EXPERIMENTAL STUDY OF DROPLET SPLASHING ON VISCOELASTIC SUBSTRATE

The impact of a liquid droplet on a solid substrate and the subsequent splash-defined as the detachment of small satellite droplets from the spreading drop\u{2014}is a common everyday phenomenon. Despite splashing''s ubiquity, both in our personal experience, and in such diverse applications as ink-jet printing, fuel combustion, industrial spray coating, and pesticide delivery, the mechanism of such splashes is not yet fully understood. For some applications, such as fuel combustion, splashing is beneficial, while for others, such as pesticide delivery, splashing should be minimized. For these reasons, it is important to understand the splash mechanism, and which parameters can be used to effectively control splashing behavior. The role of the compliance of soft substrates in drop splashing is important to understand it, since it is involved in many common applications, such as pesticide spray on leaves and spray cooling of flexible surfaces. When a liquid drop hits a solid surface, it often splashes and breaks into thousands of smaller droplets. Splashing occurs as two distinct types of prompt and corona. Prompt splashing takes place under the influence of surface roughness and corona splashing happens under the influence of the surrounding air of drop. In this study, splashing of droplet in impact on soft viscoelastic PDMS substrate with different film thicknesses using high-speed cameras was investigated experimentally. A mixture of ethanol water droplets was used in the millimeter size with different physical properties for impact velocities of 3.5 to 5 m/s. Advancing and receding contact angles and roughness of substrate of different softnesses and thicknesses of the film were measured to determine the wettability of surfaces. It can be observed that, at a specified time, from the moment of impact on, the substrate with greater film thickness has lager corona height. The experimental results showed that the an increase in surface softness and thickness of the film, increases the contact angle hysteresis. Moreover, the softness and thickness of substrate have little effect on splashing deposition threshold.