Bidirectional motion of droplets on gradient liquid infused surfaces

Abstract

The current paradigm of self-propelled motion of liquid droplets on surfaces with chemical or topographical wetting gradients is always mono-directional. In contrast, here, we demonstrate bidirectional droplet motion, which we realize using liquid infused surfaces with topographical gradients. The deposited droplet can move either toward the denser or the sparser solid fraction area. We rigorously validate the bidirectional phenomenon using various combinations of droplets and lubricants, and different forms of structural/topographical gradients, by employing both lattice Boltzmann simulations and experiments. We also present a simple and physically intuitive analytical theory that explains the origin of the bidirectional motion. The key factor determining the direction of motion is the wettability difference of the droplet on the solid surface and on the lubricant film.

Apparent contact angle of drops on liquid infused surfaces: geometric interpretation

“…ios, to first approximation, our previous works in 14,23,24 suggest that the apparent contact angles in the vanishing ridge limit are in reasonably good agreement with our model (see Eq. (2)) if (i) we replace the drop-gas surface tension by an effective tension corresponding to the sum of drop-lubricant and lubricant-gas tensions, and (ii) we use θ CB og = θ CB ow = 0 • .…”

“…Here γ αβ is the surface tension between phases α and β (drop w, lubricant o and gas g). Note that if the lubricant is encapsulating the drop, recent experimental results 9,23 suggest that the suitable effective drop-gas surface tension can be taken as the sum of drop-lubricant and lubricant-gas surface tensions γ eff wg = γ wo + γ og . Substituting the expressions for γ eff αs to Eq.…”

“…classical superhydrophobic surfaces) because they are robust against low surface tension liquids and pressureinduced failures. These features make LIS potentially transformative for a wide range of applications [4][5][6][7][8][9] , including for marine and medical coatings, product packaging, heat exchanger, water harvesting and drop microfluidics.…”

Control of droplet evaporation on smooth chemical patterns

“…This limit is particularly relevant for applications as this type of surfaces are easier to implement experimentally. While chemical patterning typically leads to pinning points on a solid surface, recent progress has been made in creating liquid-like surfaces [18] and liquid-infused surfaces [19] with very low pinning. Such surfaces could be used to study a continuous variation of the surface wettability as an experimental system to explore the ideas presented in this paper.…”

Continuous Droplet-Actuating Platforms via an Electric Field Gradient: Electrowetting and Liquid Dielectrophoresis

“… 32 The bidirectional droplet motion based on the gradient liquid-infused surface was demonstrated for long-distance droplet actuation. 33 There are, likewise, new methods to transport microscopic liquid layers based on a unique topological structure. 34 The topological fluid diode enabled long-distance directional liquid transport.…”