Magali Aubert
email: magali.aubert@inra.fr

Geoffroy Enjolras
email: geoffroy.enjolras@iae-grenoble.fr

Which incentives for direct selling? An analysis of French farms

Keywords: Direct selling, sustainable agriculture, FADN, France, panel data analysis Vente directe, agriculture durable, RICA, France, données de panel

This article focuses on factors that incite farmers to sell their production directly to consumers. Based on data from the Farm Accountancy Data Network 2006-2014, we establish profiles of direct selling farms. They are smaller and propose an increased range of produces compared to other farms. Beyond sectorial differences, their common point is to use fewer pesticides, which is consistent with the quality signal sent to consumers. Implications in terms of public policy are suggested.

Résumé

Cet article identifie les facteurs clefs de la vente en circuit court. Sur la base du Réseau d'Information Comptable Agricole 2006-2014, une caractérisation des exploitations vendant en circuit court est établie. Moins étendues, elles offrent une plus large gamme de produits. Par-delà des différences sectorielles, elles utilisent moins de produits phytosanitaires, ce qui est cohérent avec l'image de qualité qu'ils véhiculent. Des implications en termes de politiques publiques sont développées.

Introduction

In recent years, short food supply chains (SFSCs) have enjoyed renewed interests, from both practical and theoretical perspectives [START_REF] Kneafsey | Short Food Supply Chains and Local Food Systems in the EU. A State of Play of their Socio-Economic Characteristics[END_REF]. Their development fundamentally improves information on agricultural produce, a concern, which has become all the more important in the wake of food scandals that have weakened consumer confidence. Within SFSCs, any exchange should involve at most one clearly identified intermediary [START_REF] Galli | Short Food Supply Chains as drivers of sustainable development. Evidence Document[END_REF]. Without an intermediary, the sale is said to be 'direct', while it becomes 'indirect' as soon as an intermediary is involved (Agreste [START_REF] Primeur | Un agriculteur sur cinq vend en circuit court[END_REF].

SFSCs, and among them direct selling, are usually attributed several virtues closely associated with the three pillars (social, environmental and economic) of sustainable development. At the social level, short food supply chains promote close relationships between producers and consumers that induce confidence. SFSCs are also associated to a lesser use of phytosanitary products [START_REF] Enjolras | Do short food supply chains go hand in hand with environment-friendly practices? An analysis of French farms[END_REF]. Finally, in economic terms, both parties are supposed to find a mutual interest. The producer retains much of his added value thanks to the absence or limited number of intermediaries while the consumer is supposed to benefit from better-quality products sold at prices not too dissimilar from prices charged through long food supply chains [START_REF] Martinez | Local Food Systems: Concepts, Impacts, and Issues[END_REF].

The development of SFSCs has been encouraged in most developed countries. At the European level, the second pillar of the Common Agricultural Policy encourages the development of SFSCs as a means of fostering rural development [START_REF] Goodman | Alternative food networks: knowledge, practice and politics[END_REF]. At the domestic level in France, the Ministry of Agriculture initiated an action in June 2009 aiming to develop direct sales. This effort was embodied in the article 230-1 of the Act No. 2010-874 dated July 27, 2010, relating to the modernization of agriculture and fishing. This law encourages actions related to the "development of short food supply chains and […] geographical proximity between producers and processors". France is the largest European producer of agricultural commodities in terms of acreage and production value (Eurostat, 2015). The country also plays the leading role with regard to the development of direct selling. In 2010, nearly 84,000 farmers (about one fifth) sold all or parts of their production through short food supply chains (Agreste [START_REF] Primeur | Un agriculteur sur cinq vend en circuit court[END_REF]. However, the study of the French context reveals some sectorial disparities regarding the level of activity and success factors [START_REF] Draaf Limousin | Légumes: la vente en circuit court peut encore se développer[END_REF].

Because of the interest shown in them, SFSCs have been the subject of many empirical contributions but very little attention has been paid to producers and the choices they make in favour of alternative food networks, and especially direct selling [START_REF] Martinez | Local Food Systems: Concepts, Impacts, and Issues[END_REF]. The few studies on this subject are qualitative, using small data volumes [START_REF] Chiffoleau | Impacts sociaux des circuits courts alimentaires sur les exploitations agricoles[END_REF]Verhaegen and Van Huylenbroeck, 2001) or quantitative when a survey of agricultural farms can be used [START_REF] Detre | Linkage between Direct Marketing and Farm Income: Double Hurdle Approach[END_REF]. The common point between these studies is that they restrict their analyses to the individual and structural parameters of farms incorporated within SFSCs. Consequently, several crucial points such as the performance or the sustainability of farms involved in short supply chains are not studied in sufficient detail [START_REF] Blanquart | The Logistic leverages of short food supply chains performance in terms of sustainability[END_REF].

In order to complement the existing literature on SFSCs and direct selling, the contribution provided by this paper is threefold. First, we propose an innovative analysis concerning the determinants of the adoption of direct selling as a marketing channel. The aim of this research is to demonstrate the existence of a typical profile of farmers, both

Comment citer ce document : Aubert, M., [START_REF] Aubert | Which incentives for direct selling? An analysis of French farms[END_REF]. Which incentives for direct selling? An analysis of French farms. Systèmes Alimentaires / Food Systems (2), 115-141. , DOI : 10.15122/isbn.978-2-406-07196-9.p.0115 3 on a global scale and with regard to agricultural specialization. Second, this analysis takes a specific account of financial and economic parameters in addition to structural ones. These aspects are likely to explain the level of farmers' commitment to direct selling, because of the risks they take and the profitability they gain. Third, we use data from the French Farm Accountancy Data Network (FADN) for years 2006-2014 because they provide a representative overview of professional French farms, particularly in terms of productive orientation, as well as individual, structural and financial aspects of farms. They make it possible to differentiate producers who are involved in direct selling.

Our article is organized as follows. In the first part, we present the specific theoretical framework of our analysis, focusing on the determinants of direct selling as well as the resulting hypotheses. In the second part, we illustrate the empirical framework including the database used and the models estimated, while in the third part, we highlight some descriptive statistics and the econometric results in order to determine the parameters associated with the adoption of direct selling. In the fourth part, we conclude by presenting a summary of the factors encouraging farms to sell their products directly to consumers and suggest further perspectives to be explored in relation to this study.

