Mapping microscale wetting variations on biological and synthetic water-repellent surfaces

Abstract

Droplets slip and bounce on superhydrophobic surfaces, enabling remarkable functions in biology and technology. These surfaces often contain microscopic irregularities in surface texture and chemical composition, which may affect or even govern macroscopic wetting phenomena. However, effective ways to quantify and map microscopic variations of wettability are still missing, because existing contact angle and force-based methods lack sensitivity and spatial resolution. Here, we introduce wetting maps that visualize local variations in wetting through droplet adhesion forces, which correlate with wettability. We develop scanning droplet adhesion microscopy, a technique to obtain wetting maps with spatial resolution down to 10 µm and three orders of magnitude better force sensitivity than current tensiometers. The microscope allows characterization of challenging non-flat surfaces, like the butterfly wing, previously difficult to characterize by contact angle method due to obscured view. Furthermore, the technique reveals wetting heterogeneity of micropillared model surfaces previously assumed to be uniform.

Surface-wetting characterization using contact-angle measurements

“…21 Recently, we introduced scanning droplet adhesion microscopy, allowing to measure droplet adhesion forces and construct wetting maps that depict microscale spatial variation in wettability. 22 The tilting plate method is sometimes used to measure the advancing and the receding contact angles, although that method is not recommended due to reasons explained below. In this approach, the tilt angle of the plate is increased, and the contact angles on the upper and the lower side of the drop are measured just before the drop starts moving.…”

Hydration lubrication of polyzwitterionic brushes leads to nearly friction- and adhesion-free droplet motion

“…We would like to point out that the polyzwitterionic brush surfaces are qualitatively different from other classes of liquid-repellent surfaces, notably the lotusleaf effect surfaces [6,34] and the Nepenthes pitcherplant inspired lubricated surfaces [37,38]. For example, for polyzwitterionic brush surfaces F fric, adh varies nonlinearly with U (∝ U 2/3 and U 0.6 , respectively), whereas for lotus-leaf effect surfaces F fric, adh is relatively insensitive to U ∼ mm/s or less [20,22,34,39]. Table I summarizes the functional forms of F fric, adh for three surface classes, as reported here and elsewhere in the literature [17,20,23,33,34].…”

Mapping micrometer-scale wetting properties of superhydrophobic surfaces

“…Recently, by greatly suppressing the environmental noise, our group has improved the resolution of such an instrument (which we named the Droplet Force Apparatus) to * daniel@imre.a-star.edu.sg about 5 nN [17]. More impressively, by combining a sensitive force sensor with a motorized sample stage, Ras and co-workers were not only able to achieve a similar nN force resolution, but were also able to map wetting variations with a lateral resolution of 10 µm [11].…”