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 surf aces. A series of well-defined model surfaces has been manufactured, a nd the dynamic wetting of silicone oil droplets on these surfaces has been studied. The surface structures are etched, parallel, V-shaped gr ooves with varying width and spacing. Spreading has been studied in tw o different time regimes with two different techniques. At ''short tim es'', the Liquid does not penetrate significantly inside the grooves a head of the edge of the main drop. At ''long times'', the penetration of liquid inside the grooves is significant and the grooves are practi cally filled up at the edge of the main drop, which we consider as the ''contact line''. In both regimes, contact Line velocity, v, and dyna mic contact angle, theta, are measured in the flat parts between groov es. 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 bet ween the grooves is smaller than approximately 30 mu m. In this case, it appears that the effect is correlated with the disturbance of the c ontact line due to the interaction of the groove ''defects''.