It has been my great pleasure to open the 15th International Congress for Stereology and Image Analysis in Aarhus, Denmark, with my keynote on:

How to find a hidden order among disorder

where I gave an introduction and overview on hyperuniformity. The study of this anomalous suppression of density fluctuations on large length scales has shed light on a variety of seemingly unrelated fields, from the eyes of chicken to exotic many-particle ensembles and random matrices. The unique properties of hyperuniform amorphous materials (with a hidden order such that the system remains macroscopically uniform, despite not being crystalline) have recently led to intense research in physics, mathematics, material science, and biology. Aiming for an intuitive understanding of the rigorous mathematical definitions, I presented both basic concepts and recent examples.

I thank the organizers for a great conference in the last week of May 2019 with many stimulating discussions across disciplines.

It has been my great pleasure to organize together with Lisa Manning, Gregory Grason, and Gerd Schröder-Turk a GSOFT Focus Session at the APS March Meeting in Boston this year:

"Hyperuniformity and Optimal Tessellations: Structure, Formation and Properties"

After recent
breakthroughs in the search for ordered optimal tessellations (for
example, including Frank-Kasper phases in copolymer melts), now
findings of the optimal properties of amorphous tessellations are
emerging, e.g., in biological tissues.

At the same time, there have been intensive studies of amorphous systems with an anomalous suppression of density fluctuations on large length scales, known as hyperuniformity. This geometric concept qualitatively and quantitatively characterizes a hidden-order in amorphous states that allows for unique physical properties – combining those of crystalline and disordered phases. Thus it offers candidates for optimal amorphous tessellations of space.

The two invited speakers of our session on Thursday, March 7, were Jasna Brujic and Salvatore Torquato.

In this paper, we analyze the evolution of random point patterns and their Voronoi diagrams as “quantizer” of space. Applying an iterative local optimization of their so-called Quantizer energy, we show that the patterns converge to the apparently same effectively hyperuniform state regardless of their initial conditions.

M.A. Klatt, J. Lovrić, D. Chen, S. C. Kapfer, F. M. Schaller, P. W. A. Schönhöfer, B. S. Gardiner, A.-S. Smith, G. E. Schröder-Turk, S. Torquato. Universal hidden order in amorphous cellular geometries. Nature Communications 10, 811 (2019)

Partitioning space into cells with certain extreme geometrical properties is a central problem in many fields of science and technology. Here we investigate the Quantizer problem, defined as the optimisation of the moment of inertia of Voronoi cells, i.e., similarly-sized ‘sphere-like’ polyhedra that tile space are preferred. We employ Lloyd’s centroidal Voronoi diagram algorithm to solve this problem and find that it converges to disordered states associated with deep local minima. These states are universal in the sense that their structure factors are characterised by a complete independence of a wide class of initial conditions they evolved from. They moreover exhibit an anomalous suppression of long-wavelength density fluctuations and quickly become effectively hyperuniform. Our findings warrant the search for novel amorphous hyperuniform phases and cellular materials with unique physical properties.

On Oct 1st 2018, I joined the Department of Physics at Princeton University as a Postdoctoral Research Associate, continuing to combine mathematics and physics in my research on amorphous geometries, material physics, and spatial statistics.

We propose a class of goodness-of-fit tests for complete spatial randomness (CSR). In contrast to standard tests, our procedure utilizes a transformation of the data to a binary image, which is then characterized by geometric functionals. Under a suitable limiting regime, we derive the asymptotic distribution of the test statistics under the null hypothesis and almost sure limits under certain alternatives. The new tests are computationally efficient, and simulations show that they are strong competitors to other tests of CSR. The tests are applied to a real data set in gamma-ray astronomy, and immediate extensions are presented to encourage further work.

How do unique geometrical features of heterogeneous materials bring forth unique physical properties? Here, we study the link between the structure of sphere packings and a variety of physical properties using analytic approximations and rigorous bounds. In particular, we are interested in the most disordered among all mechanically stable packings of hard spheres (that are monodisperse and frictionless).

