Kinetic Theory of Viscosity of Compressed Fluids

Abstract

THE VISCOSITY OF A COMPRESSED GAS IS A FUNCTION OF THREE VARIABLES: (1) the degree of crowding of the molecules; (2) their capacity, by reason of softness, flexibility, or rotational inertia, to absorb the vector momentum applied to cause flow; (3) the resistance to this vector momentum offered by the randomly oriented thermal momenta, which becomes significant when the liquid expands sufficiently to permit molecular mean free paths between binary collisions to be long enough for thermal momenta to acquire fractions of their thermal momentum in free space.The fluidity varphi of simple liquids obeys the linear equation varphi = B(V - V(0))/V(0) and its viscosity is, therefore, eta(a) = V(0)/B(V - V(0)); this accounts for components 1 and 2. The contribution of random thermal momenta, 3, obeys the equation, eta(b) = eta(0)(1 - V(t)/V). eta(0) is the viscosity of the dilute gas; V(t) is the molal volume at which the thermal contribution begins. The total momentum, eta = eta(a) + eta(b).Values of eta(0) vary linearly with T(1/2). Values of V(t) are related to heat capacities.