# Matthias SperlGerman Aerospace Center (DLR) | DLR · Institute of Material Physics in Space

Matthias Sperl

Professor

## About

178

Publications

26,640

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2,629

Citations

Introduction

Playing with sand in microgravity, establishing a microscopic basis for granular rheology, doing proper statistical phyics

Additional affiliations

March 2011 - present

April 2009 - September 2009

March 2007 - February 2011

Education

February 2000 - March 2003

January 1999 - January 2000

July 1998 - December 1998

## Publications

Publications (178)

A three-dimensional granular gas of ellipsoids is established by exposing the system to the microgravity environment of the International Space Station. We use two methods to measure the dynamics of the constituent particles and report the long-time development of the granular temperature until no further particle movement is detectable. The result...

The macroscopic response of granular solids is determined by the microscopic fabric of force chains, which, in turn, is intimately linked to the history of the solid. To query the influence of gravity on powder flow-behavior, a granular material is subjected to compression by a piston in a closed container, on-ground and in microgravity. Results sh...

We describe an experiment container with light scattering and imaging diagnostics for experiments on soft matter aboard the International Space Station (ISS). The suite of measurement capabilities can be used to study different materials in exchangeable sample cell units. The currently available sample cell units and future possibilities for foams,...

We investigate, using a recently developed model of liquid state theory describing the rheology of dense granular flows, how a yield stress appears in granular matter at the yielding transition. Our model allows us to predict an analytical equation of the corresponding dynamical yield surface, which is compared to usual models of solid fracture. In...

We present a methodology to characterize the light intensity fluctuations that arise from rotations of individual granular particles. We describe a setup for dynamic light scattering measurements on individual macroscopic particles and isolate the contribution from rotations of the individual particles to the obtained correlation functions. The res...

The Granular Integration Through Transients (GITT) formalism gives a theoretical description of the rheology of moderately dense granular flows and suspensions. In this work, we extend the GITT equations beyond the case of simple shear flows studied before. Applying this to the particular example of extensional flows, we show that the predicted beh...

Multiple studies have been performed assessing the possibility to use lunar regolith, processed in diverse methods, to store heat within it and re-use it at a proper time either for heating purposes or heat-to-electricity conversion. These studies focused on the overall system concept and its general performances assessment. The project MESG Moon E...

Multiple studies have been performed assessing the possibility to use lunar regolith, processed in diverse methods, to store heat within it and re-use it at a proper time either for heating purposes or heat-to-electricity conversion. These studies focused on the overall system concept and its general performances assessment. The project MESG Moon E...

Force networks play an important role in the stability of configurations when granular material is packed into a container. These networks can redirect part of the weight of grains inside a container to the side walls. We employ monodisperse stress-birefringent spheres to visualize the contact forces in a quasi-2D and a nearly-2D configuration of t...

The Granular Integration Through Transients (GITT) formalism gives a theoretical description of the rheology of moderately dense granular flows and suspensions. In this work, we extend the GITT equations beyond the case of simple shear flows studied before. Applying this to the particular example of extensional flows, we show that the predicted beh...

We present 3D DEM simulations of jammed bidisperse granular packings to investigate their jamming density, $\phi_J$, and bulk modulus, $K$, as a function of the size ratio, $\delta$, and concentration of small particles, $X_{\mathrm S}$. We determine the partial and total bulk modulus for each packing and obtain a transition behavior at specific de...

A magnetic thermostat employing soft-ferromagnetic particles and a varying magnetic field has been developed to investigate a homogeneous granular gas system in microgravity. While the thermostat’s mechanism of creating homogeneous distribution of the particles was shown earlier, its characteristics have not been understood well due to limited acce...

Magnetic forces are used to heat up thousands of spherical particles under low-gravity. This long range external excitation, combined with the induced particle-particle interactions, results in a homogeneous spatial distribution of the particles. Comparisons with predictions of kinetic theories can hence be carried out. Haff’s cooling law is verifi...

