Amorphous to amorphous transition in particle rafts

TL;DR

This study reveals a unique phase transition in particle rafts from a less-rigid to a more-rigid amorphous state driven by particle density, involving structural changes and altered mechanical responses.

Contribution

It uncovers the amorphous-to-amorphous transition in particle rafts and links structural anomalies to changes in mechanical behavior as density varies.

Findings

Identification of a structural anomaly at the transition point

Observation of weakening shear response near the transition

Correlation of local plastic instabilities with contact-line depinning

Abstract

Space-filling assemblies of athermal hydrophobic particles floating at an air-water interface, called particle rafts, are shown to undergo an unusual phase transition between two i.e., a low density `less-rigid and a high density `more-rigid' amorphous states as a function of particulate number density ($\Phi$). The former is shown to be a capillary-bridged solid and the later a frictionally-coupled one. Simultaneous studies involving direct imaging as well as measuring its mechanical response to longitudinal and shear stresses show that the transition is marked by a subtle structural anomaly and a weakening of the shear response. The structural anomaly is identified from the variation of the mean coordination number, mean area of the Voronoi cells and the spatial profile of the displacement field with $\Phi$. The weakened shear response is related to local plastic instabilities caused by the depinning of the contact-line of the underlying fluid on the rough surfaces of the particles.