Shearing or Compressing a Soft Glass in 2D: Time-concentration superposition

TL;DR

This study investigates the rheological behavior of dense 2D colloidal monolayers, revealing a superposition principle that suggests these systems behave like soft glasses, with implications for understanding their mechanical response.

Contribution

The paper introduces a time-concentration superposition method for 2D soft glass monolayers, enabling master curve construction of their elastic response across different concentrations.

Findings

Master curves can be formed by rescaling frequency and modulus.

Monolayers exhibit behavior close to soft glass states.

Superposition allows extrapolation of material response.

Abstract

We report surface shear rheological measurements on dense insoluble monolayers of micron sized colloidal spheres at the oil/water interface and of the protein $\beta$-lactoglobulin at the air/water surface. As expected, the elastic modulus shows a changing character in the response, from a viscous liquid towards an elastic solid as the concentration is increased, and a change from elastic to viscous as the shear frequency is increased. Surprisingly, above a critical packing fraction, the complex elastic modulus curves measured at different concentrations can be superposed to form a master curve, by rescaling the frequency and the magnitude of the modulus. This provides a powerful tool for the extrapolation of the material response function outside the experimentally accessible frequency range. The results are discussed in relation to recent experiments on bulk systems, and indicate that these two dimensional monolayers should be regarded as being close to a soft glass state.