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1. Programming active cohesive granular matter with mechanically induced phase changes

   https://doi.org/10.1126/sciadv.abe8494  

   Li, Shengkai; Dutta, Bahnisikha; Cannon, Sarah; Daymude, Joshua J.; Avinery, Ram; Aydin, Enes; Richa, Andréa W.; Goldman, Daniel I.; Randall, Dana (April 2021, Science Advances)

   null (Ed.)

   At the macroscale, controlling robotic swarms typically uses substantial memory, processing power, and coordination unavailable at the microscale, e.g., for colloidal robots, which could be useful for fighting disease, fabricating intelligent textiles, and designing nanocomputers. To develop principles that can leverage physical interactions and thus be used across scales, we take a two-pronged approach: a theoretical abstraction of self-organizing particle systems and an experimental robot system of active cohesive granular matter that intentionally lacks digital electronic computation and communication, using minimal (or no) sensing and control. As predicted by theory, as interparticle attraction increases, the collective transitions from dispersed to a compact phase. When aggregated, the collective can transport non-robot “impurities,” thus performing an emergent task driven by the physics underlying the transition. These results reveal a fruitful interplay between algorithm design and active matter robophysics that can result in principles for programming collectives without the need for complex algorithms or capabilities. 

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2. Symmetry, Thermodynamics and Topology in Active Matter

   Bowick, Mark J; Fakhri, Nikta; Marchetti, M Cristina; Ramaswamy, Sriram (January 2022, Physical review and Physical review letters index)

   The name active matter refers to any collection of entities that individually use free energy to generate their own motion and forces. Through interactions, active particles spontaneously organize in emergent large-scale structures with a rich range of materials properties. The active matter paradigm has been applied to living and non-living systems over a vast dynamic range, from the organization of subnuclear structures in the cell to collective motion at the human scale. The diverse phenomena exhibited by these systems all stem from the defining property of active matter as an assembly of components that individually and dissipatively break time-reversal symmetry. This article outlines a selection of current and emerging directions in active matter research. It aims at providing a pedagogical and forward looking introduction for researchers new to the field and a roadmap of open challenges and future directions that may appeal to those established in the area. 

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3. The 2025 Motile Active Matter Roadmap

   https://doi.org/10.1088/1361-648X/adac98  

   Gompper, Gerhard; Stone, Howard_A; Kurzthaler, Christina; Saintillan, David; Peruani, Fernado; Fedosov, Dmitry; Auth, Thorsten; Cottin-Bizonne, Cecile; Ybert, Christophe; Clement, Eric; et al (January 2025, Journal of Physics: Condensed Matter)

   Abstract Activity and autonomous motion are fundamental aspects of many living and engineering systems. Here, the scale of biological agents covers a wide range, from nanomotors, cytoskeleton, and cells, to insects, fish, birds, and people. Inspired by biological active systems, various types of autonomous synthetic nano- and micromachines have been designed, which provide the basis for multifunctional, highly responsive, intelligent active materials. A major challenge for understanding and designing active matter is their inherent non-equilibrium nature due to persistent energy consumption, which invalidates equilibrium concepts such as free energy, detailed balance, and time-reversal symmetry. Furthermore, interactions in ensembles of active agents are often non-additive and non-reciprocal. An important aspect of biological agents is their ability to sense the environment, process this information, and adjust their motion accordingly. It is an important goal for the engineering of micro-robotic systems to achieve similar functionality. With many fundamental properties of motile active matter now reasonably well understood and under control, the ground is prepared for the study of physical aspects and mechanisms of motion in complex environments, of the behavior of systems with new physical features like chirality, of the development of novel micromachines and microbots, of the emergent collective behavior and swarming of intelligent self-propelled particles, and of particular features of microbial systems. The vast complexity of phenomena and mechanisms involved in the self-organization and dynamics of motile active matter poses major challenges, which can only be addressed by a truly interdisciplinary effort involving scientists from biology, chemistry, ecology, engineering, mathematics, and physics. The 2024 motile active matter roadmap of Journal of Physics: Condensed Matter reviews the current state of the art of the field and provides guidance for further progress in this fascinating research area. 

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4. A direct link between active matter and sheared granular systems

   https://doi.org/10.1073/pnas.2019909118  

   Morse, Peter K.; Roy, Sudeshna; Agoritsas, Elisabeth; Stanifer, Ethan; Corwin, Eric I.; Manning, M. Lisa (April 2021, Proceedings of the National Academy of Sciences)

   null (Ed.)

   The similarity in mechanical properties of dense active matter and sheared amorphous solids has been noted in recent years without a rigorous examination of the underlying mechanism. We develop a mean-field model that predicts that their critical behavior—as measured by their avalanche statistics—should be equivalent in infinite dimensions up to a rescaling factor that depends on the correlation length of the applied field. We test these predictions in two dimensions using a numerical protocol, termed “athermal quasistatic random displacement,” and find that these mean-field predictions are surprisingly accurate in low dimensions. We identify a general class of perturbations that smoothly interpolates between the uncorrelated localized forces that occur in the high-persistence limit of dense active matter and system-spanning correlated displacements that occur under applied shear. These results suggest a universal framework for predicting flow, deformation, and failure in active and sheared disordered materials. 

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5. Synthetic electrically driven colloids: A platform for understanding collective behavior in soft matter

   https://doi.org/10.1016/j.cocis.2022.101603  

   Boymelgreen, Alicia; Schiffbauer, Jarrod; Khusid, Boris; Yossifon, Gilad (August 2022, Current Opinion in Colloid & Interface Science)

   Martin Bazant (Ed.)

   The collective motion of synthetic active colloids is an emerging area of research in soft matter physics and is important both as a platform for fundamental studies ranging from non-equilibrium statistical mechanics to the basic principles of self-organization, emergent phenomena, and assembly underlying life, as well as applications in biomedicine and metamaterials. The potentially transformative nature of the field over the next decade and beyond is a topic of critical research importance. Electrokinetic active colloids represent an extremely flexible platform for the investigation and modulation of collective behavior in active matter. Here, we review progress in the past five years in electrokinetic active systems and related topics in active matter with important fundamental research and applicative potential to be investigated using electrokinetic systems. 

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