An experimental study on the equilibrium shape of water drops impacted on groove-textured surfaces

Abstract

Understanding the final shape of liquid drops deposited on groove-textured surfaces is a significant aspect of many applications (for example, liquid drainage). This is lacking in literature, especially for drop impact on micro-textured surfaces. The effect of drop impact velocity and groove-texture geometry on the final shape adopted by water drops impacted on groove-textured surfaces is reported here. Water drops gently placed on groove-textured surfaces comprising trapezoidal (rectangular) pillars adopted the Wenzel (Cassie) state. Top view images of final equilibrium shape of water drops impacted on the groove-textured surfaces revealed a contrasting behavior between Wenzel and Cassie surfaces. At low impact velocity, the final drop shape on Wenzel surface is elongated more in the direction parallel to the grooves than perpendicular to grooves thereby exhibiting anisotropy whereas on Cassie surface the final drop shape is almost isotropic. As impact velocity increases, the anisotropy in final drop shape on Wenzel surface decreases whereas on Cassie surface it increases. The final drop spread factors and contact angles measured perpendicular and parallel to grooves show contrasting trends with impact velocity between Wenzel and Cassie surfaces. This is modelled through the difference between drop receding perpendicular and parallel to the grooves, resulting from a difference in liquid drop impregnation state, between the surfaces.