Wettability tuning by surface modification

Controlling the wettability of solid materials is a key issue in surface engineering. The objective of this thesis is to control the wettability of solid substrates via various surface modifications. The wettability tuning in this study includes two opposite aspects. One is to enhance the wetting between the substrates and the polymer films. The other is to enhance the hydrophobicity of the solid substrates. The wetting between silicon substrates and the polystyrene (PS) films are improved by end-grafting a layer of polystyrene brush to the substrates. The wettability and the equilibrium structure of PS-melt/PS-brush/SiO2/Si system were then studied. It was found that when the molecular weight of the melt or the grafting density of the brush decreases, the thickness of the residual film increases and the contact angle decreases, both indicating improved wettability. By comparing the measurements with recent results of the self-consistent field theory, good agreement is found. The hydrophobicity of the solid substrates is enhanced through physically roughing the solid surfaces. The well-defined microscaled periodic structures, including connected grid structure, discrete pillar structure and parallel strip structure, were prepared on silicon surfaces by photolithography. The sliding behavior, water contact angle, and self-cleaning ability are compared among different surface topologies. The correlation between sliding angle, contact angle hysteresis, drop base width and drop volume was investigated. The wettability and the self-cleaning property were found to be surface structure dependent. The water apparent contact angle of the grid patterns agrees with the prediction by Cassie equation, but that of the pillar patterns is much larger than the prediction. Nevertheless, the pillar patterns exhibit inferior self-cleaning ability, contrary to conventional conception. The contact line of water drops were found to move in a stick-jump manner on patterned surfaces both from theoretical simulation and experiment measurements. The oscillation amplitude of the contact angle decreased with decreasing strip width. In addition, the jumping distances of the contact lines, for both advancing and receding water drops, were nearly equal to the strip period. The results of wettability mechanism studied in this thesis provide a guideline for designing surfaces with controlled wettability