Regensburg 2004 – wissenschaftliches Programm

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DY: Dynamik und Statistische Physik

DY 44: Granular Systems

DY 44.1: Hauptvortrag

Donnerstag, 11. März 2004, 14:30–15:00, H2

Phase transitions and segregation phenomena in granular systems — •Christof Krülle — Experimentalphysik V, Universität Bayreuth, 95440 Bayreuth

For a collection of macroscopic particles, which are agitated by external forces to perform stochastic movements, a granular temperature can be defined as mean kinetic energy in the center-of-mass system. For describing the physical properties of this ensemble one can ask – in analogy to thermodynamic phase transitions: (i) Are there critical temperatures at which the internal structure undergoes qualitative changes? (ii) Which order parameters characterize the dynamics of the transition? (iii) What are the consequences for granular mixtures? We present results from experiments in granular systems, which are excited by vertical and horizontal vibrations [1,2]. The transitions between different dynamic states depend on internal properties of the granulate (e.g. the density of particles) as well as external parameters of the driving shaker. Peculiar to granular systems are counterintuitive phenomena, like the crystallization by increasing the vibration amplitude and thereby the energy input, or the rise of large particles in a sea of smaller ones (Brazil-nut effect). For horizontal shaking of a bidisperse system the demixing of small and large particles is found to occur at the same critical particle density as the liquid-solid transition, which leads to the conclusion that both phenomena, segregation and phase transition, are closely related. [1] A.P.J. Breu, H.-M. Ensner, C.A. Kruelle, and I. Rehberg, Phys. Rev. Lett. 90, 014302 (2003), [2] S. Aumaître, T. Schnautz, C.A. Kruelle, and I. Rehberg, Phys. Rev. Lett. 90, 114302 (2003).