I am going to run an experiment where I have a laminar flow of water with a dye solvent. It is necessary that the velocity of the dye particles is mostly in the horisontal directions. Naturally particles will diffuse but my questions is how I can ascertain that this is kept below some prerequisite. I assume there should be some dimensional number telling me for example the ratio of convection/diffusion (thinking Peclet?) and using this I can choose my flow rate and channel dimension to keep diffusion below the required number. However, will this work in practice? I am looking at particles at a nanoscopic scale where variations in y must be kept below ~5nm over a distance of ~50nm.