Mass transport characteristics and theoretical performance limits of micropillar wicks

In this study, we report a new experimental method to independently and simultaneously measure the capillary pressure and permeability of wick structures and a simulation framework to establish their theoretical limits. The maximum liquid transport rates through different micropillar wick geometries operating against gravity are determined at various wicking lengths by measuring the dryout threshold. The capillary pressure and the permeability of these wicks are obtained by fitting the mass flow rate vs. wicking length data to Darcy's law. The permeability and capillary pressure values of these geometries are subsequently used to validate literature models of these two parameters. A permeability model, based on Stokes flow past infinitely long cylinders and corrected to account for the effect of meniscus curvature and finite pillar height, closely predicts the experimental data. A capillary model relating the pressure to the wick geometry using a thermodynamic approach better predicts the experimental results. An overall model consisting of Darcy's law and the selected capillary and permeability models, and capable of predicting mass flow rates through these arrays is proposed. Genetic algorithms together with the overall mass flow rate model are used to design optimized micropillar wicks. It is shown that the dimensions of the optimum geometry vary with the wicking length for devices operating against gravity. Experimental data from a representative optimal wick are used to verify the theoretical optimization results. This overall model is then employed to ascertain the theoretical mass transport limits of micropillar wick structures.