Experimental measurement of the electric forces acting on a growing gas bubble in quasi-static conditions

This work was aimed at measuring the resulting electric forces acting on a gas bubble growing quasi-statically, with gas supplied to it through a circular orifice, and to compare them with their theoretical counterparts. An experimental apparatus has been set up, consisting essentially in an orifice drilled in a flat stainless steel plate submerged in the test fluid (FC-72). A dedicated gas injection system allowed creating slowly growing or even static bubbles of any desired volume, up to the detachment one. An electric field could be imposed by means of a washer-shaped electrode, laid parallel to the surface and centered on the orifice. The apparatus could be operated with the orifice both in upward and in downward direction, to investigate the favorable or adverse role of the buoyancy force. Data were acquired via a high resolution video camera, equipped with a microscopic lens, and were digitized and processed via dedicated software, implemented in Matlab. The resulting forces acting on the bubbles were derived from bubble shape and size. The data in the absence of electric field were compared with their theoretical counterpart to validate the method and the image processing technique, and showed an excellent agreement. In a second phase, data with electric field were acquired. The resulting electric force was evaluated as difference from the static balance of the remaining ones, and it was also possible to determine the local value of electric stress at bubble interface from the capillary equation. The values of measured electric forces showed excellent agreement with the theoretical evaluations.