Rectification of Mobile Leidenfrost Droplets by Planar Ratchets

Abstract

The self‐transportation of mobile Leidenfrost droplets with well‐defined direction and velocity on millimetric ratchets is one of the most representative and spectacular phenomena in droplet dynamics. Despite extensive progress in the ability to control the spatiotemporal propagation of droplets, it remains elusive how the individual ratchet units, as well as the interactions within their arrays, are translated into the collective droplet dynamics. Here, simple planar ratchets characterized by uniform height normal to the surface are designed. It is revealed that on planar ratchets, the transport dynamics of Leidenfrost droplets is dependent not only on individual units, but also on the elegant coordination within their arrays dictated by their topography. The design of planar ratchets enriches the fundamental understanding of how the surface topography is translated into dynamic and collective droplet transport behaviors, and also imparts higher applicability in microelectromechanical system based fluidic devices.

Droplet Retention on Superhydrophobic Surfaces: A Critical Review

“…Considering the case where the droplet should be transported laterally on a horizontally leveled surface without any tilting, it is desirable to find an effective way to directionally transport droplet without relying on the gravitational force. In this regard, there were various approaches reported to drive a droplet motion without relying on a directional force field (i.e., the gravitation force), such as the unidirectional spreading on surfaces with anisotropic structures, [ 142–144 ] conical surfaces with a Laplace‐pressure gradient, [ 145–147 ] surfaces with a wettability gradient, [ 148–150 ] surfaces with a temperature gradient (e.g., Marangoni and thermocapillary effects), [ 151–153 ] Leidenfrost effects on hot surfaces with asymmetric structures (e.g., ratchets), [ 154–157 ] and surfaces with charge density gradient. [ 158 ] However, it should be noted that those reports were not closely related to superhydrophobic surfaces or droplet retention.…”

“…[ 142–150 ] In addition, the droplet retention plays a secondary role in the directional droplet motions dictated by the temperature gradient, Leidenfrost effects, and charge density gradient. [ 151–158 ] Here, an emphasis is put on the directional droplet transport on superhydrophobic surfaces where the droplet retention plays a dominant role, in accordance to the focus of this review. Therefore, those approaches will not be detailed in this review, although they can potentially be utilized on superhydrophobic surfaces as well for the manipulation of droplet retention.…”

“…The examples of asymmetric structures include the inclined pillars (Figure 12a), curved solid structures (Figure 12c), ratchet‐like structures, and structure top with asymmetric shapes such as a triangle and a semicircle. [ 155–157,161,162 ] The contrast in depinning forces along the path for a droplet motion can be further enhanced by roughening only one side of the solid structures as the roughness increases the length of contact line and hence the pinning. Moreover, the design of surfaces for directional droplet transport by substrate vibration must consider the properties of liquid droplets (e.g., density, surface tension, and size which determine the resonance frequency of the droplet, ).…”

Inhibiting Random Droplet Motion on Hot Surfaces by Engineering Symmetry‐Breaking Janus‐Mushroom Structure

“…Thus, the droplet impinging on the half‐pillar or JMS arrays tends to deform concavely around the top of the curved sidewalls (points A and C), forming a convex shape at the lower part of the straight sidewalls (points B and D), as schematically shown in Figure 4c. [ 17 ]…”

Design of Continuous Transport of the Droplet by the Contact-Boiling Regime

“…Hence, several landmark studies have been devoted to explore the migration mechanisms of droplets on the solid surface. − For instance, droplets can spontaneously jump on superhydrophobic surfaces by the release of surface energy driving − or the high vaporization rates . Driven by the Laplace pressure difference, droplets can also directionally move on the horizontal hot ratchet, − heated topographically patterned surface, or difference gradual charge density superhydrophobic surfaces . However, understanding the fundamental principles of the directional transportation of liquid droplets is incomplete, especially in understanding the driving mechanism of the Joule-heat-driven directional transport.…”