Electro-Coalescence Effects on a Droplet at a Flat Interface

       This research was done in the Department of Mechanical and Aerospace Engineering at UCLA during the summer of 2007 in conjunction with the Summer Program for Undergraduate Research (SPUR).  It was done to determine if and how the presence of an electric field affects the coalescence of fluids. This research is useful in the petroleum industry where the separation of oil and water is crucial.  

       When dropping a water droplet through the oil to the body of water, it rest on a microscopic film of oil preventing it from merging with the rest of the water.  It could rest as a separate droplet for hours, days, or indefinitely.  The electric field alters and accelerates the process of merging. 

The Set Up for the Experiment

      The electric field is determined by the voltage applied to the brass plates and the height of the interface.  The Strength of the electric field is calculated by dividing the voltage by the distance, D between the top brass plate and the oil/water interface.  The two plates essentially constitute a capacitor where in the calculation for electric field strength is the same as that of a capacitor.    

Electric Field, E = Voltage/Distance 

Coalescence of a droplet producing a secondary droplet

      The electric field causes the primary droplets to coalesce and also leave behind a secondary droplet (or even a third sometimes).  The time of coalescence of the primary drop and the size and time of coalescence of the secondary drop varied inversely with the strength of the electric field.  The production of secondary drops is common to most liquid droplets coalescing with larger liquid bodies.  The unique discovery came when we discovered there was a threshold voltage with which, no secondary drop was created and the time it took the droplet to coalesce was minimal and hardly detectable even with a high-speed camera.  

Coalescence without producing a secondary droplet.

The high-speed camera was used to record these events which took only fractions of a second.  The footage of the phenomena were used to measure the size of the droplets and the duration of their coalescence.  

The Set Up including the High-Speed Camera.