We present numerical evidence for an additional discontinuous transition, upon compression, inside the jammed regime for an asymmetric bidisperse granular packing. This additional transition line separates jammed states with networks of predominantly large particles from jammed networks formed by both large and small particles, and the transition i...

This work generalizes the granular integration through transients formalism introduced by Kranz et al. [Phys. Rev. Lett. 121, 148002 (2018)] to the determination of the pressure. We focus on the Bagnold regime and provide theoretical support to the empirical μ(I) rheology laws that have been successfully applied in many granular flow problems. In p...

The law of $\mu(\mathcal{I})$ rheology has proven very successful to describe the rheology of granular liquids. It however lacks theoretical support. From the granular integration through transient formalism, we extract a simplified model that expresses the rheology of granular liquids as a competition between three time scales. In particular, this...

This work generalises the granular integration through transients formalism introduced by Kranz et al. [Phys. Rev. Lett. 121, 148002 (2018)] to the determination of the pressure. We focus on the Bagnold regime, and provide theoretical support to the empirical $\mu(\mathcal{I})$ rheology laws, that have been successfully applied in many granular flo...

In contrast to molecular gases, granular gases are characterized by inelastic collisions and require therefore permanent driving to maintain a constant kinetic energy. The kinetic theory of granular gases describes how the average velocity of the particles decreases after the driving is shut off. Moreover, it predicts that the rescaled particle vel...

In contrast to molecular gases, granular gases are characterized by inelastic collisions and require therefore permanent driving to maintain a constant kinetic energy. The kinetic theory of granular gases describes how the average velocity of the particles decreases after the driving is shut off. Moreover it predicts that the rescaled particle velo...

Transport properties of a hard-sphere colloidal fluid are investigated by Brownian dynamics simulations. We implement a novel algorithm for the time-dependent velocity-autocorrelation function (VACF) essentially eliminating the noise of the bare random motion. The measured VACF reveals persistent anti-correlations manifested by a negative algebraic...

We investigate the spatial structure of dense square-shoulder fluids. To this end, we derive analytical perturbative solutions of the Ornstein–Zernike equation in the low- and high-temperature limits as expansions around the known hard sphere solutions. We then discuss the suitability of perturbative approaches in relation to the Ornstein–Zernike e...

In Paper I [O. Coquand and M. Sperl, J. Chem. Phys. 152, 124112 (2020)], we derived analytical expressions for the structure factor of the square-shoulder potential in a perturbative way around the high- and low-temperature regimes. Here, various physical properties of these solutions are derived. In particular, we investigate the large wave number...

The European Astronaut Centre (EAC) is currently constructing the European Lunar Exploration Laboratory (LUNA), a large training and operations facility to be located adjacent to EAC at the DLR (German Aerospace Centre) campus in Cologne, Germany. With an estimated representative lunar testbed area of approximately 660 m2, a large volume of lunar r...

Elastic properties of a granular packing show a nonlinear behavior determined by its discrete structure and nonlinear inter-grain force laws. Acoustic waves show a transition from constant, pressure-dependent sound speed to a shock-wave-like behavior with an amplitude-determined propagation speed. This becomes increasingly visible at low static con...

We compute the rheological properties of inelastic hard spheres in steady shear flow for general shear rates and densities. Starting from the microscopic dynamics we generalize the Integration Through Transients formalism to a fluid of dissipative, randomly driven granular particles. The stress relaxation function is computed approximately within a...

Elastic properties of a granular packing show nonlinear behavior determined by its discrete structure and nonlinear inter-grain force laws. Acoustic waves show a transition from constant, pressure-dependent sound speed to a shock-wave like behavior with amplitude-determined propagation speed. This becomes increasingly visible at low static confinem...

We present numerical evidence for an additional discontinuous transition inside the jammed regime for an asymmetric bidisperse granular packing upon compression. This additional transition line separates jammed states with networks of predominantly large particles from jammed networks formed by both large and small particles, and the transition is...

