  We present results of Molecular Dynamics (MD) calculations on the behavior of
liquid nanodroplets on rough hydrophobic and hydrophilic solid surfaces. On
hydrophobic surfaces, the contact angle for nanodroplets depends strongly on
the root mean square roughness amplitude, but it is nearly independent of the
fractal dimension of the surface. Since increasing the fractal dimension
increases the short-wavelength roughness, while the long-wavelength roughness
is almost unchanged, we conclude that for hydrophobic interactions the
short-wavelength (atomistic) roughness is not very important. We show that the
nanodroplet is in a Cassie-like state. For rough hydrophobic surfaces, there is
no contact angle hysteresis due to strong thermal fluctuations, which occur at
the liquid-solid interface on the nanoscale. On hydrophilic surfaces, however,
there is strong contact angle hysteresis due to higher energy barrier. These
findings may be very important for the development of artificially biomimetic
superhydrophobic surfaces.
