The spreading of silicone oil droplets on a surface with parallel V-shaped grooves

In this paper we report on dynamic wetting studies on model rough surfaces. A series of well-defined model surfaces has been manufactured, and the dynamic wetting of silicone oil droplets on these surfaces has been studied. The surface structures are etched, parallel, V-shaped grooves with varying width and spacing. Spreading has been studied in two different time regimes with two different techniques. At "short times", the Liquid does not penetrate significantly inside the grooves ahead of the edge of the main drop. At "long times", the penetration of liquid inside the grooves is significant and the grooves are practically filled up at the edge of the main drop, which we consider as the "contact line". In both regimes, contact Line velocity, v, and dynamic contact angle, theta, are measured in the flat parts between grooves. The spreading parallel to the grooves is described by Tanner-like laws in both cases but with different characteristics: At short times, the spreading velocity increases with the area covered by grooves. At long times no effect ofthe grooves is observed until the distance between the grooves is smaller than approximately 30 mu m. In this case, it appears that the effect is correlated with the disturbance of the contact line due to the interaction of the groove "defects".