Towards Animate Droplets: Active, Adaptive, and Autonomous

Abstract

Droplets, sub-millilitre liquid volumes with at least one interface, have traditionally served as compartments for storing, transporting, and delivering materials. Beyond familiar applications in food, coatings, and consumer goods, they find cutting-edge use in energy storage, sensing, and tissue engineering. The next frontier is their integration into animate matter, emerging materials defined by their levels of activity, adaptiveness, and autonomy. Easy to produce and dispense or print into complex structures, and with enormous chemical versatility, droplets are ideal building blocks for animate matter. In this Perspective, we outline a roadmap for advancing animacy in droplets and call for a more concerted effort to integrate novel mechanisms for motility, sensing, and decision-making into droplet design. Although research on active droplets spans more than a century, achieving true autonomy, where droplets process multiple stimuli and respond without external control, remains a central challenge. We hope to inspire interdisciplinary collaboration towards applications in consumer goods, microfluidics, adaptive optics, tissue engineering, and soft robotics.

Figures (2)

• Figure 1: Animacy in droplets: the state-of-the-art. To date, droplet systems have been designed to express behaviours embodying activity, adaptiveness, and autonomy to various degrees. These behaviours can achieve various elementary functions such as motility, sensing, self-assembly, shape change, communication, and differentiation (legend symbols). Starting from a simple emulsion, where droplets have stability against coalescence, combinations of these functions (inset symbols) can be used to achieve higher-order behaviours, such as swimming (swimmers) and self- or directed assembly (structures). As these traits are combined and the emergent complexity of the behaviour increases, so in general does the extent to which the system exhibits activity, adaptiveness, and autonomy (inset spider plots). Each behaviour can be considered more "colony-like" (top row) or "tissue-like" (bottom row) depending on whether droplets tend to act more like individual units working together or as mechanically bound pieces in a greater structure.
• Figure 2: A roadmap for research in animate droplets. Our suggestions, left to right, start from simpler advances on current literature to abstract towards more complex future implementations. Sustained Energy.A: Incorporation of chloroplasts (green) and active particle propulsion into a multi-droplet structure connected by pores could create a liquid machine sensitive to light (yellow colouring). B: Droplets that produce different responses, such as division or shape change in response to different concentrations of chemical stimuli (triangles and diamonds) C: Complex droplet systems that exhibit population control, switching high energy demand units ($\upalpha$) for lower energy demand units ($\upbeta$) when nutrient sources become scarce (orange colouring). Autonomy and Adaptation in Droplet Motion:D: Active particle Pickering emulsions allowing motion and shape change. E: Internal control of a biomimetic droplet cytoskeleton for autonomous deformation. F: Adaptive droplets that seek nutrient sources to grow and divide only when favourable. Collective Motion and Shape Transformation:G: Cyclical actuation in droplet prototissues driven by reversible active pumping of solutes. H: Structural changes, such as moving from a hexagonal to a cubic pattern could be implemented by changes in the nature of inter-droplet adhesion, as opposed to relying on specialised actuators, as in (G). I: Cilia-like transport of cargoes driven by controlled waves of droplet contraction in synchronised chemical oscillators. Communication and Information Processing:J: Droplet structures reversibly switching between colony-like and tissue-like behaviour by varying the strength of mechanical coupling in the system. K: Plastic self-rearrangement of prototissue structure, allowing a change in subunit distribution and/or prototissue function. For example, separate compartments (dark blue) in a supporting matrix (light blue) could be reorganised into a connected channel to allow transport of materials. L: Information exchange leading to a change in droplet phenotype (from $\upalpha$ to $\upbeta$) and collective decision-making (from quiescent to active) through droplet quorum sensing.