Superhydrophobicity on Hairy Surfaces

Abstract

We investigate the wetting properties of surfaces patterned with fine elastic hairs, with an emphasis on identifying superhydrophobic states on hydrophilic hairs. We formulate a 2D model of a large drop in contact with a row of equispaced elastic hairs and, by minimizing the free energy of the model, identify the stable and metastable states. In particular, we concentrate on partially suspended states, where the hairs bend to support the drop--singlet states, where all hairs bend in the same direction, and doublet states, where neighboring hairs bend in opposite directions--and find the limits of stability of these configurations in terms of the material contact angle, hair flexibility, and system geometry. The drop can remain suspended in a singlet state at hydrophilic contact angles, but doublets exist only when the hairs are hydrophobic. The system is more likely to evolve into a singlet state if the hairs are inclined at the root. We discuss how, under limited circumstances, the results can be modified to describe an array of hairs in three dimensions. We find that now both singlets and doublets can exhibit superhydrophobic behavior on hydrophilic hairs. We discuss the limitations of our approach and the directions for future work.

Geometry-induced capillary emptying

“…he interaction of liquids with solids has been a fertile topic for physicists, chemists, mathematicians, and engineers, providing fundamental insights into the properties of matter, for example, the necessity of attractive molecular forces (1)(2)(3), explaining the rich phenomenology of wetting (4-7) and superhydrophobicity (8)(9)(10)(11), and leading to the development of modern microfluidic devices (12)(13)(14)(15). One of the prototypical examples of this is the capillary rise of a liquid in a vertical tube, where the shape and location of the meniscus (the interface separating the liquid from the gas) are determined by the surface tensions, the geometry of the capillary, and the strength of the gravitational force, as firmly established by comprehensive studies spanning several centuries (16).…”

Surface Structure and Wetting Characteristics of Collembola Cuticles

“…[22], [39] If the hair-cover, rather than the cuticle structure, is quantified and used with existing wetting models different values for the contact angles would be predicted. A cover of curved hairs may act in much the same way as structures with overhang and provide robust superhydrophobicity [40]. However, for Collembola the hairs are of microns to tens of microns in scale while the granules are on a scale of hundreds of nanometers, models that incorporate the contact line energy will therefore differ between the two and would be more likely to predict high contact angles for the granules.…”

The superhydrophobicity of polymer surfaces: Recent developments

“…The second type is particularly interesting for polymeric materials as the sizes required are larger and the shapes are more robust. 95 have simulated hairs and show that moderate flexibility allows them to partially align with the interface, stabilizing the bridging state. Our research has shown that such hairy surfaces are more resistant to condensation driven drop penetration than other superhydrophobic surfaces.…”