  We report contactless measurements of the viscoelastic rheological properties
of soft gels. The experiments are performed using a colloidal-probe Atomic
Force Microscope (AFM) in a liquid environment and in dynamic mode. The
mechanical response is measured as a function of the liquid gap thickness for
different oscillation frequencies. Our measurements reveal an
elastohydrodynamic (EHD) coupling between the flow induced by the probe
oscillation and the viscoelastic deformation of the gels. The data are
quantitatively described by a viscoelastic lubrication model. The
frequency-dependent storage and loss moduli of the polydimethylsiloxane (PDMS)
gels are extracted from fits of the data to the model and are in good agreement
with the Chasset--Thirion law. Our results demonstrate that contactless
colloidal-probe methods are powerful tools that can be used for probing soft
interfaces finely over a wide range of frequencies.
