Stick–Slip Friction Reveals Hydrogel Lubrication Mechanisms

Abstract

The lubrication behavior of the hydrated biopolymers that constitute tissues in organisms differs from that outlined by the classical Stribeck curve, and studying hydrogel lubrication is a key pathway to understand the complexity of biolubrication. Here, we have investigated the frictional characteristics of polyacrylamide (PAAm) hydrogels with various acrylamide concentrations, exhibiting Young's moduli (E) that range from 1 to 40 kPa, as a function of applied normal load and sliding velocities by colloid probe lateral force microscopy. The speed-dependence of the friction force shows an initial decrease in friction with increasing velocity, while, above a transition velocity V*, friction increases with speed. This study reveals two different boundary lubrication mechanisms characterized by distinct scaling laws. An unprecedented and comprehensive study of the lateral force loops reveals intermittent friction or stick-slip above and below V*, with characteristics that depend on the hydrogel network, applied load, and sliding velocity. Our work thus provides insight into the closely tied parameters governing hydrogel lubrication mechanisms, and stick-slip friction.

Molecular Mechanisms Underlying Lubrication by Ionic Liquids: Activated Slip and Flow

“…In IL-0% (Figure 2a, triangles), the friction force varies non-monotonically with the sliding velocity; at the slowest velocities, it slightly decreases with velocity, whereas it rapidly increases with speed above a transition velocity that we call V * (see dashed line). This non-monotonic trend in friction is not unique to this system but it has been observed before for other ILs [14] and lubricants [23,37]. In IL-44% ( Figure 2b, squares), friction is quasi-velocity independent at the slowest sliding velocities, before it increases with velocity; a transition velocity V * is thus also identified in this case (see dashed line).…”

Molecular Mechanisms Underlying Lubrication by Ionic Liquids: Activated Slip and Flow

“…In IL-0% (Figure 2a, triangles), the friction force varies non-monotonically with the sliding velocity; at the slowest velocities, it slightly decreases with velocity, whereas it rapidly increases with speed above a transition velocity that we call * (see dashed line). This non-monotonic trend in friction is not unique to this system but it has been observed before for other ILs 16 and lubricants 24,38 . In IL-44% (Figure 2b, squares), friction is quasi-velocity independent at the slowest sliding velocities, before it increases with velocity; a transition velocity * is thus also identified in this case (see dashed line in Figure 2b).…”

Probing the frictional properties of soft materials at the nanoscale

“…5f ). 112 Initially, the friction decreases with increasing speed until it reaches a transition velocity where the friction starts to increase with increasing speed. Below the transition speed, the decrease in friction with increasing sliding speed is related to the continuous adsorption and desorption of the polymer chains onto the sliding body; the polymer chains that were adsorbed on the counter surface require more time to re-adsorb once the contact breaks due to the sliding motion.…”

“…32,42,68, On the right side of the figure are shown the FFM studies using a colloidal probe made from: collagen; 96 latex; 97 polyethylene (PE); 25,28,45,98 polyethylene glycol (PEG); 99 poly(methyl methacrylate) PMMA; 26,27,95,100 polystyrene (PS); 33,71,101 cellulose; 31,[102][103][104][105][106][107][108][109] and silica. 35,36,98,[110][111][112][113][114][115][116][117] The volume of the symbol corresponds to the number of studies using the specific system of materials. Colloidal probes of the same material are in groups, while the green-shaded area represents the knowledge gap on friction between soft surfaces that requires future research attention.…”

“…A. Lopez-Berganza, R. Muro-Barrios, S. A. Egner and R. M. Espinosa-Marzal, Langmuir, 2018, 34, 756-765. 112 Copyright 2018 American Chemical Society). While initially friction is decreased with increasing velocity due to reducing slip-stick ( polymer chain adsorption and release is a kinetic process), when it reaches a transition velocity the friction increases with speed, possibly due to increased deformation of the hydrogel.…”