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1. Dependency of active pressure and equation of state on stiffness of wall

   https://doi.org/10.1038/s41598-021-01605-8  

   Pirhadi, Emad; Cheng, Xiang; Yong, Xin (November 2021, Scientific Reports)

   Abstract Autonomous motion and motility are hallmarks of active matter. Active agents, such as biological cells and synthetic colloidal particles, consume internal energy or extract energy from the environment to generate self-propulsion and locomotion. These systems are persistently out of equilibrium due to continuous energy consumption. It is known that pressure is not always a state function for generic active matter. Torque interaction between active constituents and confinement renders the pressure of the system a boundary-dependent property. The mechanical pressure of anisotropic active particles depends on their microscopic interactions with a solid wall. Using self-propelled dumbbells confined by solid walls as a model system, we perform numerical simulations to explore how variations in the wall stiffness influence the mechanical pressure of dry active matter. In contrast to previous findings, we find that mechanical pressure can be independent of the interaction of anisotropic active particles with walls, even in the presence of intrinsic torque interaction. Particularly, the dependency of pressure on the wall stiffness vanishes when the stiffness is above a critical level. In such a limit, the dynamics of dumbbells near the walls are randomized due to the large torque experienced by the dumbbells, leading to the recovery of pressure as a state variable of density. 

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2. Dynamics of Active Defects on the Anisotropic Surface of an Ellipsoidal Droplet

   https://doi.org/10.1103/PhysRevX.14.031049  

   Clairand, Martina; Mozaffari, Ali; Hardoüin, Jerôme; Zhang, Rui; Doré, Claire; Ignés-Mullol, Jordi; Sagués, Francesc; de_Pablo, Juan J; Lopez-Leon, Teresa (September 2024, Physical Review X)

   We investigate the steady state of an ellipsoidal active nematic shell using experiments and numerical simulations. We create the shells by coating microsized ellipsoidal droplets with a protein-based active cytoskeletal gel, thus obtaining ellipsoidal core-shell structures. This system provides the appropriate conditions of confinement and geometry to investigate the impact of nonuniform curvature on an orderly active nematic fluid that features the minimum number of defects required by topology. We identify new time-dependent states where topological defects periodically oscillate between translational and rotational regimes, resulting in the spontaneous emergence of chirality. Our simulations of active nematohydrodynamics demonstrate that, beyond topology and activity, the dynamics of the active material are profoundly influenced by the local curvature and viscous anisotropy of the underlying droplet, as well as by external hydrodynamic forces stemming from the self-sustained rotational motion of defects. These results illustrate how the incorporation of curvature gradients into active nematic shells orchestrates remarkable spatiotemporal patterns, offering new insights into biological processes and providing compelling prospects for designing bioinspired micromachines. Published by the American Physical Society2024 

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3. Optimal transport and control of active drops

   https://doi.org/10.1073/pnas.2121985119  

   Shankar, Suraj; Raju, Vidya; Mahadevan, L. (August 2022, Proceedings of the National Academy of Sciences)

   Understanding the complex patterns in space–time exhibited by active systems has been the subject of much interest in recent times. Complementing this forward problem is the inverse problem of controlling active matter. Here, we use optimal control theory to pose the problem of transporting a slender drop of an active fluid and determine the dynamical profile of the active stresses to move it with minimal viscous dissipation. By parametrizing the position and size of the drop using a low-order description based on lubrication theory, we uncover a natural “gather–move–spread” strategy that leads to an optimal bound on the maximum achievable displacement of the drop relative to its size. In the continuum setting, the competition between passive surface tension and active controls generates richer behavior with futile oscillations and complex drop morphologies that trade internal dissipation against the transport cost to select optimal strategies. Our work combines active hydrodynamics and optimal control in a tractable and interpretable framework and begins to pave the way for the spatiotemporal manipulation of active matter. 

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4. The crumpling transition of active tethered membranes

   https://doi.org/10.1039/D3SM00403A  

   Gandikota, M. C.; Cacciuto, A. (July 2023, Soft Matter)

   Active fluctuations of tethered membranes with no self-avoidance can be encapsulated by a simple rescaling of the background temperature. The self-avoiding membrane preserves its extended phase even in the presence of very large active fluctuations. 

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5. Polarity and chirality control of an active fluid by passive nematic defects

   https://doi.org/10.1038/s41563-022-01432-w  

   Sciortino, Alfredo; Neumann, Lukas J.; Krüger, Timo; Maryshev, Ivan; Teshima, Tetsuhiko F.; Wolfrum, Bernhard; Frey, Erwin; Bausch, Andreas R. (February 2023, Nature Materials)

   Abstract Much like passive materials, active systems can be affected by the presence of imperfections in their microscopic order, called defects, that influence macroscopic properties. This suggests the possibility to steer collective patterns by introducing and controlling defects in an active system. Here we show that a self-assembled, passive nematic is ideally suited to control the pattern formation process of an active fluid. To this end, we force microtubules to glide inside a passive nematic material made from actin filaments. The actin nematic features self-assembled half-integer defects that steer the active microtubules and lead to the formation of macroscopic polar patterns. Moreover, by confining the nematic in circular geometries, chiral loops form. We find that the exact positioning of nematic defects in the passive material deterministically controls the formation and the polarity of the active flow, opening the possibility of efficiently shaping an active material using passive defects. 

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