Sure, you can observe the hydrodynamic motion of a colloid particle in water solution upon application of DC electric field.

You also can experimentally measure Disjoning Pressure as a Repulsion Force between 2 Solid plates at the condition of Overlapping Electric Double Layers.

(to your first comment) I know, but I am not sure what it tells us. The electric double layer in total should be electrically neutral.

I would assume that a DC field can induce some polarisation, that's fine, but is this an experimental proof for the double layer?

Again, IF this double exists, and IF the zeta potential exists as described, then it would create repulsion forces which are even in all x, y, z directions in the volume, hence the particles would be statistically evenly distributed which is characteristic of a system in thermodynamic equilibrium.

But this is not the case.

to your 2nd one, please direct me to some reference. Thanks.

Yes, it does induce the repulsion force, which is a VECTOR, has 3 components.

The particle + double layer is a neutral system,, the diffused part of EDL resides in LIQUID phase, so you have relative motion between the liquid and solid particle.

Second Question: KEY WORDS: DISJOINING PRESSURE, ELECTRIC DOUBLE LAYERS, EXPERIMENTAL OBSERVATIONS. Google them in.

I can see that you are a Consultant in China, speak some Mandarin, I AM IMPRESSED

In the case you are interested to measure simultaneously the size and ZETEA potential of the colloidal particles, the high frequency Electric Field is being used.

COLLOIDAL DYNAMICS, the company in the US manufactures and sells instruments.

also, I would like to add this question: what is the structure of colloidal dispersions in organic solvents? Is there any model comparable to "electric double layer" or "zeta potential"?

- if yes, I would be interested to see references

- if not, why should colloids in organic solvents behave differently than in water?

Sure, stable colloidal systems in organic liquids exist, but they can not be stabilized by overlapping electric double layers. The ionic concentration is low in organic solvents in comparison to water, so, the surface charges are insignificant, forces are too small for effect of stabilization.

SURFACTANTS are used for stabilization of Colloids in Organic Media. The Mechanism of stabilization is different, the Quantitative Theory is only recently developed. You may wish to see my paper on this site: "OSMOTIC REPULSIVE FORCE DUE TO ADSORBED SURFACTANTS", Colloids and Surfaces B, 2012.

sure, I know that stable colloidal systems in organic liquids exist, I have made a lot of them myself. I assume(d) all the time that surface charges are insignificant in such systems, and I am asking: maybe they are as well in water-born systems? Not in all organic born systems surfactants are used, and on the other side, in many water-born colloids, surfactants are used as well.

I am asking this because we all know that colloidal systems in water are very sensitive and unstable if ion concentration increases.

Actually, I doubt that the stabilisation mechanism is different. Why should colloids in organic solvents behave differently than in water?

Moreover, again, IF in water-born systems this double layer exists, and IF the zeta potential exists as described, then it would create repulsion forces which are even in all x, y, z directions in the volume, hence the particles would be statistically evenly distributed which is characteristic of a system in thermodynamic equilibrium.

I will study your paper.

Yes, read this paper, you will find different mechanisms Colloidal Stabilization.

Particles may cluster, due to long range Van der Waals forces, but when you shake the system, they are dispersed again. Not in the case of real coagulation, shaking will not help.

More and more people come to Colloid Science from nano-technology. But Colloid Science exist over 100 years, good textbooks are available. You are the modest man, you just express your doubts about fundamentals of CS. Worst cases is when nono-technology people re-discover these fundamentals, think they come with great invention, make noise in the press, then fall flat.

For this very reason the Editor of Colloids and Surfaces B asked me and Dr. Schramm to supplement our paper with mini-review of other stabilization factors, as reading the paper is less time consuming, than study text book. ,

Alexander, thanks for your comments, thanks for discussing with me. I am quite experienced in colloids (I have developed lots of different dispersions, in all kind of media, many of them are in mass produciton, and, as you may see in my RG pages, also published quite a bit including a revolutionary non-equilibrium theory about heterogeneous polymer systems).

As I also had found dissipative structures in colloidal "solutions", which convinces me that these are also non-equilibrium systems, I already long time ago started to doubt about "zeta potential" and "double layer". "doubt" does not mean, I reject, I am probably not capable enough to reject, but I have doubts and questions which I have expressed here in my starting question.

After I will have completely read your paper several times and hopefully fully digested, I will send some comments / questions.

