CPM Seminar

Solvation and Squeeze-out Mechanism of Confined Molecular Liquids

Nitya Nand Gosvami

Department of Mechanical Engineering
National University of Singapore

Liquids confined in a narrow gap between two surfaces can form ordered layers, which may lead to oscillatory-type solvation forces. The importance of studying confined liquids lies in understanding lubrication, friction, fluid flow in confined geometries, biological interactions etc. An understanding of the mechanics of the confined liquids at the nanometer scale can be obtained using the Atomic Force Microscope (AFM). However, it remains experimentally challenging to study the detailed mechanics of the confined material, and here conducting-AFM (C-AFM) can be used to reveal subtle changes in the contact junction which are not observed in standard force measurements. Furthermore, the ability to measure conductivity allows for an evaluation of the tip-sample contact area, a difficult problem in AFM.

I will present simultaneous C-AFM force and current measurements as the AFM tip presses down on the graphite surface, squeezing out the confined liquids. Experiments were performed on linear and branched alkanes. Solvation layering occurs for both types of liquid, but marked differences in the squeeze-out mechanics are observed depending on whether the monolayer is ordered or disordered. It is found that continuum elastic models are well suited to describe the squeeze-out mechanism of ordered, solid-like materials. However, when the confined molecules are disordered, the data is qualitatively very different. Simple elastic models cannot be applied anymore and comparison with recent simulations suggests that the liquid molecules remain trapped within the junction.