The force network in emulsions and the role of external stress*

Direct imaging of emulsion droplets labeled with fluorescent nanoparticles using confocal microscopy is a valuable experimental tool for studying granular materials in three dimensions. By measuring individual droplet-droplet contacts inside the frictionless emulsion piles, we visualize the force network and calculate the orientations, positions, and magnitudes of forces and their statistical distributions. We find that large forces are more likely to align parallel to each other, leading to long-range, chain-like correlations of magnitude and direction of contact force. Furthermore, we investigate how the force network evolves with time and how it changes under various external stresses. We also measure the contact force at the bottom of emulsion piles and compare to previous surface measurements and the measurement inside the bulk. This work may shed light on the aging and macroscopic viscoelasticity of granular systems.