Probing the Crystal-to-Glass Transition using the Mechanical Properties of 2D Clusters of Droplets

Lightly attractive stabilized oil droplets (R approximately 10 microns), suspended in an aqueous solution, are used to prepare model quasi-2D aggregates (tens of droplets). We investigate the response of the clusters ranging from a perfectly ordered crystal (monodisperse aggregate) to a disordered glass (bidisperse aggregate). Disorder in the system is continuously varied by mixing two monodisperse populations of droplets in different proportions. The mechanical properties of the clusters are assessed by compressing the aggregates between two parallel boundaries, one of which acts as a force sensor. The forces acting on the aggregate are directly measured and correlated with the internal rearrangements observed by optical microscopy. The force measurement reveals a signature of the cluster composition and provides insight into the crystal-to-glass transition. We find that introducing even a small amount of disorder in the clusters has a strong impact on the measured yield stress. Finally, a statistical model fully supports our experimental results.