was not counted in the balance as emitted because it was hard to distinguish from which pile it arose (it was even hard to separate the sand from the piles). For the same reason, a rigourous comparison of the emitted mass from piles p 2 (with 0.9h gap)

and p 1 is difficult; however, we noticed that the values were close. Nevertheless, emissions from pile p 2 were slightly lower for stockpiles separated by a gap distance of 1.8h than from an isolated stockpile. Therefore, the emissions of each pile in the configuration with two piles depend on the orientation of the flow. It was only the upstream pile of a pair in the 90 orientation that experienced less erosion that an isolated pile.

Figure 12 shows the total emitted mass amount from successive stockpiles compared to twice the amount from an isolated stockpile (which could mimic a

situation in an open industrial yard in which the piles are far enough away from each other to not interfere) for all tested configurations. The total amount from the two successive stockpiles was greater for both orientations, and in some cases, the emitted mass could be more than three times higher, compared to two times the value from an isolated pile.

Furthermore, Figure 12 highlights that erosion was greater in configurations with a main wind flow direction oriented 60 • than in those oriented 90 • to the piles, especially if the piles are separated by a gap distance of 0.9h. In addition, for the same value of the free stream velocity, emissions with α N E = 20% were approximately half that of emissions with α N E = 10%. Therefore, the proportion of non-erodible particles has a considerable effect on particle emissions.

Conclusion

The purpose of this study was to investigate the aeolian erosion of stockpiles with a bimodal granulometry formed by erodible and non-erodible sand using wind tunnel experiments. Numerical simulations were performed to supply wind flow data (i.e., shear stress distribution and pathlines) in order to support an understanding of the phenomenon.

Temporal evolution of the pavement phenomenon was analysed using top view photographs of the piles, studying the progress of wind erosion exposure zones.

It has been verified that the phenomenon occurred faster for larger amounts of non-erodible particles and higher velocities.

Wind flow modifications and their effects on wind erosion were numerically and experimentally assessed for two pile orientations to the main wind flow direction (60 • and 90 • ) and three different arrangements: an isolated stockpile and two successive stockpiles separated by 0.9h and 1.8h gaps. For the perpendicular arrangement it was observed highly eroded zones in which the incident flow accelerated on the crest and lateral sides and zones of weaker erosion downstream of the piles, after the flow separation. For the orientation of 60 • , the erosion was stronger due to higher shear stress values and the effects of the main vortex on the leeward wall of the piles. A strong impact on the downstream pile was observed for both orientations. The downward wall of these piles had a highly eroded region, even though the shear stress values were low (zone A 2 ) due to the impact of the eroded particles from the upstream pile. Therefore, it is important to consider this behaviour in future numerical modelling studies. With the exception of zone A 2 , good agreement was found between the numerical distribution of the wall shear stress and the agglomeration of non-erodible particles on the stockpile surface.

The results indicate that, in addition to the wind velocity, the orientation of the pile to the prevailing wind direction also plays an important role to limit aeolian erosion in actual storage yards. Industries yards may redesign their piles arrangements according to the most frequent wind direction based on local meteorological data. Oblique piles shown to be more eroded when located near to each other. On the other hand, perpendicular piles presented a protective effect. It is a general recommendation that the stockpiles should be oriented perpendicular to the wind direction.

The experimental quantification of the mass emitted revealed that the emissions from two consecutive stockpiles are greater than twice the emissions from an isolated stockpile for both orientations. The presence of a successive pile had a strong effect and increased erosion. In other words, the total emitted mass would be underestimated if it was taken as twice the emissions calculated for an isolated stockpile. In addition, the piles oriented 60 • emitted larger amounts of mass than piles oriented 90 • in all configurations, especially for the 0.9h gap. Therefore, the results may suggest that a larger single pile has less impact than two parallel piles.

However, experiments with gaps larger than 1.8h should be conducted.

The emission models of wind erosion are based on the local wall friction velocity which are well transposable at the real scales. Based on the work of Chew et al. [START_REF] Chew | Flows across high aspect ratio street canyons: Reynolds number independence revisited[END_REF] the Reynolds number of our experiments and numerical simulations (about 25000) satisfies the independence criterion. However, it is important to note that for studies involving atmospheric particle transport to describe the physics of particulate matter or sediment transport, scaling parameters considering particles characteristics should be considered. We propose for future works, studying the dispersion of emitted particles (multiphase flow), that the drag force and gravitational force on particles should satisfy similarity conditions. These days, few works are devoted to these investigations.

