Enhanced Laplace Pressures for Functional Surfaces: Wicking, Switchability, and Selectivity

Abstract

Wetting functionalities of rough surfaces are largely determined by the Laplace pressure generated across liquid–gas interfaces formed within surface structures. Typically, rough wetting surfaces create negative Laplace pressures, enabling capillary wicking, while rough non‐wetting surfaces create positive Laplace pressures, exhibiting fluid repellency. Here, with microfabricated reentrant structures, it is shown that the same surface can exhibit either a negative or positive Laplace pressure, regardless of its intrinsic wettability. This material‐independent Laplace pressure duality enables or enhances a range of wetting functionalities including wicking, switchability, and selectivity. On the same surface, capillary rise, capillary dip, and the combination of the two which leads to further enhancement of the total sustainable capillary height and Laplace pressure, the driving force for wicking is demonstrated. Further, active switching of wetting states between the hemiwicking and the repellent Cassie state on reentrant structures is shown. Moreover, with a water‐hexane mixture system, selective wetting of reentrant structures are demonstrated, that is, water can be selectively wicked or repelled in the presence of hexane, and vice versa. These functionalities are achieved, which would typically require complex chemical coatings, solely using surface structures, thus largely expanding the design space for a wide range of thermofluidic applications.

Teflon AF–Coated Nanotextured Aluminum Surfaces for Jumping Droplet Thermal Rectification

“…[ 43 ] The combination of roughness (the boehmite nanostructures) and chemical hydrophobicity (the Teflon AF‐silane promoter coating) discourages wetting within the surface structures, thereby enabling “Fakir” wetting states in which droplets rest on the tips of solid surface structures. [ 44–46 ]…”

“…[43] The combination of roughness (the boehmite nanostructures) and chemical hydrophobicity (the Teflon AF-silane promoter coating) discourages wetting within the surface structures, thereby enabling "Fakir" wetting states in which droplets rest on the tips of solid surface structures. [44][45][46] Our choice of this Teflon AF coating is motivated by recent reports of impressive durability with SHPB CuO nanostructured surfaces with a silane monolayer and polymer film coating. [47,48] The postulated mechanism for improved durability in this prior work is that attractive interactions between the silane monolayer and polymer film increases the adhesion between the promoter coating and the substrate to resist coating delamination; [49] the presence of polymer films may also protect hydrolyzable sites at the monolayer-metal oxide interface from excessive water exposure.…”

Different Roles of Surface Chemistry and Roughness of Laser-Induced Graphene: Implications for Tunable Wettability

“…LIG scribed in an ambient atmosphere showed (super)hydrophilic behavior with either a wicking effect (absorption of the liquid) 55 or high pinning of the droplets at gray values of G > 30%. This effect was also observed by Nasser et al 28 The impact of surface roughness only became evident at low LIG coverage (low gray value G ), as evidenced by a decrease in the contact angle when the droplet is in a Wenzel state.…”