Direct flow visualization of colloidal gels in microfluidic channels

The behavior of colloidal gels under pressure-driven flow in square microchannels is quantified by microscopic particle image velocimetry (mu PlV) and compared to predictions of available rheological models. The gels consist of hydrophobically modified silica microspheres (phi = 0. 15-0.33) suspended in a refractive index-matched fluid along with fluorescent tracers to aid visualization. Measured velocity flow profiles show a transition from plug flow to more fluid-like behavior with increasing volumetric flow rate (Q) at all phi. This transition is not captured by theoretical predictions of the flow profile based on the Herschel -Bulkley model. Rather, a model that accounts for gel breakup into a suspension of clusters at elevated shear rates by assuming a finite viscosity at infinite shear is needed to accurately predict the flow behavior of colloidal gels at large Q.