Bubble growth characterization during fast boiling in an enclosed geometry

Microboiling is commonly used in thermal inkjet atomizers (TIJ) and microelectromechanical (MEM) devices. The TIJ and MEM devices performance is closely related to the dynamics of the bubble used to operate them: therefore, it is important to determine the conditions of input energy and power leading to specific bubble dynamics. The objective in this work is the characterization, in a confined space, of the bubble dynamics on a range of input conditions of energy and power and what is the effect of the input conditions on the bubble extractable mechanical efficiency. Mechanical efficiency is defined by the ratio of the integral of the mechanical work (work done by the bubble expansion due to the elevated internal pressure relative to atmospheric pressure minus the increase in bubble surface energy) to the total energy input to the microheater. Bubbles are generated with energies of 7-17 mu J under high heating rates and short pulses in deionized water. Resulting nucleation temperature measurements are consistent with homogeneous nucleation. The bubble lifecycle shows strong dependence on the input heater energy and input heating rate. This work presents new results in bubble growth where growth-shrink-growth derived from specific energy conditions. The bubble growth-shrink-growth may be due to subcooled fluid, local variation in the pressure field, and by the surface tension driven change in curvature of the bubble. Mechanical bubble efficiencies result in small values suggesting most of the energy applied to the heater is distributed in other processes which may include increasing the internal energy of the heater film and the fluid. (C) 2009 Elsevier Ltd. All rights reserved.