Latent Heat Fluxes Through Soft Materials With Microtruss Architectures

Microscale truss architectures provide high mechanical strength, light weight, and open porosity in polymer sheets. Liquid evaporation and transport of the resulting vapor through truss voids cool nearby surfaces. Thus, microtruss materials can simultaneously prevent mechanical and thermal damage. Assessment of promise requires quantitative understanding of vapor transport through microtruss pores for realistic heat loads and latent heat carriers. Pore size may complicate exegesis owing to vapor rarefaction or surface interactions. This paper quantifies the nonboiling evaporative cooling of a flat surface by water vapor transport through two different hydrophobic polymer membranes, $112–119μm$ (or $113–123μm$⁠) thick, with microtruss-like architectures, i.e., straight-through pores of average diameter of $1.0–1.4μm$ (or $12.6–14.2μm$⁠) and average overall porosity of 7.6% (or 9.9%). The surface, heated at $1350±20Wt∕m2$ to mimic human thermal load in a desert (daytime solar plus metabolic), was the bottom of a $3.1cm$ inside diameter, $24.9cm3$ cylindrical aluminum chamber capped by the membrane. Steady-state rates of water vapor transport through the membrane pores to ambient were measured by continuously weighing the evaporation chamber. The water vapor concentration at the membrane exit was maintained near zero by a cross flow of dry nitrogen $(velocity=2.8m∕s)$⁠. Each truss material enabled $13–14°C$ evaporative cooling of the surface, roughly 40% of the maximum evaporative cooling attainable, i.e., with an uncapped chamber. Intrinsic pore diffusion coefficients for dilute water vapor $(<10.4mole%)$ in air (⁠$P$ total $∼112,000Pa$⁠) were deduced from the measured vapor fluxes by mathematically disaggregating the substantial mass transfer resistances of the boundary layers $(∼50%)$ and correcting for radial variations in upstream water vapor concentration. The diffusion coefficients for the $1.0–1.4μm$ pores (Knudsen number $∼0.1$⁠) agree with literature for the water vapor-air mutual diffusion coefficient to within $±20%$⁠, but for the nominally $12.6–14.2μm$ pores (Kn $∼0.01$⁠), the diffusion coefficient values were smaller, possibly because considerable pore area resides in noncircular, i.e., narrow, wedge-shaped cross sections that impede diffusion owing to enhanced rarefaction. The present data, parameters, and mathematical models support the design and analysis of microtruss materials for thermal or simultaneous thermal-and-mechanical protection of microelectromechanical systems, nanoscale components, humans, and other macrosystems.