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I am considering a suspension of a fine powder in a high-viscosity liquid. The suspension shall be stable over long time scales, i.e. the particles only move a few mm per year, and I have the following boundary conditions:

  • The particles sizes shall be between 0.1 and 1.0 microns, so the suspension is colloidal
  • The concentration of dispersed particles in the medium is high (the phases have approximately equal volume).
  • The viscosity of the medium must be high. I am considering >10000 cSt, which is comparable to thick honey or shower gels
  • The medium will also be free of water; possibly long hydrocarbons

In a first naïve treatment with Stokes' law I thought that such a suspension should be sufficiently stable. Then I read about interparticle effects, which can lead to agglomeration of the particles and sedimentation (mainly on wikipedia). I don't know yet which material will be used as the solid powder, so I have to consider the worst case of attracting forces between the particles, so that there will be agglomeration and sedimentation in my case. This has to be prevented, but for several reasons I would like to avoid steric stabilization.

However, all examples of agglomeration considered a "thin" liquid like water, while polymeric stabilization technique is based on long polymer chains in the medium. Since in my case the viscosity of the medium is very high in my case, I wondered if this might be enough to prevent agglomeration. The high viscosity is due to long molecular chains, so I'm thinking that this might count as a case of polymeric stabilization. Is it possible to prevent agglomeration and sedimentation of microparticles just by increasing the viscosity of the medium?

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If you can make this, it will be stable. At equal volume of the phases, this will already be solid-like (0.5 is above percolation threshold, which is in the range of 0.25-0.3, depending on the geometry of your particles.).

See https://en.wikipedia.org/wiki/Percolation_threshold#Thresholds_for_3D_continuum_models

The absolute particle size is irrelevant, only the size distribution matters. When you reach the percolation threshold, the only thing that matters about the continous phase is that it's incompressible, which is the case for practically all condensed matter. If it's viscosity is very low (and the solid particles large, > ~100 µm), it could flow out of the compound, but surface tension would usually preclude that.

Also a very lowly viscous continuous phase could mean that the compound yields already to gravity. https://en.wikipedia.org/wiki/Yield_(engineering)

However it is not so simple to make such a suspension. If you try to mix powder and polymer, your stirrer (of whatever kind) will stop or break long before you get a homogeneous mixture, because your sample becomes solid, and there is no way of melting it or anything. A helper solvent can lower the solid fraction, below the percolation threshold, but is hard to get rid of later.

So you'll have to find out where the percolation threshold actually is, and just use a bit more. It varies a bit with temperature (your polymer phase will have a larger expansion coefficient, not sure if that is enough to make the difference). Anyway the yield stress is still tolerable just above the threshold, and processing is possible.

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  • $\begingroup$ This is a very interesting aspect I didn't think of before. But I cannot really make much sense of the link. I can't quite relate the mathematical concept to the physical suspension. In particular, there is no dependence on the particle size or on the viscosity of the medium, which seems counterintuitive to me. $\endgroup$ – Sentry May 21 '18 at 8:29
  • $\begingroup$ I updated the answer. $\endgroup$ – Karl May 21 '18 at 13:01
  • $\begingroup$ Thank you, these were valuable additions. I think I'll have to dig a bit deeper into the matter. $\endgroup$ – Sentry May 21 '18 at 16:36

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