Effective crystallization in confined soft granular media*

We use a densely packed double emulsion as a model soft granular material to produce freely-floating clusters of sizes of 20-40 grains (droplets). Despite the lack of an energetic potential, we report the occurrence of crystalline-like structures within such clusters. These structures can occur spontaneously during the formation of the clusters, but can also form within previously amorphously arranged structures due to flow, an effect we attribute to the input of noise and shear to the clusters from the surrounding flow. We find differences between the behaviours of two groups of droplets: those within the rim of the cluster and those within the inner part of the cluster. This largely stems from an intriguing competition between the tendency of the grains within the cluster to form the preferential crystal structure and effects on the surface of the cluster, such as the tendency of the cluster to minimize its surface area with the surrounding fluid due to surface tension and an attraction between the grains and the surface of the cluster, which leads to a series of consequences, including crowding and elongation of grains at the surface of the cluster.