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1. Spontaneous crumpling of active spherical shells

   https://doi.org/10.1039/D4SM00015C  

   Gandikota, M C; Das, Shibananda; Cacciuto, A (May 2024, Soft Matter)

   The existence of a crumpled Flory phase for equilibrated self-avoiding elastic surfaces has remained contentious. Here, we show that a crumpled phase develops reliably upon subjecting a thin spherical self-avoiding shell to active fluctuations. 

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2. Spontaneous self-propulsion and nonequilibrium shape fluctuations of a droplet enclosing active particles

   https://doi.org/10.1038/s42005-022-00872-9  

   Kokot, Gašper; Faizi, Hammad A.; Pradillo, Gerardo E.; Snezhko, Alexey; Vlahovska, Petia M. (April 2022, Communications Physics)

   Abstract Active particles, such as swimming bacteria or self-propelled colloids, spontaneously assemble into large-scale dynamic structures. Geometric boundaries often enforce different spatio-temporal patterns compared to unconfined environment and thus provide a platform to control the behavior of active matter. Here, we report collective dynamics of active particles enclosed by soft, deformable boundary, that is responsive to the particles’ activity. We reveal that a quasi two-dimensional fluid droplet enclosing motile colloids powered by the Quincke effect (Quincke rollers) exhibits strong shape fluctuations with a power spectrum consistent with active fluctuations driven by particle-interface collisions. A broken detailed balance confirms the nonequilibrium nature of the shape dynamics. We further find that rollers self-organize into a single drop-spanning vortex, which can undergo a spontaneous symmetry breaking and vortex splitting. The droplet acquires motility while the vortex doublet exists. Our findings provide insights into the complex collective behavior of active colloidal suspensions in soft confinement. 

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3. Asymmetric fluctuations and self-folding of active interfaces

   https://doi.org/10.1073/pnas.2410345121  

   Zhao, Liang; Gulati, Paarth; Caballero, Fernando; Kolvin, Itamar; Adkins, Raymond; Marchetti, M_Cristina; Dogic, Zvonimir (December 2024, Proceedings of the National Academy of Sciences)

   We study the structure and dynamics of the interface separating a passive fluid from a microtubule-based active fluid. Turbulent-like active flows power giant interfacial fluctuations, which exhibit pronounced asymmetry between regions of positive and negative curvature. Experiments, numerical simulations, and theoretical arguments reveal how the interface breaks up the spatial symmetry of the fundamental bend instability to generate local vortical flows that lead to asymmetric interface fluctuations. The magnitude of interface deformations increases with activity: In the high activity limit, the interface self-folds invaginating passive droplets and generating a foam-like phase, where active fluid is perforated with passive droplets. These results demonstrate how active stresses control the structure, dynamics, and break-up of soft, deformable, and reconfigurable liquid–liquid interfaces. 

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4. Embedding vertex corrections in GW self-energy: Theory, implementation, and outlook

   https://doi.org/10.1063/5.0139117  

   Weng, Guorong; Mallarapu, Rushil; Vlček, Vojtěch (April 2023, The Journal of Chemical Physics)

   The vertex function (Γ) within the Green’s function formalism encapsulates information about all higher-order electron–electron interaction beyond those mediated by density fluctuations. Herein, we present an efficient approach that embeds vertex corrections in the one-shot GW correlation self-energy for isolated and periodic systems. The vertex-corrected self-energy is constructed through the proposed separation–propagation–recombination procedure: the electronic Hilbert space is separated into an active space and its orthogonal complement denoted as the “rest;” the active component is propagated by a space-specific effective Hamiltonian different from the rest. The vertex corrections are introduced by a rescaled time-dependent nonlocal exchange interaction. The direct Γ correction to the self-energy is further updated by adjusting the rescaling factor in a self-consistent post-processing cycle. Our embedding method is tested mainly on donor–acceptor charge-transfer systems. The embedded vertex effects consistently and significantly correct the quasiparticle energies of the gap-edge states. The fundamental gap is generally improved by 1–3 eV upon the one-shot GW approximation. Furthermore, we provide an outlook for applications of (embedded) vertex corrections in calculations of extended solids. 

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5. Dissipation controls transport and phase transitions in active fluids: mobility, diffusion and biased ensembles

   https://doi.org/10.1088/1367-2630/ab6353  

   Fodor, Étienne; Nemoto, Takahiro; Vaikuntanathan, Suriyanarayanan (January 2020, New Journal of Physics)

   Abstract Active fluids operate by constantly dissipating energy at the particle level to perform a directed motion, yielding dynamics and phases without any equilibrium equivalent. The emerging behaviors have been studied extensively, yet deciphering how local energy fluxes control the collective phenomena is still largely an open challenge. We provide generic relations between the activity-induced dissipation and the transport properties of an internal tracer. By exploiting a mapping between active fluctuations and disordered driving, our results reveal how the local dissipation, at the basis of self-propulsion, constrains internal transport by reducing the mobility and the diffusion of particles. Then, we employ techniques of large deviations to investigate how interactions are affected when varying dissipation. This leads us to shed light on a microscopic mechanism to promote clustering at low dissipation, and we also show the existence of collective motion at high dissipation. Overall, these results illustrate how tuning dissipation provides an alternative route to phase transitions in active fluids. 

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