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Evaporating salty droplets are ubiquitous in nature, in our home and in the laboratory. Interestingly, the transport processes in such apparently simple systems differ strongly from evaporating "freshwater" droplets since convection is partly inverted due to Marangoni stresses. Such an effect has crucial consequences to the salt crystallization process and to the deposits left behind. In this work we show unprecedented measurements that, not only confirm clearly the patterns of the flow inversion, but also elucidate their impact on the distribution of non-volatile solutes. Contrary to what has been often reported in the literature, such a flow reversal does not prevent the formation of ring-shaped stains: particles accumulate at the contact line driven solely by the interfacial flow. We can therefore conclude that the classical "coffee-stain effect" is not the only mechanism that can generate ring-shaped stains in evaporating droplets.

It is important to understand how interfacial composition influences the digestion of coated interfaces in order to rationally design emulsion based food products with specific digestion profiles. This study has been designed to investigate the effects of gastrointestinal digestion on protein covered interfaces. In this work, we have used a new apparatus fully designed and assembled at the University of Granada: the OCTOPUS. This new device enables the design of a customised static sequential in vitro digestion process in a single droplet. Physiological conditions of each compartment/step of the digestion process are met through subphase exchange of artificial digestive media, hence mimicking the transit through the gastrointestinal tract. We can measure in situ the evolution of the interfacial tension throughout the whole simulated gastrointestinal transit and the mechanical properties of the interfacial layer (interfacial dilatational modulus) after each digestion stage (mouth, stomach, small intestines). The in vitro digestion model used here focuses on pepsinolysis and lipolysis of two dairy proteins: b-lactoglobulin (BLG) and b-casein (BCS) adsorbed at the olive oil-water interface. The results show different susceptibilities of interfacial layers of BLG and BCS to pepsinolysis; while pepsinolysis of adsorbed BLG weakens the interfacial network, pepsinolysis of adsorbed BCS strengthens it as measured by the dilatational moduli. These numbers provide an interfacial scenario for previous findings on emulsification of these proteins, which was found to improve BLG pepsinolysis but somehow protected BCS from pepsinolysis in the stomach. The desorption profiles provide quantification of the extent of lipid digestion in subsequent simulated intestinal fluid containing lipase. The extent of lipid hydrolysis was found to be similar in BLG and BCS covered interfaces and comparable to that in the absence of coverage (pure oil-water interface) indicating that proteins do not comprise a barrier to lipolysis. This similar susceptibility is attributed to the similar interfacial properties of the interfaces reaching the duodenum despite the structural differences between native BCS and BLG, thus demonstrating the impact of the transit through the gut on lipolysis. This research allows identification of the interfacial mechanisms affecting enzymatic hydrolysis of proteins and lipolysis. The results can be exploited in tailoring novel food matrices with improved functional properties such as decreased digestibility, controlled energy intake and low allergenicity.

The rheological behavior of beta-casein adsorption layers formed at the air-water and tetradecane-water interfaces is studied in detail by means of pendant drop tensiometry. First, its adsorption behavior is briefly summarized at both interfaces, experimentally and also theoretically. Subsequently, the experimental dilatational results obtained for a wide range of frequencies are presented for both interfaces. An interesting dependence with the oscillation frequency is observed via the comparative analysis of the interfacial elasticity (storage part) and the interfacial viscosity (loss part) for the two interfaces. The analysis of the interfacial elasticities provides information on the conformational transitions undergone by the protein upon adsorption at both interfaces. The air-water interface shows a complex behavior in which two maxima merge into one as the frequency increases, whereas only a single maximum is found at the tetradecane interface within the range of frequencies studied. This is interpreted in terms of a decisive interaction between the oil and the protein molecules. Furthermore, the analysis of the interfacial viscosities provides information on the relaxation processes occurring at both interfaces. Similarly, substantial differences arise between the gaseous and liquid interfaces and various possible relaxation mechanisms are discussed. Finally, the experimental elasticities obtained for frequencies higher than 0.1 Hz are further analyzed on the basis of a thermodynamic model. Accordingly, the nature of the conformational transition given by the maximum at these frequencies is discussed in terms of different theoretical considerations. The formation of a protein bilayer at the interface or the limited compressibility of the protein in the adsorbed state are regarded as possible explanations of the maximum.