  Scattering scanning near-field optical microscopy (s-SNOM) is a powerful
technique for mid-infrared spectroscopy at nanometer length scales. By
investigating objects in aqueous environments through ultrathin membranes,
s-SNOM has recently been extended towards label-free nanoscopy of the dynamics
of living cells and nanoparticles, assessing both the optical and the
mechanical interactions between the tip, the membrane and the liquid suspension
underneath. Here, we report that the tapping AFM tip induces a reversible
nanometric deformation of the membrane manifested as either an indentation or
protrusion. This mechanism depends on the driving force of the tapping
cantilever, which we exploit to minimize topographical deformations of the
membrane to improve optical measurements. Furthermore, we show that the tapping
phase, or phase delay between driving signal and tip oscillation, is a highly
sensitive observable for quantifying the mechanics of adhering objects,
exhibiting highest contrast for low tapping amplitudes where the membrane
remains nearly flat. We correlate mechanical responses with simultaneously
recorded spectroscopy data to reveal the thickness of nanometric water pockets
between membrane and adhering objects. Besides a general applicability of depth
profiling, our technique holds great promise for studying mechano-active
biopolymers and living cells, biomaterials that exhibit complex behaviors when
under a mechanical load.
