Insights into capillary-driven motion of micro-particles interacting with advancing meniscus on a substrate.

Liquid-particle interactions at the micro-scale are quite different from the corresponding macro-scale interactions due to the substantial role of capillary forces. Herein, we explore the interaction of a single micro-particle with an air-liquid-substrate contact line. The interaction features ballistic-like motion of micro-particles toward the interacting three-phase contact line with velocities as high as 0.46 m s. Through high-speed optical imaging, we elucidate the interaction mechanism and associated intertwined dynamics, including evolution and backward dragging of the transient air-liquid-particle contact line, capillary-inertial launch of micro-particles and its subsequent trapping at the air-liquid-substrate contact line. Based on the force analysis, we build a model to predict the particle velocity profile during the interaction. Our experimental results show that both hydrophilic and hydrophobic micro-particles exhibit capillary-driven motion. The maximum velocity of the hydrophobic particle, as well as its total displacement, is smaller than that of the hydrophilic one with the same particle size. Micro-particle lifting, like dust removal from self-cleaning surfaces, is observed when the dynamic contact angle of the air-liquid-substrate contact line is sufficiently high ( >100°). We also develop criteria for the capillary-driven motion of particles and predict the critical size for particle motion. These findings are valuable to various applications including capillary-driven self-cleaning, pickering emulsions, micro-scale fluid structure interactions and capillary dynamics in porous media.