Anisotropic capillary-driven evaporation performance on laser textured prismatic micropillars

Abstract

Wicking is an essential characteristic in the operation of passive phase-change cooling devices, as it provides efficient thin film evaporation through micro/nano structures and improves liquid rewetting via capillary pumping. However, there is no consensus on how anisotropic wicking affects evaporation and droplet hydrodynamics at varying substrate temperatures. In this work, we fabricated prismatic micropillar on AA6063 surfaces with different intersection angle by laser texturing to analyze their effect on wicking dynamics and heat transfer during droplet evaporation via high-speed and infrared camera. The experiments revealed that when the droplet wetted the prismatic micropillar surfaces with different intersection angle, there was a significant difference in wicking speed between the long and short axes, and the maximum speed difference in the first 1 s could reach 25.1 mm/s for PM-15. Due to the guiding effect of the intersection angle and the balance between capillary force and viscous resistance, the wicking distance ratio between the long and short axes was approximately equal to tan(α/2) during the bulk existence stage. The laser-induced surface realized a maximum evaporation rate of 19.4 μL/s at 90 °C, achieving an enhancement factor f_e of 30.95 compared to a smooth surface. Evaporation enhancement on prismatic micropillar surfaces cannot be attributed solely to the wicking direction or wicking area, but rather to a function of roughness and wickability. When the surface temperature is too high, droplets undergo nucleate boiling, and the main reason for mass loss of the droplet is primarily splashing rather than evaporation. Further, we introduce the primary force interactions involved in high-temperature droplet evaporation process and emphasize the influence of the anisotropic surface structures on droplet dynamics.