Multilayer formation in self-shaping emulsion droplets.

In several recent studies we showed that micrometer sized oil-in-water emulsion droplets from alkanes, alkenes, alcohols, triglycerides or mixtures of these components can spontaneously "self-shape" upon cooling into various regular shapes, such as regular polyhedrons, platelets, rods and fibers (Denkov et al., Nature 2015, 528, 392; Cholakova et al., Adv. Colloid Interface Sci. 2016, 235, 90). These drop-shape transformations were explained by assuming that intermediate plastic rotator phase, composed of ordered multilayers of oily molecules, is formed beneath the drop surface around the oil freezing temperature. An alternative explanation was proposed by Guttman et al. (PNAS 2016, 113, 493; Langmuir 2017, 33, 1305) which is based on the assumption that the oil-water interfacial tension decreases to very low values upon emulsion cooling. Here we present new results, obtained by differential scanning calorimetry (DSC), which quantify the enthalpy effects accompanying the drop-shape transformations. Using optical microscopy we related the peaks in the DSC thermograms to the specific changes in the drop shape. Furthermore, from the enthalpies measured by DSC we determined the fraction of the intermediate phase involved in the processes of drop deformation. The obtained results support the explanation that the drop-shape transformations are intimately related to formation of ordered multilayers of alkane molecules with a thickness varying between several and dozens of layers of alkane molecules, depending on the specific system. The new results provide the basis for a rational approach to the mechanistic explanation and to the fine control of this fascinating and industrially relevant phenomenon.