MOTION OF SMALL PARTICLES IN NON-NEWTONIAN FLUIDS

There are many problems in which the motion of a small particle, bubble or drop in a non-Newtonian fluid differs in an important qualitative way from its corresponding motion in a Newtonian fluid. From a theoretical point of view such problems are conveniently separated into two groups. In the first, some aspect of the particle's motion only exists, for small Reynolds number, because the suspending fluid is non-Newtonian. The second major class of problems is that in which the observed difference between Newtonian and non-Newtonian behavior is due to an important, O(1) change in the fluid motion at all times. In this case, the only possible theoretical description which is valid in more than an asymptotic sense, is one based on a full nonlinear constitutive model, including ″memory″ , and thus a solution of the equations of motion is generally possible only via numerical methods. Unlike the first class of problems, an important determining factor in successful match between experiment and theory is therefore a judicious choice of the constitutive model. In the second part of this paper, some examples of numerical and experimental studies are discussed which pertain to particle motions in the regime of strongly viscoelastic flows.