I totally agree with you that in colloids (including and more so in paint and polymer compounds / composites area) not too many people are seriously addressing fundamentals, but approach this extremely complex area quite superficially, and quickly believe they have invented the wheel ... which I regret a lot.

Please let me know when your mini-review will appear, or also a pre-print would interest me.

My homepage: http://www.bernhard-wessling.com

Bernhard, mini-review is incorporated within the paper you read, appropriate references are provided.

When you address a Colloidal System, first question is either system is hydrophobic or hydrophilic (lyophobic/lyophilic). My paper is for PHOBIC case only. Water soluble polymers are also considered to be Colloid Systems, but traditianal Colloid Scientists of my breed are not the experts in this field.

I see (I had not looked at the attachment yet :-)) After having read your paper in Colloids & Surfaces, I can say: very interesting, but I need to deeper think about it and digest better.

As your paper is for hydrophobic cases, my starting question remains open. I do not doubt that one can measure / observe some phenomena which *could* be understood when *assuming* an electric double layer and by introducing some term called "Zeta potential", but are these real?

Is there any direct evidence?

How to explain the fact that (by the way, also in organic liquids colloidal systems) *structures* can be observed, while electrostatic double layer / zeta potential would result in isotropic repulsion forces which would lead to a statistical distribution of the colloidal particles.

This is what I am struggling with.

dissipative structures in colloidal systems:

https://www.researchgate.net/publication/245535674_Dissipative_Structures_in_Colloidal_Systems?ev=prf_pub

Article Dissipative Structures in Colloidal Systems

also this one may be of interest:

https://www.researchgate.net/publication/233979584_Dispersion_hypothesis_and_non-equilibrium_thermodynamics_key_elements_for_a_material_science_of_conductive_polymers._A_key_to_understanding_polymer_blends_or_other_multiphase_polymer_systems?ev=prf_pub

Article Dispersion hypothesis and non-equilibrium thermodynamics: ke...

In the last link, please have a look especially into par 4., p 133 - 137

For a Colloid Scientist EDL and ZITA potential are as real as the SUN and MOON to common folks. But only in water based systems.

Structures are due to molecular attraction forces. I would advise you to read short Textbook on Colloids by A. SHELUDKO. Very good book.

:-), ok, I believe so, and for me it was as well; but for SUN and MOON in the meantime we have direct evidence, independant from distant observation or personal belief. recently, I started thinking again about that I found dissipative structures in "my" colloidal systems, in liquids and polymers, and I also got evidence that water based colloids exhibit such structures as well, but the stabilisation mechanisms described or assumed should have isotropic repulsion forces.

For heterogeneous polymer systems (dispersions of some solid or another polymer in a polymer matrix) I discovered the mechanism which causes the structures to form (https://www.researchgate.net/publication/229816762_Electrical_conductivity_in_heterogeneous_polymer_systems._V_(1)_Further_experimental_evidence_for_a_phase_transition_at_the_critical_volume_concentration),

for organic-solvent based colloidal dispersions with polyaniline (neutral or conductive / doped form), I described a qualitative model which also allows to understand the structure formation,

but for water-based systems (if EDL and Zeta potential exist), I can not understand how structures would be formed;

(also for your theory, which is very intriguing, I can not immediately see how structures can develop, there must be an additional structure formation mechanism, but I am not deeply enough in your theory, I need some more time for deeper digestion, but I very much like your approach!)

Article Electrical conductivity in heterogeneous polymer systems. V ...

You consistently disregard molecular attraction forces, responsible for structures. They exist in any solvent and they are long range. F = A/ d^3 for 2 plated, d is the distance between plates, A is Hamaker Constant, F is the force per unit area.

I compare attraction and repulsion in my paper, but the formula is misprinted.

PRL 77, 1897 (1996) reports direct measurements of the electrostatic interactions between pairs of charge-stabilized colloidal spheres in water. The results for isolated pairs of spheres are quantitatively consistent with predictions of the DLVO theory, including the superposition approximation. The approximations underlying the DLVO theory break down for more strongly interacting systems such as colloids in polyvalent electrolytes.

yes, that's what I also found in literature. Also when more particles are involved, there are additional (attractive) forces as well. What are these forces, what kind of forces? Colloids never consist of 2 particles interacting with each other only, right?

My main thought roadblock is, as all the theories I see describe forces which are isotropic, how can dissipative structures emerge which are not isotropic (at least locally not)? very highly diversified structures!

