Printing of soft-granular threads and clusters.*

We use microfluidics to generate monodisperse emulsion droplets at high volume fraction (>80%) and extrude such droplets one-by-one using either external flow [1] or direct deposition ('printing') at a substrate. Upon extrusion, the continuous phase forms capillary bridges between the droplets which leads to their capillary arrest and stabilizes the emerging droplet patterns. In the printing experiments, depending on the applied types of fluids, the interaction with the substrate (wetting/non-wetting etc.) and on the printing speed, we find various patterns ranging from seperate clusters to granular threads including single-file chains of droplets. We study long-term relaxation of such patterns including coalescence of quasi-2D granular clusters which proceeds via avalanches of rearrangements associated with transitions between different metastable states. Upon fine-tuning of the printing speed we also observe generation of chains which randomly self-fold resulting in the formation of regular yet stochastic super-patterns along the chains. We discuss the perspective of using of such patterns to encode information, e.g., for the purpose of droplet 'labeling', potentially useful in applications such as high-throughput screening [2].