(142y) Effect of Attraction Strength On Microchannel Flow of Colloid-Polymer Mixtures

Flows of dense colloidal suspensions through very thin geometries appear frequently in technological applications, such as inkjet printing and direct write assembly, crude oil recovery, and drug delivery. While the flow properties of hard-sphere suspensions in micro-scale geometries have been studied extensively, the effects of inter-particle attraction on the confined flow properties are not well understood. In this study, we investigated the flow properties of colloidal suspensions at a volume fraction of ɸ=0.15 with varying strengths of attraction using bulk rheology and confocal microscopy. We used a slightly charged model system of poly(methylmethacrylate) particles that are suspended in a refractive-index and density-matched solvent mixture of decahydronaphthalene and cyclohexylbromide, and induced a controlled short-range depletion attraction between the particles by adding non-absorbing linear polystyrene. As the strength of attraction was increased, the steady shear viscosity increased for all values of the shear rate. Fits of the Herschel-Bulkley model to the shear stress-shear rate data showed that the strongly attractive suspension yielded at higher stresses, consistent with the formation of an interconnected network of particles. We directly imaged the flow of these suspensions in microchannels using confocal microscopy. Increasing the strength of attraction between the particles resulted in changes in the shape of the flow velocity profile that reflected the emergence of wall slip and rotational motion of clusters of particles. In addition, the formation of strong bonds between particles suppressed two mechanisms for densification present in the weakly-attractive suspension, shear-induced migration and consolidation.