Abstract

MOLECULAR PHYSICS, 1976, VOL. 32, No. 4, 1171-1176 by DENIS J. EVANS Physical Chemistry Laboratory, South Parks Road, Oxford OXl 3OZ, England t (Received 4 June 1976) 1. INTRODUCTION Over the years a steady stream of papers have concerned themselves with the transport processes in fluids consisting of rigid molecules as opposed to those consisting of ' point ' particles. The key feature of this generalization is that rigid molecules can exchange internal angular momentum in addition to transla- tional momenta and energies. In this note we make two points concerning the generalized hydrodynamics of such fluids. Recently there has been uncertainty concerning the statistical mechanical expression for the angular momentum flux tensor, or couple tensor. A series of papers has appeared [1-4] giving conflicting statistical mechanical expressions for the couple tensor. In this note we give a direct derivation of the c6uple tensor using a method which avoids the well-known ambiguities of the indirect Irving and Kirkwood scheme [5] or any of its variations [6]. In the second part of this note we show, using statistical mechanical argu- ments, that in a fluid where the single particle distribution function is isotropic in orientation space, the pressure tensor is