  We report on the poroelastic indentation response of hydrogel thin films
geometrically confined within contacts with rigid spherical probes of radii in
the millimeter range. Poly(PEGMA) (poly(ethylene glycol)) methyl ether
methacrylate), poly(DMA) (dimethylacrylamide) and poly(NIPAM)
(\textit{N}-isopropylacrylamide) gel films with thickness less than 15 $\mu$m
were grafted onto glass substrates using a thiol-ene click chemistry route.
Changes in the indentation depth under constant applied load were monitored
over time as a function of the film thickness and the radius of curvature of
the probe using an interferometric method. In addition, shear properties of the
indented films were measured using a lateral contact method. In the case of
poly(PEGMA) films, we show that poroelastic indentation behavior is adequately
described within the framework of an approximate contact model derived within
the limits of confined contact geometries. This model provides simple scaling
laws for the characteristic poroelastic time and the equilibrium indentation
depth. Conversely, deviations from this model are evidenced for poly(DMA) and
poly(NIPAM) films. From lateral contact experiments, these deviations are found
to result from strong changes in the shear properties as a result of glass
transition (poly(DMA)) or phase separation (poly(NIPAM)) phenomena induced by
the drainage of the confined films squeezed between the rigid substrates.
