The movement of particles in Taylor-Couette flow of complex fluids

A drilling process comprises a drill-pipe rotating within a borehole where fluid is pumped down the pipe and returns, with drilled cuttings, along the annulus. Predominantly the axis of the system is horizontal. Thus, in the absence of axial flow the process geometry is that of a Taylor-Couette flow. Formulated drilling fluids themselves are usually regarded as Bingham or Hershel-Bulkley in nature, but nevertheless encompass elastic behaviour. We have thus studied the distribution of dense (i.e. sedimenting) non-Brownian solid particles in Taylor-Couette flow of model drilling fluids as a function of center body rotation speed. In all cases Taylor vortices are formed above some critical, fluid dependent, Taylor number. However, depending on the fluid properties, particles decorate the vortices differently: particles in a polymeric fluid move to the centroids of the vortices, whereas in a colloidal fluid they move to the outer periphery of the vortices, as previously observed for Newtonian fluids. With a mixed fluid, a clear transition between the two regimes is found. We postulate that this behaviour is a result of a balance between elastically derived lift forces and inertially driven Saffman lift forces acting antagonistically on the particles.