1

Theoretical framework: the determinants of direct selling

In this section, we develop the theoretical approach used in our study in order to determine factors associated with direct selling practices. The resource-based view appears as an appropriate framework to understand the practice of direct selling by farms. According to [START_REF] Penrose | The Theory of Growth of the Firm[END_REF], resources and capabilities within the firm can be developed in order to create external competitive advantages. This theoretical framework has been largely used to understand choices made in supply chain management [START_REF] Halldorsson | Complementary theories to supply chain management[END_REF][START_REF] Carter | A framework of sustainable supply chain management: moving toward new theory[END_REF].

Applied to our study, the general assumption made is that the more resources and skills the farm and the farmer have, the more they are able to adopt alternative marketing channels. This strategy thus aims to extend the farmers' business capacity and outcome [START_REF] Feenstra | Entrepreneurial outcomes and enterprise size in U.S. retail farmers markets[END_REF]. The literature which is developing on the topic of short food supply chains and alternative food networks allows to define research hypotheses which provide an overview of the different keys allowing to understand the development of alternative marketing channels.

Level of activity of the farm

The activity of the farm indicates its production level and thereby its ability to adopt different marketing channels [START_REF] Blanquart | The Logistic leverages of short food supply chains performance in terms of sustainability[END_REF]. The value of sales represents both a direct measure of the economic activity of the farm and an indicator of the physical dimension of the farm: the larger the farm, the higher its sales for a specific production. The positive influence of farm size has been emphasized in works highlighting the ability of large farms to adopt alternative marketing channels [START_REF] Perrier-Cornet | La diversification des activités dans les exploitations viticoles françaises ?[END_REF]. A high value of sales also motivates the farmer to adopt direct selling because he can retain most of the added value from his production [START_REF] Chiffoleau | Impacts sociaux des circuits courts alimentaires sur les exploitations agricoles[END_REF].

H1a. Large and productive farms are more likely to adopt direct selling

Comment citer ce document : Aubert, M., [START_REF] Aubert | Which incentives for direct selling? An analysis of French farms[END_REF]. Which incentives for direct selling? An analysis of French farms. Systèmes Alimentaires / Food Systems (2), 115-141. , DOI : 10.15122/isbn.978-2-406-07196-9.p.0115 4 Moreover, the European subsidies, which increase total farm revenues, provide the farm with larger financial resources, thus providing incentives for the adoption of alternative practices [START_REF] Enjolras | Direct payments, crop insurance and the volatility of farm income. Some evidence in France and in Italy[END_REF]. More specifically, direct selling is supported within the second pillar of the Common Agricultural Policy. Measure 311 of the Rural Development Program for the 2007-2013 period promotes the "diversification into non-agricultural activities" whose main focus is to create alternative on-farm employment opportunities in non-agricultural activities and services.

H1b. Subsidies increase the likelihood to adopt direct selling

Financial assets of the farm

Insofar as direct selling is deemed complementary to traditional channels, only a farm with sufficient financial assets [START_REF] Chiffoleau | Impacts sociaux des circuits courts alimentaires sur les exploitations agricoles[END_REF] and revenue (Aubert and Perrier-Cornet, 2009) can manage alternative marketing channels. The stock of financial assets includes fixed and current assets, which are the counterpart to the invested capital (equity and debt). Working capital is a structural indicator computed as the difference between current assets and current liabilities.

H2a. Long-term financial assets increase the likelihood to adopt direct selling

The working capital requirement is more a seasonal indicator, which characterizes the money needed by the farmer to finance delayed customer invoices and stocks. Direct selling is supposed to provide increasing cash flows, and therefore decrease the working capital requirement, thanks to the one-to-one relationship between farmers and consumers. However, this effect may be offset by increasing stocks the farmer must set aside to satisfy his customers' needs in terms of produce diversity.

H2b. Short-term financial assets increase the likelihood to adopt direct selling

Profitability and financial risk of the farm

One of the basic aims of a farm is to maximize the value of sales and thereby its profitability and alternative marketing channels may contribute to this objective [START_REF] Ahearn | Direct-to-Consumer Sales of Farm Products: Producers and Supply Chains in the Southeast[END_REF]. Conversely, farmers adopting traditional marketing channels and observing low or negative profitability of their business may also be incited to change their marketing channels, thus adopting direct selling [START_REF] Conner | Locally Grown Foods and Farmers Markets: Consumer Attitudes and Behaviors[END_REF][START_REF] Praly | Nouvelles formes de valorisation territoriale en agriculture, le cas de l'arboriculture de la Moyenne Vallée du Rhône[END_REF].

H3a. Unprofitable farms are more likely to adopt direct selling

The adoption of alternative marketing channels can also be examined through the financial risk of the farm, i.e. the risk associated with its indebtedness [START_REF] Ahearn | Direct-to-Consumer Sales of Farm Products: Producers and Supply Chains in the Southeast[END_REF]. Indebted farms need to generate enough cash to payback both debt and interests. Because selling produces directly to consumers offers a convenient way to generate cash flows, indebted farms may be incited to adopt alternative marketing channels.

H3b. Indebted farms are more likely to adopt direct selling

Farm risk management

Comment citer ce document : Aubert, M., [START_REF] Aubert | Which incentives for direct selling? An analysis of French farms[END_REF]. Which incentives for direct selling? An analysis of French farms. Systèmes Alimentaires / Food Systems (2), 115-141. , DOI : 10.15122/isbn.978-2-406-07196-9.p.0115

5

The adoption of alternative marketing channels responds to consumers' demand for alternative agricultural products [START_REF] Lanciano | Le développement des circuits courts alimentaires : un nouveau souffle entrepreneurial dans l'agriculture ?[END_REF]. Consequently, this strategy offers the farmer an opportunity to reach more customers compared to standard food supply chains in which the farmer trades with a small number of partners. In return, the farmer faces uncertainty in terms of the volume sold. Farmers may choose to diversify, particularly by exercising several activities within the farm such as mixed crop and livestock production. Diversification is then measured through the number of different types of production on the farm.

H4a. Diversified farms are more likely to adopt direct selling

We can also assume that the concerned farmers are looking for direct ways to protect their production against the risks, which traditionally affect crops (e.g. diseases, adverse climate). Several means are available, such as crop insurance policies that help to secure economic yields [START_REF] Enjolras | Crop insurance policies and purchases in France[END_REF]. However, insurance policies are quite costly. Phytosanitary products (fertilizers or pesticides) may be used as flexible and quite cheap tools for the preservation of crop yields. However, excessive use would be at odds with the quality image conveyed by short food supply chains (Ilberry and Maye, 2005;Verhaegen and Van Huylenbroeck, 2001).