For polymeric systems (nanoparticles in polymer matrix), I have discovered the mechanisms and at least semi-quantitatively described them from non-equilibrium standpoint of view, but for colloidal systems (which I also have developed a lot) and also for microemulsions (which I only read about) I am still puzzled how to understand them.

thanks, but I am not looking for textbooks nor theory; I am looking for original publications showing experimental evidence for answering my questions. Do you have, or anybody else?

It depends on what you call direct evidence. Do you want a measurement that shows you the distribution of ions around a colloidal particle directly? There is no such experiment at the moment because it is not currently technically unfeasible. For me the hundreds of measurements of surface forces made in aqueous systems that show an exponentially decaying repulsion consistent with an electrostatic double layer is direct evidence. Furthermore the decay length agrees with the predicted Debye length. The theory has many limitations but at low salt concentrations it is generally correct. Then there is TIRM, streaming potential, electroacoustics and electrosmosis which all depend on the electrical double layer. There is not reason to suspect these forces are not isotropic. However when two particle adhere due to an attraction (eg Van der Waals forces, a third particle joining them will have a preferred orientation if it is free to move to the lowest energy configuration.

Vincent, thanks for your comment. I would call "direct evidence" if there is any experiment which delivers data which ONLY can be explained by electrostatic double layer. I do not doubt that many experiments MAY beor CAn be explained, at least partially, by this asumption, but as far as I see, it is still an assumption, a hypothesis, or?

There are a few aspects which make me doubt there is only such double layer capable to explain the observations:

- the FACT that colloidal systems are highly diversily structures in form of "dissipative structures" (look at some publication on my site) - this contradicts DLVO theory

- instead of electric double layer, it could be "simply" polarisation (which could also account for structures)

- colloidal systems in other cases are much more complex and highly structures, why not in water / organic solvents? (please have a look at my respective publications)

Just questions and doubts so far

Original DLVO theory is correct for surface potentiales below 50 mV and dilluted colloids,

with the dispersed phase of about 1 % of total volume. There are plenty of correctionsin the litearature, both for higher potentials and higher volume fractions.

Concept of the Electric Double Layer works for Electrochemistry as well, predicting the cappacitance of the polarised electrode and the rate of Electrochemical reactions. Polarization itself is due to EDL. Otherwise Colloidal System would have the net charge, electroscope near the glass, conteining Colloid System, would CRY about it.

Alexander, please show me direct evidence. From where it is concluded that DLVO theory is correct for diluted colloids? I am just asking, I am not at a position it is wrong, I am simply asking which experimental evidence is supporting this theory, an evidence which can not be explained by other "assumptions" than DLVO.

DLVO is one of finest theories within Physical Chemistry, but, sadly, their authors were never awarded the Nobel Prize in Chemistry. Derjaguin made erroneous statement on so called "Polywater", which he honestly admitted, but the damage was done.

Another example of great work, never formally recognised, is the "REHBINDER EFFECT".

During his life time, P.A. Rebinder was short of theoretical explanation for great forces induced by surfactants. And Nobel Prize is not given post-mortally..

Worse yet is Nobel Prize in PHYSICS. Einstein was granted Nobel Prize in 1921 for his work on Photo-electric effect and other works in Physics. RELATIVITY was not mentioned.

The nice side of the story, is that RELATIVITY is above any work honored by any PRIZE,

Newton basic laws of Mechanics is another example that real value of work is not measured by PRIZE.

Bernhard, it appears to me that you are not really asking a question but seeking to advertise your point of view. I think all the experiments I mentioned can only be described by DLVO theory, certainly polarisation cannot explain the measured Debye length. Can you give an alternative explanation for the Debye length? You must remember as stated by Alexander above that DLVO is only applicable under certain circumstances. Of course if you compare it to examples where the assumptions break down it will not work. The systems that you are talking about are such examples. It does not meant the theory is misguided or not useful, rather it has a limited range of applicability.

Every one has full rights to be against, not to believe and fight with any Theory. Most of simple folks never heard about DLVO. But almost anyone heard about RELATIVITY. Most of simple folks don't understand the concept, but are too shy to critisize. Yet, no theory in existance had and has so many enemies, including people with PhD in Physics. And sure, no one is capable to bring together 2 sets of clocks to compare the time readings. As on E=mC^2,

these are indirect experiments. Not unlike monitoring rate of coagulation, within conditions of DLVO applicability, and comparison with theoretical predictions.