Where, t

  represents the time [s]; ρ is the specific mass of the fluid [kg/m 3 ]; u i is the instant component of the velocity in the i direction [m/s 1 ]; x i identifies the coordinate in the direction i [m].

  µ is the molecular viscosity [kg/m.s] and S ij is strain rate tensor [1/s]. A variant of the k-ω model is the k-ω SST (Shear Stress Transport) model. Boussinesq hypothesis is used in the turbulence closure model. The k-ω SST is indicated for flow with adverse pressure gradients. This model includes two modifications: (i) The turbulent kinematic viscosity is modified according to equation 10 which best represents the shear stress transport effects for this type of flow and (ii) the addition of a term in the equation 9 of the diffusion of ω is a specific function that allows the validity of the equations in the zones near the wall or distant.

4. 1

 1 Comparison between basal shear stress maps and erosion patternsAs previously mentioned, the erosion patterns were identified by the agglomeration of non-erodible (black) particles in the final experimental images, after the pavement phenomenon. The surface distribution of the black particles enabled the identification of pile zones in which the shear stress was great enough to erode fine particles (i.e., white particles) and offers support to the investigation of the effect of a second stockpile on the erosion pattern. This section mainly focuses on the experiment in which the wind erosion was more evident (U ∞ = 8 m/s and α N E = 20%) to highlight the erosion patterns in different areas of the piles surface.The numerical results are presented in dimensionless form as local shear stress values were divided by a reference shear stress value (τ ref ) obtained for an undisturbed zone where the flow was not affected by the stockpile. Although the shear stress increased with an increase of the wind flow velocity, the flow pattern characteristics given by τ /τ ref remained quite similar for each velocity tested. Similarly, the mean fluid flow pathlines around the piles were coloured based on the ratio of the velocity to a reference free stream velocity (U ∞ ).

A 1 ,

 1 Figures6 and 7show the photographs taken after the pavement phenomenon, the mean wall shear stress distribution and pathlines, respectively, for an isolated stockpile and two stockpiles oriented 90 • to the main flow direction (separated by gaps of 0.9h and 1.8h).

Figure 6 (

 6 Figure 6(a) shows that no erosion occurred in the stagnation zone A 1 . Indeed, this zone was characterised by intense deceleration and wall shear stress levels near zero (see Figures 6(b) and 6(c)). The incident flow was progressively accelerated towards the crest and the lateral sides of the pile forming region B 1 , where the highest levels of shear occurred due to a strong velocity gradient (Figures 6(b)and 6(c)). Therefore, these areas were the most vulnerable to wind erosion, which explains the accumulation of non-erodible black particles seen in Figure6(a).

Figure 6 (

 6 Figure 6(c) shows the flow separation from the pile surface on the crest and the lateral sides, as a result of a strong adverse pressure gradient. The separation led to a wake region downstream of the pile (zone C 1 ), with low wall shear stress levels and weak erosion (see Figures 6(a) and 6(b)).Figure 6(c) reveals two counter-

Figure 6 (

 6 c) reveals two counterrotating vortices on the lateral sides that interact with the separation from the crest. The complex three-dimensional structures formed in this recirculating region created a small zone with higher shear (zone D 1 ), in which a slight accumulation of black sand was noticeable (see Figures6(a) and 6(b)). Between the two main counter-rotating vortices there is a flow acceleration region that slightly affects the bottom of the leeward wall. Similarly, Figures 7(b) and 7(c) show a deceleration upstream pile p 2 (region A 2 , with low levels of shear) and a flow acceleration towards the slope and sides

  (a) shows that a very high concentration of black particles in this area, which indicates a zone vulnerable to erosion, particularly with the 0.9h gap. In pile p 2 , zone B 2 is the region with the highest shear stress values because of the impact of the vortex structure shown in Figure7(c). On the other hand, zone A 2 has low shear level and the pathlines velocities were lower (see Figures7(b) and 7(c)). Unexpectedly, the erosion patterns demonstrated that this zone was characterised by high erosion (see Figure7(a)). Two possible mechanisms could explain this unexpected observation: the intermittent behaviour of the vortices due to bi-stability and the impact of the emitted particles from pile p 2 .

