Spontaneous particle desorption and "Gorgon" drop formation from particle-armored oil drops upon cooling

Abstract

Drop "self-shaping" is a phenomenon in which cooled oily emulsion drops undergo a spectacular series of shape transformations (Denkov et al., Nature 528, 2015, 392). Solid particles adsorbed on the oil-water interface could affect this drop self-shaping process in multiple ways which have not been studied. We prepared Pickering emulsions stabilized by spherical latex particles and afterwards added surfactant of low concentration which enabled drop self-shaping. Next we observed by optical microscopy the processes which occur upon emulsion cooling. Several new processes were observed: (1) Adsorbed latex particles rearranged into regular hexagonal lattices upon freezing of the surfactant adsorption layer. (2) Spontaneous particle desorption from the drop surface was observed at a certain temperature - this phenomenon is rather remarkable, as the solid particles are known to irreversibly adsorb on fluid interfaces. (3) Very strongly adhered particles to drop surfaces acted as a template to enable the formation of tens to hundreds of semi-liquid fibers, growing outwards from the drop surface, thus creating a shape resembling the Gorgon head from Greek mythology. We provide mechanistic explanations of all observed phenomena using our understanding of the rotator phase formation on the surface of cooled drops.