Sure, why we should honor Derjaguin, while EINSTEIN is GUILTY in WITCHCRAFT !

FOLKS, TRUST ONLY YOUR OWN RESULTS, others exist or existed just for the reason to fool you !

As a final argument of the existance of the Diffues Electric Double Layer, its derivation can be done on the level of Contemporary PHYSICO - CHEMICAL KINETIS, without employing Debye - Huckel Theory of Electrolite solution. These Laws of Kinetics is the same, that Newton Laws of Mechanics. Regecting these Laws, the person reject PHYSICAL CHEMISTRY as a SCIENSE. I finally rest my case.

Vincent and Alexander, believe it or not, I am really asking and I have no fixed opinion about my question. Also, I am not against DLVO theory. Moreover, discussing about it is not a topic of religious character (nor a question of belief).

One of the key questions I have is how it is possible that colloidal systems are structured (i.e., the particles are not randomly / statistically evenly distributed), and tehreofre I am asking for help: have the key elements of DLVO theory anywhere be directly proven? what's wrong with this question? (Einstein's theory has been proven by independant methods, i.e., his predictions are falsifiable and experiments had been designed to falsify, so far, no limitation has been found; if I am not wrong, even the time difference for identical timers at different speed).

I don't mind nor critisize that DLVO is only applicable for certain limited, but this may tell us that a more widely applicable theory is required which could include DLVO as a special case. However, it seems to me that an explanation needs to be found how structures in colloidal systems are generated.

Bernhard, I just typed in "COLLOIDAL STRUCTURES" into GOOGLE search engine.

There are over 9 Million references. I am sure that a single scenario does not exist. So, you have to specify disperced and continues phases, their chemistry, initial size of particles and the fraction volumes. There is no subject to discuss unless proper formulation is provided.

Alexander, here is some link to one of my publications, leading to more references:

https://www.researchgate.net/publication/245535674_Dissipative_Structures_in_Colloidal_Systems?ev=prf_pub

Article Dissipative Structures in Colloidal Systems

Bernhard, I don't know the reason, but your text appears blurred and is not readable. And even in the better form, I will not start reading literature on the problem, which is not formulated yet. Come with problem formulation, including geometrical form of particles.

Alexander, I don't know what is your reading problem there. Fact is that colloidal systems are structured, and I am by far not the only one who found them, here attached is some other publication which I cited in my paper "dissipative stuctures".

EL double layer as the key model and DLVO as the theory to explain stability of colloidal systems, fail to explain how structures in colloidal systems are formed.

And this is my question which I started above: I am looking for independant experimental evidence of EL double layer, and in other words: I am looking for the mechanism which forms structures in colloidal systems.

DLVO theory would consequently lead to a statistically even distribution of nanoparticles (or emulsion droplets) in colloidal systems. In case I am wrong with this statement, please let me know which term in DLVO describes structure formation.

Bernhard, DLVO has nothing to do with structuration of highky concentrated Collois Systems, please, leave DLVO in peace. The unified theory for colloid structures does not exist. And I am too old to get involved in the new complicated theory. However, in the case the structure has DIRECTOR (anisotropic) I can be of help to you. I just posted full text of the paper: RHEOELECTRIC EFFECT....." on my RG site. Please, take a look. Similar equetion of state can be worked out for other reasons of anisotropy.

But, otherwise, BERNHARD, I like your brute force in insisting for explanation of what Colloil Science fails in explaining today.

Best regards to GENOSSE from TOVARISCH !

haha, thanks, Alexander! I don't attack DLVO, I let it have peace. What you said (" The unified theory for colloid structures does not exist.") is exactly what I think as well. Actually, there are many explanations, models, theories for special colloidal systems, and DLVO is one of them for a special case, but none can explain all of them in a more general way, and so that predictions can be derived for special cases.

This caused the questions I am asking.

For your "director" (I will look at your paper later) topic, I want to comment: also dilute colloids, and colloidal systems of all different kind are structured, even without a director (if no flow, then the structures are isotropic on the bigger scale, but highly structured on the micron and submicron scale).

How can this be if the systems were in thermodynamic equilibrium? I think: not possible.

Apart from that, thanks for your greetings. 谢谢。再见！老卫

You are probably, Bernhard, from former East Germany, Socialist people were more open and sencere. Provide the system with non-spherical particlcles, say elongated elipsoids, we will get director even in the absence of the external field, at the case of their strong interactions. On the unified theory I just pass.

