Physics of Fluid Transport in Hybrid Biporous Capillary Wicking Microstructures

The mass transport capacity (i.e., the capillary limit,) of homogeneous wicks is limited by the inverse relation between the capillary pressure and permeability. Hybrid wicks with two or more distinct pore sizes have been proposed as alternative geometries to enhance the capillary limit. In this study, the impact of the two hybridization schemes-in-plane and out-of-plane-on the capillary transport of hybrid wicks is studied. Experimental data from in-plane hybrid wicks in conjunction with a theoretical model show that local changes in the curvature of the liquid-vapor meniscus (i.e., pore size) do not result in a higher mass flow rate than that of a comparable homogeneous wick. Instead, a global change in the curvature of the liquid-vapor meniscus (as occurring in out-of-plane hybrid wicks) is necessary for obtaining mass flow rates greater than that of a homogeneous wick. Therefore, the physics of capillary limit and dryout in out-of-plane hybrid wicks is investigated using a hybrid wick consisting of a 1-μm-thick highly porous mesh suspended over a homogeneous array of micropillars. A study of the dryout process within the structure revealed that the presence of the mesh strongly alters the dryout mechanism. Visualization studies showed that out-of-plane hybrid wicks remain operational only as long as the liquid is constrained within the mesh pores; recession of the meniscus just below the mesh results in instantaneous local dryout. To maintain liquid within the mesh structure, the mesh thickness was increased, and it was determined that the mesh thickness plays the key role in the performance of an out-of-plane hybrid wick.