A sedimenting or sedimented system can remain colloidal. For example, unaggregated particles that happen to be large enough to be influenced by gravity will eventually form a sediment. But the particles will remain discrete and all that has happen is that a high solid concentration colloid has formed at the bottom of the container.

Also...what if a suspension that is classically dismissed as not being a colloid was taken to the international space station? Is it now a colloid? What's changed? All the interparticle/interfacial forces remain. Except for an external field.

You've seen my opinion about the use of "nano" terminology and how it is destroying the foundations of colloid science :)

For me, it doesn't matter is something is a colloid or not according to some arbitrary definition. What matters is if the principles of colloid science can help you improve whatever it is you are working with.

Yes Ofcourse

A sedimenting or sedimented system can remain colloidal. For example, unaggregated particles that happen to be large enough to be influenced by gravity will eventually form a sediment. But the particles will remain discrete and all that has happen is that a high solid concentration colloid has formed at the bottom of the container.

Also...what if a suspension that is classically dismissed as not being a colloid was taken to the international space station? Is it now a colloid? What's changed? All the interparticle/interfacial forces remain. Except for an external field.

You've seen my opinion about the use of "nano" terminology and how it is destroying the foundations of colloid science :)

For me, it doesn't matter is something is a colloid or not according to some arbitrary definition. What matters is if the principles of colloid science can help you improve whatever it is you are working with.

Thank you for your input, Dr. Miller.

I agree with you that the term "nanofluid" is ambiguous, at best (also, it bears significant resemblance to the term "nanofluidics", which is something different altogether. But I have seen many confuse both terms). At the same time, what is the alternative? Probably something like "colloidal dispersion with nano-sized particles". Eventually, isn't that how all definitions really work? They are all "arbitrary", one way or another.

I have two more comments I would like to share:

1) In a colloid, the solid phase remains dispersed by means of random thermal motion, right? At least that's my understanding. In fact, by comparing the thermal energy of a colloidal particle with the gravitational energy exerted on said particle, it can be seen that a critical particle size exists above which sedimentation takes over and the dispersion is no longer a colloid. This critical particle size is inversely proportional to the constant "g". So on a space station, I expect that the critical particle size in order for the dispersion to remain a colloid will be larger.

2) I also agree with your last statement. When "nanofluids" are perceived as heterogeneous two-phase systems, which is what they really are, it all boils down to the principles of colloidal science to understand their fluid-particle, particle-particle, particle-wall, and particle-surfactant-fluid interactions. Only then can we gain some real insightful physics to justify their (if any) unique physical properties. This task is complicated by the fact that quantum effects have to be taken into account for very small nanoparticles. On the other hand, when nanofluids are perceived as homogeneous single-phase systems, that's, in my honest opinion, when things start being "speculative" with very little solid scientific background to support these speculations. I fear the majority of workers on the subject matter follow the latter approach.

(part 1)

Omar, first a comment to your opening question:

In my view "nanofluid" is just a newly invented term because people like to use names which should help their research work to appear up to date and modern. I don't like that. Research should talk for itself by *results*, not by project titles.

So I would even not "define" the word "nanofluid", I think it is senseless. It is absolutely sufficient to talk about "colloidal dispersions" or "emulsions".

Even worse, to address such systems as "suspensions", also in high-level magazines, is totally wrong. You are right by saying " in suspensions, particles settle on standing. "

And also this is correct: " While in colloids, the solid phase remains dispersed and does not separate on standing. "

(part 2 to follow)

John Francis Miller I somewhat disagree with your comment, at least as far as I know and as far as I am using the terms:

- for me, a colloidal system (a dispersion of particles or liquids in another liquid) does not sediment, does not flocculate because as soon as an (unstable) colloidal dispersion or emulsion is sedimenting / flocculating, the particles settling down on the bottom are not any more nanoscopic but much bigger than 1 micron in size, often tens of microns big;

- that does not change in space neither: such es system starting to (apparently) be a colloidal system = transparent (!), while flocculating it will become hazy or at least you start being able to see the flocculating particles even with your eyes, no need to have a microscope: so these particles are big, not nanoscopic any more!

I agree that all this fancy wording is destroying colloid science, too many people prefer to use fancy words rather than doing solid research.

(part 3 to follow)

(part 3)

Omar Z. Sharaf Thanks for your further comments and questions.

re " 1) In a colloid, the solid phase remains dispersed by means of random thermal motion, right? "

hhhmmm, random thermal motion is also active, but that's not what keeps the particles dispersed!

