Hyperuniformity

Disordered hyperuniformity is a type of liquid which has crystal properties. It greatly suppresses variations in the density of particles, like a crystal, and the particles have the same physical properties in all directions at shorter distances, like a liquid. It was discovered in the eyes of chickens. This is thought to be the case because chicken eyes cannot support the ordered, complex system best for eyesight. This may eventually be used for self-organizing colloids or optics with the ability to transmit light with crystal efficiency while still retaining liquid flexibility.

Unique optical properties have been uncovered in dense hyperuniform materials, wherein light of a wavelength specific to the material is able to propagate forward despite high particle density due to microscopic order. The uniformly spaced particles scatter light as it propagates through the material, but most of the scattering self-interferes except that in the direction of propagation. In conditions where light is propagated through an uncorrelated disordered material of the same density, the material would appear opaque due to multiple scattering. Such materials can be theoretically designed for light of any wavelength, and the applications of the concept cover a wide variety of fields of wave physics and materials engineering.

The term hyperuniformity was coined by chemist and packing expert Salvatore Torquato, co-author of a pioneering 2003 paper on the topic. Torquato says that another example of this ordering is that found in a shaken box of marbles, which fall into an arrangement, called maximally random jammed packing. Two classes of hyperuniformity include equilibrium systems - such as quasicrystals - and non-equilibrium systems, which include shaken marbles, emulsions, colloids and ensembles of cold atoms. It's also thought to emerge on the mysterious biological patterns known as fairy circles - circle and patterns of circles that emerge in arid places.

The challenge of creating disordered hyperuniform materials lies on successfully suppressing the density fluctuations over long scales in the material. Thus many proposed hyperuniform systems are composed of particles that are large in size so that not be influenced by thermal fluctuations. Recently Chremos has proposed a design rule for the practical creation of hyperuniform materials at the molecular level. Specifically, hyperuniformity is achieved by having one component fluid composed of molecules having a substituent of their particles (e.g., the core of the star polymers or the backbone chains in the case of bottlebrush polymers) localized within their own molecular structure, which tends to increase the magnitude of Sp in the partial structure factor correlations. The combination of these features leads to molecular packing that is highly uniform at both small and large length scales.

Disordered hyperuniformity implies long-range direct correlations in the system. In equilibrium, this requires delicately designed long-range interactions, which are not necessary for the dynamic self-assembly of non-equilibrium hyperuniform states. Ni and co-workers found that a non-equilibrium strongly hyperuniform fluid phase exists in systems of circularly swimming active hard spheres. This new hyperuniform fluid features a special length scale, i.e., the diameter of the circular trajectory of active particles, below which large density fluctuations are observed. Moreover, based on a generalised random organising model, Lei and Ni formulated a hydrodynamic theory for non-equilibrium hyperuniform fluids, and the length scale above which the system is hyperuniform is controlled by the inertia of the particles. The theory generalises the mechanism of fluidic hyperuniformity as the damping of the stochastic harmonic oscillator, which indicates that the suppressed long-wavelength density fluctuation can exhibit as either acoustic (resonance) mode or diffusive (overdamped) mode.

See also

• Quasicrystal

External links

• A Bird’s-Eye View of Nature’s Hidden Order, Natalie Wolchover, Quanta Magazine