Lunar regolith simulant is exposed to a fast neutron flux of a spallation source. The samples consisted of a set of as-received, traditionally sintered, and solar sintered JSC-2A lunar regolith simulant. Aluminium samples were also irradiated for comparison. The ChipIr beamline of the ISIS neutron and muon source was used to provide a flux of atmos...

We investigate the spatial structure of dense square-shoulder fluids. To this end we derive analytical perturbative solutions of the Ornstein-Zernike equation in the low- and high-temperature limits as expansions around the known hard sphere solutions. We then discuss the suitability of perturbative approaches in relation to the Ornstein-Zernike eq...

In a companion paper, we derived analytical expressions for the structure factor of the square-shoulder potential in a perturbative way around the high- and low-temperature regimes. Here, various physical properties of these solutions are derived. In particular, we investigate the large wave number sector, and relate it to the contact values of the...

A thermostat utilizing a varying magnetic field has been developed to agitate soft ferromagnetic particles in microgravity platforms for an investigation of an energy-dissipative granular gas. Although the method has experimentally realized a reasonably homogeneous spatial distribution of particles, the physics behind the magnetically excited parti...

Additive manufacturing (AM) is one of the most promising techniques for on-site manufacturing on extraterrestrial bodies. In this investigation, layerwise solar sintering under ambient and vacuum conditions targeting lunar exploration and a moon base was studied. A solar simulator was used in order to enable AM of interlockable building elements ou...

Additive manufacturing (AM) is one of the most promising techniques for on-site manufacturing on extraterrestrial bodies. In this investigation, layerwise solar sintering under ambient and vacuum conditions targeting lunar exploration and a moon base was studied. A solar simulator was used in order to enable AM of interlockable building elements ou...

The bond orientational order parameters originally introduced by Steinhardt et al. (Phys. Rev. B 28, 784 (1983)) are a common tool for local structure characterization in soft matter studies. Recently, Mickel et al. (J. Chem. Phys. 138, 044501 (2013)) highlighted problems of the bond orientational order parameters due to the ambiguity of the underl...

Transport properties of a hard-sphere colloidal fluid are investigated by Brownian dynamics simulations. We implement a novel algorithm for the time-dependent velocity-autocorrelation function (VACF) essentially eliminating the noise of the bare random motion. The measured VACF reveals persistent anti-correlations manifested by a negative algebraic...

Ordered two-dimensional arrangements of triboelectrically oppositely charged granular particles have been reported several times, but observations of bulk ordered binary granular particle packings are singular. We attribute this suppression of triboelectrically induced order to the concurrent behaviour of granular particles to pack densest due to g...

The bond orientational order parameters originally introduced by Steinhardt \emph{et. al.} [Phys. Rev. B \textbf{28}, 784 (1983)] are a common tool for local structure characterization in soft matter studies. Recently, Mickel \emph{et. al.} [J. Chem. Phys. \textbf{138}, 044501 (2013)] highlighted problems of the bond orientational order parameters...

The capability to manufacture items in space is an exploration enabling advancement, and will be crucial for sustainable human exploration as we progress beyond Earth orbit. The extrusion based Fused Filament Fabrication (FFF) method using thermoplastics represents a robust and simple methodology applicable to printing parts for both current and fu...

Microwave heating has emerged as a feasible sintering and melting method for lunar regolith simulant. Unlike conventional heating methods, microwaves operating at 2.45 GHz penetrate the material and drive a dielectric heating process within the material volume itself. This property makes microwave sintering a viable and ideal candidate for localize...

Due to the undesired impact of gravity, experimental studies of energy-dissipative gaseous systems are difficult to carry out on ground. In the past several years, we developed a series of experimental devices suitable for various kinds of microgravity platforms. The central idea adopted in our devices is to use long-range magnetic forces to excite...

In this paper, the thermal characterization of lunar regolith simulant, sintered using a conventional oven under ambient and vacuum conditions is presented. Additionally, thermal characterization of samples is performed for the parts manufactured using solar, laser and microwave processing. Samples for oven sintering are prepared using regolith bul...

