Hybrid microfluidic device for on-demand control of droplet coalescence and testing of emulsion stability

Abstract

We present a new microfluidic method for the characterization of emulsion stability against coalescence within a microfluidic droplet-making device. In our device, the merging of droplets can be actively controlled under a wide range of flow conditions, using a simple structure. The new method combines features of already existing passive and active methods, hence we refer to it as a “hybrid” method. Our hybrid method allows for a relatively easy integration within any PDMS (polydimethysiloxane)-based microfluidic device comprising a droplet-making structure, since it employs a simple single-layer PDMS geometry. The main device structure comprises 2 pressure chambers, placed symmetrically along a locally expanded main channel, through which the droplets are flowing. By inducing an overpressure in the pressure chambers, a change of the cross-sectional area of the main channel is reached, which influences the droplets’ velocities and their mutual distance, and thus the coalescence rate. To test our hypotheses on the working principle of the device and to test its performance, we carry out systematic experiments at varying flow rates and applied pressures. These experiments confirm the hypothesized working principle of the device and indicate that the method is suitable for characterizing emulsion stability. Moreover, we show that the hybrid method is capable of actively controlling the creation of on-demand coalescence patterns and even of triggering specific single coalescence events. This indicates that the hybrid method, when integrated into a droplet-making device, can offer a promising approach both for characterizing the stability of emulsions and for controlling on-demand droplet coalescence.