Pancake Jumping of Sessile Droplets

Abstract

Rapid droplet shedding from surfaces is fundamentally interesting and important in numerous applications such as anti‐icing, anti‐fouling, dropwise condensation, and electricity generation. Recent efforts have demonstrated the complete rebound or pancake bouncing of impinging droplets by tuning the physicochemical properties of surfaces and applying external control, however, enabling sessile droplets to jump off surfaces in a bottom‐to‐up manner is challenging. Here, the rapid jumping of sessile droplets, even cold droplets, in a pancake shape is reported by engineering superhydrophobic magnetically responsive blades arrays. This largely unexplored droplet behavior, termed as pancake jumping, exhibits many advantages such as short interaction time and high energy conversion efficiency. The critical conditions for the occurrence of this new phenomenon are also identified. This work provides a new toolkit for the attainment of well‐controlled and active steering of both sessile and impacting droplets for a wide range of applications.

Droplet Impact on Asymmetric Hydrophobic Microstructures

“…Surfaces with asymmetric ratchets and spikes allow directing a droplet in a desired direction, and such anisotropic surfaces are useful particularly in self-cleaning, water harvesting, and cell directing. ,, Here, hydrophobic surface properties are advantageous to increase the mobility of a droplet. On such hydrophobic surfaces, upon impact, droplets bounce off toward the direction in which the surface structures are oriented. ,− …”

Hovering spreading rebound on porous superhydrophobic surface with active air plastron for rapid drop detachment

“…Recently, researchers used superhydrophobic surface with magnetically controlled blades to actively induce drop rebound or jumping. 26 Here, the active air plastron applied through a porous superhydrophobic surface would provide a convenient strategy to control the liquid–solid contact. 17 With an unprecedentedly low contact time (Table S1, ESI†), active air plastron on a porous superhydrophobic surface will provide a robust and effective strategy for ultra-fast drop detachment or controlling the liquid–solid contact, which could be useful in a wide range of applications such as drag reduction in marine transportation for the air-cushion vessel or the amphibious aircraft, 46 manipulation or directional transportation of liquid drops (such as antigen/anti-body drops 41 ), self-cleaning, 43,47 anti-icing 45 and energy harvesting.…”

“…18,19 On a flat superhydrophobic surface, impinging drops would laterally spread to the maximum diameter, recoil back and rebound at the end of retraction, following the conventional bouncing pathway, 1,2,20 with the contact time t c bounded by the inertial-capillary time τ ( t c ≈ (2.6 ± 0.1) τ and τ = ( ρR 3 / γ ) 1/2 where R is drop radius, ρ is liquid density and γ is the surface tension coefficient). 1,21–29…”

“…Under high impacting speeds, impinging drops would wet the bottom surface of macrotextures, leading to local wet and drop adhesion. Recently, rapid shedding of sessile or impacting drops was proposed by engineering the superhydrophobic surface with magnetically controlled blades, 26 demonstrating a minimal contact time of 0.504 τ (about 2.9 ms for a 10 μl water drop).…”

“…However, further reduction in contact time beyond 0.5 τ has not been reported (see the literature review in Table S1, ESI†). 3,22,26–29 Under high impacting speeds 3,28 or impacted by large drops, 1,29 ineffectual plastron at the liquid–solid interface would lead to local wetting and drop adhesion. Thus, rapid rebounding and potent repelling of drops with high viscosity or high impacting speeds is still challenging.…”