ROTATING-DISK-SAMPLER MEASUREMENT OF AEROSOLS N(r) =f- n(r) dr 223 (1) For this work it is convenient to divide the size spectrum into two regions: the upper end for particles larger than 0.2 » in diameter and the lower end for those smaller than 0.2 , in diameter. A simple new method was used to sample the lower end of the distribution, Particles were collected from room air by Brownian diffusion to an electron- microscope grid placed at the center of a l-in. stainless-steel disk rotated about the axis normal to its face. The results of the rotatingdisk study are reported in this paper. THEORY The theory of convective diffusion to a rotating disk has been worked out for the laminar flow regimeby Levich’ based on the velocity distribution derived from the theory of von Karman, This velocity dis- tribution represents one of the few exact solutions of the Navier—Stokes equations of fluid motion. A striking result of the theory of the fluid motion is that the component of the velocity normal to the disk is a function only of distance from the disk surface and not of distance from the axis of rotation or angular position, As a result, Levich noted, there exists a solution to the equation of convective diffusion in which the concentration is a function only of distance from the disk surface. Moreover, the diffusion flux is uniform over the surface of the disk and is given by the following expression: oe = 0.62 v-"D4we (2) where J = flux of matter, moles or particles/cm?/sec C,, concentration at infinity C, concentration at disk surface v = kinematic viscosity D = diffusion coefficient of the species with flux J wW = rotational speed of the disk This expression is derived for the limiting-case Schmidt number = v/D— , but Eq. 2 is accurate® to within 7% of a more exact solution whenv/D is greater than 100. This limit corresponds to particles larger than about 0.0035 u in radius under ordinary atmospheric conditions. The theory is also limited to the laminar flow regime, which has been found by experiment to exist below a Reynolds number, wR, of 10‘ or 10°, where R is the disk radius. The theory has been checked experimentally by several investigators. Litt and Serad* rotated disks of various materials in water and