Local velocity of thermoresponsive colloidal gels in rate-driven flow

Abstract

The interplay between flow and attractive interactions in colloidal gels results in complex particle trajectories and velocity profiles that are not evident from bulk rheological measurements. We use high-speed confocal microscopy to investigate the local velocity of a low volume fraction (ϕ = 0.20) thermogelling nanoemulsion system as it flows through a cylindrical capillary at temperatures below and above the gel point. The nanoemulsions are composed of poly(dimethyl siloxane) droplets in a continuous phase of sodium dodecyl sulfate, de-ionized water, and a gelator molecule, poly(ethylene glycol diacrylate). The trajectories of fluorescent polystyrene tracer beads in the oil-rich domains are tracked using two-dimensional image processing. While the velocity profiles agree with those computed from rheometry measurements for nanoemulsion suspensions below the gel point temperature, increasing attractive interactions above the gel point results in statistically significant deviations. Specifically, the velocity measurements indicate a higher yield stress and a larger degree of shear thinning than expected from bulk rheology measurements, resulting in a more plug-shaped velocity profile as temperature and associated interdroplet attraction increase. These deviations from theoretical predictions are likely due to structural heterogeneity. Confocal microscopy images show that small, fluidized clusters are found in high shear rate regions near the capillary walls, while large dense clusters form in low shear rate regions closer to the center of the capillary.