H4b. The purchase of insurance policies and phytosanitary products decreases the likelihood to adopt direct selling

Skills available on the farm

Beyond the level of activity, individual skills within the farm are key factors in explaining the adoption of alternative marketing channels. It is widely acknowledged that direct selling requires additional workforce in order to perform both productive and commercial activities on the farm [START_REF] Chiffoleau | Impacts sociaux des circuits courts alimentaires sur les exploitations agricoles[END_REF][START_REF] Galli | Short Food Supply Chains as drivers of sustainable development. Evidence Document[END_REF]. Some authors showed that some favorable conditions, such as increased workforce, were a prerequisite to the development of new activities on the farm [START_REF] Perrier-Cornet | La diversification des activités dans les exploitations viticoles françaises ?[END_REF].

H5a. An increased workforce increases the likelihood to adopt direct selling

More educated and experienced farmers seem more able to sell their own production because they can manage the different aspects of short food supply chains, including the production, transformation and commercialization processes [START_REF] Chiffoleau | Impacts sociaux des circuits courts alimentaires sur les exploitations agricoles[END_REF][START_REF] Gilg | The role of household factors in direct selling of farm produce in France[END_REF]. Alternative marketing channels, including the adoption of direct selling, requires more labour [START_REF] Lanciano | Le développement des circuits courts alimentaires : un nouveau souffle entrepreneurial dans l'agriculture ?[END_REF]) and more specifically a waged and qualified workforce [START_REF] Perrier-Cornet | La diversification des activités dans les exploitations viticoles françaises ?[END_REF].

H5b. Educated and experienced farmers are more likely to adopt direct selling

Proximity between farmers and consumers

The farm location appears as a key factor in the adoption of alternative marketing channels. Some studies especially consider the spatial proximity between farmers and consumers [START_REF] Capt | Demande de biens différenciés, comportements spatiaux et diversification de l'activité des exploitations agricoles[END_REF][START_REF] Capt | Economie spatiale et agriculture : les dynamiques spatiales de l'agriculture contemporaine[END_REF][START_REF] Low | Direct and intermediated marketing of local foods in the United States[END_REF], while others also take into account the number of intermediaries between farmers and consumers (Martinez [START_REF] Jarosz | The city in the country: Growing alternative food networks in Metropolitan areas[END_REF] specifies that alternative food networks are characterized by the existence of retail venues such as local markets, community supported agriculture and a commitment to sustainable food production and consumption.

H6. A farm location close to consumers increases the likelihood to adopt direct selling

Empirical framework

In this section, we present the specific database used and its importance in understanding farmers' motivations. We also illustrate the econometric model to be estimated.

Database

In order to examine the characteristics of farms involved in direct selling, we use data from the European Farm Accountancy Data Network (FADN) for the period 2006-2014. These data are both the most precise available at the individual level and the most complete and recent that we have. They allow to examine closely the structural features of the farm and the individual characteristics of the owner.

One of the major advantages of our database is that it allows us to identify the intensity of direct selling using a ternary variable. This variable distinguishes farms that do not sell through direct selling from farms that adopt such a practice. The latter are differentiated according to whether this marketing channel represents more or less than 75% of the value of sales (Figure 1). In general, farms involved in direct selling do not consider this marketing strategy as an exclusive channel. One should note that the FADN database allows to examine the adoption of direct selling at the farm level, but without mentioning the first year of adoption.

Comment citer ce document : Aubert, M., [START_REF] Aubert | Which incentives for direct selling? An analysis of French farms[END_REF]. Which incentives for direct selling? An analysis of French farms. Systèmes Alimentaires / Food Systems ( 2

Figure 1. Distribution of farms according to the marketing channel in 2014

Our analysis considers the determinants of direct selling at the global level for all professional farms regardless of their activities and for the farms' main specializations. Market gardeners, wine growers, fruit producers as well as sheep and goat breeders are more involved in short supply chains. For these specializations and whatever period considered, one to three farmers out of five sell through this marketing channel while they are less than one in ten for the other specializations (Figure 2).

Figure 2. Dynamics of direct selling according to the economic and technical orientation

Comment citer ce document : Aubert, M., [START_REF] Aubert | Which incentives for direct selling? An analysis of French farms[END_REF]. Which incentives for direct selling? An analysis of French farms. Systèmes Alimentaires / Food Systems (2), 115-141. , DOI : 10.15122/isbn.978-2-406-07196-9.p.0115

9

The distribution of farms adopting direct selling shows the importance of wine growing involved in this marketing channel (Figure 3). An explanation can be found in the specific characteristics of the concerned productions. Winemaking is a long-term process, which requires a high level of investment (e.g. cellars, maceration vats and presses). Producers are very often involved in cooperatives, leading to higher integration of marketing channels (Traversac et al., 2011). Fruit production, market gardening and cattle breeding are perishable productions, which face somehow the same financial challenges, while experiencing difficulties related to volatile weather conditions or drop in prices. Therefore, direct selling constitutes a credible alternative to conventional marketing channels.

Source: French FADN 2014.

Figure 3. Distribution of farms, which adopted direct selling in 2014

Econometric models

The aim of our study is to understand what are the individual, structural and financial characteristics that lead to a higher probability to sell directly to consumers. To do so and take into account the individual and temporal dimensions associated with direct selling, we estimate a multinomial logit model. This cross-section model allows us to appreciate the intensity of the adoption of direct selling. The model is specified with clusters in order to underline that a same farmer can be present from one year to the next. Such an approach is more suitable than a panel data analysis given the fact that farms adopting direct selling stay involved in this marketing channel overtime (Figure 1).

Formally, the model considered can be defined as follows: Where α represents the constant, β, γ, δ, θ, ξ, ρ and τ the coefficients associated with each group of variables and ε the residuals.

Direct selling = ∝ + β*
We summarize the model (1) as follows:

Y=X' ρ+μ (2)
Where Y corresponds to the practice of direct selling detailed above. The adoption of direct selling is a ternary variable, which takes the value 0 if the farmer does not sell through this marketing channel, 1 if this activity represents less than 75 % of its sales and 2 otherwise. This choice is conditioned by a continuous effect that is not observed. This decision is modelled as shown below:

Y=0 if Y*< ξ Y=1 if ξ<Y*< ζ (3) Y=2 if Y*> ζ
Where Y* is the latent variable that conditions the decision to adopt direct selling (Y).

