Hi, first rule, do not always trust what other researchers say. We could not have CuO in water sstabilised without a dispersant.  It was impossible. We could get it dispersed in ethylene glycol, but that was for less than an hour, after sonication. Sonication does help. 

Hi, first rule, do not always trust what other researchers say. We could not have CuO in water sstabilised without a dispersant.  It was impossible. We could get it dispersed in ethylene glycol, but that was for less than an hour, after sonication. Sonication does help. 

Absolutely @Marc,  But Prof. i don't think achieving stability for less than an hour would be practically acceptable especially when potential benefits of using nanofluids are to be harnessed! 

You are absolutely correct my friend!! There are some systems that seems to be stable but not CuO and water. Furthermore, keep in mind that it is not only the thermal conductivity enhancement that is important, but the change in viscosity too. The Prandtl number enters calculations, so the small amount gained by the conductivity might be lost by the viscosity... See our paper by Tertsinidou, you will find it intereseting

We have used the nano Cu in paraffin wax to produce nanocomposite for thermal energy storage. the settlement of particles is not happening that fast. We tested the settlement after 360 thermal cycle. there is settlement, but still the particles are suspended even in the top layer. Here, I talk about Cu in paraffin wax, not in water, where the story is differ. Well, Akilu: do you need the nanofluid in static use, or it is circulated by pumping or any other means. such imposed shear by rotating unit may re-mix the particles in the base fluid. As far as I know that you use it in thermal system. Go to Afolabi and discuss with him his experimental findings on nano Cu in water/Clycole mixture. 

Prof Assael: May I contact you through your email concerning possible contribution from your end in my special issue on "Nano  enhancement of the thermal properties", please?

2 important points

CuO has a high density of about 6.3 g/cm³ - stability against sedimentation and stability against agglomeration are 2 different things.

When you try to disperse starting with dry powder, nanoparticles are likely aggregated. What is the size of the particles/aggregates. From sedimentation velocity you may calculate their effective size based on Stokes law.

I appreciate all of your insightful comments. @Asael, Prof.  Quite interesting. Thanks once again for being so helpful. 

The dispersion phases for solubility of CuO nanoparticles can be utilized in the form of ethylene glycol, surfactant and water.

' dispersion phases for solubility' something is either dispersed or dissolved - only one option!

For CuO, particle size must be very small (monodisperse 5 nm or below) and suspension must be 1 mmol or less to be appreciably stable. Such suspensions may be stable for some days. Adding surfactant like PVP can prolong this to months. Using longer chain PVP grade like K90 can increase viscosity which will significantly reduce settling velocity. Small PVP capped CuO are as difficult to separate as small size Ag or Au. Lab centrifuges will take atleast an hour for complete settling

employing nano Cuo particles and water/EG usually used for heat transfer purpose.But this question rise that these surfactants are still effective at high temperatures?

PVPs may give a solution to the problem as Jaydeep B. Deshpande said above. PVP is Polyvinylpyrrolidone and is water-soluble polymer made from the monomer N-vinylpyrrolidone. PVP polymers are available in several viscosity grades, ranging from low to high molecular weight (4,000–3,000,000). PVP acts more as a protective colloid and thickener. I know that PVPs are used in emulsion polymerization (EP) as protective coloid. EP is carried at high temperature together with strong oxidation reagents (initiator) so PVPs they are very stable and effective at high temperatures they may give a solution to the problem. Take also a look at the following papers:

Dalton Trans. 2015 Nov 7;44(41):17883-905. doi: 10.1039/c5dt02964c. Epub 2015 Oct 5. Polyvinylpyrrolidone (PVP) in nanoparticle synthesis.

Antioxidation Properties and Surface Interactions of Polyvinylpyrrolidone-Capped Zerovalent Copper Nanoparticles Synthesized in Supercritical Water

ACS Appl. Mater. Interfaces 2016, 8, 3, 1627-1634 https://doi.org/10.1021/acsami.5b07566

Some information about different grades of PVPs available in the industry and their properties like Viscosities of the various grades, Viscosities in different solvents, Radius of Hydration etc can be found in attached brochure from Ashland.