(552e) Understanding Electrocoalescence of A Two-Dimensional Arrangement of Water Droplets

In an electrocoalescence process, conducting fluid droplets dispersed in an insulating liquid are forced to coalesce using an electric field.  The process is based on the polarization of the droplets by an external electric field, which induces an attractive force required for the coalescence.  Electrocoalescence has applications in a number of processes where conducting water droplets are dispersed in oil.  In digital lab-on-a-chip applications, micron-sized water droplets containing different reagents and analyte samples are merged by electrocoalescence to undertake diagnostic assays.  In petroleum refining, electrocoalescence has been applied for breaking emulsions in the desalting process since 1911.  Despite the importance of the electrocoalescence process, a fundamental understanding of this phenomenon is still lacking, and previously reported studies focused only on one-dimensional geometries.  To gain a better fundamental understanding of the process, we have studied the mechanics of electrocoalescence of a two-dimensional dispersion of water droplets in oil using a microfluidic cell.  Flow focusing was used to form a monodisperse emulsion of water droplets in oil.  The droplets were then directed into a microfluidic chamber whose depth was only slightly larger than the droplet diameter, causing the droplets to form a two-dimensional monolayer.  A uniform electric field was applied across the monolayer and individual coalescence events were imaged using optical microscopy.  Flow focusing allowed the droplet size and the emulsion number density in the chamber to be controlled. The reduced depth of the chamber allowed for the direct observation of the configuration of all the droplets, and of the merging events as a function of time.

To develop a predictive model for droplet coalescence, hydrodynamic drag coefficients were obtained by study of one-dimensional, pair-wise droplet dielectrophoresis.  Finite element (FEM) numerical solutions for the quasi-static electric field around two-dimensional droplet configurations were then obtained.  The sequence of observed droplet configurations, generated using video recordings, was mapped directly to the droplet location in the FEM computational domain.  From these calculations, the electrostatic and hydrodynamic forces acting on all the droplets were obtained, and the force of mutual attraction or repulsion for each droplet pair was computed as a function of the droplet separation distance.  These mutual forces were then correlated with the coalescence events to determine the local field strengths and pairwise droplet separation distances necessary to induce coalescence. A compilation of these individual events allowed us to construct global criteria for electrocoalescence.  This model can also be used to evaluate the effect interfacial rheology, particularly the effect of surface tension on the critical electric field strength.