Effect of Surface Structure on the Spreading of a PDMS Droplet

The effect of surface structures on the dynamics of wetting was investigated through measurements of the dynamic contact angle and spreading velocity of a PDMS droplet spreading over a surface with a gridlike pattern of V-shaped surface channels. A rim of liquid was observed to spread out in the channels ahead of the drop front after a short time of spreading (seconds). The characteristics of the rim were investigated as well, with respect to channel depth and spacing, and compared to the spreading characteristics in channels with no intersections, that is, parallel channels. The macroscopic droplet appears to be dependent on the rim for spreading; before the rim had appeared the drop front spread stepwise between the perpendicular channels and did not advance until the channel was liquid filled. The spreading on the filled channels was correlated to the surface area covered by channels rather than the depth and width of the individual channel. The spreading of the rim follows the expected scaling with (time)1/2 and (channel depth)1/2.