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Wetting on flexible and anisotropic surfaces

By: Matthew Blow
From: CFTC
At: Instituto de Investigação Interdisciplinar, Anfiteatro
[2011-04-07] 11:30

Topographically patterned surfaces occur on the bodies of many plants and animals and can also be microfabricated. They exhibit exotic wetting behaviour, including superhydrophobicity and imbibition. We quantitatively assess the feasibility of, and transitions between, wetting states on some specific substrates, using analytical free-energy calculations, and numerical techniques comprising of lattice Boltzmann and lattice spring simulations, and the Surface Evolver program.

We investigate the wetting properties of surfaces patterned with fine elastic hairs, with an emphasis on identifying superhydrophobic states on hydrophilic hairs. We analytically solve a two-dimensional model, identifying singlet states where all hairs bend in the same direction, and doublet states where neighbouring hairs bend in opposite directions, and find limits of stability of these configurations in terms of material contact angle, hair flexibility, and system geometry. Suspended singlet states exist for hydrophilic contact angles, but doublets exist only when the hairs are hydrophobic. Inclined hairs are more likely to evolve into a singlet state. Under limited circumstances, the results can be extended to a three-dimensional array of hairs.

We model rigid hairs of various shapes, and compare their effectiveness for plastron respiration by aquatic arthropods. Hairs with a section tangential to the interface can withstand a high Laplace pressure whilst providing a large interfacial area for respiration. The plastron is vulnerable to depinning from the tips of the hairs but this can be suppressed by making the hairs more hydrophobic.

We show that a film advancing amongst hydrophilic triangular posts is connected along the post corners, but punctuated by the faces. The spreading threshold contact angles differ with direction, allowing for anisotropic spreading. Triangular posts can inhibit spreading in one direction or create a triangular film when arranged in square or hexagonal arrays respectively. Hexagonal posts can facilitate stripe formation.