Super-stable parallel flows of multiple visco-plastic fluids

We consider the stability of a multi-layer plane Poiseuille flow of two Bin gham fluids. It is shown that this two-fluid flow is frequently more stable than the equivalent flow of either fluid alone. This phenomenon of super-s tability results only when the yield stress of the fluid next to the channe l wall is larger than that of the fluid in the centre of the channel, which need not have a yield stress. Our result is in direct contrast to the stab ility of analogous flows of purely viscous generalised Newtonian fluids, fo r which short wavelength interfacial instabilities can be found at relative ly low Reynolds numbers. The results imply the existence of parameter regim es where visco-plastic lubrication is possible, permitting transport of an inelastic generalised Newtonian fluid in the centre of a channel, lubricate d at the walls by a visco-plastic fluid, travelling in a stable laminar flo w at higher flow rates than would be possible for the single fluid alone. ( C) 2001 Elsevier Science B.V. All rights reserved.