Wetting of Nano-Structured Substrates

Wetting The behavior of fluids at macroscopic length scales is determined mainly by gravity and inertia. On the other hand, at microscopic length scales the effects of surface forces, viscosity, and diffusion are dominant, resulting in remarkable and sometimes counter-intuitive behaviors of fluids. For example, a microscopic water droplet can climb up an inclined surface of decreasing hydrophobicity. The recent resurgence of interest in fluid adsorption and wetting phenomena has been triggered by technological breakthroughs in micro- and nano-fabrication. Presently available techniques allow one to fabricate substrate surfaces with designed chemical and geometrical patterns on the nanoscale.

In the context of wetting such microstructured substrates demonstrate so-called superhydrophobic behavior, meaning a water droplet sitting on a microtextured hydrophobic substrate has a contact angle which is much larger compared to a similar droplet placed onto a 'smooth' surface. Droplets deposited on a microstructured substrate can be either in the Wenzel state where the texture beneath a drop is filled by the liquid, or in the Cassie state in which the texture beneath a drop is partially or completely filled by air. Both states correspond to an increased surface hydrophobicity. The Cassie state is also characterized by a small contact angle hysteresis and weak droplets sticking to it. This effect is a nice manifestation how the underlying micro-/nanoscale surface structure influences the macroscopic behavior (contact angle) of liquids spread over it.

Liquid films adsorbed on patterned surfaces are characterized by their fluid interface which exhibits a wealth of equilibrium morphologies with phase transitions between them. We study the morphologies of equilibrated wetting films on geometrically or chemically structured substrates by using density-functional-based effective interface models. For specific examples these results are compared with x-ray scattering data and with data obtained by atomic force microscopy . Thus, the theoretically obtained structure of thin liquid films adsorbed on such substrates agrees quantitatively with the experimental observations. We also are researching the relation between the superhydrophobic effect and the characteristic length scale of the substrate aiming at answering the question down to which length scales this effect persists.
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