This maximally random jammed (MRJ) state exhibits an anomalous suppression of large-scale density fluctuations, known as hyperuniformity. Although the system is isotropic and locally disordered, it appears uniform on large scales. We compare the diffusion in the pore-space of the MRJ spheres to that of an equilibrium hard-sphere liquid or non-interacting spheres. Moreover, we study flow properties and effective conductivity; the latter also for anisotropic packings of spheroids. Therefore, we provide a comprehensive overview of rigorous bounds that connect these seemingly unrelated physical properties.

The most surprising result is found for electromagnetic waves that propagate through the MRJ sphere packings, where the wavelengths are much larger than the radii of the spheres. Usually disorder causes dissipation, but because of the unique property of hyperuniformity, the MRJ state forms, to a very good approximation, a dissipationless isotropic heterogeneous medium. This is demonstrated using an analytic strong-contrast expansion. It holds for any phase dielectric contrast ratio. The anomalous suppression of density fluctuations also suppresses the scattering of the electromagnetic waves.

This attribute could be useful for the design of photonic materials with novel structural color characteristics or color-sensing capabilities. Additive manufacturing fabrication techniques offer a simple production of samples for experiments with microwaves. So, our analytic results call for an experimental testing of these predicted qualitative trends in the physical properties associated with the MRJ structure.

The “Kulturpreis Bayern 2017” a price for culture and science awarded on October 26th 2017 by the Bayernwerk and the Federal State of Bavaria (Press Release in German).

We characterize the fluctuations in disordered systems of overlapping grains by calculating the second moments of a quite general class of robust shape descriptors. These include volume, surface area, Euler characteristic, and all other Minkowski functionals and tensors.

D. Hug, M. A. Klatt, G. Last, M. Schulte. Second order analysis of geometric functionals of Boolean models. In Lecture Notes in Mathematics Tensor Valuations and their Applications in Stochastic Geometry and Imaging (eds. E. B. Vedel Jensen and M. Kiderlen) 2177:339–383 (Springer International Publishing, 2017)

We especially focus on models of anisotropic heterogenous materials, that is, on anisotropic grain distributions.

How does the behaviour of the second moments and probability distributions differ for various models in finite systems for specific examples like aligned or isotropically
oriented rectangles? We derive explicit formulae, which are compared to the results of simulations. We show analytically that the asymptotic formulae for the thermodynamic limit are actually exact even for finite systems with periodic boundary conditions.

In the thermodynamic limit, the geometric functionals follow normal distributions, where the first and second moments are (explicitly) known, which could be used to construct hypothesis tests for random heterogeneous media.

Packings of hard, impenetrable spheres are useful models of granular media, low-temperature states of matter, suspensions and biological systems. What is the structure of the most disordered among all mechanical stable packings?

A unique property of this maximally random jammed (MRJ) state is that despite the local disorder, similar to a liquid, there is a hidden long-range order that anomalously suppresses density fluctuations on large length scales, more like in a crystalline solid. In a series of papers, we describe both the local and global structure of such disordered sphere packings using a variety of different structural characteristics.

In this second article, we derive explicit formulas but also apply Monte Carlo methods. By comparing the structure of MRJ packings to common models of disordered materials, our shape analysis helps to distinguish, despite seemingly similar features in all of those systems, their distinctly different structure.

Moreover, these structural characteristics are related to a host of different effective physical behavior, for example, flow or diffusion in these systems as well as their elastic moduli or electromagnetic properties. Our analysis thus links problems from material science, chemistry, physics, mathematics and biology.

The gyroid is an ordered network-like labyrinth bounded by minimal surfaces. It has become a house-hold name in soft materials with order on the nanometer scale, for example in the nanoporous photonic crystals of some green butterflies.

We here find by simulation that the same spatial gyroid structure forms spontaneously in nuclear matter at finite temperatures, as is prevalent in supernova explosions. While the structure of the gyroid in nuclear matter is the same as in soft materials, the length scale of a few femtometers is radically different, making this the discovery of the smallest reported gyroid found in dynamical simulations. The state of nuclear matter at this high nuclear density will greatly affect the neutrino transport during and after a supernova-explosion and is thus important to understand the production of heavy elements.