Collapse dynamics of confined liquid films

Liquids in a conned geometry have attracted much interestin science and technology in recent years. Conned liquidlms with a thickness in the range of a few molecular diameters exhibit dierent mechanical properties than in the bulk. In this work a newly designed surface forces apparatuswith two-dimensional imaging capability is presented. Dueto the unprecedented sensitivity of the two-dimensional imaging technique the dynamics of the layer by layer thinning ofa conned liquid lm can be investigated in great detail.
Different expulsion scenarios depending on the substrate elasticity and the approach rates are presented. Applying slow approach rates to both stiff and soft substrates results in similar dynamics of the expulsion processes. A comparison
of the dynamics with a simple hydrodynamic model shows good agreement. In the case of particularly thin substrates with fast approach rates the formation of small trapped liquid droplets and their subsequent squeeze-out was observed. Fast approaches on thick substrates resulted in a mixed drainage scenario.
While it is generally accepted that the viscosity of confined liquids increases with decreasing thickness, the order of magnitude is highly debated. The viscosity of a model lubricant (OMCTS) is measured as a function of the thickness of the
confined film and found to increase by a factor of ten with decreasing lm thickness from 6 to 2 layers. This is in contrast to results reported by others where an increase of several orders of magnitude was observed. Importantly, in our experiments, a method to improve the standard preparation procedure was used. In order to describe the increase in the eective friction observed in our work a new hydrodynamic model is presented. In terms of this model it is shown that the sliding friction of liquid layers on top of the solid substrates is approximately 35 times higher than the mutual friction between adjacent liquid layers. The latter is independent of lm thickness and in close agreement with the bulk viscosity. The mentioned variations in mechanical properties of thin
liquid films compared to the bulk properties arise from structural changes. Making use of recent advances in synchrotron radiation sources and beam shaping techniques allows the investigation of the in-plane structure of confined liquids with x-ray scattering experiments. Preliminary x-ray scattering data from thin liquid crystalline films (8CB) are shown and the domain structure of the liquid crystal and the anchoring of the liquid crystal with respect to the mica lattice is discussed.