The Role of Wettability on the Shedding of Drops*

The study explores how a small change in the contact angle (CA) of a drop makes a difference in the shedding from a surface with similar wettability. Three oleophobic surfaces, polystyrene (static CA of 93\textpm 3\textdegree ), tin (104\textpm 4\textdegree ) and glass (118\textpm 4\textdegree ), were chosen. Silicon oil drops (350cSt) of various volumes were shed due to water shear flow. The velocity of shedding of a drop when the adhesion is balanced by the drag provided by the shear-flow, is termed as the critical velocity (Ucr). The results show that Ucr for a given drop on the polystyrene surface is~higher than that of the tin and glass surfaces.~The higher contact length for drops on polystyrene surfaces, makes the adhesion higher. The lower frontal area makes less drag to be applied for the same velocity of the shear-flow. Therefore, higher drag becomes necessary for the shedding. So,~the Ucr increases with the wettability (polystyrene \textless tin \textless glass). The CAs of a drop predominantly found to control the shedding. The Ucr is found to decrease with the increase in the volume for a given surface. Bigger drops provide higher frontal area and makes the drag higher. The flow map is discussed from a modified Weber and Ohnesorge numbers.