Characterization of microstructures for heat transfer performance in passive cooling devices

The topology and geometry of microstructures play a crucial role in determining heat transfer performance in passive cooling devices such as heat pipes. It is therefore important to characterize microstructures based on their wicking performance, the thermal conduction resistance of the liquid filling the microstructure, and the thin-film characteristics of the liquid meniscus. In the present study, the free-surface shapes of the static liquid meniscus in common microstructures have been modeled using the program, Surface Evolver, for zero Bond number. Four well-defined topologies, viz., surfaces with parallel rectangular ribs, horizontal parallel cylinders, vertically aligned cylinders, and spheres (the latter two in both square and hexagonal packing arrangements), have been modeled. Non-dimensional capillary pressure, average distance of the free liquid surface from solid walls (a measure of the conduction resistance of the liquid), total exposed area and thin-film percentage of surface area of the liquid meniscus have been computed. These parameters are presented as functions of the non-dimensional geometrical parameters of the microstructures, volume of the liquid filling the structure, as well as the contact angle between the liquid and solid. Based on these non-dimensional performance parameters, the microstructure, contact angle and non-dimensional liquid volume for the best performance are identified.