The answer is 'it depends'.... If the material is sterically stabilized with, say, 50 kDa PEG or PEI, then 0 mV for zeta potential (ZP) can be perfectly stable.

I suspect you're referring to charge (often called electrostatic) stabilization and the repeated (incorrect) mantra that a ZP roughly greater than 30 mV confers stability on water-based systems. This would not be the case for say, 300 nm (not even nano!) TiO2, where closer to 70 mV is needed. You have to search the literature and ResearchGate where I have described the origin of this oft-quoted nonsense (the original literature of Riddick) where 30 mV or greater confers 'moderate stability'. You'd need 100 mV or greater for 'excellent stability' and this is closer to nanobased (< 100 nm) materials rather than the micron-plus sized blood cells that Riddick was referring to.

For charged nanoparticles above 40 mV or below -40 mV is generally accepted.

Generally higher than +/- 30 mV is enough for disaggregation. However, the exceptions exist. If you introduce the counterions to stable nanodiamonds dispersion, they precipitate, but zeta potential value may not drop below 30 mV.

The answer is 'it depends'.... If the material is sterically stabilized with, say, 50 kDa PEG or PEI, then 0 mV for zeta potential (ZP) can be perfectly stable.

I suspect you're referring to charge (often called electrostatic) stabilization and the repeated (incorrect) mantra that a ZP roughly greater than 30 mV confers stability on water-based systems. This would not be the case for say, 300 nm (not even nano!) TiO2, where closer to 70 mV is needed. You have to search the literature and ResearchGate where I have described the origin of this oft-quoted nonsense (the original literature of Riddick) where 30 mV or greater confers 'moderate stability'. You'd need 100 mV or greater for 'excellent stability' and this is closer to nanobased (< 100 nm) materials rather than the micron-plus sized blood cells that Riddick was referring to.

As indicated above, there is no general rule of thumb since both charge and steric effects may keep nanoparticles from aggregating. You may find http://www.materials-talks.com/blog/2017/07/27/isoelectric-points-of-nanomaterials-qa/#steric of interest. As a rule of thumb, the stronger the charge the more stable - and most often zeta is used as a relative tool for comparing different batches, pH, buffers, additives, etc.

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thank you dear Qaysar M Al-Hashemi