Allowable Repetition Frequency of Pulse Heating in Microactuators Using Rapid Boiling

High frequency pulse heating of heaters in microactuators utilizing the rapid expansion of boiling bubbles causes a gradual increase in the temperature of the heater and the substrate material in the immediate vicinity of the heater, resulting in a loss of reproducibility of the boiling phenomena. In the present report, the maximum frequency at which the time-averaged increase in the heater temperature can be maintained within allowable limits (allowable frequency) is obtained from numerical simulations of heat conduction from the heater to adjacent materials using a model in an axisymmetrical system. Instead of direct calculation of the entire repetition process, calculations for two different heating conditions, single pulse heating and steady heating at a time-averaged power during repetition of pulses, are performed. The effects of heater size, pulse width and substrate properties on the allowable frequency are examined and approximate correlations for the allowable frequency are derived in dimensionless form based on an analytical examination. The results show that the allowable frequency increases significantly with a decrease in heater size for prescribed conditions of pulse width and temperature increases during both pulsed heating and steady heating at a time-averaged power. The simulated results are verified by comparison with those from experiments both with and without boiling.