Thin film and high shear rheology of multiphase complex fluids

We explore the high-shear and gap-dependent rheological properties of multiphase complex fluids using narrow-gap parallel-plate rheometry. This technique has been developed to explore the apparent rheological properties of such fluids when they are confined to length scales comparable to that of their underlying microstructure. This is particularly relevant to processes such as lubrication and microfluidics, whereby complex fluids are typically confined to length scales of below 100 μm and subjected to shear rates well in excess of 1000 s−1. We demonstrate that the parallel-plate geometry is capable of accessing extremely high shear rates (e.g. 105 s−1) using narrow gap heights (5-100 μm) for Newtonian, shear-thinning, and elastic fluids. In order to obtain meaningful measurements, numerous errors that arise must be accounted for. The most apparent error is that the measured viscosity decreases with gap height at gaps below a few hundred microns. This results from an error in the gap that is typically 5-30 μm and usually occurs due to misalignment of the parallel plates, although there is also a contribution from the squeeze flow of air during the gap-zeroing procedure for very accurately aligned plates. The effect of microscale-confinement on the apparent viscosity and viscoelastic properties of microstructured fluids and suspensions is also considered, whereby confinement to gaps that are approaching that of the characteristic microstructure length scale causes a solid-like response with a substantially enhanced storage modulus and apparent yield stress. Despite confinement and jamming effects at low stresses, at high stress the multiphase fluids flow with a viscosity similar to that of bulk fluid and continuous phase even when the gap height is similar to the particle size. Slip and depletion effects are particularly apparent at narrow gaps and must be considered in order to obtain reliable rheological measurements. It is anticipated that the utilisation of these techniques to explore the dynamics of confined microstructures will lead to new insights into the behaviour of such systems under the extreme conditions of narrow gaps and/or high shear rates that are experienced during many processes and/or applications.