Temperature-based analysis of droplet cooling and freezing on femtosecond laser textured surfaces

Abstract

• Sessile droplet freezing on the femtosecond laser textured hydrophobic surfaces is experimentally investigated. • A temperature-based quantitative analysis method is used. • The variation of micro-structure only affects the droplet initial cooling and nucleation onset. • A 3.34–12.62 wt% solid fraction is formed from recalescence. • Increasing micro-pillar width and mutual distance are suggested for anti-icing surface design. The droplet behaviors on sub-freezing surfaces are crucial for air-precooler frosting prediction. Experimental investigations are conducted to explore droplet cooling and freezing on femtosecond laser textured surfaces. Test samples with eight different microstructures are used, the initial contact angles (iCA) of which are in the range of 121.7° ± 0.4°–146.9° ± 2.2°. The sample surface temperature is −16.8 °C ± 0.16 °C. Deionized water droplet cooling and freezing under natural convection are studied. The water droplet wetting states on various samples are evaluated. The temporal surface temperature of the droplet is obtained, based on which the cooling and freezing stages are identified. A temperature-based analysis of the droplet cooling rate, nucleation onset, recalescence crystal production, and internal solidification is conducted. The effects of the surface structure including the micro-pillar width ( a ), inter-pillar distance ( b ), and the pillar height ( h ) are examined. It is found that the variation of micro-structure only affects the initial cooling and onset of nucleation. The temperature increment and crystal production at recalescence show no dependency on the micro-pillar geometric parameters. A calculated 3.34–12.62 wt% solid fraction is found following recalescence. The droplet ice nucleation is suppressed with a larger b / a ratio and a lower a / h ratio. A 138% increase of freezing delay time (FDT) is found when b / a increases from 0.5 to 4.0 and a / h decreases from 2 to 0.25. However, for b / a increases from 0.5 to 4, only an 8.7% and 15.6% increment of the average recalescence and internal freezing duration is achieved respectively. Increasing the micro-pillar width and the mutual distance are suggested for anti-icing surface design.