Experimental Study on the Breakdown of Oil Film in the Application of Water Drop Electrocoalescence

Electrocoalescence is the use of electrokinetics of drops to separate different fluid phases such as water drops in the oil phase. One of the important processes in electrocoalescence is the merging of water drops into a bulk water plane. When a water drop approaches a water plane, the electric field results in thinning the oil film that separates the drop from the plane. Eventually, the breakdown of the oil film takes place, leading to the coalescence of the drop to the water plane. Existing studies mainly focused on the factors affecting electrocoalescence patterns, and the use of a pulsed electric field enhances the efficiency of the separation process. However, it is important to investigate the fundamental characteristics of the oil film breakdown under an electric field. In this work, the experiment of the breakdown of oil film in relation to the coalescence between the water drop and the water plane under applied dc electric pulse is performed. The purpose is to determine the time required for the oil film breakdown and the coalescence time between the water drop and the water plane for the variation of applied electric field magnitudes, drop sizes, and interfacial tension coefficients. The results show that for a certain drop volume, the minimal field duration ${T} _{\min }$ needed for the oil film breakdown decreases exponentially with electric field magnitude. Under the same applied electric field magnitude, the larger water drops require longer ${T} _{\min }$ . The coalescence time ${T} _{C}$ of the water drop increases with the drop volume, but hardly depends on the magnitude and duration of the electric field. The results indicate that the interfacial tension is the dominant factor for both the breakdown of the oil film and coalescence between the water drop and the water plane. A lower interfacial tension results in longer ${T} _{\min }$ and ${T} _{C}$ .