@ Fazlay, Generally two techniques are used to measure the stability of nanofluids. They are visual inspection and zeta potential measurement of nanofluids. Beside zeta potential, the visual inspection method is used to measure the stability period of nanofluids for different particle concentration, sonication time and pH values. Beside this, from measurement of settling velocity during centrifugation you may get good indication of stability.

Dear J. C. Tarafdar

Thank you for your answer.

However, I have studied literatures claiming ZP is not effective for dark NF samples. Many suggested DLS method to evaluate stability for more accurate measurement. What do you recommend for measuring stability of a dark nano suspension such as 0.15 wt.% Graphene in synthetic oil?

@ Fazlay, please have a look of the attached file.

Not all NFs rely on electrostatic repulsion for stability. I recommended reading a standard textbook on colloid science to learn about stability mechanisms and the ways to measure stability.

"Stability" can mean different things to different people. No charge-stabilized NF is truly stable - just the kinetics are slowed down. So you have to relate the kinetics of aggregation etc to the application of the NF. An NF that aggregates and sediments in a day will be of little value for a product requiring a shelf-life of months but okay if it is used within a few minutes.

Also, the comment about ZP and dark samples isn't accurate. It is true that some techniques that measure ZP rely on optical transparency but there are other techniques that do not. Some can measure ZP on optically opaque liquids.

Note also that ZP cannot be directly measured. It is always estimated through an appropriate electrokinetic theory from some other measurable property. Hence, comparing ZP from one technique is not necessarily valid.

Dear J. C. Tarafdar , This article does explain a few methods to improve and evaluate the stability of NF however, it doesn't conclude which technique is suitable for all classes of NF. Thank you for your response to my questions.

Dear John Francis Miller

Thank you for your very informative answer.

Can I get your recommendation for a standard textbook on the stability and colloidal science (would be convenient if it is available online)?

Introduction to Colloid and Surface Chemistry, Duncan J. Shaw. I think you can find the fourth edition online, this is an old, but really good book!

Luiz Fernando de Sousa Lima My first colloid book back in 1985 or so. I still refer to it. Excellent recommendation!

Hi Fazlay,

There are many characterization techniques to assess the stability of a nanofluid. This largely depends on the type of destabilizers your dispersion will be exposed to (which could be as simple as storage time).

As Dr. Miller explains, and contrary to common practices in the nanofluid literature, zeta potential should not be interpreted as a definitive indicator of colloidal stability/instability in a nanofluid, unless this nanofluid is purely stabilized by electrostatic repulsion between nanoparticles. In other words, zeta potential can be a quantitative measure of charge-induced stability.

In Section 6 of Article On the colloidal and chemical stability of solar nanofluids:...

In many cases, multiple characterization techniques can be used to complement one another. In other cases, a single technique can be conclusive. But it is hard to label a single technique as 'universal'.

John Francis Miller Everytime I see some nano-novelty and I go back to this book, I can find the same concept as colloid hahaha.... It's amazing how people rediscover and rename what is already known and published in a book which should be the first step for everyone working with colloids.

If we remember about the "old lady" of colloidal chemistry, then there have long been terms that help to understand this issue. Over time, all disperse systems coagulate. They are all thermodynamically unstable in general. The minimum of the Gibbs energy of the dispersed system is reached when the particles settle. Coagulation consists of two simultaneous processes of particle aggregation and particle settling. They compete with each other. The term stability is not desirable. How can an unstable system be called stable? A solution of sodium chloride in water is thermodynamically and kinetically stable.

The coagulation of a dark suspension can be measured by a settling process in a cylinder, taking samples at regular intervals or by centrifugation.

The term nanofluid came from technicians for dispersed systems with magnetic properties and has never been used in colloidal chemistry.

If anyone wants to remember the terms of colloidal and physical chemistry, he can look at short reference books

https://www.researchgate.net/publication/309732185_YuAMirgorod_RV_Grebennikova_Glossary_on_Colloid_Chemistry

https://www.researchgate.net/publication/309733414_Yu_A_Mirgorod_RV_Grebennikova_Glossary_on_Physical_Chemistry?_sg%5B0%5D=-IPh7lbulilvyzCxoyWx9Z7ZA_OvhkJNB1KMT32n_xnmOIDMN-mDasqJVjlhVD1CkCCiA4duDV1ywwyId4mWu8ngA43R4GyNkyI5dUgF.5R_UUgeqf0u3szWwrSlDD4l08LcGJ9Jcf10JvzKR0EBZ26tB1I5YQsoBxdmdXBrkZaGT4r6DlLTrIv1Nv-4bkA

Yuri Mirgorod : Thank you very much for information. Please provide with the English version of referred materials, if possible.

