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Register a new userTransient sliding of thin hydrogel films: the role of poroelasticity
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Added by
Antoine Chateauminois
Publication date
2020
fields
Physics
and research's language is
English
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We report on the transient frictional response of contacts between a rigid spherical glass probe and a micrometer-thick poly(dimethylacrylamide) hydrogel film grafted onto a glass substrate when a lateral relative motion is applied to the contact initially at rest. From dedicated experiments with textit{in situ} contact visualization, both the friction force and the contact size are observed to vary well beyond the occurrence of a full sliding condition at the contact interface. Depending on the imposed velocity and on the static contact time before the motion is initiated, either an overshoot or an undershoot in the friction force is observed. These observations are rationalized by considering that the transient is predominantly driven by the flow of water within the stressed hydrogel networks. From the development of a poroelastic contact model using a thin film approximation, we provide a theoretical description of the main features of the transient. We especially justify the experimental observation that the relaxation of friction force $F_t(t)$ toward steady state is uniquely dictated by the time-dependence of the contact radius $a(t)$, independently on the sliding velocity and on the applied normal load.
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We report on the frictional behaviour of thin poly(dimethylacrylamide) (PDMA) hydrogels films grafted on glass substrates in sliding contact with a glass spherical probe. Friction experiments are carried out at various velocities and applied normal loads with the contact fully immersed in water. In addition to friction force measurements, a novel optical set-up is designed to image the shape of the contact under steady-state sliding. The velocity-dependence of both friction force $F_t$ and contact shape is found to be controlled by a Peclet number Pe defined as the ratio of the time $tau$ needed to drain the water out of the contact region to a contact time $a/v$, where $v$ is the sliding velocity and $a$ is the contact radius. When Pe<1, the equilibrium circular contact achieved under static normal indentation remains unchanged during sliding. Conversely, for Pe>1, a decrease in the contact area is observed together with the development of a contact asymmetry when the sliding velocity is increased. A maximum in $F_t$ is also observed at Pe~$approx$~1. These experimental observations are discussed in the light of a poroelastic contact model based on a thin film approximation. This model indicates that the observed changes in contact geometry are due to the development of a pore pressure imbalance when Pe>1. An order of magnitude estimate of the friction force and its dependence on normal load and velocity is also provided under the assumption that most of the frictional energy is dissipated by poroelastic flow at the leading and trailing edges of the sliding contact.
Hydrogel coatings absorb water vapor - or other solvents - and, as such, are good candidates for antifog applications. In the present study, the transfer of vapor from the atmosphere to hydrogel thin films is measured in a situation where water vapor flows alongside the coating which is set to a temperature lower that the ambient temperature. The effect of the physico-chemistry of the hydrogel film on the swelling kinetics is particularly investigated. By using model thin films of surface-grafted polymer networks with controlled thickness, varied crosslinks density, and varied affinity for water, we were able to determine the effect of the film hygroscopy on the dynamics of swelling of the film. These experimental results are accounted for by a diffusion-advection model that is supplemented with a boundary condition at the hydrogel surface: we show that the latter can be determined from the equilibrium sorption isotherms of the polymer films. Altogether, this paper offers a predictive tool for the swelling kinetics of any hydrophilic hydrogel thin films.
We report on memory effects involved in the transient frictional response of a contact interface between a silicone rubber and a spherical glass probe when it is perturbed by changes in the orientation of the driving motion or by velocity steps. From measurements of the displacement fields at the interface, we show that observed memory effects can be accounted for by the non-uniform distribution of the sliding velocity within the contact interface. As a consequence of these memory effects, the friction force may no longer be aligned with respect to the sliding trajectory. In addition, stick-slip motions with a purely geometrical origin are also evidenced. These observations are adequately accounted for by a friction model which takes into account heterogeneous displacements within the contact area. When a velocity dependence of the frictional stress is incorporated in this the model, transient regimes induced by velocity steps are also adequately described. The good agreement between the model and experiments outlines the role of space heterogeneities in memory effects involved in soft matter friction.
Crumpling of a thin film leads to a unique stiff yet lightweight structure. The stiffness has been attributed to a complex interplay between four basic elements - smooth bends, sharp folds, localized points (developable cones), and stretching ridges - yet rigorous models of the structure are not yet available. In this letter we show that adhesion, the attraction between surfaces within the crumpled structure, is an important yet overlooked contributer to the overall strength of a crumpled film. Specifically, we conduct experiments with two different polymers films and compare the role of plastic deformation, elastic deformation and adhesion in crumpling. We use an empirical model to capture the behaviour quantitatively, and use the model to show that adhesion leads to an order of magnitude increase in effective modulus. Going beyond statics, we additionally conduct force recovery experiments. We show that once adhesion is accounted for, plastic and elastic crumpled films recover logarithmically. The time constants measured through crumpling, interpreted with our model, show an identical distribution as do the base materials measured in more conventional geometries.
We report on the effect of intermolecular forces on the fluctuations of supported liquid films. Using an optically-induced thermal gradient, we form nanometer-thin films of wetting liquids on glass substrates, where van der Waals forces are balanced by thermocapillary forces. We show that the fluctuation dynamics of the film interface is strongly modified by intermolecular forces at lower frequencies. Data spanning three frequency decades are in excellent agreement with theoretical predictions accounting for van der Waals forces. Our results emphasize the relevance of intermolecular forces on thermal fluctuations when fluids are confined at the nanoscale.
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