Thermodynamic analysis of the intrinsic stability of superheated liquid ina micromechanical actuator with elastic walls

The rise in internal pressure and / or the increase in volume that results from heating of a liquid in a closed chamber can be used as a thermally dri ven actuator mechanism in MEMS components. The conditions at which phase in stability and subsequent homogeneous nucleation occur in such systems is of ten of central importance, either because bubble formation is undesirable o r because bubble formation is a desired part of the design behavior. This a rticle examines the thermophysics of the onset of nucleation in a superheat ed liquid in a chamber with an elastic wall. Classical limits of thermodyna mic intrinsic stability, which are usually derived for a system held at con stant pressure, are not directly applicable to a system of this type. A mod el analysis is developed from the results of statistical thermodynamics the ory and a Redlich-Kwong equation of state that can be used to predict the o nset of nucleation in a liquid chamber with an elastic wall. The model pred icts that the elastic modulus of the wall material together with the thermo dynamic properties of the fluid dictate whether the thermodynamic limit of superheat will be reached during heating of liquid in the actuator chamber.