Applying in-situ resources combined with Additive Manufacturing allows for on-demand fabrication in space. Regarding the powder-based Additive Manufacturing, wettability of the melt material affects the properties of the final product. In this study, viscosity temperatures, dilatometric sintering curves and contact angles of JSC-2 lunar simulant ar...

The Fredrickson-Andersen (FA) model---a kinetically constrained lattice model---displays an ergodic to non-ergodic transition with a slow two-step relaxation of dynamical correlation functions close to the transition point. We derive an asymptotically exact solution for the dynamical occupation correlation function of the FA model on the Bethe latt...

The Moon, largely considered as the next stepping stone on the path of extending mankind’s reach beyond the ISS, is the focus of many national and international campaigns for manned exploration of the solar system. To enable and safeguard the survival of habitation and technological assets on the surface (facing nights lasting up to 14 Earth days a...

Assess the potential of thermal energy storage systems to store heat and re-use it for in-situ electricity production as means of supporting future lunar exploration scenarios.

The authors demonstrated the feasibility of sintering lunar regolith layer-by-layer solely using concentrated sunlight. First trials using actual concentrated sunlight led to an inhomogeneous sintering of individual layers of lunar regolith simulant, due to the significant flux density variations caused by atmospheric fluctuations. Tests focusing X...

Energy dissipation is one of the most important properties of granular gas, which makes its behavior different from that of molecular gas. In this work we report our investigations on the freely-cooling evolution of granular gas under microgravity in a drop tower experiment, and also conduct the molecular dynamics (MD) simulation for comparison. Wh...

The project MESG: Moon Energy Storage and Generation, under development for ESA, targets the thermally challenging missions on the surface of the Moon, investigating the possibility to use in situ resources, i.e. processed lunar regolith using available source of radiation and energy to "build" an efficient thermal mass to be used to store within h...

A new experimental facility has been designed and constructed to study driven granular media in a low-gravity environment. This versatile instrument, fully automatized, with a modular design based on several interchangeable experimental cells, allows to investigate research topics ranging from dilute to dense regimes of granular media such as granu...

MESG project aims to assess the usage of processed Moon regolith to store heat, to produce electricity and to release heat during the lunar night, allowing extended operations on the Moon surface or simplified designs (reduced batteries, masses) for users such as lunar rovers, landers and human outposts.

The mechanisms underlying triboelectric charging have a stochastic nature. We investigate how this randomness affects the distributions of charges generated on granular particles during either a single or many collisions. The charge distributions we find in our experiments are wider than normal distributions with an exponential decay of the probabi...

Handling and transport of granular media are inevitably governed by the settling of particles. Settling into a dense state is one of the defining characteristics of granular media, among dissipation and absence of thermal agitation. Hence, settling complicates the adaptation of microscopic theories from atomic, molecular, or colloidal media to gran...

Future missions to the Moon will require the utilization of the local resources in order to make them affordable. The EU-funded RegoLight project advances existing 3D printing technologies and methodologies for the purpose of shaping lunar regolith, a readily available resource on the Moons surface, through the means of concentrated sunlight that s...

We discuss the uniform shear flow of a fluidized granular bed composed of monodisperse Hertzian spheres. Considering high densities around the glass transition density of inelastic Hertzian spheres, we report kinetic theory expressions for the Newtonian viscosity as well as the Bagnold coefficient. We discuss the dependence of the transport coeffic...

The Integration Through Transients (ITT) formalism provides a kinetic theory to go beyond the linear response regime of low density and shear rate. Its main advantage is to express observables under shear as expectation values in the quiescent system. Here we show that the quiescent fluid does not have to be in thermal equilibrium. Considering a fl...

Considering a granular fluid of inelastic smooth hard spheres we discuss the conditions delineating the rheological regimes comprising Newtonian, Bagnoldian, shear thinning, and shear thickening behavior. Developing a kinetic theory, valid at finite shear rates and densities around the glass transition density, we predict the viscosity and Bagnold...