Simultaneous normal and shear measurements of nanoconfined liquids in a fiber-based atomic force microscope.

We have developed an atomic force microscopy (AFM) technique that can perform simultaneous normal and shear stiffness measurements of nanoconfined liquids with angstrom-range amplitudes. The AFM technique is based on a fiber-interferometric, small-amplitude, off-resonance AFM. This AFM is capable of providing linear quasistatic measurements of the local mechanical properties of confined liquid layers while only minimally disturbing the layers themselves. A detailed analysis of the measurement geometry reveals that shear stiffness measurements are extremely challenging, as even small deviations from perfect orthogonality can lead to data that is very difficult to interpret. We will show ways out of this dilemma and present results that show simultaneous measurement of the shear and normal stiffness of confined liquid layers.