Hydrophobicity of model surfaces with closely packed nano- and micro-spheres

Studies of model surfaces with orderly arranged nano- and micro-spheres in a closely packed structure were conducted to pursue a quantitative understanding of the effect of surface topography on surface hydrophobicity. Polystyrene (PS) and silica spheres with size ranging from 15 nm to 2000 nm were used in fabricating the model surfaces. The hydrophobicity of all surfaces was characterized by the means of water contact angle measurement. Compared with flat reference surfaces on which the contact angle was 108-110°, the hydrophobicity of surfaces with closely packed spheres was improved which had contact angle of 123-124°. Further increases in the sphere size did not help to further improve the surface hydrophobicity as the contact angle measurements showed the similar values (i.e. 123-124°) for different sphere sizes. The relationship between the water hydrophobicity and the surface structure were elaborated by a modified Cassie formulation. A good correlation was found between the model elaboration and experimental measurements.