As stated previously, farms specializing in field crops, market gardening, winegrowing, fruit production, sheep and goat breeding as well as cattle breeding are those which are most likely to adopt the practice of direct selling. Thus, we consider seven different models: the first is general and encompasses all French professional farms, while the other models consider each specialization separately. It should be noted that, to avoid endogeneity problems, financial variables are lagged by one year. Similarly, potential size effects are neutralized for financial variables, e.g. the amounts in euros are divided by total farm sales.

Results

In this section, we present the results of the descriptive statistics and the econometric model which enable us to understand the factors associated with the adoption of direct selling by farmers. Among farms that adopted direct selling, we separate those that are very involved into this marketing channel (more than 75% of sales) and those that are less involved (less than 75% of sales).

Descriptive statistics

Farmers who adopted direct selling in 2014 seem to declare a higher value of sales than other farms (Figure 4). The explanation may lie in the fact that farmers involved in short food supply chains have the ability to determine selling prices by themselves. Moreover, farmers who adopted direct selling appear to receive fewer subsidies than those who sold their entire production through another channel. Such a result may indicate that overall subsidies are not enough of an incentive to promote the adoption of direct selling. 12 Keys: The null hypothesis considers equality of means between the population and the reference "No direct selling". Means are significantly different at the 10% (*), 5% (**) and 1% (***) thresholds.

Figure 4. Farm characteristics according to the marketing channel

Comment citer ce document : Aubert, M., [START_REF] Aubert | Which incentives for direct selling? An analysis of French farms[END_REF]. Which incentives for direct selling? An analysis of French farms. Systèmes Alimentaires / Food Systems (2), 115-141. , DOI : 10.15122/isbn.978-2-406-07196-9.p.0115 13 Direct selling requires fewer investments, but this is compensated with higher current assets. We notice actually a significantly higher return on capital employed and higher working capital only for farm highly involved in direct selling. Moreover, farms which are more weakly involved in direct selling benefit from slightly higher cash-flow levels compared to farms not involved in this marketing channel.

We also observe that the return on capital employed is higher for farms highly involved in direct selling. All these results seem to indicate that direct selling is associated with profitability. Moreover, direct selling farms do not get more indebted compared to other farms and they pay on average the same amount of interest. Direct selling is therefore not associated with a higher financial risk.

Farmers who adopted direct selling seem to offer a smaller range of produces to their customers. However, we notice an increased value of stocks, which is necessary to meet the customers' demand1 . Consequently, the working capital requirement of their farm is significantly higher. This last indicator is almost doubled when compared to farms that did not practice direct selling. Such a stock level requires consistent financing, which is not offset by the fact that customers pay for their purchases in cash.

Because they are associated with higher investment, products sold directly to consumers have to be covered upstream at the time of production. However, the profile of these farmers denotes an orientation towards risk. Firstly, they seem to decrease their insurance. Secondly, they spend smaller amounts of fertilizers and pesticides. These are in line with the image of quality associated with direct selling.

Finally, the literature points to skills as a key element in the adoption of direct selling (Figure 5). We note that farmers who adopted direct selling seem to be younger and benefit from a higher level of education, both in terms of agricultural and general education. These elements appear to confirm the literature, which asserts that younger and better-educated farmers are more likely to adopt direct selling. Keys: The null hypothesis considers equality of proportions between the population and the reference "No direct selling". Proportions are significantly different at the 10% (*), 5% (**) and 1% (***) thresholds.

Figure 5. Farmer's characteristics according to the marketing channel

Comment citer ce document : Aubert, M., [START_REF] Aubert | Which incentives for direct selling? An analysis of French farms[END_REF]. Which incentives for direct selling? An analysis of French farms. Systèmes Alimentaires / Food Systems (2), 115-141. , DOI : 10.15122/isbn.978-2-406-07196-9.p.0115 15 Direct selling is associated with an increased workforce working on the farm. Additional waged labour is required because of the various skills needed at the different stages of the productive process, e.g. production, processing and marketing. This workforce represents an additional operating cost, which increases according to the share of the farm's production sold through this marketing channel.

Econometric models

We complement these descriptive statistics by econometric models that explain the adoption of direct selling practices considering the main kinds of variables exposed earlier. The results are presented in Figure 6.

Before considering the main specializations, the global model highlights two main results. The first one is the existence of some specificities among specializations in terms of marketing strategy. The probability of selling directly to consumers is greater for farms specializing in market gardening, wine-growing and fruit production, while lesser for farms specializing in cattle breeding, compared to farms specializing in field crops (the reference). Indeed, specific produces, such as fruits and wine, are more suitable for direct selling than generic ones, such as cereals. The second one is that the degree of involvement in direct selling (percentage of sales) does not fundamentally change the nature and the extent of the results. Once adopted, direct selling leads to the adoption of a specific pattern.

Figure 6. Econometric models explaining direct selling

In terms of the level of activity, it appears that the value of sales is an important decisive factor for the adoption of direct selling. The global model shows that the smallest farms are more likely to adopt this marketing channel. These farms may adopt direct selling as a convenient way to sell a small production. On the other side of the spectrum, a high level of activity may also incite farms to get very involved in direct selling, probably because of the larger range of produces they can offer to consumers. We observe an opposite situation among wine-growing farms, probably because of the specificities of this specialization. H1a is mostly validated. Furthermore, we find that direct selling is not conditioned by the level of operating subsidies. Except for cattle breeding, subsidies do not seem to provide enough of an incentive to help producers adopt this marketing channel. H1b is not validated.

For cattle breeding, market gardening and field crops, capital accumulation leads to increased likelihood to adopt widely direct selling. This result suggests that farms, which reinforce their productive activity, are able to adopt alternative marketing channels, thus bringing some validity to this hypothesis. We also notice that fixed assets do not determine the adoption of direct selling. H2a is partially validated. Consequently, direct selling farmers seem to have to find short-term resources to carry out their marketing and sales activities. The study of the financial situation of farms shows that, for all professional farms, the working capital requirement plays a significant and positive role in the adoption of direct selling in the global model. Availability of stocks thus appears to be decisive. However, for wine-growers and cattle breeders very involved in this marketing channel, an opposite effect is noticed. These farmers may find in traditional marketing channels the most convenient way to sell their products. H2b is also partially validated.

