Superhydrophobicity and Contact-Line Issues

Abstract

The wettability of several superhydrophobic surfaces that were prepared recently by simple, mostly single-step methods is described and compared with the wettability of surfaces that are less hydrophobic. We explain why two length scales of topography can be important for controlling the hydrophobicity of some surfaces (the lotus effect). Contactangle hysteresis (difference between the advancing, θ A , and receding, θ R , contact angles) is discussed and explained, particularly with regard to its contribution to water repellency. Perfect hydrophobicity (θ A /θ R = 180°/180°) and a method for distinguishing perfectly hydrophobic surfaces from those that are almost perfectly hydrophobic are described and discussed. The Wenzel and Cassie theories, both of which involve analysis of interfacial (solid/liquid) areas and not contact lines, are criticized. Each of these related topics is addressed from the perspective of the three-phase (solid/liquid/vapor) contact line and its dynamics. The energy barriers for movement of the three-phase contact line from one metastable state to another control contact-angle hysteresis and, thus, water repellency.

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“…36,37 (Roughness is required for an ultrahydrophobic surface. 38,39 ) The advancing contact angles of our spin-coated films were 94°-105°against water, which is consistent with a hydrocarbon or a mixed hydrocarbon-fluorocarbon surface, not a purely fluorocarbon surface. Likewise contact angles of <10°against hexadecane indicate a hydrocarbon-like surface of all films prepared with 10% of a fluoromonomer.…”

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“…They are irregularly distributed on the leaf surface with distances between each other ranging from 10 to 100 m. The cones also displayed nanometer sized hairy features, which are reported to be critical in achieving superhydrophobic surfaces with low contact angle hysteresis [11][12][13][14][15][16][17]. Some of the pioneering work in the field was performed by McCarthy and co-workers [8,9,[11][12][13][14][46][47][48][49][50][51][52][53][54][55][56][57][58] and other groups [4,10,[15][16][17]28,41,[59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74], which ...…”

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