Alexander, sorry to disappoint you, but I am from the former West Germany :-)

re the "director": also colloidal systems with spherical particles / emulsion droplets are structured; I would call them "dissipative structures", I would assume such systems are not in thermodynamic equilibrium, and if yes, then structure formation is not surprising (non-equilibrium systems all exhibit a rich diverse structure - so-called "dissipative structures").

Bernhhard, Colloidal Systems are too different in nature, including their structures, to be described by UNIFIED THEORY. For example, suspentions of Kaolinites stay alone case, violates DLVO enen at small volume fraction, the final structure is independent on the chemistry of liquid phase, it can be even organic solvent.. I published results of the unique experiments in the PROCEEDINGS, found them and posted the FULL TEXT on my RG site. Its entitled: "SETTLING of SYNTETIC SLUGES and KAOLINITES", you can take a look. Probably some of your materials may have similar behavior.

Alexander, personally, I *believe* that colloidal systems in ANY matrix (= continuous phase), whether solid colloids or emulsions, are governed by the same basic laws, by the same basic principles, irrespective of whether it is in water, polar or unpolar organic solvents or solvent mixtures.

For "colloids" I understand that the dispersed phase should be significantly below 1 µm. Sure, "1 µm" is a kind of artificial boundary, but from

*believe* means: I believe so, I assume so, it is my hypothesis (my approach in any research is to start with a hypothesis, consider alternative hyptheses, collect facts / design experiments for proving or disproving the one or the other hypothesis.

I assume that the wide variety of properties and behaviour of colloids is due to their non-equilibrium nature which is the basis of an incredibly rich variety of dissipative self-organized structures.

(please note, that I am here still talking in a hypothetical, assumption mode, guided by my experience in polymer matrix based colloidal systems which definitely are non-equilibrium systems, and they exhibit extremely diverse dissipative structures, so I started thinking about whether or not maybe other colloidal systems a reas well non-equilibrium systems? just asking first, beginning to think about it ...)

Bernhard, you keep dreaming about impossible, as Colloid Sytems is only the NAME

for very different disperse multiphase and multicomponent systems. For example, the solution of surfactants is the real ONE - PHASE Chemical solution. The Phase Transition at CMS is not the PHASE TRANSITION of FIRST ORDER, as no surface tention exist between MICELLS and the solvent. But physics treat internal molecular restructuring as PHASE TRANSITION of SECOND ORDER.

And CLAYS stay alone, as ther particles are QUADRUPPLES due to the space separation of internal electric charges. Plese see the reference I provided to you.

By insisting on unified theory, you are just irritating professional Colloid Scientists,

they will refuse any kind of associetion on this basis. Please, don't be offended, I provide the best advise to you.

Alexander, thanks for your advice. I will not be able to draft such a unified colloid theory, but I hope to motivate other scientists (younger than me) to work on it. In the meantime, I will continue to ask questions and collect experimental facts.

No one can provide a united theory, for Different Physical and Chemical System united by NAME only, but not by subsence. In the case you motivate some one, he will just lose his time. But sure, PERPETTOM MOBILES are still being invented, no harm is done.

Thanks, Vincent, for supporting my point of view. I am an old man, have over 50 years of professional experience, largely in theoretical modelling of different Colloid and Surface Phenomena. Here I name basic Attraction Forces between Colloid particles: Molecular Forces as in DLVO, Dipol-Dipol and Quadruple-Quadruple Ineractions as in the case of Kaolinites, or in Electro-Reological Suspentions, Magnetic Forces as in the case Magneto-Rheological Suspentions, bridging effect as for surfactant induced coalesense, etc.

Repulsion Forces: Disjoining Pressure as in DLVO, Osmotic Repulsion Forces as in the case of adsorbed surfactant monolayers on the surfaces, Steric Barrier as in protection by adsorbed polymers, I can go on and on. But most common repulsion forces are either as in DLVO, or as in the case os ADSORBED SURFACTANTS. Only OSMOTIC PRESSURE unites these 2 cases, but it fails with POLYMERS. and with other possible cases.

Now, go and provide the UNIFIED THEORY FOR ALL POSSIBLE CASES OF COLLOIDAL STRUCTURES. In the case anyone can post such a UNIFIED THEORY

on his RG site, I announce the PRIZE $10,000 paid out of my bank account. Sorry for this small number, but I SERVED SCIENCE, not the MONEY.