Advancing and receding contact lines on patterned structured surfaces

An experimental investigation of advancing and receding contact lines on patterned surfaces was performed in a controlled environment. Hydrophobic polymers were used to create patterned surfaces to mimic defects and the working fluid was water. Surfaces were prepared with holes or pillars every 200 μm and depth/height from 1 to 11 μm. An optical technique was used to measure contact angle. On smooth (control) surfaces, an advancing or receding contact angle was observed. On the patterned surfaces, pinning and depinning at the defects (holes or pillars, respectively) was observed, with advancing or receding contact lines occurring between these depinning/pinning events. The observed pinning/depinning phenomenon of the contact line was investigated to demonstrate the dynamics of the contact line motion over rough surfaces for a small range of contact line velocity. The competition between the Young unbalanced force and the anchoring forces of the defects is thought to dominate the pinning/depinning process. Stick-slip behaviour of the contact line is observed for larger structures and the results show a strong pinning of the contact line on surfaces with larger defects. The datum contact angle and its deviation were measured and a new concept of scaled energy barrier was calculated for advancing contact lines. This was strongly dependent on defect size. An estimation of the unbalanced Young force per unit length was also made for comparison, which also depended on defect size. This new approach allows new insights into this wetting phenomenon.