Survey of Micro/Nanofabricated Chemical, Topographical, and Compound Passive Wetting Gradient Surfaces

Abstract

Surface wetting gradients are desirable due to their ability to passively transport liquid droplets without the aid of gravity. Such surfaces can be prepared through topographical or chemical methods or a compound approach involving both methods. By altering the surface free energy across a surface, a droplet that contacts such a surface will experience an actuation force toward the hydrophilic region. Such transport properties make these surfaces attractive for a range of applications from thermal management to microfluidics to the investigation of biomolecular interactions. This paper reviews passive wetting gradients that have been demonstrated over the last three decades, focusing on the types of surfaces that have been developed to date along with the materials that have been used. The corresponding wetting ranges and physical lengths over which droplet mobility has been achieved on these various types of gradient surfaces are compared to guide future developments.

Development of a Coating-Less Aluminum Superhydrophobic Gradient for Spontaneous Water Droplet Motion Using One-Step Laser-Ablation

“…Number of publications on topographical wetting gradients per year. 24 The orange bars represent gradients obtained on a metal surface without coatings, which is a relatively unexplored area. (coating-less) aluminum gradient based on a simple topographic modification approach as opposed to the most popular approach of chemical treatment to produce superhydrophobicity in metals.…”

“…They showed that there was enough force provided by the gradient of surface energy to passively transport a small droplet along a surface. After those initial studies, numerous different ideas of gradients based on various coatings and/or topographical modifications have appeared . However, aiming to introduce spontaneous droplet motion based only on topographic gradients, we here investigate coating-less methods based on laser-etched and micromachined aluminum.…”

Superhydrophobic 304 Stainless Steel Mesh for the Removal of High-Density Polyethylene Microplastics

“… 25 , 26 Additionally, superhydrophobic surfaces can be classified into different wetting states according to the adhesive force, such as the rose petal effect (high adhesive force) or lotus leaf effect (low adhesive force). 27 , 28 Three models have been used to explain the surface wettability: 29 Young’s model that connects the WCA with the surface free energy; 30 − 32 Wenzel’s model that considers the contribution of the surface roughness leading to an homogeneous regime; 33 and the Cassie–Baxter model that explains the heterogeneous regime where an air interface is found between the solid and the liquid. 34 Moreover, these surfaces can be used to separate oil from water mixtures as well as oil-in-water emulsions with extremely low oil contact angles (OCAs) close to 0°.…”

“…In this scenario, superwettable materials, particularly superhydrophobic/superoleophilic surfaces, present a key role in the capture and removal of these solid pollutants. It is well-known that superhydrophobic materials are defined by a WCA > 150° as well as a sliding angle (SA) and contact angle hysteresis (CAH) of less than 10°. , Additionally, superhydrophobic surfaces can be classified into different wetting states according to the adhesive force, such as the rose petal effect (high adhesive force) or lotus leaf effect (low adhesive force). , Three models have been used to explain the surface wettability: Young’s model that connects the WCA with the surface free energy; − Wenzel’s model that considers the contribution of the surface roughness leading to an homogeneous regime; and the Cassie–Baxter model that explains the heterogeneous regime where an air interface is found between the solid and the liquid . Moreover, these surfaces can be used to separate oil from water mixtures as well as oil-in-water emulsions with extremely low oil contact angles (OCAs) close to 0°. − Recently, the ability of superhydrophobic materials to remove microplastics is attracting attention.…”

Virtual Issue: Wettability Gradient Surfaces

“…Such surfaces have been developed over the last three decades . Typically, the wettability gradients are created through topographical or chemical methods. ,− ,, Surfaces involving artificial patterns, bioinspired shapes, or hierarchical textures have been created. − ,, In addition to experimental studies, the behaviors of droplets on wettability gradient surfaces were also explored theoretically, including condensation heat transfer and the fascinating dynamics of drop impact and rebound. − , Potential applications are indicated: in thermal management applications such as heating, ventilation, air-conditioning, and refrigeration systems; in biomedical systems where the lack of an external power supply may be advantageous, including micropump needles and biochips, including diagnostic devices; and in sustainable environmental material interfaces. We hope that you will enjoy reading this sample of a collection of Langmuir publications that present recent and exciting work on this topic.…”