Electrorheoimaging of a nonequilibrium emulsion: Microstructures and multistate switchable viscosities

Complex fluids such as colloids and emulsions are ubiquitous in biopharmaceuticals, manufacturing, and the oil industry, but controlling their viscosity can pose a significant challenge. Low viscosities are ideal for mixing, high viscosities are helpful for pharmaceuticals, and switchable viscosities are useful for smart materials. One strategy to make multipurpose fluids is to make nonequilibrium modifications to the microstructure of colloids and emulsions using external fields, which can result in emergent changes in their rheological characteristics. We introduce an experimental technique, electrorheoimaging (ERI), which directly visualizes microstructure during rheological measurements while simultaneously applying an electric field. We also introduce a continuous phase that lowers electrohydrodynamic thresholds and enables experiments at modest field strengths, revealing behaviors previously inaccessible in laboratory settings. Using this combination of technique and material design, we observe shear-induced banding that only appears in the presence of electric depending on the frequency of the electric field, at either high or low shear rates. These microstructural transitions produce dramatic, reversible viscosity modulation—not only increases, but also decreases, including negative intrinsic viscosities. Frequency control enables instantaneous switching among multiple effective viscosities spanning a 30-fold range, establishing ERI as a powerful tool for probing and controlling rheology in field-responsive soft materials.