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
structured substrates by using density-functional-based effective interface models. For specific
examples these results are
compared with x-ray scattering
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.