Eric Dickson

Several points:

• The sample may be better measured with laser diffraction at these sizes for an orthogonal light scattering technique - one expects significant multi-angle dependency at 450 nm
• Your PDI is low, so that's a good sign that your system is homogeneous. This is supported by the quality report
• The international (ASTM & ISO) standards recommend a dilution ladder as you may have particle-particle interactions at high concentration reducing the Brownian motion and thus an apparent larger size may be the outcome
• The dilution should be in the 'mother liquor' and not in DI water. The latter is about the worst medium to dilute a sample as all the surfactants and stabilizers can be washed out. At 450 nm your (starting suspension at least) should be milky. Transparency is normally an indicator of a system < 100 nm
• In your system I suspect that dispersion is not a major issue
• 3 measurements have no statistical validity whatsoever. Throw away the 2 outliers and you're left with a perfect measurement... Use groups of 5 or 10
• Take care with the number of decimal places you quote. Find out the diameter of a hydrogen atom...

yes you can

Fria Hossein

Thanks for your answer.

But the original dispersion was prepared in PEG and for diluting purposes, I used water.. Any chance the particles aren't dispersed in the original dispersion but dispersed well in water during the diluting process?

Just curious. Thanks

It actually depends on particle characterisations such as shape and size and also how you disperse it, rather than the volume.

You can dilute in specific proportion and check.

Eric Dickson

Several points:

• The sample may be better measured with laser diffraction at these sizes for an orthogonal light scattering technique - one expects significant multi-angle dependency at 450 nm
• Your PDI is low, so that's a good sign that your system is homogeneous. This is supported by the quality report
• The international (ASTM & ISO) standards recommend a dilution ladder as you may have particle-particle interactions at high concentration reducing the Brownian motion and thus an apparent larger size may be the outcome
• The dilution should be in the 'mother liquor' and not in DI water. The latter is about the worst medium to dilute a sample as all the surfactants and stabilizers can be washed out. At 450 nm your (starting suspension at least) should be milky. Transparency is normally an indicator of a system < 100 nm
• In your system I suspect that dispersion is not a major issue
• 3 measurements have no statistical validity whatsoever. Throw away the 2 outliers and you're left with a perfect measurement... Use groups of 5 or 10
• Take care with the number of decimal places you quote. Find out the diameter of a hydrogen atom...

Hi Eric,

I would say that if your particles were aggregated in the concentrated stock, diluting them in DI water would not cause them to redisperse. So, there is a relation between the dispersion state of your concentrated and diluted samples.

I would probably try less dilution and see if I get similar results.

Also, I really like the organized breakdown of your issue.

Best of luck with your work!

Omar Z. Sharaf Thanks.

Yes, I will dilute the original dispersion to a series of samples and measure every one of them. Hope I can get similar results.

Alan F Rawle Thanks for your comments.

Actually, I was trying to make the shear thickening fluid (STF) reported by Lee et al in the journal of materials science, 38 (2003) 2825-2833.

My colloidal dispersion is an STF prepared by mixing 69.5wt% SiO2 and 30.5wt% PEG. This STF is a thick, sticky, non-transparent substance which thickens from 3Pa.S to 160Pa.S at 20s^(-1) shear rate.

However my STF does not thicken as much as Lee's. That's why I wanted to check the particle dispersing situation in my STF.

Eric Dickson

One difficulty battled with in our ISO TC24/SC4 Particle Characterization committee is to define 'good dispersion'...

Alan F Rawle I see.

Hi Eric,

I noticed that in the article your refer to, the dispersing medium is actually ethylene glycol diluted with ethanol. If I correctly understand from your description, you seem to have used a PEG solution instead.

If you want to get the exact same shear-thickening behavior as that in the referred article, it would be important to first carefully replicate how they prepared their STF. Then look into carefully replicating their testing conditions during your rheological characterization measurements (e.g., shearing time per point).

Hope this helps!

Omar Z. Sharaf , thanks for the advice.

Eric Dickson

Here are several checkpoints you can do.

1. The reason for using PEG200 for silica is to make index-matched suspension for minimizing the van der Waals interaction. In this condition, since the refractive index is matched, you sample should be close to transparent. If you can still see some white aggregates on the wall, that means it's still not well-dispersed. I personally recommend high speed blade mixer or ultrasonication for better dispersity.

2. Check the measuring range of your DLS. Note that DLS can hardly pick large aggregates larger than 10 microns. Laser diffraction will be a better choice.

3. If you still didn't see shear thickening effect, try a longer higher shear stress preshear protocol to break down the aggregates. You can find the protocol in the following references.

1. Cwalina and Wagner, Journal of Rheology, 2014

2. Lee et al., Journal of Rheology, 2020

Yu-Fan Lee , many thanks for your comments and the papers you shared. They're helpful.

When I read the 2003 paper by Lee and Wagner, I noticed the following statement

• 'the index of refraction of ethylene glycol is close to that of the silica particles, providing enhanced colloidal stability'

I didn't pay attention to the statement until you mentioned 'to make index-matched suspension for minimising Van Der Waals interaction'. Now I begin to understand the use of a glycol medium as the dispersing phase. Thanks.

You mentioned 'since the refractive index is matched, you sample should be close to transparent'. Would the STF be close to transparent even the particle loading reaches 65% by weight? I've ball-milled my STF (Silicon dioxide + PEG200) for 24 hours, and the STF looks relatively transparent only when it forms a layer several millimetres in thickness.

Also, what method do you recommend for the removal of the gas micro-bubbles in the STF? Do you vacuum the bubbles out?

By the way, wow! I finally found someone from the university of Delaware working on STFs. Great! I always think Prof Norman Wagner's STF amazing, and would like to know the preparation details.

Best wishes, Eric Dickson

Eric Dickson

The transparency is more about the scattering of the light which should be independent to the amount of the loading. I would say relatively transparent should be fine because we can never perfectly index-matched the particles. Keep in mind that make sure you check there is no chunks/aggregates on the vial wall, if you can literally see it, that means it's still not well-dispersed.

Yes, you can for sure just vacuum out the bubbles, as the PEG should be non-volatile solvent, which shouldn't alter the concentration.

Hope this helps.