Visualizing failure in arrested hard and sticky spheres

Our understanding of the mechanism by which the viscosity of supercooled liquids increases by many orders of magnitude is often described as a major challenge in condensed matter physics [1]. While understanding the static glass transition is challenging, prediction of mechanical failure of amorphous soft materials takes this to a new level and moreover has profound implications in a wide range of industrial settings, from cosmetics to argichemicals [2].

Among the key developments in analysing colloidal systems is to resolve the particle coordinates in 3D [3], allowing a level of analysis usually only possible in computer simulation. However in amorphous materials, relating mechanical properties to microscopic structure remains problematic. This makes it rather hard to understand, for example, mechanical failure. Here we introduce a new colloidal model system to address this challenge by studying the contacts and the forces between particles, as well as their positions. To do so, we use an emulsion in which the interparticle forces and local stress can be linked to the microscopic structure. We demonstrate the potential of our method to reveal insights into the failure mechanisms of soft amorphous solids by determining local stress two examples: firstly, a colloidal gel, which collapses under its own weight and secondly a colloidal glass under shear stress. In particular, we identify “force chains” of load–bearing droplets, and local stress anisotropy, and investigate their connection with locally rigid packings of the droplets [4].