Role of oil conductivity in electrocoalescence of a droplet

Electrocoalesence is a widely adopted method in droplet microfluidics, used for the controlled merging of droplets to manipulate the drop content.

We aim to experimentally and quantitatively study this mechanism. After the droplet pairs are statically trapped, we vary the properties of the electric field applied across the droplet pair (voltage, frequency), and we vary the properties of the fluids (electrical conductivity, surfactant concentration).  

We find that the efficiency of electrcoalescence is highly dependent on the frequency of the applied electric field, higher frequencies require a lower applied voltage. Furthermore, we find a variation of this dependency when using different oils to create the emulsion. To understand these results, we developed an equivalent electrical model for our system. In this model we analyze the resulting voltage over the liquid film between the two drops as compared to the applied voltage. A parameter scan of this model shows that the droplet conductivity indeed does not play major role as was observed in the experiments. However the electrical resistivity of the oil is an essential parameter – although usually neglected – of the model that could explain our experimental results.

These findings may generate a better understanding on the mechanism of droplet electrocoalescence. Electrocoalescence can then be optimized, and electric fields can be minimized to avoid unwanted biological effects of the electric field on drop content for biotechnology applications.