Comment citer ce document : Aubert, M., [START_REF] Aubert | Which incentives for direct selling? An analysis of French farms[END_REF]. Which incentives for direct selling? An analysis of French farms. Systèmes Alimentaires / Food Systems (2), 115-141. , DOI : 10.15122/isbn.978-2-406-07196-9.p.0115 16 Except for farms specializing in field crops and wine-growing, the model highlights that the return on capital employed has no impact on direct selling. This result demonstrates that a higher level of returns does not specially motivate farmers that adopt direct selling. Except for field crops, the indebtedness level and interest paid do not influence the choice of a marketing channel. Self-financing, however, has a negative influence on direct selling for field crops and wine-growing sectors, and a positive influence for sheep and goat breeding. The ability to use internal resources provides a disincentive to adopt alternative marketing channels for farmers in the former group while it is the opposite for the latter. H3a and H3b are therefore not validated.

Direct selling producers are more diversified than farmers who sell through traditional channels because they have to meet consumer demand in terms of produce diversity, which validates H4a. Irrespective of the production, subscribing crop insurance policies has generally no impact on the farmer's marketing strategy. Conversely, most models highlight the fact that increased expenses of pesticides and fertilizers lead to a lower probability of selling directly to consumers. This very significant result confirms the image of quality associated with products sold using short food supply chains. Thus, H4b is partially validated.

Contrary to what the descriptive statistics suggested, the study of the farmers' skills reveals that their age is generally not decisive in the choice to sell any part of their production directly to consumers. The level of agricultural and general education does not seem to be relevant either. The relative weight of the waged workforce in the total workforce has a contrasted influence on the decision to sell directly to consumers. H5a and H5b are not validated.

Because of its characteristics, the FADN database allows to analyze only the farm and its owner's characteristics (hypotheses H1a to H5b). Therefore, one of the main dimensions associated to direct selling, the proximity between farmers and consumers, could not be taken into account in our analysis. H6 was not testable.

Conclusion

In this article, we have proposed a study of economic and financial factors encouraging farmers to sell their production directly to consumers. This analysis is intended to complement a literature that traditionally focuses on the analysis of the individual or structural determinants of the adoption of direct selling. Moreover, with FADN data we considered the main sectors in which farmers are most commonly engaged in direct selling.

One salient result of this study is to highlight key features of French farms and farmers practicing direct selling, despite differences in production. In general, these farmers appear to operate on smaller farms, and this small size is an incentive to adopt such a marketing channel. Direct selling implies the mobilization of a set of human (labour) and financial resources (working capital requirement and current assets) in order to face the short-term challenges of this marketing channel. These farms are then able to offer an increased range of products to meet the consumers' expectations. Such diversification is indeed a strong prerequisite to direct selling. By contrast, direct selling farms do not need a large amount of fixed assets to operate, which results in lightweight financial and operating structures.

Comment citer ce document : Aubert, M., [START_REF] Aubert | Which incentives for direct selling? An analysis of French farms[END_REF]. Which incentives for direct selling? An analysis of French farms. Systèmes Alimentaires / Food Systems (2), 115-141. , DOI : 10.15122/isbn.978-2-406-07196-9.p.0115 17 This comprehensive knowledge of direct selling allows us to discuss the implications in terms of public policy. Our analysis underlines that farmers practicing direct selling are more respectful of the environment, which results in reduced expenses of chemical inputs (fertilizers and pesticides) for almost all productions considered. It therefore appears relevant for a farmer to adopt direct selling with an ecological objective, and vice versa.

Future research should confirm these results by studying in more detail the dynamics of direct selling. We could notice that many farms belonging to our database have adopted direct selling for years. Given these aspects, an interesting question would be to understand if this adoption is temporary or permanent, and in all cases which kinds of producing patterns are adopted.

Comment citer ce document : Aubert, M., [START_REF] Aubert | Which incentives for direct selling? An analysis of French farms[END_REF]. Which incentives for direct selling? An analysis of French farms. Systèmes Alimentaires / Food Systems (2), 115-141. , DOI : 10.15122/isbn.978-2-406-07196-9.p.0115 19 Traversac J., Rousset S. and Perrier-Cornet P., 2011, "Farm resources, transaction costs and forward integration in agriculture: Evidence from French wine producers", Food Policy, vol. 36, p. 839-847. Verhaegen I. and Van Huylenbroeck G., 2001, "Costs and benefits for farmers participating in innovative marketing channels for quality food products", Journal of Rural Studies,vol. 17,n° 4,

  ), 115-141. , DOI : 10.15122/isbn.978-2-406-07196-9.p.0115 7 Source: French FADN 2006-2014.

Because the FADN database focuses only on farm entities considered as profit centers, we were not able to consider multi-activity in this analysis.

Nationale de la Recherche dans le cadre du projet ANR-11-ALID-0006ILLIAD.

Comment citer ce document : Aubert, M., [START_REF] Aubert | Which incentives for direct selling? An analysis of French farms[END_REF]. Which incentives for direct selling? An analysis of French farms. Systèmes Alimentaires / Food Systems (2), 115-141. , DOI : 10.15122/isbn. 978 Keys: Estimates significant at the 10 % (*), 5 % (**) and 1 % (***) thresholds, -1 denotes a lagged variable.


Mathieu Nespoulous
email: mathieu.nespoulous@univ-amu.fr

Mickaël Antoni

Carine Chassigneux

Renaud Denoyel

Porous silica beads produced by nanofluid emulsion freezing

Keywords: emulsion freezing, silica nanoparticle, double-scale structure

INTRODUCTION

Porous materials are ubiquitous with many applications in separation, purification or catalysis. Intense research activity has been devoted to their characterization for decades [1][2][3][4]. One still challenging problem is the synthesis of porous objects with controlled size and porosity. The production of spherically shaped microparticles is for example of first importance for separative analytical techniques, like chromatography, where transport properties of columns must be carefully controlled. In situ chemical templating techniques have been used to this end with silica non-porous spheres produced by the Stöber method [5]. Spherical porous materials are also commonly synthetized using polymers as templates.

In addition to molecules, latex particles have been used as template to produce small mesoporous spheres with radius in the range 200-800 nm while the pore size is controlled by triblock copolymers [6]. In industrial processes, spherical porous particles have long been produced by spray drying methods with suspensions of non-porous particles [START_REF] Masters | Spray Drying Handbook[END_REF]. Several levels of porosity can be achieved depending upon the properties of the initial suspensions. The use of mesoporous particle suspensions allows for example the preparation of porous spheres with two typical porosity scales [START_REF] Urata | [END_REF].

