Analysis of drop spreading upon impact on dual-textured surface

Abstract

Practical applications involving lab-on-a-chip devices in microfluidics demand a variety of manipulation to be done on liquid drops on a solid surface including drop transport from one location to another. Wettability gradient surfaces, featuring a spatial variation of surface wetting along a particular direction on the surface, are commonly used to achieve this goal. In this study, we investigate the spreading process of liquid drops impacting, with velocity Uo in the range 0.3-1.5 m/s, on the junction of a dual-textured surface comprising a textured portion and a smooth portion of different wetting characteristics. Comparisons with the results for drop impact under same impact velocities on the homogeneous (far away from the junction) textured and smooth portions of dual-textured surface are also made to understand the effect of the dual-texture nature of target surface on drop spreading process. The drop spread factor, β increases with normalized time, τ as τ0.5 in the initial kinematic phase on all the surfaces whereas for the entire spreading process the power reaches 0.5 as impact velocity is increased. Even though the average spreading velocity, which shows a slight decrease with Uo, does not show any significant difference between the surfaces, the drops impacted on textured surfaces (homogeneous and dual-textured) show a slightly higher deceleration than on the corresponding smooth surfaces. The maximum drop spread and the time taken to attain it are seen to be lower on the textured surfaces; however no difference is seen between the homogeneous and dual-texture portions of the corresponding surfaces.