4. 1 . 2

 12 Figures 8 and 9 show the photographs after the pavement phenomenon, the mean fields of the wall shear stress distribution and the flow pathlines, respectively, an isolated stockpile and two stockpiles oriented 60 • to the main flow and separated by 0.9h and 1.8h gaps. The wind exposure patterns were similar on piles p 1 and p 2 for the configurations oriented 60 • to the main flow direction. Progressive flow acceleration up the slope and towards the lateral sides was observed on the windward wall of these piles (see Figures 8(c) and 9(c)). Zones A 1 and A 2 had low levels of shear and ineffective erosion whereas the opposite occurred in zones B 1 and B 2 (see Figures 8(a), 8(b), 9(a) and 9(b)). The highest friction levels in zones B 1 and B 2 were found on the lateral sides of the pile facing the wind. In addition, the shear peak values were higher for the 60 • orientation than for the 90 • orientation. Figures 8(c) and 9(c) show that a single main helical vortex was formed downstream of the stockpiles p 1 and p 2 . The flow separation near the crest of these piles led to a wake regions on the leeward wall (zone C 1 and C 2 , respectively), with low wall shear stress levels and weak erosion (see Figures 8(a), 8(b), 9(a) and 9(b)).

Fig. 2 :

 2 Fig. 1: Wind tunnel scheme

Fig. 3 :

 3 Fig. 3: (a) Device used for the construction of the sand stockpile model and the mean dimensions of the sand stockpile in (b) Top and (c) Side views

Fig. 5 :Fig. 6 :

 56 Fig. 5: (a) Perspective view of the mesh over the two piles, (b) Zoom of the mesh near the crest, and (c) Transversal cut of the domain over the pile

Fig. 7 :

 7 Fig. 7: Experimental and numerical results for two parallel stockpiles oriented 90 • separated by 0.9h and 1.8h gaps: (a) Top view of the eroded sand stockpiles after the pavement phenomenon, (b) Mean wall shear stress on the pile surfaces and on the ground and (c) Mean flow pathlines over the piles. Four wind erosion regions are highlighted on each pile, according to the degree of wind exposure: zones A 2 , B 2 , C 2 and D 2 in the upstream pile (pile p 2 ) and zones A 2 , B 2 , C 2 and D 2 in the downstream pile (pile p 2 ).

Fig. 8 :Fig. 9 :

 89 Fig. 8: Experimental and numerical results for an isolated stockpile (pile p 1 ) oriented 60 • to the main flow: (a) Top view of the eroded sand stockpile after the pavement phenomenon, (b) Mean wall shear stress on the pile surface and on the ground and (c) Mean flow pathlines over the pile. Four wind erosion regions are highlighted according to the degree of wind exposure: zones A 1 , B 1 , C 1 and D 1 .

Fig. 10 :Fig. 11 :

 1011 Fig. 10: Impact of the emitted particles from the upstream pile p 2 in two stockpiles oriented 60 • with α N E = 20% and separated by the 1.8h gap for (a) U∞ = 8 m/s and (b) U∞ = 6 m/s

Table 1 :

 1 Tested configurations. The pile dimensions are: 7.7 cm of height, 23.6 cm of length and 57.9 cm of width

	Configuration	Velocity (m/s)	Gap	α N E
			6, 7, 8	-	10%, 20%
	𝜃 = 60° Wind direction	𝜃 = 90°
			6, 7, 8	0.9h, 1.8h	10%, 20%
	𝜃 = 60° Wind direction	𝜃 = 90°

Table 2 :

 2 Emitted mass measurements for an isolated stockpile and for two successive piles with gaps of 0.9h and 1.8h (h is the stockpile height)

	Configuration	Emitted mass pile p 1 (g)		Emitted mass successive piles (g)
	α N E (%)	U∞ (m/s)	90 •	60 •	Gap	pile p 2	90 •	pile p 2	pile p 2	60 •	pile p 2
		6	83.2	212.9	0.9h 1.8h	27.8 22.2		477.1 609.0	376.3 350.3		581.6 418.7
	10	7	261.6	451.8	0.9h 1.8h	160.4 191.5		647.9 801.0	464.6 521.7		816.7 677.1
		8	399.2	626.6	0.9h 1.8h	438.4 393.1		811.8 922.9	690.5 714.3		1029.8 989.9
		6	46.3	88.2	0.9h 1.8h	3.2 18.7		252.0 285.4	186.4 137.5		358.2 218.1
	20	7	140.3	190.1	0.9h 1.8h	135.4 103.6		345.2 420.7	197.1 238.7		451.7 365.7
		8	247.8	356.4	0.9h 1.8h	250.7 192.9		448.3 569.1	421.9 389.0		604.1 483.7

Acknowledgements The authors acknowledge the financial support from Capes/Cofecub, Fapes and CNPq.


