Experimental Study of Pulsatile Electric Field Effect on a Single Drop Rupture Through the Response Surface Methodology: Critical Electric Field Estimation

Electro-coalescence has been an environmentally friendly technology for decades. However, electric field strength should not exceed a critical value (Ecrit) to inhibit droplets from disintegrating during coalescence. Response surface methodology (RSM) with a D-optimal design was utilized to develop a model to achieve the maximum Ecrit of a single drop. The p-values showed that all studied variables were statistically significant, including Waveform, frequency, drop diameter, and interfacial tension. The results showed that by increasing the drop diameter,  decreases at all frequencies in the presence and absence of surfactant. Frequency change revealed Ecrit increases with a moderate slope for all waveforms. Because the change in field periodicity at higher frequencies becomes more frequent, this observation was attributed to a lower degree of drop deformation due to shorter on-time intervals of pulsatile electric field and non-compliance of drop vibration with field frequency. Moreover, the critical electric field declined by decreasing the interfacial tension for all waveforms and the entire frequency range examined. Adding SDS surfactant diminishes the force of surface tension against electric force and the critical field is reduced accordingly. Following  the revelation of the interaction between diameter and frequency, elevated frequencies significantly impact larger droplets, and the sensitivity of Ecrit to the diameter decreases with frequency. This suggests higher frequencies as a valuable and fast controllable variable to compensate for the effect of droplet size distribution. Optimization suggested a minimum drop diameter and a maximum frequency that can be used as two essential limits for the robust electro-coalescer design. The maximum critical electric field was obtained for Pulse 90 at a frequency of 1000 Hz for a drop diameter of 2.12 mm in the absence of the surfactant. These findings can be used to attain the appropriate ranges of variables to design a robust electro-coalescer.