Besides spray drying, emulsion-based synthesis of porous materials with particles has also drawn a considerable attention over past years [9,10]. Emulsified suspensions are known to involve complex adsorption and wetting mechanisms. It has been for example demonstrated already more than a century ago by Ramsden and Pickering that such emulsions can be stabilized only by surface active particles [START_REF] Ramsden | Proc. R. Soc. Lond[END_REF][START_REF] Pickering | [END_REF]. In this context, intense activity has been devoted to nanofluid emulsions that consist of emulsions where particles are nanometer sized and dispersed in either the continuous or the dispersed phase.

Surfactant molecules have been further used to tune interfacial properties of these nanoparticles. Studies on synergistic mechanisms [13], liquid/liquid interfacial properties of drops [14] as well as emulsion rheology [15] and drop morphology [16][17][18] have been reported for a large variety of nanoparticles and surfactant molecules. In the case of silica nanoparticles, both oil in water and water in oil nanofluid emulsions have been studied for porous material elaboration. For oil in water emulsions, it has been evidenced that such nanoparticles get polymerized along the drop interfaces [9] while for water in oil ones, mesoporous silica spheres were obtained [19]. Hierarchical porosity systems can be further elaborated by multiple O/W/O emulsions where the dispersed oil droplets create macropores surrounded by mesopores produced by silica polymerization [20].

In the context of the present work, nanofluid emulsions have the great interest to offer a broad range of possibilities for sol-gel preparation of porous materials in well controlled conditions. But one drawback here is the use of large additives concentrations, mainly surfactant molecules, for chemical templating that have to be eliminated by calcination and/or complex washing. Time consuming procedures, not always easy to manage regarding environmental standards, are therefore necessary. Recent studies have shown that this problem can be overcome using foaming techniques or ice templating [21]. This latter has been successfully used to produce controlled porous polymeric [22,23] or inorganic [24] materials with very limited amounts of additives and a rather reduced use of solvents. Ice templating has been used for example to structure porous polymers in the case of water in oil emulsions, where both phases contain polymers [25]. The ice templating may also occur in both water and oil phases: the porous spheres, produced in oil droplets, are then obtained after dissolution of the porous polymer produced in the water continuous phase [26]. The synthesis of macro-mesoporous inorganic powders with the principle of ice templating has also been addressed in the case of ceramic microspheres [27] but using cyclohexane as freezing solvent and high temperature subsequent treatments. To our knowledge, the use of ice templating, to synthesize porous inorganic powders starting directly from a suspension of nanoparticles in oil emulsion, has not been developed.

The aim of this work is to contribute to fill this gap. The formation of porous structures has been recently observed when applying freeze-thaw cycles to nanoparticle suspensions in a pendent drop configuration [28]. Based on this observation, it is proposed hereafter to use aqueous nanofluids of silica nanoparticles emulsified in a heptane continuous phase with SPAN80 as stabilizer. Micrometer sized droplets are easily formed in such systems.

Different emulsion volume fractions are studied for the same emulsification protocol and yield modifications in both droplet sizes and shapes. After drying, spherical micrometric particles consisting of flocculated silica nanoparticles are evidenced for low volume fraction emulsions. For large volume fractions, flocculated particles turn deformed and exhibit unexpected macroporosity. Both cases are discussed at the light of known water freezing mechanisms. Conversely to conventional sol-gel chemical synthesis, the production of flocculated particles in this work can be seen as the result of a physical synthesis. Minimum amount of chemicals is necessary and almost all the solvent can be reused for subsequent synthesis rounds.

EXPERIMENTAL

Emulsion preparation -The continuous phase of the emulsions is a solution of heptane (Sigma-Aldrich) with SPAN80 (Sigma-Aldrich) at 10 g/L concentration. The dispersed phase is an aqueous nanofluid of amorphous silica nanoparticles (Ludox SM 30 wt.% or 16% volume, provided by Sigma-Aldrich). The latter was used without further purification and dilution. 10 mL samples are prepared in 16 mL glass vials by introducing first the heptane/SPAN80 solution and then, on the top of it, the nanofluid with a micropipette (see Figure 1(a)). Five different emulsion volume fractions, ϕ = 1%, 2%, 5%, 10% and 20%, are prepared: this is the ratio of nanofluid introduced volume to the total volume. W/O emulsions are finally created with a high-speed homogenizer (IKA T18 Ultra-Turrax) for 2 minutes at 8000 rpm (see Figure 1(b)). After mixing, the emulsion is immediately introduced in a freezing chamber. Freezing and drying -Two freeze-thaw cycles are applied with a SP Scientific VirTis Sentry 2.0 lyophilizer freezing chamber. The temperature is cycled at an average cooling rate of 2 °C/min from room temperature down to -40 °C and back to room temperature. At -40 °C the dispersed aqueous nanofluid is frozen whereas the heptane continuous phase remains liquid (as the heptane freezing is about -91 °C). After the two freeze-thaw cycles, flocculated particles, denoted as beads in the following, are created and sediment in a bed at the bottom of the vials (see Figure 1(c)). Most of the heptane is then carefully removed with a Pasteur pipette before sample drying in an oven at 80 °C overnight.

Mercury porosimetry -Pore size distribution is determined with a Quantachrome Poremaster apparatus. Applied pressure runs from 4 kPa to 348 MPa and a contact angle of q = 140° is used as recommended for silica-mercury interface [29]. This allows to probe pore diameters ranging from 300 µm down to 4 nm. Two pressure domains are successively explored: low pressure (4 to 345 kPa) and high pressure (138 kPa to 345 MPa). Pore diameter is computed from the intrusion experiments with the Laplace-Washburn equation.

The mass of sample is about 100 mg and experiment temperature 22 °C.

Nitrogen sorption -The surface area and mesopore size distribution of the beads are derived from gas adsorption analysis, using a Micromeritics ASAP 2010 apparatus. The samples, again of mass close to 100 mg, were first outgassed in stable vacuum (~0.05 mbar) conditions at 150 °C for 12h. N2 adsorption/desorption isotherms are then measured at 77 K.

Scanning electron microscopy (SEM) -Microstructure morphology is studied using a Philips XL30 SFEG STEM scanning electron microscope associated to an Oxford energydispersive spectrometry analysis system. Samples are carefully deposited on a carbon film before gold-palladium metallization.

