An asymptotic analysis of two-dimensional free-surface cusps associated
with flows
at low Reynolds numbers is presented on the basis of a model which, in
agreement
with direct experimental observations, considers this phenomenon as a particular
case of an interface formation–disappearance process. The model was
derived from
first principles and earlier applied to another similar process: the moving
contact-line
problem. As is shown, the capillary force acting on a cusp from the free
surface, which
in the classical approach can be balanced by viscous stresses only if the
associated
rate of dissipation of energy is infinite, in the present theory is always
balanced by the
force from the surface-tension-relaxation ‘tail’,
which stretches from the cusp towards
the interior of the fluid. The flow field near the cusp is shown to be
regular, and the
surface-tension gradient in the vicinity of the cusp, caused and maintained
by the
external flow, induces and is balanced by the shear stress. Existing approaches
to the
free-surface cusp description and some relevant experimental aspects of
the problem
are discussed.