Long-term nanofluid stability can be achieved by using functionalized nanoparticles. The technique is shown without the surfactant. The fact that no deposition layer forms on the heated surface is one of nanofluids' most outstanding qualities. The resulting nanofluids exhibit good fluidity, low viscosity, and high viscosity after a pool boiling technique that is free of contamination. Modern thermal systems might benefit from coolants with superior thermal conductivity and stability. It's a mechanochemical that works in the wet state. Using double- and single-walled carbon nanotubes, the procedure was utilized to make surfactant-free nanofluids. According to the findings of the infrared spectrum and zeta potential experiments, the hydroxyl groups were imported into the treated CNT surfaces. Chemically modifying carbon nanotube surfaces to functionalize them is a common technique for enhancing the stability of carbon nanotubes in solvents.
I've made nanofluids for heating/cooling applications and good stability was a bad idea. The particles were 100nm zinc oxide but these were sufficiently dense to cause sedimentation and fouling of valves and radiators etc. I proved that controlled instability was better, yielding the same thermal transfer properties but without the problems associated with the formation of dilatant sediments in valves and radiators.
When I read papers in this field, I am saddened not only by the lack of knowledge of some basic principles of colloid chemistry, but that the reviewers and editors of the journals have no idea either.
Hi John Francis Miller ,
I am intrigued by the idea of 'controlled instability' that you have mentioned.
For the particular case you mentioned, how does 'controlled instability' aid in solving the problem of sedimenting 100-nm particles? Wouldn't any form of instability, controlled or otherwise, result in clusters of particles that would just sediment even faster?
Omar Z. Sharaf Consider a 2% w/v suspension that is highly stable against aggregation but sediments. It will very slowly form a tightly packed sediment that is difficult to resuspend. The sediment will occupy perhaps 3% of the volume of the whole fluid. If you intentionally promote instability, the aggregation can be made to occur faster than the sedimentation, leading to a porous, loose sediment that could occupy 50% or more of the volume (by measuring sediment height). Indeed, given a high enough solids content and fast enough aggregation, the "sediment" could occupy the entire volume and not sediment at all. This makes for a system that is much easier to agitate following extended periods of quiescent standing.
You can finely tune the aggregation kinetics and the ease with which the aggregates redisperse under shear by selecting an appropriate molecular weight for a suitable adsorbing polymer. That's why I call it "controlled" aggregation, as opposed to just making the system as unstable as possible. In one approach, the idea is to create a small secondary energy minimum in the particle-particle interaction energy such that there is sufficient attraction under quiescent conditions for translation diffusion of the particles to quickly lead to aggregation but sufficiently weak that externally applied shear (e.g., pumping the fluid) allows the particles to separate. A similar concept is used to formulate non-drip paints.
Of course, I'm using the term "aggregation" in a way that may not sit well with others!
Thank you for the detailed answer, John! I think this is brilliant!
But I assume it highly depends on obtaining porous clusters of particles, as opposed to compact (or linear?) ones.
Given that this is meant for a heating/cooling application, I wonder if you can still maintain the desired 'controlled' cluster structures under all expected operation conditions. Variations of temperature can lead a handful of changes in the dispersing medium and adsorbing polymers, potentially leading to different clustering kinetics (and structures).
One other issue I can think of is the viscosity penalty and how it will affect pumping power requirements in this case. Assuming that this is not an issue, what about abrasion problems over prolonged use in the piping and pumping system?
A final issue that crosses my mind is relating to orthokinetic clustering during fluid flow. Given that the particles are now intentionally less stable against clustering, I expect the clustering kinetics during flow due to fluid shear will be entirely different relative to perikinetic clustering due to random diffusion under stagnant conditions. This, once again, may lead to non-porous structures.
Again, thank you for sharing your experience and insights.
Omar Z. Sharaf All very good and wise questions, Omar! And that's why a good idea can take a long time to become a commercially viable product. Of course, a solid understanding of colloid science can help reduce the trial and error involved. You also have to get creative with marketing. For example, you may find a formulation with one polymer could give fair performance under heating and cooling conditions, but better performance would be achieved using separate (though similar) formulations for each. Simply market one as "general purpose" and the others as "high performance" so that the customer believes that changing the fluid between seasons is a benefit. Or use catchy names like "EasyStart" or "NoClog" to help overcome the obvious reactions when a customer sees an aggregated fluid :)
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