Length Scale Selects Directionality of Droplets on Vibrating Pillar Ratchet

Abstract

Directional control of droplet motion at room temperature is of interest for applications such as microfluidic devices, self‐cleaning coatings, and directional adhesives. Here, arrays of tilted pillars ranging in height from the nanoscale to the microscale are used as structural ratchets to directionally transport water at room temperature. Water droplets deposited onto vibrating chips with a nanostructured ratchet move preferentially in the direction of the feature tilt while the opposite directionality is observed in the case of microstructured ratchets. This remarkable switch in directionality is consistent with changes in the contact angle hysteresis. To glean further insights into the length scale dependent asymmetric contact angle hysteresis, the contact lines formed by a nonvolatile room temperature ionic liquid placed onto the tilted pillar arrays were visualized and analyzed in situ in a scanning electron microscope. The ability to tune droplet directionality by merely changing the length scale of surface features all etched at the same tilt angle would be a versatile tool for manipulating multiphase flows and for selecting droplet directionality in other lap‐on‐chip applications.

Directional bouncing of droplets on oblique two-tier conical structures

“…Anisotropic structures of surfaces to control water transportation [1][2][3] have been of interest for application in the elds of heat-transfer, 4,5 fog-harvesting, [6][7][8] digital microuidic devices [9][10][11] and so on. In recent years, many studies have been focused on designing anisotropic structures (oriented, 12 tilted pillars, 13 ratchets, 14 overlapping scales, 15 etc.) to control the directional transportation of droplets, [16][17][18] due to the introduction of differences in the solid-liquid-gas three phase contact lines (TCLs) along different orientations of the surfaces.…”

Modeling transpiration in synthetic trees

“…As a result, the ambient humidity was approximately equivalent to the local humidity immediately above the water menisci. This was confirmed by directly measuring the negative Laplace pressure ( P L ) of the synthetic tree's water with a microtensiometer, equating it to the Kelvin pressure ( P k ), and solving the Kelvin equation for the local humidity which was roughly equivalent to that of the ambient [24] . Remarkably, beneath the critical humidity where the resulting Fig.…”

Droplet leaping governs microstructured surface wetting

“…However, the rapid wetting that precedes this late spreading is not well understood. Rapid wetting is crucial for droplet impact on solid surfaces 22,23 , or in situations where time scales are imposed externally, such as wetting phenomena in the presence of vibrations 24 .…”