Droplet leaping governs microstructured surface wetting

Abstract

Microstructured surfaces that control the direction of liquid transport are not only ubiquitous in nature, but they are also central to technological processes such as fog/water harvesting, oil-water separation, and surface lubrication. However, a fundamental understanding of the initial wetting dynamics of liquids spreading on such surfaces is lacking. Here, we show that three regimes govern microstructured surface wetting on short time scales: spread, stick, and contact line leaping. The latter involves establishing a new contact line downstream of the wetting front as the liquid leaps over specific sections of the solid surface. Experimental and numerical investigations reveal how different regimes emerge in different flow directions during wetting of periodic asymmetrically microstructured surfaces. These insights improve our understanding of rapid wetting in droplet impact, splashing, and wetting of vibrating surfaces and may contribute to advances in designing structured surfaces for the mentioned applications.

Droplet Impact on Surfaces with Asymmetric Microscopic Features

“…Then, the spreading in both directions is expected to follow the same mechanisms: the combination of “stick” and “leap”. 64 Therefore, the spreading can be symmetric for the hydrophobic asymmetric microstructures.…”

“…In this work, we investigate the same microstructured surface as studied in ref ( 64 ), but now for impacting drops, which introduces the impact velocity V 0 as an additional parameter. Here, we postulate that the impact velocity V 0 determines the characteristic speed of “leaping”.…”

Droplet impact on surfaces with asymmetric microscopic features

“…We perform experiments of a droplet impacting on asymmetric microstructures and quantify the spreading radius in different surface-parallel directions. We explain the emergence of asymmetric droplet spreading after impact using mechanisms of slip, stick (pinning) and leap [21]. Droplet impact can roughly be divided into three stages; (i) free falling drop before impact at a nearly constant impact velocity V 0 ; (ii) the initial interaction between the drop and surface, which involves rapid wetting of surface; (iii) late stage spreading of the drop with a potentially retraction of the wetting line on the surface.…”

“…For non-smooth surfaces, one may define an effective line friction parameter that takes geometric surface details into account. Based on this parameter, one may normalize time such that the spreading curves of different droplets on microstructures exhibit nearly the same scaling [21,31,32].…”

“…To the best of our knowledge however, no study has linked the geometrical features of a particular surface to the spreading resistance imposed by line friction after impact. In our previous work [21], the spontaneous spreading mechanisms of a droplet on slanted microstructures were investigated. The spreading in the direction against the inclination was driven by the capillary spreading (i.e.…”