Electro-coalescence of two charged droplets under pulsed direct current electric fields with various waveforms: A molecular dynamics study

Abstract

Electrocoalescence has been extensively used to remove water droplets in water-in-oil emulsions. Pulsed direct current (DC) electric fields were found to be more effective than DC electric fields. However, as an essential process in electro-dehydration, coalescence dynamics of two charged droplets and corresponding coalescence mechanisms under pulsed DC electric fields have remained poorly understood. In this work, such coalescence dynamics behaviors are investigated via molecular dynamics simulations. The contact time of droplets, deformation ratio of coalescing droplets, and critical field strength are compared among square, sinusoidal, and triangular waves. The results show that, with the same pulse amplitude, the contact time is the shortest for the square wave, the following are sinusoidal and triangular waves. Sinusoidal and triangular waves have similar waveforms and they significantly differ from the square wave. However, it is astonishing that the contact times are very close for the square and sinusoidal waves, which are far less than that for the triangular wave. This seemingly abnormal result is reasonably explained by the equivalent field strength or RMS (root mean square) field strength. The square and sinusoidal waves have the same RMS field strength, whereas the triangular wave has a smaller RMS field strength. After two droplets coalesce into a large droplet, the coalescing droplet undergoes periodic deformation. The deformation ratio significantly depends on waveforms. As compared with sinusoidal and triangular waves, the deformation is much drastic for the square wave. The critical field strengths are also evaluated for the three waveforms. The result shows that the square and sinusoidal waves have almost the same critical field strength, whereas the triangular wave has a much higher critical field strength. Further analysis demonstrates that the RMS critical electric fields are actually the same for the three waveforms.