(310d) High Throughput Measurement of Droplet Surface Tension in Confined Flows

We present a novel microfluidic device to collect high-throughput measurements of droplet surface tension in-line with a droplet generator, without requiring manual adjustment to input flow rates or driving pressures. Drops flow at constant volume flow rate through increasingly narrower channels and thereby experience increasingly higher shear stress. Video analysis provides simultaneous measurements of drop velocity and deformability index. We calculate surface tension using its relationship to the viscous drag forces described in the capillary number (Ca), which is in turn related to deformability. We validate our tool using standard pendant drop measurements, in which one drop is measured at a time after reaching steady state, using a system of water drops in mineral oil stabilized by Span-80 over a range of concentrations. Our system has several key potential advantages over the pendant drop technique. Rather than measuring a single static image, as in pendant drop, we perform high-throughput, dynamic measurements to analyze thousands of individual drops in a several-second video. Our results reveal oscillatory dynamics in the deformation of droplets subject to high shear, which can be averaged to obtain surface tension measurements within 5-10% of those obtained via pendant drop.