Aggregation of particles settling in shear-thinning fluids - Part 1. Two-particle aggregation

It is well known that particle aggregation can occur in a non-Newtonian fluid during sedimentation but this effect has not yet been documented in a quantitative way. We present an experimental study of the behaviour of a few non-Brownian particles settling along their line of centres in a weakly elastic and strongly shear-thinning fluid at low Reynolds numbers. Instantaneous velocities of the settling particles have been measured in three polymeric fluids that present different rheological properties. According to previous works, the behaviour of the two particles reveals the existence of a critical initial separation distance under which the particles form a chained doublet. At small separation distances we show that both particles experience an effective constant viscosity but of lower value for the second particle. Particle interactions are successfully described by analogy with the classical approach in a Newtonian creeping flow. Our analysis allows a quantitative prediction of the particle positions with time. We also demonstrate that the critical distance for particle aggregation is linked to the fluid relaxation characterised by transient shear viscosity measurements. The same approach shows a good agreement in the more complex case of three particles that will be detailed in part 2 of this work.