(358a) Searching for Volume Diffusion

At low pressure, the amount sublimated of a large organic molecule varies with the square of the diameter of the tube in which sublimation occurs. This implies that the velocity profile in the tube is nearly flat, consistent with volume diffusion, but not with laminar flow. However, molecular simulation calculations show that under the conditions used, the velocity profile is near parabolic when there is no deposition on the tube wall, inconsistent with volume diffusion but in agreement with laminar flow. The profile is almost flat when there is deposition on the wall, even though the flow is laminar. But if volume diffusion does not occur here, when if ever will it occur? This paper begins that search.