Experimental study on bubble detachment under variable heat load and the action of electric field

This paper is aimed at studying single bubble growth and detachment in controlled conditions and under the action of an electric field. An experimental set-up was built, consisting mainly of a stainless steel lamina with a nucleation site etched in its center, facing the testing fluid. The required heating power was provided by an SMD microresistor, stuck by means of epoxy resin on the back side of the plate. A signal generator, connected to the resistor, allowed for heating power modulation and thus for variation of the bubble size. Two different fluids, FC-72 and HFE-7100 were tested. An external electrostatic field was applied by imposing up to 15 kV dc to a ring of 4mm inner diameter suspended 3mm above the plate: in this way, a very clear axisymmetric configuration was created, suitable for two-dimensional analyses. High speed images were taken by means of a high-speed camera, operated at 1000 frames per second with a resolution of about 100 pixel/mm. Images were treated with a dedicated image processing software to obtain bubble significant geometrical parameters. The preliminary results reported here show elongation of the detached bubble under the action of the electric field, confirming former results obtained with nitrogen bubbles originating from an orifice in the same liquids. The detachment volume under the action of the electric field either increases or decreases, for low and high electric permittivity fluid, respectively, evidencing the complex nature and role of the electric forces. First results under variable heat load evidence a reduction in detachment diameter. The dynamical forces outside the microlayer region seem to have a minor role, compared to the static ones, in vapor bubble growth.