Spark discharges at water-hydrocarbon interface: application to emulsion production

Discharge at the interface of two immiscible liquids is a relatively recent research field with an important potential for applications and for fundamental plasma physics. In this study, spark discharges are produced at the water-heptane interface. High voltage pulses of several kilovolts were applied to pin-to-pin electrode placed near or at the water-heptane interface. When the electrodes are placed at the interface, we found that the needed voltage to achieve breakdown is lower than the one needed when the electrodes are above the interface (i.e. in heptane) or below the interface (i.e. in water). This finding is correlated to the formation of an emulsion, i.e. droplets of heptane in water. The analysis of liquid samples by optical microscope allowed the confirmation of emulsion formation with droplet size distribution of 5-10 μm. The formation mechanism is related to the cavitation bubble dynamic and/or the strong acoustic waves propagating in the medium, as evidenced by high speed imaging. By performing a static E-field calculation, using the generalized Poisson’s equation, we found that the E-field is intensified at the surface of heptane droplet due to the discontinuity of the dielectric permittivity: ε_water = 80 vs. ε_heptane = 2. Overall, this study shows that spark discharge at the heptane/water interface and emulsion formation are interrelated, and that one promotes the other.