Search

The experimental discovery of materials known as higher-order topological insulators corroborates theoretical predictions and expands the toolbox for integrated optics and mechanical devices.

The dynamics of a viscous liquid undergo a dramatic slowdown when it is cooled to form a solid glass. Recognizing the structural changes across such a transition remains a major challenge. Machine-learning methods, similar to those Facebook uses to recognize groups of friends, have now been applied to this problem.

The development of active solids based on centimetre-scale building blocks incorporating odd elasticity shows that they can spontaneously undergo limit cycles of shape changes, leading to adaptive locomotion such as rolling and crawling.

Mechanical metamaterials are artificial structures whose properties originate from their geometry. In such structures, it is now shown that topological modes can exist that are robust against a range of structural deformations.

Most soft materials, such as sand, can be in either a solid-like or a liquid-like state. New experiments probe the surprisingly rich nonlinear physics that can occur in between these two states. See Letter p.355

Broken and tailored symmetries have a fundamental role in wave phenomena and their applications. This Review surveys the recent progress in the domain of artificial phononic media with an emphasis on the role of symmetry breaking, in both space and time, for advanced wave phenomena.

Keeping viral vaccines cold from the manufacturers to patients is problematic and costly. Here, Krol and others show additives that can significantly improve at very low concentrations the storage of adenovirus type 5 at ambient and elevated temperature.

Turbulent energy cascades can be arrested by non-dissipative viscosities, resulting in pattern formation at intermediate length scales.

Active fluids exhibit properties reminiscent of equilibrium systems when their degrees of freedom are statistically decoupled. A theory for the fluctuating hydrodynamics of these fluids offers a probe of their anomalous transport coefficients.

Mechanical metamaterials can be engineered with properties not possible in ordinary materials. Here the authors demonstrate and study an active metamaterial with self-sensing characteristics that enables odd elastic properties not observed in passive media.

Curvature in elastic sheets can be used to control material failure and guide the paths of cracks.

Hexagons can easily tile a flat surface, but not a curved one. Defects with topological charge (such as heptagons and pentagons) make it easier to tile curved surfaces, such as soccer balls. Here, a new type of defect is reported that accommodates curvature in the same way as fabric pleats. The appearance of such defects on the negatively curved surfaces of stretched colloidal crystals are observed. The results will facilitate the exploration of general theories of defects in curved spaces, the engineering of curved structures and novel methods for soft lithography and directed self-assembly.

Soft filamentous bundles, including F-actin, microtubules or bacterial flagella, can experience large frictional forces that scale logarithmically with sliding velocity, and such frictional coupling can be tuned by modifying lateral interfilament interactions.

Dualities—mathematical mappings between different systems—can act as hidden symmetries that enable materials design beyond that suggested by crystallographic space groups.

Active, non-conservative interactions can give rise to elastic moduli that are forbidden in equilibrium and enter the antisymmetric part of the stiffness tensor. The resulting solids function as distributed elastic engines that can perform work on their surroundings through quasistatic strain cycles.

A theoretical study of non-reciprocity in collective phenomena reveals the emergence of time-dependent phases heralded by exceptional points in contexts ranging from synchronization and flocking to pattern formation.

A theoretical framework for the design of so-called perturbative metamaterials, based on weakly interacting unit cells, has led to the experimental demonstration of a quadrupole topological insulator.

Interfaces between non-excitable tissues can be electrically excitable, suggesting a possible bioelectrical mechanism for interface sensing.

Active chiral fluids are a special case of active matter in which energy is introduced into rotational motion via local application of torque. Here Banerjee et al. develop a hydrodynamic theory of such active fluids and connect it with odd viscosity which was previously considered an abstract concept.

Nucleation is the fundamental relaxation mechanism that leads to the emergence of a new phase or structure via first-order phase transitions. Here, the authors study nucleation and growth of two-dimensional phases on curved surfaces, and show how the curvature influences its inhomogeneity and speed.

Light-responsive myosin motors enable spatial and temporal control over the dynamics and transport of active nematic liquid crystals composed of actin filaments and myosin at an oil–water interface.

Ensuring topological protection of the edge states in candidate topological insulators is complicated by the need to break time-reversal symmetry. Polar active liquids present an innovative solution to this problem, as a new metamaterial design shows.

Active matter encompasses various non-equilibrium systems in which individual constituents convert energy into non-conservative forces or motion at the microscale. This Review provides an elementary introduction to the role of topology in active matter through experimentally relevant examples.

Acoustically levitated granular particles offer a platform on which to study self-assembly in a macroscopic system. Precise acoustic tuning reveals how cluster statistics and assembly pathways change as the system moves out of equilibrium.

Mechanical metamaterials exhibit exotic properties that cannot be realized in conventional materials. In this Review, recently developed shape-morphing, topological and nonlinear metamaterials are discussed together with their functionalities and design principles, and future challenges for the field are outlined.