Nonwetting of impinging droplets on textured surfaces

Abstract

This paper studies the impinging droplets on superhydrophobic textured surfaces and proposes a design guideline for nonwetting surfaces under droplet impingement. A new wetting pressure, the effective water hammer pressure, is introduced in the study to clearly define wetting states for the impinging droplets. This approach establishes the design criteria for nonwetting surfaces to impinging droplets. For impingement speed higher than raindrop speed, the surfaces need to have sub-100-nm features to generate a large enough antiwetting pressure for the droplets to take a nonwetting state after impingement.

Physicochemical Characteristics and Droplet Impact Dynamics of Superhydrophobic Carbon Nanotube Arrays

“…Since in this case P L > P WH >> P D , the SH-CNT array is always in the total non-wetting state. 28 This prediction is in a good agreement with the experimental result where no droplet pinning has ever been observed on the SH-CNT array, even at a very high impact velocity of v i = 2.51 m/s (equivalent to We = 335.32).…”

Droplet mobility on lubricant-impregnated surfaces

“…[1][2][3][4][5] Inspired by nature, 6,7 these properties originate 40 from the diminished contact of the surface with the liquid by trapping air pockets within the texture; as long as these air pockets are stable, the surface continues to exhibit non-wetting behaviour. 8 Maintaining stable air pockets, however, is challenging: the air pockets can be collapsed by external wetting 45 pressures [8][9][10][11] , can diffuse away into the surrounding liquid 12-14 , can lose robustness upon damage to the texture 1,5 and may be displaced by low surface tension liquids unless special texture design is implemented. 3 Furthermore, condensation or frost nuclei, which can form at the nanoscale throughout the texture, 50 can completely transform the wetting properties and render the textured surface highly wetting [15][16][17][18] .…”

Design of a robust superhydrophobic surface: thermodynamic and kinetic analysis

“…The value of k approaches a maximum value of 0.5 for nearly elastic collisions 48 . The experiments on droplet impingement typically use a droplet speed of the order of ms -1 , for which the water hammer coefficient is typically approximated as 0.2 20,22,49 . For low velocities and high droplet volumes, the collision is known to be inelastic, which lowers the coefficient k to the order of 0.001 (equation 23) 18 .…”

“…The velocity driven deposition involves forcible impingement of a drop onto a surface. Both the aforementioned cases are known to cause an irreversible wetting transition from the Cassie to the Wenzel state [18][19][20][21][22] . If the pressure imparted by the drop on the surface (wetting pressure) exceeds the surface energy required in penetrating a unit volume of the roughness valleys (antiwetting pressure), a wetting transition can be observed.…”