In this paper, you can find the ideal way to prepare nanofluids that will remain stable for several months without nanoparticles aggregation.

Good luck

basic ideas are

• making particles as small as possible, starting from very dilute solutions where particle precipitation starts from
• making the solution viscous and nonreactive to particles
• attaching chemical, possibly hydrophobic groups onto the particle surface, if sought to be used in aqueous environment.

But I still doubt indefinite stability period of nanoparticles. You may check out how it is done in Magnetic Nanofluids (ferrofluids)

The combination of mechanical and chemical stability techniques can give a long-term stability (6-12 months) depending on the optimum concentrations and the properties of nano-particles, base-fluid and surface-agent. However, "Forever" is not achievable but the on-going research in this field may lead to "Forever stability" some day.

Hopefully, rigorous testing and optimization to find the best concentration and continuous agitation is the only way to get long term stability.

in my experience. alumina+ DI water nanofluid will show its stability for more than six month. This is applicable for 4 % v/v concentration, Above this volume fraction sedimentation will start shortly.

The biggest bottleneck for large scale implementation of nanofluid. Oil based nanofluids are generally more stable than water based nanofluid. Selection of Nanoparticle and basefluid plays a critical role in determination of long term stability. Depending on your Nanoparticle and basefluid different stabilization can be implemented such as steric, electrostatic, electrosteric stabilization. However, stability is also largely dependent on its field of application as different stabilization techniques are suitable for different application. Kindly have a look into the following review article which might be of some use to you.

Stability of nanofluid : A review.

April, 2020, Applied thermal engineering

Link: 10.1016/j.applthermaleng.2020.115259

My first thought when coming across this (and any similar) question is: stable against what?

Assuming the purpose of preparing the nanofluid is not to just indefinitely store it, you need to identify (or at least anticipate) the types of destabilizers your dispersion will be exposed to (e.g., heating and cooling cycles, fluid shear). Of course, this highly depends on the intended application for your nanofluid.

Only then can you systematically start looking into possible stabilization strategies.

To get good stabilty*

1, Suitable concentration( wt% or vol%)

2, making same functional groups for both nanoparticle and base fluid.

3, Adding suitable surfactant (better used for low viscous base fluids like water and glycols)

4, less size of the nanoparticle (high surface area)

5, Designing equavalent density for both nanoparticle and base fluids.

Even my question is still on the measurement of stability of nanofluid.

Through PLASMA FUNCTIONALIZATION, a great dispersion stability of more than 20 months have been obtained. Chemical modification of nanoparticles through plasma treatment has been shown to produce better dispersion and interfacial bonding than traditional methods. Coupled with the fact that it is solvent free, time efficient, and versatile, it has become the main focus of a number of recent studies. In this process, nanoparticles are functionalized through exposure to a continuous wave capacitively coupled radio frequency glow discharge plasma.

For clear methodology, refer to the following paper:

Article Plasma Functionalization of Carbon Nanotubes for the Synthes...

For different aspects of stability improvement, refer to the below paper

Article Comprehensive study on nanofluid and ionanofluid for heat tr...

Infinitely stable nanofluid: it's completely theoretical or not? This is also my question.

Entropic decay makes this almost physically impossible. Therefore I always propose a mechanical solution of intermittent ultrasonication. The drawback of this is energy loss to keep the fluid stable.

A perfect example of stable nanofluid is circulating blood. The blood circulatory system can tell us more about its stability.

At the time of writing this, it appears to be no permanent guarantee for Nanofluid stability. However, there are some stabilizers (surfactants) that can really keep suspended nanoparticles for a good number of days until nanoparticles sediment. The trick is to get the right pairings in terms of quantities (nanoparticles, base fluid and surfactants).