Open Problems in Wetting Phenomena: Pinning Retention Forces

Abstract

We review existing explanations for drop pinning and the origin of the force required to initiate the sliding of a drop on a solid surface (depinning). Theories that describe these phenomena include de Gennes', Marmur's, Furmidge's, the related Furmidge−Extrand's, and Tadmor's theory. These theories are all well cited but generally do not address each other, and usually papers that cite one of them ignore the others. Here, we discuss the advantages and disadvantages of these theories and their applicability to different experimental systems. Thus, we link different experimental systems to the theories that describe them best. We describe the force laws that can be deduced should these theories be united and the major open problems that remain. We describe a physical meaning that can be extracted from retention force measurements, specifically, the interfacial modulus that describes the tendency of a solid to conform to the liquid. This has implications for various wetting phenomena such as adhesion robustness, drug penetration into biological tissues, and solid robustness/resilience versus solid degradation over time as a result of its contact with a liquid.

Synchronized oscillations in swarms of nematode Turbatrix aceti

“…However, the simplified eqn (3) does not take into account the pinning force acting on the contact line. 43 Nevertheless, in the regime that we are studying, the droplet diameter has already started to move and the pinning force is negligible. This is proven by the fact that we find the value of g sl À g sg , equivalent to the one in previous experiments, which indicate that an additional term for the pinning force is not needed.…”

Nano-optical designs enhance monolithic perovskite/silicon tandem solar cells toward 29.8% efficiency

“…On the nanotextured sample, we observe a larger roll-off angle (25°) than for the planar reference (18°), indicating the improved ability of the surface to retain the perovskite solution. This observation can be explained from the approach of de Gennes et al, in which the resulting droplet retention force is enhanced by surface roughness pinning the three-phase contact line 32,33 .…”

An experimental and theoretical study of the inward particle drift in contact line deposits

“…3,4,9–32 In a typical sessile drop placed on a solid substrate that loses solvent due to evaporation, a three phase contact line of the drop is often pinned throughout the drying process due to surface roughnesses or inhomogeneities. 33,34 As a result of the pinned contact line, solute particles in the droplet have a propensity to amass at the drop edge due to the outward fluid flow that is established to replenish the solvent loss from the drop periphery. 3 This is the well accepted mechanism of coffee-ring formation.…”