I have an experimental apparatus with an inner cross-section of 15 x 1.5 centimeters and an height of 1 meter. Inner walls (front and rear walls) are made of common glass whereasplexiglas is used for side walls. In this apparatus some glass microbeads (diameter range between 200 and 800 microns) of different colors (red and green) are suspended in an air stream injected from the bottom by means of a microporous plastic distributor plate. The microbeads have a central core of glass, a superficial silver layer (with a thickness of 1 micron) and an external layer of paint (red or green). In an undesiderable manner, when suspended in the air stream inside the apparatus, the particles tend to stick to the glass walls (and also in a more pronounced way to the plexiglas ones). I need to capture with a video camera what happens 'near the wall' and this undesiderable phenomenon doesn't allow me to obtain reliable results for the mixing extent of the particles. What is the actual nature of this phenomenon and how can I solve this tedious problem? I tried to warm the particles heating them at 100°C for about 30 minutes just before putting them in the apparatus, but I observed the same kind of phenomenon: the particles, especially the smallest ones, when suspended in the air stream, tend to stick to the glass walls. The phenomenon takes place also if only superficially painted glass microbeads are used (in place of the silver-under-paint covered glass particles). If electrostatic charges are involved, are there any anti-static spray products? Thanks in advance.