Mechanisms of pinning accompanying evaporation of colloidal dispersion droplets

Experiments have been performed to clarify the mechanism of pinning, i.e., the phenomenon of fixing menisci of evaporating dispersion droplets, related to the formation of ring-shaped deposits (coffee ring effect). The influence of particle concentration in dispersions and the degree of hydrophilicity (hydrophobicity) of substrates on pinning has been studied. It has been shown that there are three main mechanisms of pinning, with the first one being due to the hysteresis of droplet contact angle, the second mechanism resulting from the adhesion of particles to substrates, and the third one being caused by the formation of a dense adsorption layer on a substrate. A relation has been revealed between the pinning mechanism that occurs and the degree of substrate hydrophilicity.