The pore radius (i.e., capillary radius) and contact angle determine the capillary pressure generated in a porous medium. The most common method to determine these two parameters is through measurement of the capillary pressure generated by a reference liquid (i.e., a liquid with near-zero contact angle) and a test liquid. The rate of rise technique, commonly used to determine the capillary pressure, results in significant uncertainties. In this study, we utilize a recently developed technique for independently measuring the capillary pressure and permeability to determine the equivalent minimum capillary radii and contact angle of water within micropillar wick structures. In this method, the experimentally measured dryout threshold of a wick structure at different wicking lengths is fit to Darcy's law to extract the maximum capillary pressure generated by the test liquid. The equivalent minimum capillary radii of different wick geometries are determined by measuring the maximum capillary pressures generated using n-hexane as the working fluid. It is found that the equivalent minimum capillary radius is dependent on the diameter of pillars and the spacing between pillars. The equivalent capillary radii of micropillar wicks determined using the new method are found to be up to 7 times greater than the current geometry-based first-order estimates. The contact angle subtended by water at the walls of the micropillars is determined by measuring the capillary pressure generated by water within the arrays and the measured capillary radii for the different geometries. This mean contact angle of water is determined to be 54.7°.

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.

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