Anisotropic Motion of Aqueous Drops on Lubricated Chemically Heterogenous Slippery Surfaces

Abstract

Hydrophobic or superhydrophobic surfaces have commonly been used to produce such kind of motion, but they are not very efficient due to a large amount of pinning sites on dry surfaces. [9] Alternatively, inspired by Nepenthes pitcher plants, slippery liquid infused porous surfaces (SLIPs) provide almost frictionless motion to a variety of liquid drops. [10] Such slippery surfaces have broad-ranging applications for instance anti-icing, [11] enhanced condensation, [12] antibiofouling, [13] selfcleaning, [14] oil-water separation. [5] Due to the isotropic nature of underneath roughness or pattern (ordered or random) of a solid surface, liquid-infused slippery surfaces show uniform drop mobility in all directions. [10b,15] Studies reported earlier used anisotropic micro-or nanopatterning of the underlying solid surfaces to obtain directional motion of aqueous drops on lubricating fluid-infused slippery surfaces. [16] On such surfaces, the motion of drops is restricted only along the direction of the fabricated pattern. Recently, researchers demonstrated on-demand reversible liquid transport on lubricating fluid-infused slippery surfaces using different external stimuli, e.g., electric field, magnetic field, temperature, light, and mechanical strain on patterned lubricated surfaces. [4,14a,17] In all these examples, the pattern of underlying topography governs the direction of drop motion, and changing that direction requires repatterning of the surface. Alternatively, chemically heterogenous (CHet) surfaces, with the pattern of hydrophilic and hydrophobic regions, have been used in microfluidics for directional liquid transport. [18] Due to the anisotropic wetting behavior and directional motion, chemical heterogenous surfaces have recently gained a significant interest among the researchers' community and opens up a new scope to smartly control the drop motion. [18a,19] Anisotropic wetting behavior depends on both the wettability contrast and the dimensions of the alternating wettable regions. [18c,20] The major problem associated with CHet surfaces is strong pinning sites for moving drops at chemical heterogeneities, which results in reduced drop mobility on them. [19c,21] Using a lubricating film on top of chemically patterned surfaces could be helpful in addressing this problem. But one has to also worry about the stability of lubricating film on such surfaces. It has been shown that on hydrophilic surfaces, lubricating films underneath water Conventional slippery surfaces show isotropic drop mobility in all directions, whereas anisotropic drop motion may often be required to guide drops in a particular direction. In most cases, topographically structured substrates are employed to provide anisotropic drop motion, but this technique is neither efficient nor cost-effective. Current findings elucidate a novel approach to control the drop motion by virtue of designing lubricated chemically heterogenous (LCHet) surfaces. Upon depositing aqueous drops on such surfaces, the underneath lubricating film dewets only from...

Bionic surface diode for droplet steering

“…[37][38][39][40][41][42][43][44] The SLIPS patterns show strong affinity toward the deposited droplet but exhibit ultralow sliding resistance without absorption due to the immiscible droplet-lubricant interface, which is similar to the hydrophilic patterns from the aspect of stress condition but has no risk of mass loss. [45][46][47] As a result, it is possible to patch SLIPS patterns on the roll-off superhydrophobic background to precisely control the droplet motion. A few notable studies succeeded in simultaneously transferring multiple droplets by taking the advantage of anisotropic droplet sliding on SLIPS patterns.…”

“…A slippery lubricant‐infused porous surface (SLIPS) inspired by Nepenthes alata plants has been widely applied for droplet motion control due to its intrinsic properties of ultralow contact angle hysteresis, self‐healing, pressure stability, and being immiscible with manipulated droplets 37–44 . The SLIPS patterns show strong affinity toward the deposited droplet but exhibit ultralow sliding resistance without absorption due to the immiscible droplet–lubricant interface, which is similar to the hydrophilic patterns from the aspect of stress condition but has no risk of mass loss 45–47 . As a result, it is possible to patch SLIPS patterns on the roll‐off superhydrophobic background to precisely control the droplet motion.…”

Droplet Memory on Liquid-Infused Surfaces

“…One particularly promising application of these surfaces, among many notable applications, is their use for precise droplet manipulation . As a matter of fact, droplet mobility on LIS can be tuned with an appropriate selection of surface topography and lubricant properties, − and the direction of motion can be controlled with the introduction of surface anisotropy and texture gradients. − …”