SHAPE AND SIZE OF THE CURRENT HEAD IN CREEPING FLOWS

We investigate the shape and the size of the current head formed when a viscous liquid driven by gravity spreads on a horizontal smooth subs trate. A characteristic roll-shaped current head is observed during th e experiments, which evolves towards a wedgelike shape when the (decre asing) capillary number Ca goes below the unity. The lower part of the advancing roll observed when Ca much greater than 1 may be described as half a cusped entrance of rile free liquid surface, centered on the spreading surface. The size of the roll structure during this stage i s proportional to the square of the front velocity which, in turn, is determined by the global dynamics of the flow.