Xuan An

Wan Fang
email: yfang@snnu.edu.cn

Zhaolong Wang

Kaiqiang Liu

Liping Ding

Junxia Peng

Taihong Liu

Haonan Peng

Lionel Salmon
email: lionel.salmon@lcc-toulouse.fr

Yu Fang

Supramolecular gel strategy-based nanomaterials

Keywords: spin crossover, thermochromism, self-assembly, gel

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

Mobile electronics, the Internet, artificial intelligence, big data and high-performance computing have posed a major challenge on data storage and processing: the volume of digital information across the globe doubles every two years, which has reached 44 zettabytes (4.4 × 10 10 terabytes) by 2020. For information storage, the strategy based on the concept of molecular bistability that switches between two electronic states of a molecule in the same way as a binary switch is one of the most promising strategies among the competing candidates. A typical example of molecular bistability is the spin crossover (SCO) phenomenon, also called spin transition (ST) phenomenon. The two states of SCO materials, high spin (HS) and low spin (LS) states, can be converted by external stimuli such as temperature, light, pressure, magnetic field and/or inclusion of a guest molecule, [1][2][3] which makes them a fascinating class of materials with potential applications in molecular electronics, [4][5][6] sensors, [7][8][9] molecular actuators, [10][11][12][13] etc. The related characterization technology is diverse and advanced, 14 which provides a solid foundation for the application of SCO materials. To satisfy these applications, it is of prominent importance to design and synthesize SCO materials with a size of nano-scale and room temperature spin transition. 15,16 J o u r n a l P r e -p r o o f At present, several methods have been developed to prepare SCO nanomaterials, 17 including reverse micelle technique, 18 polymer and biopolymer stabilization method 19,20 and homogeneous acid medium technique. 21,22 Nevertheless, it is still challenging to prepare SCO materials with controllable sizes and morphologies. Reverse micelle method was applied to synthesize NPs with different sizes through adjusting the surfactant concentrations, while the reproducibility is poor due to uncontrolled parameters. 23 Moreover, owing to the strong matrix effect, the reverse micelle technique and polymer stabilization method will undoubtedly introduce excess surfactants or polymers on the surface of SCO materials, leading to a weakening of their SCO response. 24,25 In addition, there are only few reports about room temperature SCO materials, since it depends not only on the characteristic of SCO complexes but also on their molecular interactions. Such as hydrogen bonding 26,27 or π-π stacking interactions 28,29 can achieve the cooperative SCO transitions. Within supramolecular gels, the gelators self-assemble 30,31 into three-dimensional (3D) networked structures with different morphologies through noncovalent intermolecular interactions including hydrogen bonding, π-π stacking, van der Waals, coordination, and other interactions. 32 These special morphologies and interactions make supramolecular gels applicable in a wide range of areas. 33,34 However, to our knowledge, there are only limited reports for the preparation of SCO nanomaterials by using the supramolecular gel strategy that could exhibit remarkable properties. 35,36 Meanwhile, there is no report for the preparation of nanoparticles with different size and morphology through the supramolecular gel strategy.

Herein, the tris(dodecyloxy)-benzamide modified triazole SCO derivative [Fe(Rtrz)3](SO3CF3)2 was designed and synthesized. (Triazole)Fe(II) complexes are an important family of compounds that could exhibit switching properties at around room temperature. 37,38 Through the modification of 4substituent on the triazole ligand and/or the choice of the counter-anion, the SCO characteristics of (triazole)Fe(II) complexes could be well modulated. 39,40 In this work, the tris(dodecyloxy)-benzamide was chemically modified at the 4-J o u r n a l P r e -p r o o f substituent of the triazole ligand to improve the solubility in common organic solvents and the gelation ability of the SCO complex. Interestingly, by using the supramolecular gel strategy, surfactant/polymer-free SCO nanomaterials could be fabricated with as-synthesized [Fe(Rtrz)3](SO3CF3)2 complex. The size and morphology of the SCO nanomaterials could be controlled by varying the concentration, the solvent and the temperature. Moreover, their SCO behavior was studied by optical, magnetic and Mössbauer measurements.

