Electro-coalescence of water drops in oils under pulsatile electric fields

Electric fields are commonly used to enhance the coalescence of water drops in oils. However, this process could cause some undesirable phenomena such as secondary droplets formation, reducing the separation efficiency. Here the effect of pulsatile electric fields (PEF) on the secondary droplets formation has been investigated. In the presence of a very low frequency PEF or DC electric field three distinct drop-drop and drop-interface interaction patterns are observed: complete coalescence, partial coalescence and rebound without coalescence. The first is the ideal pattern not leaving any secondary droplets. It has previously been shown that an increase in the electric field strength and/or a decrease in the interfacial tension result in non-ideal patterns in drop-interface coalescence. The application of PEF shifts the coalescence pattern from a non-ideal to an ideal one in both drop-drop and drop-interface coalescences. Three waveform types, i.e. square, half-sinusoidal and sawtooth waves have been applied to the coalescence process. It is shown that the sawtooth waveform is the most effective in reducing the secondary droplets formation in drop-interface coalescence, followed closely by the half-sinusoidal one. The observation of videos sequences suggests that a threshold frequency exists above which a non-ideal pattern switches to an ideal one. For drop-drop coalescence this threshold frequency depends on the PEF amplitude and the size of primary drop pairs, as for bigger primary drop pairs and larger amplitudes of PEF the threshold frequency would be higher. When using pulsatile electric fields higher field strengths can be applied for systems having high water content without causing field breakdown, as compared to the constant DC field. This is useful in optimizing the electro-coalescence process.