  A liquid drop moves on a solid surface if it is subjected to a gradient of
wettability or temperature. However, the pinning defects on the surface
manifested in terms of a wetting hysteresis, or first-order nonlinear friction,
limit the motion in the sense that a critical size has to be exceeded for a
drop to move. The effect of hysteresis can, however, be mitigated by an
external vibration that can be either structured or stochastic, thereby
creating a directed motion of the drop. Many of the well-known features of
rectification, amplification, and switching that are generic to electronics can
be engineered with such types of movements. A specific case of interest is the
random coalescence of drops on a surface that gives rise to self-generated
noise. This noise overcomes the pinning potential, thereby generating a random
motion of the coalesced drops. Randomly moving coalesced drops themselves
exhibit a directed diffusive flux when a boundary is present to eliminate them
by absorption. With the presence of a bias, the coalesced drops execute a
diffusive drift motion that can have useful applications in various water and
thermal management technologies.
