Thanks Farid,

The experiment that I'm doing is ph sensitive, so it would be best if ph remained neutral. Polystyrene latex is the best option for me but the iep is below 1, and glass is around 3~4. It would be really grateful if you can suggest me a material that has iep around 7. Thanks again!

ZrO2 particles have an isoelectric point near 7. But if you add particles with an isoelectric point near zero that means that the particles will attract each other due to van der Waals forces. So the particles will form aggregates of small particles. Larger aggregates  will the tend to sediment due to gravity if the flow is slow enough. At higher flows the aggregates may break up due to shear forces from the liquid. If you instead use particles with a high electric charge they will repel each other and follow the flow as single particles. It they are small enough they will remain suspended by Brownian motion. If you have an electric field present the charged particles will move in this field.  It might be possible to find a dispersant with pure steric stabilization to keep your particles from flocculating but most dispersants that work in water are charged polymers and will charge the particles. 

Lysozyme Microparticles?

See: http://pubs.acs.org/doi/abs/10.1021/la9023426

Thanks Elis and Antonio for the insightful answers!

My guess is that you have figured one fundamental characteristic of a tracer in a fluid. The tracer needs to be of the same density of the fluid, also very small and be highly dispersed. My guess is that the potential issue will need to be removed, after the first problem is solved. 

There are lot of example of microparticles with almost 0 zeta potential, but ...

be careful in considering the data from zetameter determinations ...

Most of particles have isoelectric point while dispersion phase  is titrated.