From adaptive self-assembly to avalanching instabilities in driven soft-granular matter.*

Self-assembly of close-packed deformable grains into ordered structures is of basic interest in developmental biology as well as in tissue engineering. A significant challenge associated with manipulation of soft-granular materials is their inherent metastability associated with deformability of the grains which leads to complex energy landscapes. Here, we demonstrate that the control over rearrangements can be established via precisely balancing the internal elastic stresses with external viscous forces. We use microfluidics to generate monodisperse droplet structures and superstructures such as linear chains, multi-chains or chains with recurring folds adapting to the external flow. We stabilize the generated structures via adding a small volume fraction of a third immiscible phase to the system (besides the droplet and external phases) which engulfs the droplets and leads to their capillary arrest. We not only study the stability of the structures (against uncontrolled rearrangements) but also demonstrate that they can be ‘printed’ at a substrate which opens a range of applications, not only in tissue engineering but also (upon miniaturization) in photonics and/or microelectronics.