Drying of complex fluids such as colloidal suspensions or emulsions has been used in practical production processes. However, fundamental aspects in drying kinetics of those fluids are still challenging topics. In this review, we introduce recent research topics on drying kinetics of silica colloidal particles and that of both water-in-oil and oil-in-water emulsions. Drying generally causes compression to dispersants in drying solutions. In colloidal suspensions, suspended particles are packed and particulate films spontaneously formed. Dispersed liquid droplets are also compressed in drying emulsions by evaporating continuous phase and deformed droplets affects drying kinetics of the continuous phase in turn. Drying kinetics of complex fluids are discussed using several examples.

Titanium dioxide and zinc oxide particles have been used in sunscreens as UV filters. Generally, these nanoparticles are formulated as dispersions with an oil medium. Flocculation of the particles has been regarded as “taboo” in the formulations of sunscreens because it results in the reduction of the UV protection efficiency and the problem of an unnatural white cast of the skin. In this study, we developed a new dispersion composition of the mineral UV filters as well as discovered a dynamic structural change of the flocculation system. An applied film of the newly developed dispersion exhibited a dramatic optical transition with a dynamic de-flocculation of the particles, where the UV protection efficiency and the visual transparency were enhanced over the drying time of the film. This was triggered by the evaporation of a poor solvent and the recovery of the solvent affinity to the dispersant. In addition, the original flocculation of the dispersion suppresses the spontaneous flow of the applied liquid into the skin furrows and pores, which leads to the formation of a uniform film on the skin. As a result, the new dispersion system had excellent UV protection performance compared to a system with originally well-dispersed UV filters.

Powders are the ingredients that make cosmetics “eye-catching”. Furthermore, there is powder surface design that makes powders even more attractive. In this report, I will first introduce surface treatment methods that directly change the powder surface and surface treatment methods that coat the surface with other substances. I will then explain the effects obtained through surface design. Specifically, these features give cosmetics new functions and appeal, such as “improved stability”, “make-up effect”, “improved usability and makeup durability”, “effects on the skin”, and “deodorizing effect”.Finally, I will also explain the impact of cosmetics regulations such as “microplastics regulations” on powder surface design.