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We report on the transient frictional response of contacts between a rigidspherical glass probe and a micrometer-thick poly(dimethylacrylamide) hydrogelfilm grafted onto a glass substrate when a lateral relative motion is appliedto the contact initially at rest. From dedicated experiments with \textit{insitu} contact visualization, both the friction force and the contact size areobserved to vary well beyond the occurrence of a full sliding condition at thecontact interface. Depending on the imposed velocity and on the static contacttime before the motion is initiated, either an overshoot or an undershoot inthe friction force is observed. These observations are rationalized byconsidering that the transient is predominantly driven by the flow of waterwithin the stressed hydrogel networks. From the development of a poroelasticcontact model using a thin film approximation, we provide a theoreticaldescription of the main features of the transient. We especially justify theexperimental observation that the relaxation of friction force $F_t(t)$ towardsteady 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.

Transient sliding of thin hydrogel films: the role of poroelasticity