Effect of the Surface Peak–Valley Features\non Droplet Impact Dynamics under Leidenfrost Temperature

This\nstudy explores the kinetic behavior of droplets impacting\nmicrotextured surfaces under a Leidenfrost temperature, employing\nhigh-speed photography and picosecond laser micromachining techniques.\nThe investigation focuses on two types of microtextured surfaces with\ntotally different surface peak–valley features: a negatively\nskewed surface with micropit arrays (<i>Ssk</i> < 0)\nand a positively skewed surface with micropillar arrays (<i>Ssk</i> > 0). The results indicate that both microtextured surfaces contribute\nto a higher Leidenfrost temperature compared with the original smooth\nsurface, which is consistent with previous studies. However, it is\nworth noting that the Leidenfrost points of the micropit and micropillar\nsurfaces showed opposite trends with the microtexture area occupancy.\nSpecifically, the Leidenfrost temperature on micropit surfaces increases\nwith greater micropit area occupancy, while it decreases on micropillar\nsurfaces under similar conditions, which is mainly attributed to the\ndifferential impact of area occupancy on droplet heat transfer efficiency.\nWhen the microtexture area occupancy is 50%, it is worth noting that\nthe micropit and micropillar surfaces have nearly same roughness (<i>Sa</i>), but the Leidenfrost temperature was notably higher\non the micropit surface with negative skewness (<i>Ssk</i> < 0), which was related to differences in vapor flow dynamics.\nWe further find that the Weber number (<i>We</i>) significantly\ninfluences the Leidenfrost point, with the droplet impact wall behavior\ngoing through the states of film bounce back, ejecting tiny droplets\nand bounce back, and ultimately droplet breakup as the <i>We</i> increases. The dynamic Leidenfrost point was found to be generally\nhigher than the static point and increases with the <i>We</i>. Finally, we compare the cooling efficiency of these surfaces, and\nit is found that the micropit surfaces with a negative skewness exhibit\nsuperior heat dissipation performance under the same conditions, which\nproved that the negatively skewed surface may have great potential\nin high-density heat dissipation technology.