  Clarifying the factors that control the contact angle of a liquid on a solid
substrate is a long-standing scientific problem pertinent across physics,
chemistry and materials science. Progress has been hampered by the lack of a
comprehensive and unified understanding of the physics of wetting and drying
phase transitions. Using various theoretical and simulational techniques
applied to realistic fluid models, we elucidate how the character of these
transitions depends sensitively on both the range of fluid-fluid and
substrate-fluid interactions and the temperature. Our calculations uncover
previously unrecognised classes of surface phase diagram which differ from that
established for simple lattice models and often assumed to be universal. The
differences relate both to the topology of the phase diagram and to the nature
of the transitions, with a remarkable feature being a difference between drying
and wetting transitions which persists even in the approach to the bulk
critical point. Most experimental and simulational studies of liquids at a
substrate belong to one of these previously unrecognised classes. We predict
that while there appears to be nothing particularly special about water with
regard to its wetting and drying behavior, superhydrophobic behavior should be
more readily observable in experiments conducted at high temperatures than at
room temperature.
