Droplet deformation under pulsatile electric fields

The deformation of a water droplet in a dielectric oil phase in the presence of externally pulsatile electric fields is numerically analysed with the finite element method. The proprietary software Comsol Multiphysics is used to conduct the simulation and the motion of the interface is captured by the Level-Set method. Experimental work is conducted to validate the model, and found to be in good agreement with the numerical results. The effects of electric field type, electric field intensity, electric field frequency, droplet size, surface tension, and bulk phase viscosity have been systematically assessed. The electric field strength induces droplet deformation, opposed by surface tension and viscosity. The ratio of Weber Number (describing electric field effects) and Ohnesorge Number (describing physical properties) is found to describe the droplet deformation well for the low frequency range, where the time for the droplet to reach stationary shape is shorter than the electric field half-period. Here a linear relationship is found to prevail between the RMS value of deformation ratios DR as a function of We/Oh. At higher frequencies, where the electric field half period becomes much shorter than the mechanical response time the functional dependence becomes first non-linear and then eventually approaches that of the constant electric field at equivalent RMS strength. The outcome of this work is potentially useful for optimizing the design of oil-water separation devices.