  Equilibrium and motion of a contact line are viewed as analogs of phase
equilibrium and motion of an interphase boundary. This point of view makes
evident the tendency to minimization of the length of the contact line at
equilibrium. The concept of line tension is, however, of limited applicability,
in view of a qualitatively different relaxation response of the contact line,
compared to a two-dimensional curve. Both the analogy and qualitative
distinction extend to a non-equilibrium situation arising due to coupling with
reversible substrate modification. Under these conditions, the contact line may
suffer a variety of chemo-capillary instabilities (fingering, traveling and
oscillatory), similar to those of dissipative structures in nonlinear
non-equilibrium systems. The preference order of the various instabilities
changes, however, significantly due to a different way the interfacial
curvature is relaxed.
