Numerical modeling and quantification of droplet mixing using mechanowetting

Abstract

Capillary forces are often found in nature to drive fluid flow, and methods have been developed aimed to exploiting these forces in microfluidic systems to move droplets or mix droplet contents. Mixing of small fluid volumes, however, is challenging due to the laminar nature of the flow. Here, we show that mechanowetting, i.e., the capillary interaction between droplets and deforming surfaces, can effectively mix droplet contents. By concentrically actuating the droplet, vortex-like flow patterns are generated that promote effective mixing. To quantify the degree of mixing, we introduce two strategies that are able to determine mixer performance independent of the initial solute distribution within a droplet, represented by single scalars derived from a matrix-based method. We compare these strategies to existing measures and demonstrate the full decoupling from the initial condition. Our results can be used to design efficient mixers, featuring mechanowetting as a new enabling technology for future droplet mixers.