Abstract
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.
Original language | English |
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Awarding Institution |
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Supervisors/Advisors |
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Award date | 27 May 2011 |
Place of Publication | Enschede |
Publisher | |
Print ISBNs | 978-90-365-3198-6 |
DOIs | |
Publication status | Published - 27 May 2011 |