Philip Born

Enhanced granular medium-based tube and hollow profile press hardening

Abstract Active and passive control strategies of internal pressure for hot forming of tubes and hollow profiles with granular media are described. Force transmission and plastic deformation of granular medium is experimentally investigated. Friction between tube, granular medium and die, and the external stress field are shown to be essential for the process understanding. Wrinkling, thinning and insufficient forming of the tube establishes the process window for the active pressure process. By improving the punch geometry and controlling tribological conditions, the process limits are extended. Examples for the passive pressure process reveal new opportunities for hot forming of tubes and hollow profiles.

Preface: Focus on imaging methods in granular physics

Axelle Amon, Philip Born, Karen E. Daniels, Joshua A. Dijksman, Kai Huang, David Parker, Matthias Schröter, Ralf Stannarius, and Andreas Wierschem Institut de Physique de Rennes, UMR UR1-CNRS 6251, Université de Rennes 1, 35042 Rennes, France Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luftund Raumfahrt, 51170 Cologne, Germany Department of Physics, North Carolina State University, Raleigh, NC, 27695 USA Physical Chemistry and Soft Matter, Wageningen University & Research, Wageningen, The Netherlands Experimentalphysik V, Universität Bayreuth, 95440 Bayreuth, Germany School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK Institute for Multiscale Simulation, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 91052 Erlangen, Germany. Institut für Experimentelle Physik, Otto-von-Guericke-Universität, 39106 Magdeburg, Germany Institute of Fluid Mechanics, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 91058 Erlangen, Germany

Particle characterization using THz spectroscopy

THz extinction spectroscopy extends UV–Vis and NIR-spectroscopy to characterize particles from fine powders and dust to sand, grains and granulated materials. We extract particle sizes from the spectral position of the first peak of the interference structure and size distributions from the visibility of the fine ripple structure in the measured extinction spectra. As such, we can demonstrate a route for a quick determination of these parameters from single measurements.

Granular structure determined by terahertz scattering

Light scattering from particles reveals static and dynamical information about the particles and their correlations. Such methods are particularly powerful when the wavelength of the light is chosen similar to the sizes and distances of the particles. To apply scattering to investigate granular matter in particular —or other objects of similar submillimeter size— light of suitable wavelength in the terahertz regime needs to be chosen. By using a quantum cascade laser in a benchtop setup we determine the angle-dependent scattering of spherical particles as well as coffee powder and sugar grains. The scattering from single particles can be interpreted by form factors derived within the Mie theory. In addition, collective correlations can be extracted as static structure factors and compared to recent computer simulations.

Analysis of granular packing structure by scattering of THz radiation

Scattering methods are widely used to characterize the structure and constituents of matter on small length scales. This motivates this introductory text on identifying prospective approaches to scattering-based methods for granular media. A survey to light scattering by particles and particle ensembles is given. It is elaborated why the established scattering methods using X-rays and visible light cannot in general be transferred to granular media. Spectroscopic measurements using terahertz radiation are highlighted as they probe the scattering properties of granular media, which are sensitive to the packing structure. Experimental details to optimize a spectrometer for measurements on granular media are discussed. We perform transmission measurements on static and agitated granular media using Fourier transform spectroscopy at the THz beamline of the Bessy II storage ring. The measurements demonstrate the potential to evaluate degrees of order in the media and to track transient structural states in agitated bulk granular media.

Probing density waves in fluidized granular media with diffusing-wave spectroscopy

Density waves are characteristic for fluidized beds and affect measurements on liquidlike dynamics in fluidized granular media. Here the intensity autocorrelation function as obtainable with diffusing-wave spectroscopy is derived in the presence of density waves. The predictions by the derived form of the intensity autocorrelation function match experimental observations from a gas-fluidized bed. The model suggests separability of the contribution from density waves from the contribution by microscopic scatterer displacement to the decay of correlation and thus paves the way for characterizing microscopic particle motions using diffusing-wave spectroscopy as well as heterogeneities in fluidized granular media.

Drop Tower Setup for Dynamic Light Scattering in Dense Gas-Fluidized Granular Media

Investigation of dynamics in dense granular media is challenging. Here we present a setup that facilitates gas fluidization of dense granular media in microgravity. The dynamics is characterized using diffusing wave spectroscopy. We demonstrate that agitated granular media reach a steady state within fractions of a second in drop tower flights. The intensity autocorrelation functions obtained in microgravity show a remarkable dependence on sample volume fraction and driving strength. A plateau in correlation emerges at low volume fractions and strong driving, while correlation decays only very slowly but continuously at high packing fractions. The setup allows to independently set sample volume fraction and driving strength, and thus extends the possibilities for investigations on dynamics in dense granular on ground.

Dense fluidized granular media in microgravity

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 granular media. It is desirable to provide experiments in which selectively one of the granular characteristics is tuned to test suitable adaptation of a theory. Here we show that gas fluidization of granular media in microgravity is a suitable approach to achieve steady states closer to thermally agitated systems free of settling. We use diffusing-wave spectroscopy to compare the spatial homogeneity and the microscopic dynamics of gas-fluidized granular media on the ground and in drop tower flights with increasing packing densities up to full arrest. The gas fluidization on the ground leads to inhomogeneous states as known from fluidized beds, and partial arrest occurs at packing fractions lower than the full arrested packing. The granular medium in microgravity in contrast attains a homogeneous state with complete mobilization even close to full arrest. Fluidized granular media thus can be studied in microgravity with dynamics and packing fractions not achievable on the ground.Granular matter: Coming to rest in low gravityMicrogravity provides a better environment to study how grains move when agitated, scientists in Germany show. Philip Born and co-workers from the Institute of Materials Physics in Space demonstrate that microgravity analogue conditions enable studies on particle dynamics at packing densities not achievable on the ground. Dense granular media under external agitation tends to partially arrest on the ground due to particle collision and settling caused by gravity. This diverseness of the medium makes it hard to develop a theory that can describe the general behavior. Born and colleagues use diffusing-wave spectroscopy to compare the spatial homogeneity and microscopic dynamics of granular media on the ground and in drop towers. They show that unlike on the ground, granular media in microgravity conditions can reach a homogeneous state without partial arrest at high packing densities.