Results and discussion

The triazole derivative [Fe(Rtrz)3](SO3CF3)2 (Scheme 1) was prepared according to the synthesis route (Scheme S1, ESI †), the composition of which was verified by IR spectra and elemental analysis (see the ESI † for details).

Unfortunately, in this family of 'Fe-triazole' coordination polymers, crystals structures are very difficult to obtain. 41 To study the self-assembly property of as-synthesized compound, the gelation behavior of [Fe(Rtrz)3](SO3CF3)2 in 18 organic solvents was investigated systematically, as shown in Table 1 assemblies changed a lot as the temperature increased from 283 K (Fig. 2) to 288 K (Fig. S8). Uniform structures formed at 283 K disappeared, instead, large and irregular aggregates were generated. This phenomenon could be explained that the molecules might slowly transform into equilibrium aggregation state of the uniform nanostructure at low temperature. The TEM particle size distribution of [Fe(Rtrz)3](SO3CF3)2 assembled at 283 K (Fig. S9)

shows that the size of the square morphology is about 200-400 nm. However, J o u r n a l P r e -p r o o f when the temperature was increased, the molecules tended to an increased disorder, and TEM images showed the existence of aggregate structures with uneven and irregular size. 43 In addition, we have also studied assembly behavior of [Fe(Rtrz)3](SO3CF3)2 in poor solvents (such as ethyl acetate), but no nanomaterials with regular morphologies have been obtained (Fig. S10).

For the SCO complexes, drastic color change often occurs accompanying the spin transition, since the changes in the bond length between the metal center and the ligand could affect the electronic state and symmetry of metal complex.

Therefore, the SCO phenomenon of [Fe(Rtrz)3](SO3CF3)2 was monitored by the variable-temperature UV-Vis spectra and the optical reflectance measurements. Fig. 3 presents the variable-temperature UV-Vis of the complex in n-heptane.

At a low temperature of 293 K, it was observed that the 1 A-1 T absorption of [Fe(Rtrz)3](SO3CF3)2 centered at 520 nm. Switching from 293 K (LS state) to 343 K (HS state), it led to the bleaching of the absorption band at 520 nm. This process was reversible, and the absorption centered at 520 nm would recover

J o u r n a l P r e -p r o o f

when the temperature was turned back to low temperature again (Fig. 3c).

Moreover, optical reflectance of [Fe(Rtrz)3](SO3CF3)2 varying with temperature cycled between 200 K and 390 K was recorded. For the bulk sample (Fig. 4a), the transition temperatures were T1/2(↓) = 298 K and T1/2(↑) = 309 K. A narrow hysteresis centered at room temperature was observed. This result was consistent with the data of variable-temperature UV-Vis. As shown in Fig. 4b and Fig. 4c, the transition became gradual and the transition temperatures were centered at 302 K for sample 1 with fiber network structure, while 301 K for the sample 3 with cubic structure. From these results, it can be concluded that as-synthesized [Fe(Rtrz)3](SO3CF3)2 nanomaterials showed excellent thermochromic behavior and room temperature spin transition.

In order to further confirm the SCO property of as-synthesized [Fe(Rtrz)3](SO3CF3)2, the magnetic measurements were performed. As shown in Fig. 5 (black) for the bulk sample, the value of χmT ca. 3.5 cm 3 •K•mol -1 at high temperature corresponded well to Fe(II) complex in the high spin state.

