You did nto tell us anything about speed but I suspect a plastic bearing might be in order: http://catalog.buntingbearings.com/c...astic-bearings
I have a rotor with 3 mm OD axle which is always submerged in water / oil or chemical.
The function of the rotor is to rotate with respect to the flow of fluid.
Which type of bearing shall I use to for this application?
"water / oil or chemical" That's a wide range! Includes hydrochloric acid!
I would check out www.igus.com/. They have a very wide selection of materials and applications, and good selection assistance.