Passive and active particle dynamics in microhydrodynamic flows of complex fluids

There is a rich history in the literature on the effects of particles in Newtonian flows, but when the suspending fluid is also complex, as it is in many applications from environmental flows such as avalanches or mudslides, to energy applications such as down-hole scenarios in the oilfield, to biophysical flows such as cells in the human body, the rheology of these suspensions is far less understood. Particles can interact with nonlinear non-Newtonian stresses in these complex suspensions to significantly alter flows and an ongoing effort is to characterize and understand these effects.

In this talk, I will discuss a number of instances where particles suspended in complex fluids lead to markedly different dynamics and rheology for microhydrodynamic flows. For example, particles suspended in a Newtonian fluid will increase the apparent viscosity of the fluid due to the additional dissipation caused by the disruption of streamlines, called the Einstein viscosity; however, if instead the suspending fluid is shear-thinning, the higher strain-rates induced by the particles will also produce a competing apparent reduction of the viscosity and for a dilute suspension of particles in a weakly shear-thinning fluid, these combined effects lead to modified Einstein viscosity. Anisotropic particles can undergo complex degenerate dynamics, even in steady shear of Newtonian fluids (known Jeffery orbits for spheroids), here we discuss how shear-thinning rheology can affect these dynamics. Finally we discuss how active particles (biological or otherwise) can display completely different behaviour in complex fluids, such as reciprocal swimmers that in a Newtonian fluid would not produce net motion but in a complex fluid move and whose motion is directly coupled with the non-Newtonian rheology of the fluid and hence act as a nonlinear microrheometer.