Protein-stabilized emulsions and nanoemulsions

This presentation will summarize briefly our current understanding of the scientific and technological basis for the formation of emulsions containing micrometer and nanometer sized droplets, with applications in food, cosmetic and pharma industries [1-3]. The focus will be on protein-stabilized emulsions. Their specific features will be summarized in comparison to the surfactant- and particle-stabilized emulsions [3]. The subtle relations between the material characteristics of the emulsions (oil type, emulsifier, pH, etc.) and the optimal hydrodynamic conditions for emulsification will be discussed [3] in the context of obtaining emulsions with desired properties. Examples of appropriate mixtures of proteins with other emulsifiers (lipids, lysolipids, natural gums) will be given. The differences between oil-in-water and water-in-oil emulsions will be briefly discussed [4,5]. Special focus will be given on the recent advance in production of nanoemulsions using high pressure homogenizers, high viscosity of the continuous phase, and/or high oil volume fraction during emulsification [6,7]. Some new methods for self-emulsification will be briefly reviewed [8,9]. The basic physicochemical and hydrodynamic concepts will be illustrated by multiple examples with real systems. References: David Julian McClements, Food Emulsions: Principles, Practices, and Techniques, 3rd Edition, CRC Press, 2015. Andreas Håkansson, Emulsion Formation by Homogenization: Current Understanding and Future Perspectives, Annu. Rev. Food Sci. Technol. 10 (2019) 239–258 (review article). S. Tcholakova, N. D. Denkov, and A. Lips, Comparison of Solid Particles, Globular Proteins and Surfactants as Emulsifiers. Phys. Chem. Chem. Phys. 10 (2008) 1608-1627 (review article). N. Politova, S. Tcholakova, N. D. Denkov, Factors Affecting the Stability of Water-oil-water Emulsion Films. Colloids Surfaces A 522 (2017) 608–620. N. Politova, S. Tcholakova, S. Tsibranska, N. D. Denkov, K. Muelheims, Coalescence Stability of Water-in-Oil drops: Effects of Drop Size and Surfactant Concentration. Colloids Surfaces A 531 (2017) 32–39. S. Tcholakova, I. Lesov, K. Golemanov, N. Denkov, S. Judat, R. Engel, T. Daner, Efficient Emulsification of Viscous Oils at High Drop Volume Fraction. Langmuir 27 (2011) 14783-14796. D. Gazolu-Rusanova, I. Lesov, S. Tcholakova, N. Denkov, B. Ahtchi, Food grade nanoemulsion preparation by rotor-stator homogenization. Food Hydrocolloids (2019) under review. S. Tcholakova, Z. Valkova, D. Cholakova, Z. Vinarov, I. Lesov, N. D. Denkov, K. Smoukov, Efficient Self-Emulsification via Cooling-Heating Cycles. Nature Comm. 8 (2017) 15012. Zh. Valkova, D. Cholakova, S. Tcholakova, N. Denkov, S. K. Smoukov, Mechanisms and Control of Self-Emulsification upon Freezing and Melting of Dispersed Alkane Drops. Langmuir 33 (2017) 12155−12170

Shake-induced order in nanosphere systems

Self-assembled patterns obtained from a drying nanosphere suspension are investigated by computer simulations and simple experiments. Motivated by the earlier experimental results of Sasaki and Hane and Schope, we confirm that more ordered triangular lattice structures can be obtained whenever a moderate intensity random shaking is applied on the drying system. Computer simulations are realized on an improved version of a recently elaborated Burridge-Knopoff-type model. Experiments are made following the setup of Sasaki and Hane, using ultrasonic radiation as source for controlled shaking.Comment: 7 pages, 10 figure

Structure of rotator phases formed in C13_{13}-C21_{21} alkanes and their mixtures: in bulk and in emulsion drops

Crystallization of alkane mixtures has been studied extensively for decades. However, majority of the available data consider the behaviour of alkanes with chain length of 21 C-atoms or more. Furthermore, important information about the changes of the unit cell structure with temperature is practically absent. In this work, the phase behavior of several pure alkanes $C_{n}H_{2n+2}$, with $n$ ranging between 13 and 21, and their binary, ternary or multi-component equimolar mixtures are investigated by X-ray scattering techniques. Both bulk alkanes and oil-in-water emulsions of the same alkanes were studied. The obtained results show formation of mixed rotator phases for all systems with chain length difference between the neighbouring alkanes of $\Delta n \leqslant 3$. Partial demixing is observed when $\Delta n = 4$, yet the main fraction of the alkane molecules arranges in a mixed rotator phase in these samples. This demixing is suppressed if an alkane with intermediate chain length is added to the mixture. Interestingly, a steep temperature dependence of the interlamellar spacing in mixed rotator phases was observed upon cooling at temperatures down to 10{\deg}C below the melting temperature of the mixture. The volumetric coefficient of thermal expansion of the rotator phases of mixed alkanes (${\alpha}_{V} \approx 2$ x $10^{-3} {\deg}C ^{-1}$) is around 10 times bigger compared to that of the rotator phases of pure alkanes. The experiments performed with emulsion drops containing the same alkane mixture while stabilized by different surfactants, showed that the surfactant template also affects the final lattice spacing which is observed at low temperatures. In contrast, no such dependence was observed for drops stabilized by the same surfactant while having different initial diameters - in this case only the initial temperature of the crystallization onset was affected

The Jamming Perspective on Wet Foams

Amorphous materials as diverse as foams, emulsions, colloidal suspensions and granular media can {\em jam} into a rigid, disordered state where they withstand finite shear stresses before yielding. The jamming transition has been studied extensively, in particular in computer simulations of frictionless, soft, purely repulsive spheres. Foams and emulsions are the closest realizations of this model, and in foams, the (un)jamming point corresponds to the wet limit, where the bubbles become spherical and just form contacts. Here we sketch the relevance of the jamming perspective for the geometry and flow of foams --- and also discuss the impact that foams studies may have on theoretical studies on jamming. We first briefly review insights into the crucial role of disorder in these systems, culminating in the breakdown of the affine assumption that underlies the rich mechanics near jamming. Second, we discuss how crucial theoretical predictions, such as the square root scaling of contact number with packing fraction, and the nontrivial role of disorder and fluctuations for flow have been observed in experiments on 2D foams. Third, we discuss a scaling model for the rheology of disordered media that appears to capture the key features of the flow of foams, emulsions and soft colloidal suspensions. Finally, we discuss how best to confront predictions of this model with experimental data.Comment: 7 Figs., 21 pages, Review articl

Dissipative flows of 2D foams

We analyze the flow of a liquid foam between two plates separated by a gap of the order of the bubble size (2D foam). We concentrate on the salient features of the flow that are induced by the presence, in an otherwise monodisperse foam, of a single large bubble whose size is one order of magnitude larger than the average size. We describe a model suited for numerical simulations of flows of 2D foams made up of a large number of bubbles. The numerical results are successfully compared to analytical predictions based on scaling arguments and on continuum medium approximations. When the foam is pushed inside the cell at a controlled rate, two basically different regimes occur: a plug flow is observed at low flux whereas, above a threshold, the large bubble migrates faster than the mean flow. The detailed characterization of the relative velocity of the large bubble is the essential aim of the present paper. The relative velocity values, predicted both from numerical and from analytical calculations that are discussed here in great detail, are found to be in fair agreement with experimental results