Active materials represent a new class of condensed matter in which motile elements may collectively form dynamic, global structures out of equilibrium. Here, we present a general strategy to reconfigure active particles into various collective states by introducing imbalanced interactions. We demonstrate the concept with computer simulations of self-propelled colloidal spheres, and experimentally validate it in a two-dimensional (2D) system of metal-dielectric Janus colloids subjected to perpendicular a.c. electric fields. The mismatched, frequency-dependent dielectric responses of the two hemispheres of the colloids allow simultaneous control of particle motility and colloidal interactions. We realized swarms, chains, clusters and isotropic gases from the same precursor particle by changing the electric-field frequency. Large-scale polar waves, vortices and jammed domains are also observed, with the persistent time-dependent evolution of their collective structure evoking that of classical materials. This strategy of asymmetry-driven active self-organization should generalize rationally to other active 2D and three-dimensional (3D) materials.

Synchronization occurs widely in the natural and technological worlds, from the rhythm of applause and neuron firing to the quantum mechanics of coupled Josephson junctions, but has not been used to produce new spatial structures. Our understanding of self-assembly has evolved independently in the fields of chemistry and materials, and with a few notable exceptions has focused on equilibrium rather than dynamical systems. Here we combine these two phenomena to create synchronization-selected microtubes of Janus colloids, micron-sized spherical particles with different surface chemistry on their opposing hemispheres, which we study using imaging and computer simulation. A thin nickel film coats one hemisphere of each silica particle to generate a discoid magnetic symmetry, such that in a precessing magnetic field its dynamics retain crucial phase freedom. Synchronizing their motion, these Janus spheres self-organize into micrometre-scale tubes in which the constituent particles rotate and oscillate continuously. In addition, the microtube must be tidally locked to the particles, that is, the particles must maintain their orientation within the rotating microtube. This requirement leads to a synchronization-induced structural transition that offers various applications based on the potential to form, disintegrate and fine-tune self-assembled in-motion structures in situ. Furthermore, it offers a generalizable method of controlling structure using dynamic synchronization criteria rather than static energy minimization, and of designing new field-driven microscale devices in which components do not slavishly follow the external field.

Biofilms are surface-associated bacterial communities that play both beneficial and harmful roles in nature, medicine, and industry. Tolerant and persister cells are thought to underlie biofilm-related bacterial recurrence in medical and industrial contexts. Here, we review recent progress aimed at understanding the mechanical features that drive biofilm resilience and the biofilm formation process at single-cell resolution. We discuss findings regarding mechanisms underlying bacterial tolerance and persistence in biofilms and how these phenotypes are linked to antibiotic resistance. New strategies for combatting tolerance and persistence in biofilms and possible methods for biofilm eradication are highlighted to inspire future development.

Biofilms are surface-associated bacterial communities that are crucial in nature and during infection. Despite extensive work to identify biofilm components and to discover how they are regulated, little is known about biofilm structure at the level of individual cells. Here, we use state-of-the-art microscopy techniques to enable live singlecell resolution imaging of a Vibrio cholerae biofilm as it develops from one single founder cell to a mature biofilm of 10,000 cells, and to discover the forces underpinning the architectural evolution. Mutagenesis, matrix labeling, and simulations demonstrate that surface adhesion-mediated compression causes V. cholerae biofilms to transition from a 2D branched morphology to a dense, ordered 3D cluster. We discover that directional proliferation of rod-shaped bacteria plays a dominant role in shaping the biofilm architecture in V. cholerae biofilms, and this growth pattern is controlled by a single gene, rbmA. Competition analyses reveal that the dense growth mode has the advantage of providing the biofilm with superior mechanical properties. Our single-cell technology can broadly link genes to biofilm fine structure and provides a route to assessing cell-to-cell heterogeneity in response to external stimuli. biofilm | single cell | self-organization | community | biomechanics

Monodisperse polyhedral metal-organic framework (MOF) particles up to 5 μm in size, large enough to enable in situ optical imaging of particle orientation, were synthesized by the strategy of simultaneous addition of two capping ligands with different binding strength during crystallization. Upon dispersing them in ethylene glycol and applying AC electric field, the particles facets link to form linear chains. We observe well-regulated crystal orientation not only for rhombic dodecahedra all of whose facets are equivalent, but also for truncated cubes with nondegenerate facets. After removing the electric field, chains disassemble if their facets contain even modest curvature, but remain intact if their facets are planar. This assembly strategy offers a general route to fabricate oriented polyhedral crystal arrays of potential interest for new applications and functions.

Biofilms, surface-attached communities of bacteria encased in an extracellular matrix, are a major mode of bacterial life. How the material properties of the matrix contribute to biofilm growth and robustness is largely unexplored, in particular in response to environmental perturbations such as changes in osmotic pressure. Here, using Vibrio cholerae as our model organism, we show that during active cell growth, matrix production enables biofilm-dwelling bacterial cells to establish an osmotic pressure difference between the biofilm and the external environment. This pressure difference promotes biofilm expansion on nutritious surfaces by physically swelling the colony, which enhances nutrient uptake, and enables matrix-producing cells to outcompete non-matrix-producing cheaters via physical exclusion. Osmotic pressure together with crosslinking of the matrix also controls the growth of submerged biofilms and their susceptibility to invasion by planktonic cells. As the basic physicochemical principles of matrix crosslinking and osmotic swelling are universal, our findings may have implications for other biofilm-forming bacterial species.

Dipolar particles are fundamental building blocks in nature and technology, yet the effect of particle anisotropy is seldom explored. Here, we fabricate colloidal silica rods coated with a hemicylindrical magnetic layer to satisfy multiple criteria: nearly monodisperse, easily imaged and magnetic interaction that dominates over gravity. We confirm long-predicted features of dipolar assembly and stress the microstructural variety brought about by shape and constituent anisotropy, especially by extrapolating knowledge learned from literal molecules. In this colloidal system, we describe analogies to liquid crystalline deformations with bend, splay and twist; an analogy to cis/trans isomerism in organic molecules, which in our system can be controllably and reversibly switched; and a field-switching methodology to direct single ribbons into not only single but also multiple rings that can subsequently undergo hierarchical self-assembly. We highlight subtle material issues of control and design rules for reconfigurable dipolar materials with building blocks of complex shape.

Biofilms are communities of bacteria adhered to surfaces. Recently, biofilms of rod-shaped bacteria were observed at single-cell resolution and shown to develop from a disordered, two-dimensional layer of founder cells into a three-dimensional structure with a vertically-aligned core. Here, we elucidate the physical mechanism underpinning this transition using a combination of agent-based and continuum modeling. We find that verticalization proceeds through a series of localized mechanical instabilities on the cellular scale. For short cells, these instabilities are primarily triggered by cell division, whereas long cells are more likely to be peeled off the surface by nearby vertical cells, creating an “inverse domino effect”. The interplay between cell growth and cell verticalization gives rise to an exotic mechanical state in which the effective surface pressure becomes constant throughout the growing core of the biofilm surface layer. This dynamical isobaricity determines the expansion speed of a biofilm cluster and thereby governs how cells access the third dimension. In particular, theory predicts that a longer average cell length yields more rapidly expanding, flatter biofilms. We experimentally show that such changes in biofilm development occur by exploiting chemicals that modulate cell length.