Yuri Mirgorod one point to correct, settling is not related with minimum of the Gibbs energy of the dispersed system, but with minimum in potential energy. Settled particles may be still individual (dispersed) particles after settling, which eventually will form larger particles or a bulk phase due to thermodynamic instability. Settling relates to mechanical stability not to colloidal stability. Even dissolved salt forms a concentration gradient over time similar to CO2 in air.

Fazlay Rubbi

The first thing that should be done, IMHO, is a simple Stokes' Law calculation - density and size are usually the things to consider before charge ('electrostatic') and steric stabilization mechanisms. The first table in ASTM E2490-09 (15) for DLS/PCS shows calculated settling times for systems of varying density and size. This will tell you whether a particle is likely to remain in suspension with Brownian motion effects alone.

Titus Sobisch

Let's get back to terms again. No terms 1. mechanical stability and 2. colloidal stability. These are your terms. 1. Apparently you mean the behavior of a dispersed system in the field of action of the Earth's gravitational forces. Therefore, classify a dispersed system as a mechanical system (a set of interacting macroscopic bodies). 2. Apparently you mean not colloidal stability, but the stability of the dispersed system.

During coagulation of the sol, the processes of aggregation and deposition of nanoparticles occur simultaneously. In the study by the thermodynamic method, the motion of nanoparticles (Brownian motion and deposition) is included in the entropy factor. When the sol nanoparticles have settled, the coagulation process reaches a minimum of the Gibbs energy. At the bottom there is a hydrogel. This dispersed system gradually turns into a gel and is accompanied by its own decrease in the change in the Gibbs energy in this process.

About the gravitational factor. If we put a dispersed system in a centrifuge, then it will be in the field of action of centrifugal forces. The centrifugal forces will be much greater than the gravitational ones and they can be neglected, as in the thermodynamic method. Determining the size of nanoparticles using a centrifuge is more accurate than in the process of settling them in a cylinder. Of course, now there are modern instruments for their determination.

Fazlay Rubbi I have no English version of dictionaries.

I am not a chemist and the last course I did in chemistry dates back more than 40 years. Hence, many the conclusions I have derived at may stand to be corrected using one or another law, but it works in practice. In the macro environment engineers have to work with emulsions comprising a mixture of material varying in size from less than nano-size to several microns in size. In addition, in the production process of bitumen emulsions, oil needs to be mixed with water using the chemical properties of the particles available. In comparison, the stability of nanofluids may be relatively easy to explain in terms of Brownian movements, gravity, etc. However, basic chemistry and bond strengths are just as relevant to explain the stability of an emulsion containing the various different particle sizes mentioned.

In the production of a bitumen emulsion a surfactant (nano-particle) is used to enable water molecules to attach to the bitumen molecules (oil). The stability of this mix is closely associated with the properties of the surfactant (also known as the emulsifying agent or commonly as “soap”). However, the very properties comprising the surfactant (crucial to the stability of the emulsion) is no-where to be specified (at least not in sub-Saharan Africa). In practice (at least in the developing world) products need to be stable for at least 4 months after delivery in the field – often subjected to extreme conditions close to the equator. In the modern era emulsions can be substantially improved thought the addition (modification) of nano-size products such as nano-silanes. These additions result in a complex mixture comprising of a high variation in particle sizes that need to remain in suspension, evenly distributed throughout the mix and stable for successful application.

Each of the different components of the nano-modified emulsion is crucial to the stability of the mix suitable for practical application in the field. I am using the basic bonds strengths at ambient temperatures (from a chemistry hand-book from more than 40 years ago) between the various elements comprising the various available surfactants (over and above the obvious length of the “carbon tail”) to explain the differences in the stability of various basic emulsions (assuming the manufacturing process is well controlled). The second emulsification process, adding a material compatible nano-silane often improve the stability of the original emulsion due to a further separation of the oil-based particles, resulting in smaller particles that enhances the stability of the end-product. The pH of the original emulsion and the pH of the nano-silane modification (incidentally the last block at the bottom right of the figure added by Omar Z. Sharaf ) have been found to have a meaningful influences the stability of the nano-modified emulsion.

With the correct quality control in place, a product can be produced that will remain stable, without visual separation and no increase in measured viscosity exceeding an on-site period of more that 4 months. With simple once-a-week circulation using an ordinary circulation pump, this “shelf-life” can be extended to more than a year. This product will remain stable and immediately disperse within a tanker containing construction water for the construction of roads using naturally available materials (gravel / soil). The successful manufacturing of a stable emulsion as well as the strong bonds achieved during stabilisation can all be explained (at least in my limited understanding) through the basics of chemistry (assuming good quality control during manufacturing).