Effect of electrowetting induced capillary oscillations on coalescence of compound droplets.

Hypothesis: Coalescence time depends on the drainage rate of the fluid-bridge separating the droplets. Drainage rate is determined by external forcing and properties of the surrounding fluid. Modulating external forcing using electrowetting induced interface motion should allow control of the drainage rate, thereby affecting the coalescence time. Hence, quick coalescence or prolonged non-coalescence can be obtained for compound droplets on the microfluidic lab-on-chip systems.

Experiments: Using high-speed imaging, we have investigated the effect of electrowetting induced capillary oscillations on the coalescence of compound droplets consisting of water core encapsulated in an oil shell. A systematic study was performed by varying the shell viscosity and actuation parameters (i.e. amplitude, frequency and waveform).

Findings: For actuated interface, we observed specific regimes of coalescence or non-coalescence, whereas in absence of actuation, coalescence was observed in finite time. Non-coalescence was attributed to the continuous modulation of the oil-bridge width, which was caused by the interface motion. Oil-bridge width modulation was seen to be dependent on the amplitude and shape of the excited capillary modes (axisymmetric and non-axisymmetric). These modes were tuned by the actuation parameters. This is the first report of controlling coalescence dynamics by using electrowetting induced interface motion.