Fei Liu, Sissi de Beer, Dirk van den Ende, and Frieder Mugele
PHYSICAL REVIEW E 87, 062406 (2013
We use atomic force microscopy to measure the distance-dependent solvation forces and the dissipation across liquid ﬁlms of octamethylcyclotetrasiloxane (OMCTS) conﬁned between a silicon tip and a highly oriented pyrolytic graphite substrate without active excitation of the cantilever. By analyzing the thermal bending ﬂuctuations, we minimize possible nonlinearities of the tip-substrate interaction due to ﬁnite excitation amplitudes because these ﬂuctuations are smaller than the typical 1A, which is much smaller than the characteristic interaction ˚length. Moreover, we avoid the need to determine the phase lag between cantilever excitation and response, which suffers from complications due to hydrodynamic coupling between cantilever and ﬂuid. Consistent results, and especially high-quality dissipation data, are obtained by analyzing the power spectrum and the time autocorrelation of the force ﬂuctuations. We validate our approach by determining the bulk viscosity of OMCTS using tips with a radius of approximately 1 μm at tip-substrate separations >5 nm. For sharp tips we consistently ﬁnd an exponentially decaying oscillatory tip-substrate interaction stiffness as well as a clearly nonmonotonic variation of the dissipation for tip-substrate distances up to 8 and 6 nm, respectively. Both observations are in line with the results of recent simulations which relate them to distance-dependent transitions of the molecular structure in the liquid.