Spontaneous breakdown of superhydrophobicity

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Spontaneous Breakdown of Superhydrophobicity, Mauro Sbragaglia, Alisia M. Peters, Christophe Pirat, Bram M. Borkent, Rob G. H. Lammertink, Matthias Wessling, and Detlef Lohse, Phys. Rev. Lett. 99, 156001 (2007).

Abstract from the original paper

In some cases water droplets can completely wet microstructured superhydrophobic surfaces. The dynamics of this rapid process is analyzed by ultrahigh-speed imaging. Depending on the scales of the microstructure, the wetting fronts propagate smoothly and circularly or—more interestingly—in a stepwise manner, leading to a growing square-shaped wetted area: entering a new row perpendicular to the direction of front propagation takes milliseconds, whereas once this has happened, the row itself fills in microseconds (‘‘zipping’’). Numerical simulations confirm this view and are in quantitative agreement with the experiments [1].

Overview

When a surface structure is introduced in a material, the roughness of the material changes wetting behavior of the surface. Wenzel derived an expression for a contact angle with the microstructured surface as follows:

                                                cosθw  = rcosθ

(θ: contact angle of a surface without microstructures, θw : contact angle of a surface with microstructures, r is the ratio of the actual area to the projected area [2])

As a result, a hydrophobic surface (a surface with a contact angle larger than 90°) becomes more hydrophobic, whereas a hydrophilic surface (a surface with a contact angle smaller than 90°) becomes more hydrophilic [3]. Thus, materials with surface structures can show superhydrophobicity (contact angle larger than 150°). Cassie and Baxter found that if a liquid is suspended on top of microstructures, the contact angle will change to a new value θcb with the following relation [4]:

                                              cosθcb = φ(cos θ + 1) – 1

(φ is the area fraction of the solid that touches the liquid.)