In situ observation of a hydrogel–glass interface during sliding friction

Abstract

Direct observation of hydrogel contact with a solid surface in water is indispensable for understanding the friction, lubrication, and adhesion of hydrogels under water. However, this is a difficult task since the refractive index of hydrogels is very close to that of water. In this paper, we present a novel method to in situ observe the macroscopic contact of hydrogels with a solid surface based on the principle of critical refraction. This method was applied to investigate the sliding friction of a polyacrylamide (PAAm) hydrogel with glass by using a strain-controlled parallel-plate rheometer. The study revealed that when the compressive pressure is not very high, the hydrogel forms a heterogeneous contact with the glass, and a macro-scale water drop is trapped at the soft interface. The pre-trapped water spreads over the interface to decrease the contact area with the increase in sliding velocity, which dramatically reduces the friction of the hydrogel. The study also revealed that this heterogeneous contact is the reason for the poor reproducibility of hydrogel friction that has been often observed in previous studies. Under the condition of homogeneous full contact, the molecular origin of hydrogel friction in water is discussed. This study highlights the importance of direct interfacial observation to reveal the friction mechanism of hydrogels.

Clogging of soft particles in two-dimensional hoppers

“…This is the same order of magnitude as prior measurements [36]. The variability is likely due both to heterogeneities of the particles and also the variability of the contact, which can sometimes trap water [37]. Likely µ varies within our clogging experiment; the main point is that it is always small [36].…”

Tough Hydrogels with Fast, Strong, and Reversible Underwater Adhesion Based on a Multiscale Design

“…This not only reduces the true contact area, but also acts as a flaw for initiating the debonding at the interface. [28,29] At the nanoscale, the hydrogels usually favour, thermodynamically, the formation of a water film at the interface owing to the strong hydrating ability of hydrophilic polymer strands, which prevents the formation of molecular bridges at the interface. [30] Here, we present a design strategy to obtain hydrogels with fast, strong, and reversible adhesion underwater by combining energy dissipative hydrogels with dynamic bonds and bioinspired surface drainage architecture.…”

“…A control sample that does not contain hexagonal facets with the same total thickness was also prepared, labelled as sample P0 (see "Experimental Section" and Table S1, Supplementary Information). To test the idea, we first observed the evolution of the contact formation underwater for flat and surface engineered PA gels using a home-made set-up using critical refraction, [29] as shown in Figure 2b. In brief, a gel disc with diameter of 35 mm and total thickness of 2.19 mm was placed on the upper side of the substrate with the engineered surface.…”

Bio-tribology of plant cell walls: measuring the interactive forces between cell wall components

“…The crucial challenge with using parallel plate geometry for friction measurements is the unknown contact area. This is highlighted explicitly by Yamamoto et al 137 , who capture images of the contact between polyacrylamide hydrogels and glass in water. As shown in Squeeze film lubrication theory has been applied to study the behaviour of a fluid film at the interface between hydrogel surfaces when they approach each other in the normal direction.…”

“…There is a region of trapped water at the interface that causes the area of contact to be less than the area of the hydrogel. (Reproduced from Yamamoto et al 137 ).…”