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In most synthetic self-assembly processes the size of the final structure grows unbound and is only limited by the number of accessible microscopic building blocks. In comparison, biological assemblies can autonomously regulate their size and shape. One mechanism for such self-regulation is based on the chirality of microscopic units. Chirality induces a twisted geometry of building blocks that is incompatible with long-ranged crystalline packing, thereby stopping the assembly’s growth at a given stage. Chiral self-regulating self-assemblies, based on thermodynamic equilibration rather than kinetic trapping, remain an elusive target that has attracted considerable attention. So far studies of chiral self-assembly processes have focused on non-responsive systems, whose equilibrium points are not easily shifted in situ, which limits their versatility and applicability. Here, we demonstrate stimuli-responsive self-regulating self-assembly. This assembly is composed of chiral and magnetically alignable nanorods, where the effective chirality is modulable by balancing chirality-induced twisting with magnet-induced untwisting alignment. Changing the magnetic field intensity, controls the strength of self-regulation, leading to assemblies whose sizes and shapes are rationally controlled. The described size/shape control mechanism is tunable, reversible, robust, and widely applicable, opening up new possibilities for generating biomimetics structures with desirable functions and properties. 

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. 

Deep learning-based optical flow (DLOF) extracts features in video frames with deep convolutional neural networks to estimate the inter-frame motions of objects. DLOF computes velocity fields more accurately than PIV for densely labeled systems. 

Microscopic structure tuned by depletant concentration dictates mesoscale dynamics in extensile kinesin-driven microtubule bundles. 

By introducing light-activated motors, we spatiotemporally pattern nematic defect structure and flow in two-dimensional microtubule nematics. 

We study how the three-dimensional shape of rigid filaments determines the microscopic dynamics and macroscopic rheology of entangled semidilute Brownian suspensions. To control the filament shape we use bacterial flagella, which are microns-long helical or straight filaments assembled from flagellin monomers. We compare the dynamics of straight rods, helical filaments, and shape-diblock copolymers composed of seamlessly joined straight and helical segments. Caged by their neighbors, straight rods preferentially diffuse along their long axis, but exhibit significantly suppressed rotational diffusion. Entangled helical filaments escape their confining tube by corkscrewing through the dense obstacles created by other filaments. By comparison, the adjoining segments of the rod-helix shape-diblocks suppress both the translation and the corkscrewing dynamics. Consequently, the shape-diblock filaments become permanently jammed at exceedingly low densities. We also measure the rheological properties of semidilute suspensions and relate their mechanical properties to the microscopic dynamics of constituent filaments. In particular, rheology shows that an entangled suspension of shape rod-helix copolymers forms a low-density glass whose elastic modulus can be estimated by accounting for how shear deformations reduce the entropic degrees of freedom of constrained filaments. Our results demonstrate that the three-dimensional shape of rigid filaments can be used to design rheological properties of semidilute fibrous suspensions. 

Living systems are intrinsically nonequilibrium: They use metabolically derived chemical energy to power their emergent dynamics and self-organization. A crucial driver of these dynamics is the cellular cytoskeleton, a defining example of an active material where the energy injected by molecular motors cascades across length scales, allowing the material to break the constraints of thermodynamic equilibrium and display emergent nonequilibrium dynamics only possible due to the constant influx of energy. Notwithstanding recent experimental advances in the use of local probes to quantify entropy production and the breaking of detailed balance, little is known about the energetics of active materials or how energy propagates from the molecular to emergent length scales. Here, we use a recently developed picowatt calorimeter to experimentally measure the energetics of an active microtubule gel that displays emergent large-scale flows. We find that only approximately one-billionth of the system’s total energy consumption contributes to these emergent flows. We develop a chemical kinetics model that quantitatively captures how the system’s total thermal dissipation varies with ATP and microtubule concentrations but that breaks down at high motor concentration, signaling an interference between motors. Finally, we estimate how energy losses accumulate across scales. Taken together, these results highlight energetic efficiency as a key consideration for the engineering of active materials and are a powerful step toward developing a nonequilibrium thermodynamics of living systems. 

Active interfaces exhibit giant fluctuations and propagating waves that power wetting transitions and droplet shape shifting.