RESULTS AND DISCUSSION

Because it is known that the size of droplets has an influence on the freezing mechanism [35] and that emulsion volume fraction has an influence on droplet size, the influence of emulsion volume fraction is the main investigated parameter of this study. Figure 2 presents typical SEM images of samples obtained from the original suspension (i.e. without emulsification) and from five emulsified samples differing only by their emulsion volume fraction ϕ. In the absence of emulsification, irregular aggregates with sharp edges are produced (top left image of Fig. 2) while for emulsified ones polydispersed beads with typical size increasing with volume fraction are evidenced. They consist of silica nanoparticles that have been aggregated by ice templating. For ϕ = 1% and 2% they appear as micrometer sized spheres with radius between 1 µm and 20 µm. For larger volume fractions, they look like deformed spheres with radius up to few tens of micrometer. Detailed visual inspection of the images for ϕ > 2% indicates that micrometer sized beads are still present and exhibit again a regular spherical geometry. Such beads are observed in all experiments and constitute the signature of the presence of sufficiently small droplets in the original emulsion for capillary forces to prevail and therefore preserve sphericity. Figure 2 also emphasizes that beads seem to organize into clusters. This is most likely a consequence of the drying procedure during which uncontrolled capillary forces may come into play as wetting films vanish in the very last evaporation phase. The way beads are deposited on the carbon film before SEM analysis has probably also to be accounted for here. All the experiments are performed with the original Ludox 30 wt.% dispersion. The reason for this is to make sure the densification processes involved in ice templating will create beads sufficiently stiff to keep memory of the droplets from which they were produced, as suggested by a preceding study with drops [28]. For diluted Ludox solutions, beads like the ones shown in Figure 2 would be instable and collapse under their own weight in the drying phase. Two freeze-thaw cycles are necessary here since only unconnected micrometer sized flocs are formed after the first cycle [28]. Optical microscopy images of the main features associated to each cycle is illustrated in figure 3 Figure 4 shows both small and large beads and illustrates the microstructures that will be characterized hereafter. One striking feature is the difference in the apparent surface textures. Micrometer sized beads turn compact and smooth while large ones exhibit a porous surface. The structure of the porous network therefore depends upon the size of droplets. Microstructures are known to be generated when solidification occurs in undercooled silica colloidal suspensions in pendent drop configurations [28]. For such undercooled systems, two successive freezing regimes have been evidenced: a dendritic growth regime that lasts for less than few milliseconds and subsequently a regime where latent heat is evacuated. This latter takes few seconds to be completed, in the case of millimetric droplets, and exhibits a spherical ice front propagating from outside to inside the drop. As just discussed, micrometric flocs are formed after the first freeze-thaw sequence and turn further flocculated with a second freeze-thaw sequence. Beads of the size of the pendent drop were produced this way and shown a porous surface as well, but the typical size was in the range of a few micrometers. Comparison of these results to the ones presented here for polydisperse emulsions clearly indicates that droplet size influences strongly the apparent porous structure of the material. The pore size distribution of the beads is evaluated by mercury porosimetry. Intrusion and extrusion curves for ϕ = 20 % are displayed in Figure 5. This figure also presents the curves of a reference Ludox sample obtained from a non-emulsified Ludox sample but that has undergone the same freeze-thaw cycles and drying procedures than the emulsified ones. At low pressures (i.e. large values of pore size d), a macroporosity shows up for both samples in the range d > 1 µm. This porosity is the usual signature of the compaction and the filling of interparticle space and corresponds here to the inter bead (IB) porosity. The corresponding porous volume is denoted as VIB hereafter. For the highest pressures (i.e. smallest values of d) mesopores are detected with typical diameter d ≈ 9 nm (resp. 5 nm) for the emulsion (resp. reference sample). This is again an interparticle signature but it now results from inter-nanoparticles (INP) residual space with porous volume given by VINP.

Surprisingly, an additional macroporosity in the range 50 < d < 200 nm is observable but only for emulsified samples. It is probably reminiscent of the size and shape of all the ice dendrites that propagated throughout the nanofluid as freezing took place. This specific bead porosity (B) has volume VB and was already visible on the surface of the large beads in Figure 4. Porous volumes VIB, VB and VINP are each associated to a specific porosity that actually depends upon ϕ. One question raised by the new macroporous volume VB evidenced in Figure 5 is the way it is modified when changing ϕ. Mercury intrusion curves as a function of ϕ are plotted in Figure 6(a) and reveal the same three-step behavior than the one of Figure 5. They also reveal that VIB increases with ϕ. From total intruded volume, silica density and VIB, it is possible to calculate the apparent densities of beads that are shown to decrease with ϕ, which is the reason for the increase of VIB. The intrusion step in the IB range shifts moreover towards smaller values of d when decreasing ϕ and indicates that bead typical size also decreases as already suggested by the SEM images of Figure 2.

A careful image processing could obviously help to complement this result, but it is rather clear from this figure that different typical bead sizes are already observable when comparing the images for ϕ = 1% and 2% with the ones for ϕ = 10% and 20%. porosity domains are therefore well separated and reveal a two-level hierarchical structure with porous domains filled and emptied independently. For smaller values of ϕ however, the separation between the two domains is less marked: for example, macropores could be connected to outside through mesopores which act as constrictions. This trend demonstrates that the structure of the porous network is strongly modified and signs a change of connectivity of the network topology. The inflexion points of the mercury cumulative intrusion curves are now further used to compute VB and the mean pore size dB in porous domain B as a function of ϕ. This latter is given by the value of d at the inflexion point within B. Intrusion curves are smoothed with a three-points running average prior to computations. Results are reported in figure 7. Both VB and dB exhibit a clear increasing trend with dB ≈ 50 nm for ϕ = 1% and dB ≈ 140 nm for ϕ = 10%. Beyond this value, dB seems to have reached a plateau. As discussed above, the emulsification protocol allows the formation of larger droplets when ϕ is increased. The increasing value of dB with ϕ therefore suggests an increasing typical size of the dendrites created in the freezing phase. This result, together with the smaller fraction of constrictions in the pores geometry just discussed, confirm the key role played by droplet size in the final pore organization. As smaller droplets support more complex porous networks, other contributions like capillarity or the achievement of larger supercooling rates for nucleation, are most likely key in the overall ice templating mechanisms occurring in the emulsion droplets. For the reference Ludox sample, the shape of the hysteresis belongs to IV(a) type with H2(a) loop [32] and is characteristic of densely packed spheres [33]. The pore size obtained from BJH method applied to the desorption branch (see Figure 8(b)) yields a typical diameter for inter-nanoparticles pores of about dINP ≈ 3.6 nm that turns consistent with densely packed spheres of 15 nm diameter [34]. This result must however be handled carefully since the BJH method is not fully accurate close to the cavitation domain (corresponding to a relative pressure range p/p° between 0.4 and 0.5 where p° is the saturation pressure) and may under evaluate dINP. The intraparticle porosity, calculated from the saturation plateau and the amorphous silica density (2.2 g/mL), is around 0.37 which is compatible with the 0.36 value of the random close packing in three dimensions [35]. In the case of emulsified samples, the hysteresis occurs at rather higher relative pressures showing larger pores: dINP = 6.1 nm for ϕ = 1%, dINP = 6.2 nm for ϕ = 10% and dINP = 7.0 nm for ϕ = 20%. This confirms that freezing generates flocculated structures with smaller densities, as already underlined with mercury porosimetry. Beads therefore present smaller number of close neighbors around each nanoparticle. Even though the saturation plateau at high relative pressures is poorly defined, an apparent porosity can still be calculated from the adsorbed nitrogen amount at the inflexion point that lies close to p/p° = 0.94. One gets about 0.46 for ϕ = 1%, 0.49 for ϕ = 10% and 0.52 for ϕ = 20%.

