Non-linear rheology of a nanoconfined simple fluid

TL;DR

This study investigates the non-linear rheological behavior of a nanoconfined simple fluid, OMCTS, revealing viscosity enhancement and shear-thinning akin to supercooled liquids near glass transition, with insights into drainage dynamics.

Contribution

It provides new experimental evidence linking nanoconfined fluid rheology to glassy dynamics and challenges existing hydrodynamic models of drainage front propagation.

Findings

OMCTS exhibits viscosity increase under confinement.

Flow properties resemble supercooled liquids near glass transition.

Drainage front propagation not governed by large-scale flow.

Abstract

We probe the rheology of the model liquid octamethylcyclotetrasiloxane (OMCTS) confined into molecularly thin films, using a unique Surface Forces Apparatus allowing to explore a large range of shear rates and confinement. We thus show that OMCTS under increasing confinement exhibits the viscosity enhancement and the non-linear flow properties characteristic of a sheared supercooled liquid approaching its glass transition. Besides, we study the drainage of confined OMCTS via the propagation of "squeeze-out" fronts. The hydrodynamic model proposed by Becker and Mugele [Phys. Rev. Lett. {\bf 91}, 166104 (2003)] to describe such front dynamics leads to a conclusion in apparent contradiction with the dynamical slowdown evidenced by rheology measurements, which suggests that front propagation is not controlled by large scale flow in the confined films.