Hollow droplet impact on a micro-trench hydrophilic surface

Abstract

Hollow droplet impact on a solid surface occurs in applications ranging from controllable bio-medicine, thermal spray coating and additive manufacturing. In this work, we experimentally study the impact dynamics of silicone oil hollow droplets on a hydrophilic solid substrate, including a smooth surface and a micro-trench surface. The size of the total droplet and the air bubble are kept constant and we investigate the influence of the location of the air bubble, impact velocity, and micro-trench structure on the hollow droplet shape evolution. The results show that a counter-jet formed after the hollow droplet impact on a solid surface and the height of the counter-jet and the spreading diameter of the lamella grow with impact velocity. Two cavity break mode, natural crashing and external crashing, are found in our experiment and the critical Weber number $We$ is around 40. When $We$ reaches 70, the counter-jet clamps off and detaches from the surface. For a given hollow droplet, the spreading characteristics do not vary significantly with different locations of the air bubble, while the height of the central counter-jet increased with the increasing eccentricity. The effects of gap width and height of the micro-trench are discussed. The structured surfaces reduce the maximum spreading diameter compared to smooth surfaces. A theoretical model considering energy conversation is established to predict the maximum spreading diameter. The deviation between the predicted value and the measured value is less than 10%, which shows excellent consistency. These findings offer critical insights into the behavior of hollow droplets and hold significant potential for applications requiring precise droplet manipulation.