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Soft Matter and Biological Physics


Eur. Phys. J. E 8, 507-515 (2002)
DOI: 10.1140/epje/i2002-10034-0

Impurity in a Maxwellian unforced granular fluid

E. Ben-Naim1 and P.L. Krapivsky2

1  Theoretical Division and Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
2  Center for Polymer Studies and Department of Physics, Boston University, Boston, MA 02215, USA

ebn@lanl.gov

(Received 5 March 2002 and Received in final form 26 June 2002 Online publication 1 October 2002)

Abstract
We investigate velocity statistics of an impurity immersed in a uniform granular fluid. We consider the cooling phase, and obtain scaling solutions of the inelastic Maxwell model analytically. First, we analyze identical fluid-fluid and fluid-impurity collision rates. We show that light impurities have similar velocity statistics as the fluid background, although their temperature is generally different. Asymptotically, the temperature ratio increases with the impurity mass, and it diverges at some critical mass. Impurities heavier than this critical mass essentially scatter off a static fluid background. We then analyze an improved inelastic Maxwell model with collision rates that are proportional to the average fluid-fluid and fluid-impurity relative velocities. Here, the temperature ratio remains finite, and the system is always in the light-impurity phase. Nevertheless, ratios of sufficiently high-order moments $\langle v^n_{\rm impurity}\rangle/\langle
v^n_{\rm fluid}\rangle$ may diverge, a consequence of the multiscaling asymptotic behavior.

PACS
05.20.Dd - Kinetic theory.
02.50.-r - Probability theory, stochastic processes, and statistics.
47.70.Nd - Nonequilibrium gas dynamics.
45.70.Mg - Granular flow: mixing, segregation and stratification.


© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2002