While at 169 K, the value of 0.08 cm 3 •K•mol -1 indicated that the sample could fully transfer to low spin state. The transition temperatures were T1/2(↓) = 306 K and T1/2(↑) = 312 K, and the hysteresis width was 6 K. As shown in Fig. 5 (red), the spin transition of sample 1 became rather gradual accompanying residual

J o u r n a l P r e -p r o o f

HS fraction and also some residual LS fraction together with an apparent hysteresis loop. While for the nano-cubic structure sample 3 in Fig. 5 (blue), the spin transition was also around room temperature, and the hysteresis width increased. Consistent with the results in optical reflectivity measurements, herein the hysteresis loop was confirmed again for the nano-cubic structure of [Fe(Rtrz)3](SO3CF3)2. These results clearly demonstrated that [Fe(Rtrz)3](SO3CF3)2 nanomaterials presented obvious size dependent SCO property. As the size of SCO materials decreased to nano-scale, the surface-tovolume ratio increased, and the amount of inactive SCO residue on the surface of the complex increased, which resulted in the formation of high residual LS fraction. While the increased hysteresis width for [Fe(Rtrz)3](SO3CF3)2 nanomaterials could mainly be attributed to the intermolecular van der Waals interactions between the introduced tris(dodecyloxy)-benzamide structure.

Besides, increased elastic interactions between SCO nanoparticles might also contribute the appreciated large hysteresis loop. In addition, water fraction in the complexs has no effect on the the spin transition behavior. It should be due J o u r n a l P r e -p r o o f to the introduction of tris(dodecyloxy)-benzamide, which could screen water away the iron center.

The room temperature spin transition behaviour was also confirmed by the calorimetric studies of the bulk and nano-sized samples. The calorimetric data was recorded in the heating and cooling modes at 10 K•min -1 in order to evaluate the enthalpy (ΔH) and entropy (ΔS) variations associated with the spin transitions. DSC studies (Fig. S11) showed a singularity at 293K and 313 K for the nano-sized sample 3. Compared with the magnetic measurement, the deviation of the transition temperature might be related to the difference of heating and cooling rates (2 K•min -1 for magnetic measurements and 10 K•min - 1 for DSC). The estimated ΔH and ΔS variations measured in the sample 3 were 13.9 kJ•mol -1 and 45.9 J•mol -1 •K -1 , respectively. These values were at the same range as that of other Fe(II) SCO complex were found in gradual-type systems. 44 With the aim to determine more precisely the fraction of iron centres involved in the spin transition, 57 Fe Mӧssbauer spectra (Fig. S12) were recorded. Taking into account the low percentage of iron (lower than 3%), only J o u r n a l P r e -p r o o f the spectrum for the bulk sample at 293 K was obtained. As expected, at 293 K the spectrum consists of one doublet with an isomeric shift of 1.29 mms -1 and a quadruple splitting of 3.09 mms -1 attributed to the HS state and two doublets with an isomeric shift of 0.40 mms -1 and a quadruple splitting of 0.226 mms -1 attributed to the LS state, in agreement with the magnetic data.

Conclusion

In summary, novel SCO nanomaterials featured with room temperature transition were prepared through the supramolecular gel strategy. The SCO derivative [Fe(Rtrz)3](SO3CF3)2 could form gels in alkane solvents, and the CGCs of the gels were 0.4% w/v for n-hexane, 0.4% w/v for n-heptane and 0.7% w/v for cyclohexane, respectively. Nanomaterials with different morphologies, including fiber network structure, lamellar shape and cubic structure, could be obtained by varying the concentrations of the gelators and the temperature of the solution. Intriguingly, as-prepared [Fe(Rtrz)3](SO3CF3)2 nanomaterials presented room temperature transition with a hysteresis loop.

The unique SCO property of nanomaterials may be envisaged as possessing potential applications in display devices, inkless paper and micro electro mechanical systems. The approach presented herein should be applicable for preparation of an extended range of SCO nanomaterials. J o u r n a l P r e -p r o o f

1 .

