Probing the properties of confined liquids

In this thesis we describe Atomic Force Microscopy (AFM) measurements and Molecular Dynamics (MD) simulation of the static and dynamic properties of layered liquids confined between two solid surfaces. Liquid molecules in the proximity of a solid surface assemble into layers. When a fluid is confined between two surfaces, the discrete molecular nature of the liquid becomes observable via the oscillatory solvation forces and can be probed with AFM spectroscopy. Upon approach of an in liquid immersed AFM cantilever – driven with a sub-angstrom amplitude – towards a solid graphite surface, we find that both the amplitude and phase response strongly oscillate as the distance is decreased. From the amplitude and phase response we extract the conservative and dissipative interaction forces. We observe that the conservative forces increasingly oscillate for a decreasing tip-surface distance, as expected for oscillatory solvation forces. For the dissipative interaction forces or the damping on the tip we find pronounced maxima positioned at the transition from 3-2, 2-1 and 1-0 layers. From these observations we conclude that the dynamic transport-properties of the confined liquid significantly change in these transition-regions. Nevertheless, in AFM measurements we only measure forces. We can not see what happens with the confined liquid molecules. To study the effect of confinement on the dynamics of the molecules and how that will affect the response on the cantilever, we also performed MD simulations. In our simulations the average force on the tip shows the same exponential decaying oscillations as we found in our experiments. Next to the average force, we also monitored the force-fluctuations on the tip. Using fluctuationdissipation we converted these force-fluctuations in the dissipative force or damping on the tip. The damping on the tip shows pronounced maxima very similar to our experimental results. The maxima are also positioned at the transition regions of 3-2, 2-1 and 1-0 layers. By monitoring the Mean Squared Displacement and the number of nearest neighbors of the molecules confined under the tip, we find that the damping is closely related to the configuration and the dynamics of the molecules. Regarding these observations one might be tempted to conclude that the confined molecules behave either liquid-like or solid-like depending on the distance between the tip and the surface. However, spectral analysis suggests that the elastic and viscous response of the confined liquid is more complex and would be better described as either a gel or a soft glassy material.