Directional Droplet Transport Mediated by Circular Groove Arrays. Part I: Experimental Findings

Abstract

Directional transport of liquid droplets is crucial for various applications including water harvesting, anti-icing, and condensation heat transfer. Here, bouncing of water droplets with patterned superhydrophobic surfaces composed of circular equidistant grooves was studied. The directional transport of droplets toward the pole of the grooves was observed. The impact of the Weber number, initial polar distance r, and geometrical parameters of the surface on the directional droplet bouncing was experimentally explored. The nature of bouncing was switched when the Weber numbers exceeded We ≅ 20−25. The rebouncing height was slightly dependent on the initial polar coordinate of the impact point for a fixed We, whereas it grew for We > 20. The weak dependence of the droplet spreading time on the Weber number was close to the dependence predicted by the Hertz bouncing, thus evidencing the negligible influence of viscosity in the process. Change in the scaling exponent describing the dependence of the normalized spreading time on the Weber number was registered for We ≅ 25. The universal dependence of the offset distance ΔL of the droplets on the Weber number ΔL/D 0 ∼ We 1.5 was established. The normalized offset distance decreased with the normalized initial polar distance as ΔL/D 0 ∼ (r/S) −1 , where D 0 and S are the droplet diameter and groove width, respectively. This research may yield more insights into droplet bouncing on patterned surfaces and offer more options in directed droplet transportation.

Directional transport of droplets impacting on superhydrophobic opening triangular groove surfaces

“…In current researches of directional droplet transport, the lateral offset distance ΔL generally increases with We [14,15]. In this work, ΔL does not monotonically increase with We, and is also related to L. As shown in Fig.…”

“…Wang et al [13] fabricated a flexible needle-like surface with controllable inclination angle by changing the magnetic field strength, and the horizontal offset distance of the droplet impacting on the surface was different under different inclination angles. Liu et al [14,15] proposed an array of equidistant circular grooves, and realized the directional transport of impinging droplets towards the center of the curvature. Nevertheless, the structures mentioned above are difficult to process and costly.…”

Aqueous Drop‐on‐Drop Impact on Super‐Repellent Surface

“…Besides Weber number, there are other dimensionless numbers that are relevant in describing the impact process, [ 21 ] including Bond number (the ratio of gravitational force to surface tension force, Bo = ρgd 2 /γ), capillary number (the ratio of viscous force to surface tension force, Ca = μU /γ), Reynolds number (the ratio of inertial force to viscous force, Re = ρdU /μ), Ohnesorge number Oh = μ/( ρdγ ) 1/2 = where μ is the viscosity of the drop. With U ≈ 0.5 m s −1 , d = 2.0 mm, and μ = 0.9 mPa s for water drop, Bo ≈ 0.54, Ca ≈ 6.3 × 10 −4 , Re ≈ 1.1 × 10 3 , and Oh ≈ 2.4 × 10 −3 , showing negligible effect of the viscosity on the impact process.…”

“…[ 1,2 ] With the development of super‐repellent surfaces, [ 3–12 ] extensive study has been focused on the impact of single drop on them. [ 13–23 ] Efforts are made to reduce one‐time‐bouncing contact time [ 24–26 ] between the impacting drop and the surface by surface structures like macrotexture, [ 25,27,28 ] tapered posts, [ 29,30 ] curved structure [ 31 ] or waterbowls structure. [ 32 ] However, continuous impact of drops causes subsequent drop impacting on static drop first, instead of the super‐repellent surface.…”

Anti‐icing polyimide thin film with microcolumns fabricated by RIE with cooling and PCVD

“…However, a 10 μL water droplet could not rebound from the PI‐S surface and retract fully after achieving maximum contact with the surface. The rebound time depends strongly on the Weber number ( We ) [20,21], which is related to the density ( ρ ), the diameter ( D 0 ) and initial impact velocity ( v 0 ) of the droplets, and the liquid–vapour interface tension ( γ LV ) as follows: …”