Probing surface wetting across multiple force, length and time scales

Abstract

Surface wetting is a multiscale phenomenon where properties at the macroscale are determined by features at much smaller length scales, such as nanoscale surface topographies. Traditionally, the wetting of surfaces is quantified by the macroscopic contact angle that a liquid droplet makes, but this approach suffers from various limitations. In recent years, several techniques have been developed to address these shortcomings, ranging from direct measurements of pinning forces using cantilever-based force probes to atomic force microscopy methods. In this review, we will discuss how these new techniques allow for the probing of surface wetting properties in far greater detail. Advances in surface characterization techniques will improve our understanding of surface wetting and facilitate the design of functional surfaces and materials, including for antifogging and antifouling applications.

Quantifying microdroplet contact-line friction using atomic force microscope

“…Finally, we would like to point out that the AFM technique described here measures surface wetting under dynamic conditions (i.e., with controlled contact-line speeds), which is different from conventional contact angle measurements which is primarily a static/quasi-static measurement with poorly controlled contact-line speeds. This is a point often ignored in the literature, which we discussed at lengths in a recent review paper [40].…”

“…5a). We have chosen to present the friction force in its nondimensional form F fric /2rγ to allow us to compare results between different surfaces, droplet sizes, and applied F N ; F fric /2rγ is also equivalent to the contact angle hysteresis ∆ cos θ ≈ ∆θ 2 /2 [28,39,40]. For underwater superoleophobic surface, we measured F fric /2rγ as low as 3×10 −5 which is equivalent to ∆θ ≈ 8×10 −3 rad or 0.4 • , a value that would be too low to measure accurately using conventional contact angle goniometry.…”

Transparent anti-fingerprint glass surfaces: comprehensive insights into theory, design, and prospects

“…Following this line of argument, existing literature classifies fingerprint-resistant surfaces into three types, based on variations in surface wettability: oleophilic, self-decomposing, and hydrophobic fingerprint-resistant surfaces. [62][63][64][65] Additionally, it should be emphasized that while certain SLIPS [66][67][68] transparent antifouling surfaces may possess anti-fingerprint properties, the prevalent lubricating oil on these surfaces poses contamination risks. Given these challenges for applications on actual transparent functional surfaces, we opted to exclude a summary in this context.…”

Droplet detachment force and its relation to Young–Dupre adhesion

“…1,2 Despite the prevalence of the phenomenon and clear importance in various technologies (from inkjet printing 3 to agricultural technologies 4,5 ), there is still no consensus on how to link the vertical force F d required to detach a droplet from a surface to its wetting properties. 6–9 For example, Tadmor et al . (2017) proposed that F d /2π r (where r is the contact radius) is equivalent to the Young–Dupre work of adhesion γ (1 + cos θ r ) where γ is the surface tension and θ r is the receding contact angle, 10 but others disagreed.…”

“…1) with very different droplet volumes ( V = 5 pL–10 μL) and detachment speeds ( U = 10 −6 –10 −3 m s −1 ). As a result, different groups reported force magnitudes that vary considerably from 38 nN (as measured using atomic force microscopy or AFM) to 1.8 μN (force microbalance), 7 and 0.36 mN (centrifugal adhesion balance or CAB). 10 We will show later that the various datasets are in broad agreement with one another once F d is normalized by the droplet radius R .…”