Directional Transportation of Impacting Droplets on Wettability-Controlled Surfaces

Abstract

Although a superhydrophobic surface could realize rapid rebounding (i.e., short contact time) of an orthogonal impacting droplet, the rebounding along the original impacting route may limit its engineering application; in contrast, the directional transportation seems to be more promising. Here, we achieve directional transportation of a droplet impacting on a wettability-controlled surface. When the droplet eccentrically impacts on the boundary between the superhydrophobic part and the hydrophilic part, it undergoes spreading, retracting, departure, throwing and breaking up stages, and finally bounces off directionally. The directional transportation distance could even reach more than ten times of the droplet size, considered the adhesion length (i.e., covering length on the hydrophilic part by the droplet at the maximum spreading) is optimized. However, there is a critical adhesion length, above which the directional transportation does not occur. To be more generalized, the adhesion length is de-dimensionalized by the maximum spreading radius, and the results show that as the dimensionless adhesion length increases, the transportation distance first increases and then decreases to zero. Under the present impacting conditions, the optimal dimensionless adhesion length corresponding to the maximum transportation distance is near 0.4, and the critical dimensionless adhesion length is about 0.7. These results provide fundamental understanding of droplet directional transportation, and could be useful for related engineering applications.

Droplet Bouncing: Fundamentals, Regulations, and Applications

“…[154,155] By designing a nonwetting solid surface with a wettability gradient, vertical momentum of impacting droplet can be transferred to horizontal momentum with the designed direction. [156][157][158][159][160][161] The phenomenon is caused by asymmetric liquid-solid adhesion, resulting in the net horizontal momentum of the impacting droplet for directional rebounding. [53,162,163] When the impacting droplet reaches its maximum diameter, the droplet starts to rebound from the surface.…”

Nonspecular Reflection of Droplets

“…[20] Nevertheless, such symmetries impede the possibility of directional transport of droplets on solid surfaces. Heterogeneously engineered surfaces have been exploited to break down the symmetry for directional transport, including those with oblique structures [21,22] and gradients in surface roughness, [6,[23][24][25][26][27] surface chemistry, [28,29] and surface charge density. [30] With surface gradients, directed droplet motion is driven by the unbalanced forces that come from the anisotropy in, for example, wetting states, contact angles, and contact lines at the leading and trailing sides.…”

“…[30] With surface gradients, directed droplet motion is driven by the unbalanced forces that come from the anisotropy in, for example, wetting states, contact angles, and contact lines at the leading and trailing sides. [6,21,23,25,29] Therefore, the direction of droplet rebounding must be parallel to the direction of gradients [6,[21][22][23][24][25][26][27][28][29][30] and thus less steerable. In most cases, droplet transport is only unidirectional.…”

“…In most cases, droplet transport is only unidirectional. [1,21,29] Moreover, some heterogeneous surfaces will have the whole droplets and/or liquid residues firmly pinned, [1,29] rendering lossless and frictionless droplet transport unachievable. Alternatively, droplets can be actively directed with external fields, such as optical, thermal, electrical, and magnetic controls, [31][32][33][34] for more flexible manipulation.…”

Liquid Film Sculpture via Droplet Impacting on Microstructured Heterowettable Surfaces

“…[ 7 ] Recent manipulation strategies are mainly divided into active method, by applying external electric fields, [ 8 ] magnetic fields, [ 9 ] temperature fields, [ 10 ] light, [ 11 ] and acoustic waves, [ 12 ] and passive method, by designing macrostructures like ridges, [ 13 ] tips, [ 14 ] ratchets, [ 15 ] grooves, [ 16 ] lattice cells, [ 17 ] or integrating wettable and less wettable regions on a solid surface. [ 18 ] Owing to its advantages of spontaneity, facility, diversity, as well as the energy‐saving property, the passive liquid manipulation method has aroused large amounts of research interest. [ 19 ] However, most of the studies focus on the regulation of single or multiple droplets, whereas the manipulation of liquid films still remains unconcerned.…”