# Browsing by Author "Schröter, Matthias"

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Item Open Access A local view on the role of friction and shape(EPJ Web of Conferences, 2017) Schröter, MatthiasLeibniz said "Naturam cognosci per analogiam": nature is understood by making analogies. This statement describes a seminal epistemological principle. But one has to be aware of its limitations: quantum mechanics for example at some point had to push Bohr's model of the atom aside to make progress. This article claims that the physics of granular packings has to move beyond the analogy of frictionless spheres, towards local models of contact formation.Item Open Access Analyzing X-ray tomographies of granular packings.(The Review of scientific instruments, 2017-05) Weis, Simon; Schröter, MatthiasStarting from three-dimensional volume data of a granular packing, as, e.g., obtained by X-ray Computed Tomography, we discuss methods to first detect the individual particles in the sample and then analyze their properties. This analysis includes the pair correlation function, the volume and shape of the Voronoi cells, and the number and type of contacts formed between individual particles. We mainly focus on packings of monodisperse spheres, but we will also comment on other monoschematic particles such as ellipsoids and tetrahedra. This paper is accompanied by a package of free software containing all programs (including source code) and an example three-dimensional dataset which allows the reader to reproduce and modify all examples given.Item Open Access Correction of beam hardening in X-ray radiograms.(The Review of scientific instruments, 2019-02) Baur, Manuel; Uhlmann, Norman; Pöschel, Thorsten; Schröter, MatthiasThe intensity of a monochromatic X-ray beam decreases exponentially with the distance it has traveled inside a material; this behavior is commonly referred to as Beer-Lambert's law. Knowledge of the material-specific attenuation coefficient μ allows us to determine the thickness of a sample from the intensity decrease the beam has experienced. However, classical X-ray tubes emit a polychromatic bremsstrahlung-spectrum. And the attenuation coefficients of all materials depend on the photon energy: photons with high energy are attenuated less than photons with low energy. In consequence, the X-ray spectrum changes while traveling through the medium; due to the relative increase in high energy photons, this effect is called beam hardening. For this varying spectrum, the Beer-Lambert law only remains valid if μ is replaced by an effective attenuation coefficient μeff which depends not only on the material but also on its thickness x and the details of the X-ray setup used. We present here a way to deduce μeff(x) from a small number of auxiliary measurements using a phenomenological model. This model can then be used to determine an unknown material thickness or in the case of a granular media its volume fraction.Item Open Access Correlation between Voronoi volumes in disc packings(EPL (Europhysics Letters), 2012-02-01) Zhao, Song-Chuan; Sidle, Stacy; Swinney, Harry L; Schröter, MatthiasWe measure the two-point correlation of free Voronoi volumes in binary disc packings, where the packing fraction $\phi_{\rm avg}$ ranges from 0.8175 to 0.8380. We observe short-ranged correlations over the whole range of $\phi_{\rm avg}$ and anti-correlations for $\phi_{\rm avg}>0.8277$. The spatial extent of the anti-correlation increases with $\phi_{\rm avg}$ while the position of the maximum of the anti-correlation and the extent of the positive correlation shrink with $\phi_{\rm avg}$. We conjecture that the onset of anti-correlation corresponds to dilatancy onset in this system.Item Open Access Influence of humidity on tribo-electric charging and segregation in shaken granular media.(Soft matter, 2017-01) Schella, André; Herminghaus, Stephan; Schröter, MatthiasWe study the effect of humidity on the charge accumulation of polymer granulates shaken vertically in a stainless steel container. This setup allows us to control the humidity level from 5% to 100%RH while performing automated charge measurements in a Faraday cup directly connected to the shaking container. We find that samples of approximately 2000 polymer spheres become highly charged at low humidity levels (<30%RH), but acquire almost no charge for humidity levels above 80%RH. The transition between these two regimes does depend on the material, as does the sign of the charge. For the latter we find a correlation with the contact angle of the polymer with only very hydrophilic particles attaining positive charges. We show that this humidity dependence of tribo-charging can be used to control segregation in shaken binary mixtures.Item Open Access Measuring the configurational temperature of a binary disc packing.(Soft matter, 2014-06) Zhao, Song-Chuan; Schröter, MatthiasJammed packings of granular materials differ from systems normally described by statistical mechanics in that they are athermal. In recent years a statistical mechanics of static granular media has emerged where the thermodynamic temperature is replaced by a configurational temperature X which describes how the number of mechanically stable configurations depends on the volume. Four different methods have been suggested to measure X. Three of them are computed from properties of the Voronoi volume distribution, the fourth takes into account the contact number and the global volume fraction. This paper answers two questions using experimental binary disc packings: first we test if the four methods to measure compactivity provide identical results when applied to the same dataset. We find that only two of the methods agree quantitatively. This implies that at least two of the four methods are wrong. Secondly, we test if X is indeed an intensive variable; this becomes true only for samples larger than roughly 200 particles. This result is shown to be due to recently measured correlations between the particle volumes [Zhao et al., Europhys. Lett., 2012, 97, 34004].Item Open Access Nucleation in Sheared Granular Matter.(Physical review letters, 2018-02) Rietz, Frank; Radin, Charles; Swinney, Harry L; Schröter, MatthiasWe present an experiment on crystallization of packings of macroscopic granular spheres. This system is often considered to be a model for thermally driven atomic or colloidal systems. Cyclically shearing a packing of frictional spheres, we observe a first order phase transition from a disordered to an ordered state. The ordered state consists of crystallites of mixed fcc and hcp symmetry that coexist with the amorphous bulk. The transition, initiated by homogeneous nucleation, overcomes a barrier at 64.5% volume fraction. Nucleation consists predominantly of the dissolving of small nuclei and the growth of nuclei that have reached a critical size of about ten spheres.Item Open Access Stationary state volume fluctuations in a granular medium.(Physical review. E, Statistical, nonlinear, and soft matter physics, 2005-03-30) Schröter, Matthias; Goldman, Daniel I; Swinney, Harry LA statistical description of static granular material requires ergodic sampling of the phase space spanned by the different configurations of the particles. We periodically fluidize a column of glass beads and find that the sequence of volume fractions phi of postfluidized states is history independent and Gaussian distributed about a stationary state. The standard deviation of phi exhibits, as a function of phi, a minimum corresponding to a maximum in the number of statistically independent regions. Measurements of the fluctuations enable us to determine the compactivity X , a temperaturelike state variable introduced in the statistical theory of Edwards and Oakeshott [Physica A 157, 1080 (1989)].Item Open Access Structural similarity between dry and wet sphere packings(New Journal of Physics) Weis, Simon; Schröder-Turk, Gerd E; Schröter, MatthiasThe mechanical properties of granular materials change significantly in the presence of a wetting liquid which creates capillary bridges between the particles. Here we demonstrate, using X-ray tomographies of dry and wet sphere packings, that this change in mechanical properties is not accompanied by structural differences between the packings. We characterize the latter by the average numbers of contacts of each sphere $\langle Z\rangle$ and the shape isotropy $\langle \beta_0^{2,0} \rangle$ of the Voronoi cells of the particles. Additionally, we show that the number of liquid bridges per sphere $\langle B\rangle$ is approximately equal to $\langle Z\rangle + 2$, independent of the volume fraction of the packing. These findings will be helpful in guiding the development of both particle-based models and continuum mechanical descriptions of wet granular matter.