The picture that shows up in this work for the beads structure is a first porous domain generated by dendrite growth surrounded by porous walls with both pore sizes depending upon droplet size. The important output of this study is the highlighting of macroporous domain B which does not exist when the nanofluid is not emulsified. This domain is a consequence of the freezing mechanisms taking place in the supercooled sequence of the nanofluid. It has been discussed above that millimeter sized pendent drops generate porous spheres having the same size than the original drop and with an apparent surface pore size of a few micrometers [28]. These latter are attributed to the "memory" of dendrites that are formed during the initial freezing step (see graphical abstract of [28]). This observation is to be compared with the surface pore sizes below 0.5 µm evidenced in the present study for the largest beads and to the fact that, for the smallest ones, pores are below the SEM resolution (see Figure 2). The freezing mechanisms in the case of pendent drops are moreover known to involve two successive steps: first a very fast formation of ice dendrites that invade the droplets followed by an advancing ice front from outside to inside the droplets. This latter is related to bulk solidification and heat transfer. In the case of small droplets of pure water, it has been further shown that the time delay between these two sequences is reduced. As heat exchanges with the surrounding continuous phase is eased for small droplets as the ones produced in emulsions, dendrite and bulk freezing sequences become most likely simultaneous. Such a transition from a two-step freezing to a single step one has been evidenced in the case of pure water droplets in air with a critical size between 0.1 and 1 µm [30]. The present work suggests the existence of a similar relationship between the size of the emulsion droplets and the size of the dendrites they can support. One important perspective of this study is to proceed with the understanding of this transition in the case of emulsified nanofluid suspensions.

CONCLUSION

Silica beads have been synthetized via the freezing and drying of nanofluid emulsions. Their shape, size and porous microstructure are shown to depend upon volume fraction. Porosity results from a two round ice templating. The porous structure of the beads consists of a macroporous domain and a mesoporous domain. This double-scale structure is the signature of the ice dendrite network that invaded the droplets in the first freezing sequence and of inter-nanoparticle spacing created in the subsequent flocculation phase. The twolevel porosity increases its compactness and complexity as volume fraction is reduced: both macropore and mesopore size reduce while the connectivity is modified. The improvement

Figure 1 :

 1 Figure 1: Preparation of porous silica microparticles by freeze-thawing a W/O emulsion. (a) heptane/SPAN80 and nanofluid solution preparation. (b) emulsion creation by Ultra-Turrax homogenization. (c) sedimentation of flocculated silica microparticles after freezing.

Figure 2 :

 2 Figure 2: SEM micrographs of beads obtained after freezing as a function of ϕ (given top left in each image). The top left picture shows silica structures obtained from the freezing of a non-emulsified suspension. The red arrow in the picture for ϕ = 10% points to the region of interest emphasized in Figure 3.

  in a pendent drop configuration. Micrometer flocs are visible in figure 3(b). The flocs completely fill the droplets and show up as a micro-suspension of floating filamentous aggregates that sediment as time runs. Applying right after the first cycle a second one acts similarly but, instead of nanoparticles aggregation, micrometer sized flocs now aggregate and come compacted together. This procedure prevents sedimentation to set in and sufficient compactness for stable bead production.

Figure 3 :

 3 Figure 3: Optical microscopy images of a nanofluid pendent drop during the freeze-thawing procedure. (a) dendriticfreezing regime. (b) After first thawing -flocs are visible in the upper section of the drop. (c) Drop shape in its solid state after second thawing. Scale bar is 250 µm.

Figure 4 :

 4 Figure 4: Zoom in the area emphasized by the arrow of Figure 2 (ϕ = 10 %). Small beads appear as compact objects while the large ones exhibit macroporosity.

Figure 5 :

 5 Figure 5: Cumulative intruded and extruded mercury volume as a function of pore diameter. Diamonds (resp. triangles) correspond to emulsion sample (resp. reference Ludox sample). Dashed lines qualitatively illustrate porous volumes VIB, VB and VINP presented on the right of this figure.

Figure 6 (

 6 Figure 6(b) shows intrusion-extrusion curves for d < 1 µm and illustrates the two steps

Figure 6 :

 6 Figure 6: (a) Cumulative intruded mercury volume as a function of pore diameter and volume fraction ϕ. (b) intrusion and extrusion curves for d < 1 µm.

Figure 7 :

 7 Figure 7: Bead porous volume VB and typical pore size dB in macroporous domain B as a function of ϕ.

Figure 8 :

 8 Figure 8: (a) Nitrogen sorption isotherms at 77 K for ϕ = 1%, ϕ = 10% and ϕ = 20% and for the reference Ludox sample. (b) Pore size distributions obtained via the BJH method.

of the control of droplet size in the emulsification procedure is one important perspective of this work in particular to produce materials with better controlled porosity, particle shape and particle size. A key point is to study the emulsification parameters such as stirring speed.

The simple methodology used also opens new synthesis routes to incorporate active ingredients in porous structures, not by post modification, but by including them directly in the emulsified nanofluid before freezing. This work could therefore lead to potential innovative approaches for target applications like catalysis or drug delivery. Finally, the proposed material is the result of a physical synthesis involving a minimum amount of chemicals and almost all the solvent can be reused for subsequent synthesis rounds.

AKNOWLEDGMENTS

The authors would like to gratefully acknowledge ESA (ESA/MAP/EDDI), CNES (project Stabilité des dispersions -mesures et modélisation) for financial support and GdR/CNRS microgravité fondamentale et appliquée.