Revisiting viscosity from the macroscopic to nanoscale regimes

TL;DR

This study experimentally confirms that molecular-level viscosity matches macroscopic shear viscosity, bridging different scales using rheometry and fluorescence techniques, with implications for understanding fluid behavior at nanoscale.

Contribution

The paper provides experimental verification that microscopic and macroscopic viscosities are equivalent, bridging the gap between different length- and time-scales in fluid dynamics.

Findings

Molecular and macroscopic viscosities are equivalent.

Fluorescence Correlation Spectroscopy and Time Resolved Fluorescence effectively measure local viscosity.

Viscosity scaling in glucose solutions resembles branched polymer behavior.

Abstract

The response of a fluid to deformation by shear stress is known as shear viscosity. This concept arises from a macroscopic view and was first introduced by Sir Isaac Newton. Nonetheless, a fluid is a series of moving molecules that are constrained by the shape of the container. Such a view begs the treatment of viscosity from a microscopic or molecular view, a task undertaken by both Einstein and Smoluchowski independently. Here we revisit the concept of viscosity and experimentally verify that the viscosity at a molecular level, which describes the drag force, is the same as the macroscopic shear viscosity; hence, bridging different length- and time-scales. For capturing the shear stress response of a fluid, we use classical rheometry; at a molecular level we use probe diffusion to determine the local viscosity from the translational and rotational motions. In these cases, we use Fluorescence Correlation Spectroscopy and Time Resolved Fluorescence, respectively. By increasing the osmolyte (Glucose-D) concentration, we change the viscosity and find that these methods provide a unified view of viscosity bridging the gap between the macroscopic and nanoscale regimes. Moreover, Glucose's viscosity follows a scaling factor more commonly associated to solutions of branched polymer because the probe dimensions are comparable to the dimensions of the osmolyte that exerts the drag.