- dispersion is done by the adsorption of the dispersion liquid inner surface onto the nanoparticle surface, see page 10 of this presentation:

Conference Paper Scientific and Technological Progress with Polyaniline and P...

by this process, the interfacial energy is reduced, see below

- a dispersion is stable if the reduction of the interfacial energy is sufficient to keep the colloidal system kinetically stable:

Article Critical Shear Rate - the Instability Reason for the Creatio...

- in addition, the formation of (frozen) dissipative structure helps to stabilize the system:

Article Dissipative structure formation in colloidal systems

Article Frozen dissipative structures in heterogeneous polymer syste...

(not only in polymer systems, that's generally the case)

- in kinetically stable colloidal systems, gravity does not play a significant role.

(end)

with a bit sense of humor I would say, Omar see what you get, several conflicting views on the matter, depending on how many experts you ask as much expert views you will get. I am thankful to you putting forward this question.

I highly value the knowledge of Bernhard and John, however, I have to disagree to some degree.

Based on how I understand classical German Colloid science, here is my version -

Dispersions are inhomogeneous systems characterized by having at least one interface where molecules feel a different environment than same molecules in the bulk, there are always molecules at interface but also in bulk. Based on this, suspensions (nothing else as solids dispersed in liquid, which may settle under given circumstances or not) and emulsions (liquids dispersed in liquids) are both dispersions beside other ones, e.g. fog.

Term colloid is unfortunately historically not clearly defined, something between characterized by slow diffusion and microscopically not resolved. Importantly properties of colloid dispersions are interface determinated. For more confusion there are also colloidal solutions (no dispersed bulk phase). If colloidal dispersions get flocculated, they are still interface determinated opposite to possible result of coalescence or Ostwald ripening, when dispersed phase grows beyond colloidal size.

In this sense nanofluids are colloidal dispersions and of course colloidal suspensions same time. And they settle with slow or slower velocity. When settling and diffusion velocity get to the same order you will get a so call 'isobaric' equilibrium. Even solutions (e.g. salt solutions) or mixtures of gases exhibit enrichment of higher density components towards the bottom, even at earth gravity.

I think the heart-felt and convincing opinions speak to the confusion that has developed in this area.

When I am asked questions like this, I often answer (without trying to ignore the question):

It's not whether or not something meets a definition of a colloid but whether it has properties that can be explained using colloidal theories and measured using colloidal techniques.

Typically, you want to measure a property so you can control a process. As long as that property is a critical one for that process, it doesn't matter your system meets the traditional definitions.

I've used basic colloid principles to develop and control the manufacture of suspensions of 5um dense particles in a pressurized liquid. A colloid? Not by any accepted definition. But the principles applied.i

Of course, when sharing scientific advances in journals, at conferences etc , you must be more rigorous and clear.

Titus, thanks for your comment, and I agree: especially in colloid science, you can find any definition (including "colloidal *solution*" which is really funny) and wording, whatever you like or dislike.

I would also admit, that my definitions are more strict (like, that I differentiate "suspensions" from "dispersions", insisting that "suspensions" are some system where you have stirred some stuff (like fine sand) into a liquid (like water), and the sand will settle quickly after you stop stirring. I just use a more strict and narrow sets of terms.

I have never studied whether or not terms in colloid science are historically poorly or well defined, at least here I see this:

https://de.wikipedia.org/wiki/Kolloid

https://en.wikipedia.org/wiki/Colloid

which both- to my regret, also mention "suspensions" - but ok, I let other people write as they like it.

And again, if people like to "enrich" their projects or publications by using the fancy word "nanofluid", so be it. I won't. I even don't understand how a fluid can be "nano":

- we have nanoparticles, ok, well understood;

- we have nanodroplets, sure, fine;

but nanofluid - a fluid which is nano in size? is this just a nano sized fluid droplet? (I know what people like to be understood by using this term, but why not using "colloidal dispersion", what's wrong with it?)

Anyway ... I agree to look at this topic with a big sense of humor. :-)

Re suspensions, in my example I deliberately used that word because my system resembled a snow globe souvenir. After agitation, aggregation was evident in seconds and sedimentation was almost complete in less than a minute. But I was able to improve the product by applying colloid science. A product is more than just the formulation - includes all the surfaces and forces it is exposed to and the methods used to test the product's performance. Many fields of science can be applied as long as you know their true limitations and those that people insist are limitations but aren't...

John Francis Miller agreed!