Do droplets always move following the wettability gradient?

Abstract

Impacting droplets on rough surfaces with a wettability gradient have been reported to rebound obliquely or migrate following the wettability gradient due to the unbalanced interfacial forces created by such heterogeneous architectures. Here we demonstrate under certain conditions, droplets can be self-propelled against the wettability gradient. We show that the local Cassie-to-Wenzel transition in the droplet is critical for such a counter-intuitive phenomenon. We believe that our letter extends our conventional understanding of wettability dynamics to heterogeneous surfaces and provides important insight for the design of micro/nanotextured surfaces for controlled droplet manipulation.

Bioinspired Interfacial Materials with Enhanced Drop Mobility: From Fundamentals to Multifunctional Applications

“…It is reported that the contact time in the case of the oblique impact is basically the same as that of the vertical impact . On another note, oblique bouncing can also occur on the horizontally placed surface but a wetting or thermal gradient …”

Bioinspired Surfaces with Superwettability for Anti‐Icing and Ice‐Phobic Application: Concept, Mechanism, and Design

“…By engineering nonuniform textures to create roughness gradients on the substrate, Wang and co‐workers achieved a gradient surface, where the droplet can be controlled to rebound axially and rebound by a prescribed lateral trajectory . Additionally, they found that, different from the previous reports, the self‐migration direction (toward or against the wettability gradient) mainly depended on the competition of the capillary pressure and the effective water hammer pressure . This is significant for some potential applications, such as antifogging, water‐harvesting, and anti‐icing.…”

“…Notably, when a droplet bounces on the substrate, how to control the direction is vital to the transportation of the droplets . Using the decorated substrates with a texture of variable density, Quéré and co‐workers investigated the impacting water drops bouncing off obliquely.…”

“…[130,132,136] Notably, when a droplet bounces on the substrate, how to control the direction is vital to the transportation of the droplets. [138][139][140][141][142] Using the decorated substrates with a texture of variable density, Quéré and co-workers investigated the impacting water drops bouncing off obliquely. They verified the transition mechanism of bouncing direction, and found that asymmetry would lead to a transportation of the liquid toward fields of high texture density.…”

A study on the dynamic behaviors of water droplets impacting nanostructured surfaces

“…These control parameters include wettability of the surface, surface tension, impact velocity of the droplet, liquid density, liquid viscosity, and surface roughness. [1][2][3][4][5][6][7][8][9][10] Understanding the behaviors of droplets impacting a nanostructured solid surface plays an important role in industrial and environmental processes, such as spray cooling, ink-jet printing, and deposition of rain on solid surfaces. 11 Droplets that impact a textured solid surface immediately spread at the moment of impact.…”