Turning traditionally nonwetting surfaces wetting for even ultra-high surface energy liquids

Abstract

We present a surface-engineering approach that turns all liquids highly wetting, including ultra-high surface tension fluids such as mercury. Previously, highly wetting behavior was only possible for intrinsically wetting liquid/material combinations through surface roughening to enable the so-called Wenzel and hemiwicking states, in which liquid fills the surface structures and causes a droplet to exhibit a low contact angle when contacting the surface. Here, we show that roughness made of reentrant structures allows for a metastable hemiwicking state even for nonwetting liquids. Our surface energy model reveals that with liquid filled in the structure, the reentrant feature creates a local energy barrier, which prevents liquid depletion from surface structures regardless of the intrinsic wettability. We experimentally demonstrated this concept with microfabricated reentrant channels. Notably, we show an apparent contact angle as low as 35° for mercury on structured silicon surfaces with fluorinated coatings, on which the intrinsic contact angle of mercury is 143°, turning a highly nonwetting liquid/material combination highly wetting through surface engineering. Our work enables highly wetting behavior for previously inaccessible material/liquid combinations and thus expands the design space for various thermofluidic applications.

Hydrophilic reentrant SLIPS enabled flow separation for rapid water harvesting

“…The pinning force at the corner of the reentrant overhang is higher than that in the regular channel. This results from the three-phase contact line movement across the discontinuous geometry (i.e., horizontal overhang to vertical wall) ( 29 , 37 , 38 ). To emerge out of the reentrant channels, the contact line first moves horizontally underneath the overhang then moves vertically on the side wall of the overhang ( SI Appendix , Fig.…”

“…The reentrant channels show a potential to lock the liquid inside due to their overhang structures. With a surface engineering approach, the reentrant structure has been modified to keep highly nonwetting liquids inside ( 29 ). However, the reentrant structures are wetted by liquids during condensation as the condensates are pinned inside the channels ( 30 ).…”

Dramatically reducing the critical velocity of air cavity generation via biomimetic microstructure effects

“…Next, we will further analyze why crown-like structures appear on the microstructure surface during droplet impact from the wetting transition of the microstructured surface (Wenzel transition to Cassie-Baxter). According to the Cassie-Baxter model: 38,39…”

“…Next, we will further analyze why crown-like structures appear on the microstructure surface during droplet impact from the wetting transition of the microstructured surface (Wenzel transition to Cassie–Baxter). According to the Cassie–Baxter model: 38,39 cos θ * = f (1 + cos θ Y ) + 1where f is the area fraction of the solid–liquid contact and θ Y is the intrinsic contact angle (Young's contact angle). 40 The relationship between apparent contact angle ( θ* ) and solid–liquid contact fraction ( f ) and intrinsic contact angle ( θ Y ) of microstructured surfaces is explored according to the above equation.…”

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

“…[ 15b,19a,24 ] Recently, we demonstrated non‐wetting surfaces can also exhibit negative Δ P L (Figure 1d‐ii) without additional functional coatings, meaning the same surface design can achieve both positive or negative Δ P L for both wetting or non‐wetting liquids. [ 25 ] This dual Laplace pressure characteristic of reentrant structures is due to their unique ability to sustain contact line pinning at the reentrant feature. Depending on the reentrance angle, α (Figure 1c‐ii,d‐ii), the surface can repel fluids with different contact angles, θ.…”