Relaxations in molecularly thin simple liquid films:linear viscoelasticity and non-linear response to shear
Demirel, Adem Levent
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https://hdl.handle.net/2142/18835
Description
Title
Relaxations in molecularly thin simple liquid films:linear viscoelasticity and non-linear response to shear
Author(s)
Demirel, Adem Levent
Issue Date
1996-05
Doctoral Committee Chair(s)
Granick, Steve
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
liquid films
linear viscoelasticity
molecularly thin simple liquid films
Language
en
Abstract
To understand the relaxations of molecules under confinement, the linear and nonlinear
response of molecularly thin simple liquid films confined between two solid surfaces
were studied. The method was to measure the resistance to applied oscillatory shear
deformation as a function of frequency and amplitude of deformation, film thickness and
normal pressure using the modified surface forces apparatus technique.
The investigation of the frequency spectra of linear viscoelastic response as a function
of the film thickness with octamethylcyclotetrasiloxane (OMCTS) films confined between mica surfaces showed more than two orders of magnitude increase in the relaxation times
from 7 to 1 molecular layers. The frequency spectra taken at different film thicknesses
were found to superpose onto a master curve when shifted horizontally and vertically. The
successful superposition of the frequency spectra of both predominantly viscous and
elastic response was concluded to show that a discontinuous phase transition was not
involved in the transition to the confinement induced elastic state, also ruling out the possibility of a phase transition from linear to non-linear response.
The transition from linear to non-linear response of elastic squalane films was
associated with the yield of structure in the thin film. The transition as a function of applied force was reversible and repeatable, but with a significant hysteresis. Within the
hysteresis loop, the time evolution of the viscoelastic parameters showed significant
fluctuations in the form of triangular spikes on a constant baseline. The durations of the spikes were much larger than the oscillation period indicating a correlation of these
changes over many periods. The dependence of the fluctuation amplitudes on the applied
shear force was concluded to show the competition between the intermolecular forces and the externally applied shear.
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