Bulk-like viscosity and shear thinning during dynamic compression of a nanoconfined liquid

TL;DR

This study investigates the viscosity of nanoconfined liquids, revealing bulk-like behavior with shear thinning at extreme confinement, and identifies oscillations indicating non-continuum effects.

Contribution

It provides detailed measurements showing that weakly interacting liquids maintain bulk viscosity under confinement, except for shear thinning below four molecular layers, and highlights non-continuum oscillations.

Findings

Viscosity remains bulk-like except under extreme confinement

Shear thinning occurs when confined to less than four molecular layers

Oscillations indicate non-continuum behavior and elastic response

Abstract

The viscosity of liquids under nanoconfinement remains controversial. Reports range from spontaneous solidification to no change in the viscosity at all. Here, we present thorough measurements with a small-amplitude linear atomic force microscopy technique and careful consideration of the confinement geometry, to show that in a weakly interacting liquid, average viscosity remains bulk like, except for strong shear thinning once the liquid is confined to less than four molecular layers. Overlaid over this bulk-like viscous behavior are stiffness and damping oscillations, indicating non-continuum behavior, as well as an elastic response when the liquid is allowed to order in the confinement gap.