Granular Character of Particle Rafts

TL;DR

This study investigates the granular behavior of particle rafts at liquid interfaces, revealing a Janssen effect and stress decay, thus highlighting their dual elastic and granular nature, with implications for two-dimensional granular physics.

Contribution

It demonstrates for the first time that particle rafts exhibit a Janssen effect, showing their granular character alongside elastic properties, which current models neglect.

Findings

Stress decays exponentially from barriers

Quantitative agreement with Janssen's theory

Rafts exhibit both elastic and granular behavior

Abstract

We consider a monolayer of particles floating at a horizontal liquid-gas interface -- a particle raft. Upon compressing the monolayer in a Langmuir trough, the particles at first pack but ultimately the monolayer buckles out of the plane. We measure the stress profile within the raft at the onset of buckling and show for the first time that such systems exhibit a Janssen effect: the stress decays exponentially away from the compressing barriers over a length scale that depends on the width of the trough. We find quantitative agreement between the rate of decay and the simple theory presented by Janssen and others. This demonstrates that floating particle rafts have a granular, as well as elastic, character, which is neglected by current models. Finally, we suggest that our experimental setup may be suitable for exploring granular effects in two dimensions without the complications of gravity and basal friction.