Magnetic resonance imaging of the rheology of ionic liquid colloidal suspensions

Abstract

The rheology, and underpinning colloidal interactions, of ionic liquid (IL) dispersions of colloidal silica have been investigated using bulk rheological measurements with magnetic resonance (MR) velocity and relaxation measurements. Two ionic liquids were investigated: tetradecyl(trihexyl)phosphonium bistriflamide ([P_6,6,6,14][NTf₂]) and 1-butyl-methylimidizolium tetrafluoroborate ([C₄mim][BF₄]), in the absence and presence of hydrophilic silica nanoparticles (Aerosil 200). Bulk rheology was probed using measurements of shear stress and viscosity as a function of shear rate in a cone-and-plate rheometer. Local rheology was probed using MR velocity imaging of flow in Couette and cone-and-plate cells. Velocity profiles were extracted from the Couette measurements and fitted using a power-law model. Newtonian rheology was observed for both ILs in the absence of dispersed silica. For the dispersion of 15% silica in [C₄mim][BF₄], bulk rheology and MR velocity imaging measurements showed Newtonian behaviour at low shear rates (<10 s⁻¹) and shear-thickening behaviour at higher shear rates (>10 s⁻¹). For the dispersion of 5% silica in [P_6,6,6,14][NTf₂], more complex rheology was observed in the flow curve, which was suggestive of shear-banding. This was investigated further using the MR velocity profiles in a Couette cell and velocity images in a cone-and-plate cell, which both showed the coexistence of regions of sheared and unsheared fluid. The sheared fluid was found to be highly shear-thinning and close inspection of the flow profile at the interface between sheared and unsheared fluid suggested that the behaviour was shear-banding rather than shear-localisation. This was further confirmed by the velocity images in the cone-and-plate rheometer, which showed sheared and unsheared fluid in a uniform shear stress environment.