2D foams above the jamming transition: Deformation matters

TL;DR

This paper shows that 2D foams near the wet limit exhibit a linear increase in contact number with gas fraction, contrasting with the square root behavior of soft disk packings, highlighting the importance of particle deformation.

Contribution

It demonstrates that 2D foam behavior near the wet limit cannot be accurately modeled by soft disks due to bubble deformability, revealing a linear relationship in contact number growth.

Findings

2D foams near the wet limit show linear Z growth with gas fraction.

Soft disk models predict a square root increase in contact number.

Deformability of bubbles significantly affects packing behavior.

Abstract

Jammed soft matter systems are often modelled as dense packings of overlapping soft spheres, thus ignoring particle deformation. For 2D (and 3D) soft disks packings, close to the critical packing fraction $\varphi_c$, this results in an increase of the average contact number $Z$ with a square root in $\varphi-\varphi_c$. Using the program PLAT, we find that in the case of idealised two-dimensional foams, close to the wet limit, $Z$ increases linearly with $\varphi-\varphi_c$, where $\varphi$ is the gas fraction. This result is consistent with the different distributions of separations for soft disks and foams at the critical packing fraction. Thus, 2D foams close to the wet limit are not well described as random packings of soft disks, since bubbles in a foam are deformable and adjust their shape. This is not captured by overlapping circular disks.