Effects of particulates on contact angles and adhesion of a droplet: A critical review

Adhesion of droplets to solid surfaces plays an important role in various applications, such as oil recovery, detergency, coatings, self-cleaning surfaces, anti-icing surfaces, pesticide deposition, and self-assembly of nanoparticles. Behaviors of a droplet on a solid surface, including static contact angles before depinning, contact angle hysteresis during depinning, and droplet boundary motion after depinning, are affected by particulates contained in the droplet. The mechanisms of particulates affecting the adhesion of a droplet to a solid surface are discussed in this review. The static contact angles are affected by the adsorption of particulates on solid-liquid, liquid-gas, and solid-gas interfaces. The contact angle hysteresis is determined by the increased roughness of the solid surface, the viscosity of liquid along the droplet boundary, and the structural disjoining pressure induced by the ordering of particulates. The droplet boundary motion behaviors, such as contact line moving velocity and stick-slip behavior, are also affected by the droplet boundary pinning due to the presence of particulates. The local concentration of particulates along the droplet boundary has an essential impact on the adhesion behavior of the droplet, which is determined by flows within the droplet and interactions amongst particulates, between particulates and the substrate, and between particulates and the liquid-gas interface. At last, several research directions for further understanding of the effects of particulates on droplet adhesion are proposed.