Particle Motion and Segregation in Piston Driven Free Surface Flows of Viscoelastic Fluids
We have investigated the motion, redistribution and segregation of solid particles dispersed in a viscoelastic fluid during piston-driven free surface flows. This phenomenon and its effect on suspension microstructure is of significant importance in a number of processes used to form composite materials, notably injection and compression moulding. In the experimental part of this study we used a 5% w/w suspension of PMMA particles dispersed in a hydrated hydroxypropyl guar gel. A reversible motor with variable speed and minimal vibration was used to move a tube up and down on a stationary piston, creating a piston-driven flow with a free surface. We observed that the particles move away from the piston and towards the free surface where they slowly accumulate. This observation confirms theoretical predictions that in the presence of a normal stress gradient perpendicular to the direction of bulk flow, as would be observed with viscoelastic fluids, suspended particles tend to gravitate towards and then remain near the free surface. Simulations show that the speed of this segregation increases with fluid elasticity, shear rate and particle size, in agreement with literature models for the lateral migration velocity of particles in viscoelastic fluids.