STEEP WAVE-FRONTS ON EXTRUDATES OF POLYMER MELTS AND SOLUTIONS - LUBRICATION LAYERS AND BOUNDARY LUBRICATION

Steep wave fronts tend to develop in many regimes of lubricated, slipp ing flows in which waves appear. Problems of slip. spurt, fracture and extrudate distortion can be framed in terms of lubrication theory wit h paradigms arising from the lubrication of heavy oil with water for s ome problems and concepts from the theory of boundary lubrication for others. In water-lubricated pipelines, high pressures are produced at the front side of a wave on the oil when water is forced through the w avecrest and the wall, low pressures develop at the back of the wave w here the gap opens. The steep waves which develop on cores of heavy oi l lubricated by water are irregular and look like melt fracture. Direc t numerical simulation of regular periodic waves give rise to sharkski n solutions in which the wave length decreases with the wave amplitude as the gap size decreases, preserving the steep wave front. Wave stee pening seems always to occur in extrusion when the polymers slip, in t he abrasion of rubber samples and in Schallamach's waves of detachment . (C) 1997 Elsevier Science B.V.