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

Abstract

For the separation of disperesed water drops from oils an electric field may be used to enhance their coalescence. 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 dropinterface coalescence. The application of PEF shifts the coalscence pattern from a non-ideal to an ideal one in both drop-drop and drop-interface coalescences. Three waveform types, i.e. square, 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 dropinterface coalescence, followed closely by the 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 a high water content without causing field breakdown, as compared to constant DC field. This is useful in optimizing the electro-coalescence process.

Linear dynamics modelling of droplet deformation in a pulsatile electric field

“…The presence of an externally applied electric field increases the rate of drainage of the oil film between two coalescing droplets (Mhatre et al, 2015). However, incomplete coalescence can occur when the field strength is excessively high (Mousavi et al, 2014;Mousavichoubeh et al, 2011a;2011b). Also, Taylor cones can form, causing electro-spraying (atomisation) of water droplets.…”

“…The electric field type also affects the pattern of coalescence, with the application of pulsed DC fields being beneficial to the enhancement of the process efficiency (Bailes andLarkai, 1981, Mousavi et al, 2014;Vivacqua et al, 2015). With respect to the mitigation of incomplete coalescence, Mousavi et al (2014) report that the volume of the secondary droplets formed in the process decreases if pulse DC fields are employed, instead of constant fields. The electric field waveform and frequency also cause differences in behaviour.…”

“…The electric field waveform and frequency also cause differences in behaviour. Mousavi et al (2014), applied square, sawtooth and half-sinusoidal waves and obtained practically total suppression of partial coalescence with an applied field frequency in the range 1-100 Hz for the range of droplet size studied. Also, they showed that the half-sinusoidal and sawtooth waves are more effective in suppressing partial coalescence than the square waves.…”

Analysis of partial electrocoalescence by Level-Set and finite element methods

“…In the presence of an electric field, the rate of film thinning between two coalescing droplets increases [1], but partial coalescence can occur if the field strength is too high [3][4][5][6]. The formation of small secondary droplets undermines the efficiency of the separation process; it would therefore be highly beneficial to understand the operating conditions that favour incomplete coalescence.…”

“…It has recently been shown [5] that the ratio between the volume of the secondary droplet formed and the initial drop volume is strongly correlated with the product of the Weber and Ohnesorge numbers. According to the same authors, the volume of the secondary droplets decreases if pulse-DC fields are used: the incidence of partial coalescence almost vanishes in the frequency range between 1 and 100 Hz using square, sawtooth and sinusoidal waves [6].…”