Successful application of explosive boiling depends on the ability to control the growth and collapse of thermal bubbles in a micro-cavity. The present research aims to provide an in-depth of stability of liquids and to measure their maximum attainable superheat during uniform and non-uniform heating.
The effect of heating rate on the maximum attainable liquid temperature was measured using the experimental system shown in Figure 1, which consists of a specially designed ultra-thin platinum wire micro-heater (1 mm length and 10 µm diameter) and state-ofthe-art monitoring equipment. A pulse of high heating rate was imposed on the liquid surrounding the micro-heater by applying direct current to the platinum wire while monitoring and visualizing the liquid conditions. Figure 2 shows the liquid response when low and high heating rates are applied. It shows that at low heating rate, nucleate boiling prevails at the heater surface after about 6 µs, while at a higher heating rate boiling is almost totally suppressed until the liquid becomes highly superheated.
Our experiments have further demonstrated that the maximum attainable liquid superheat is directly proportional to the heating rate. At atmospheric pressure and relatively low heating rate (3.6x107 K/s), the maximum temperature measured in water before boiling is initiated was 474.4 K. Increasing the heating rate to 5.9x107 K/s increases the attainable water superheat to 573.9 K, which is very close to the kinetic limit of superheat for water at atmospheric pressure (575.2 K). Superheat limits were also measured in Freon (FC-72) and in pure silicone ﬂuid (Clearco).
The initial liquid temperature was theoretically found to have strong effect on the phase stability and thus, on the characteristic time for explosive boiling initiation. For instance, at 580 K the lifetime of liquid water was found to be 4 ns while at 570 K it reaches about 170 s. A mechanistic model was also developed for estimating the spatial location and time to reach the limit of superheat in a non-uniformly heated volume of liquid. The model was found useful in developing a set of criteria for estimating the limit of superheat in practical cases relevant to some MEMS applications, where an extremely small liquid volume may be heated non-uniformly.
The various interesting phase transition phenomena described above may be applied in the design of a variety of novel explosive-boiling activated MEMS devices. understanding of the generation and control of micro-scale explosive boiling phenomenon. Both theoretical and experimental investigations have been carried out to deﬁne the thermal limit