Relaxations in molecularly thin simple liquid films:linear viscoelasticity and non-linear response to shear

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.U of I OnlyThesi