A critical review of the drag force on a sphere in the transition flow regime

Improvements in micro-fabrication techniques are enabling Micro-Electro-Mechanical-Systems to be constructed with sub-micron feature sizes. At this scale, even at standard atmospheric conditions, the flow is in the transition regime. This paper considers the range 0.1 < Kn < 1, where non-equilibrium effects can be appreciable, and specifically illustrates problems involving non-planar surfaces by analyzing the drag force for low speed, incompressible flow past an unconfined microsphere. A critical comparison is made between experimental data, analytical solutions derived from kinetic theory, Grad's thirteen-moment equations, and the Navier-Stokes equations with first- and second-order treatment of the slip boundary. The results, even for this simple geometry, highlight major problems in predicting the drag in the transition flow regime for non-kinetic schemes.