 1 Scheme 1The synthetic route for target [Fe(Rtrz)3](SO3CF3)2.

  J o u r n a l P r e -p r o o f turned to a pale yellow transparent solution at 310 K. These results indicated that the gel-sol phase transition temperature in alkane solvents could be ascribed to the introduction of tris(dodecyloxy)-benzamide, which was an important building block of self-assembling systems. The van der Waals interactions between the three long-chain alkane residues and alkane solvents might facilitate the formation of gel. It was reported that the supramolecular aggregation behavior of tris(dodecyloxy)-benzamide was extremely sensitive to the polarity of the environment,42 which could explain the differential selfassembly behavior of [Fe(Rtrz)3](SO3CF3)2 in varied organic solvents. Supramolecular gels are usually constructed by 3D networked structures of the gelators, the formation of which could be modulated by the concentrations of the gelators and the temperature of the solution. Therefore, the self-assembly behavior of [Fe(Rtrz)3](SO3CF3)2 in alkane solvents under different concentrations and temperatures was investigated in detail. As shown from the transmission electron microscope (TEM) images in Fig. 2, it was lucky to find that the microscopic morphologies of [Fe(Rtrz)3](SO3CF3)2 aggregates could be well controlled by varying the solvents and concentrations at 283 K. Fiber network structure was formed by the self-assembly of [Fe(Rtrz)3](SO3CF3)2 in n-hexane solution with a concentration of 0.1% w/v (sample 1). Increasing the concentration of [Fe(Rtrz)3](SO3CF3)2 from 0.1 w/v to 0.3% w/v, the aggregates

Fig. 1

 1 Fig. 1 The gel-sol phase transitions of single solvent gel (n-heptane) of [Fe(Rtrz)3](SO3CF3)2 in response to heat trigger. (a) 2% w/v at 283 K (gel); (b) 2% w/v at 293 K (sol); (c) 2% w/v at 310 K (solution).

  J o u r n a l P r e -p r o o f changed to lamellar shape with obvious fringes. [Fe(Rtrz)3](SO3CF3)2 selfassembled in 0.1% w/v n-heptane solution to form well organized cubic morphology with a size of hundreds of nanometers (sample 2). When the concentration of [Fe(Rtrz)3](SO3CF3)2 increases from 0.1% w/v to 0.3% w/v, crude fiber structure appeared. In addition, the cubic structure was also shown in the samples of [Fe(Rtrz)3](SO3CF3)2 in cyclohexane solution at both 0.1% w/v (sample 3) and 0.3% w/v concentrations. To the best of our knowledge, this was the first case that cubic morphology was formed by the Fe(II)-triazole derivatives. Temperature is also an important factor that affects the selfassembly behavior of the gelators. The morphology of [Fe(Rtrz)3](SO3CF3)2

Fig. 2

 2 Fig. 2 TEM images of the aggregates of [Fe(Rtrz)3](SO3CF3)2 in n-hexane : (a) 0.1% w/v (sample 1); (b) 0.3% w/v; n-heptane : (c) 0.1% w/v (sample 2); (d) 0.3% w/v; cyclohexane : (e) 0.1% w/v (sample 3) and (f) 0.3% w/v at 283 K.

Fig. 3

 3 Fig. 3 UV-Vis absorption spectra of [Fe(Rtrz)3](SO3CF3)2 recorded in n-heptane at a concentration of 1.1 ×10 -5 mol/L in the range temperature of (a) 293-343 K; (b) 343-293 K; (c) Thermal dependence of the absorbance at 530 nm in the cooling (red) and heating (black) modes for [Fe(Rtrz)3](SO3CF3)2.

Fig. 4

 4 Fig. 4 Optical reflectivity measurements of dehydrated (a) bulk sample; (b) sample 1 (assembled in n-hexane, 0.1% w/v); (c) sample 3 (assembled in cyclohexane, 0.1% w/v). Note: heating or cooling rate: 10 K• min -1 .

Fig. 5

 5 Fig. 5 Magnetic measurements of dehydrated bulk sample (black); sample 1 (red, assembled in n-hexane, 0.1% w/v); sample 3 (blue, assembled in cyclohexane, 0.1% w/v). The inset shows a zoom in the SCO region.

Table 1

 1 Gelation behavior of [Fe(Rtrz)3](SO3CF3)2 in different solvents at a concentration of 2.0% (w/v) at 283 K and CGCs (%; in parentheses) = Solution; TS = Turbid solution; I = Insoluble; G = Gel; PG = Partial gel; PS = Partial solution.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21603138, 21972084 and 21820102005), the Fundamental Research Funds for the Central Universities (No. GK202005002, GK202001005 and GK202003040), 111 project (B14041) and Program for Changjiang Scholars and Innovative Research Team in University (IRT-14R33).

We also appreciate the support from "The Youth Innovation Team of Shaanxi Universities".

J o u r n a l P r e -p r o o f

Conflicts of interest

There are no conflicts to declare.

Author Contributions

# Xuan An and Wan Fang contributed equally to this work.

J o u r n a l P r e -p r o o f

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.