## High-Shear Particle Rheology

### Quick Start

Particle-filled systems at high shear and high loading have a distinctive rheological behaviour depending on particle fraction and shear rate and just two key parameters, φc for percolation and φm for packing. The effect of shape and agglomeration are also included. With two more parameters, Yield Stress can be estimated.

These ideas apply only to hard particle systems. Things get much more complex with deformable particles.

### High-Shear Particle Rheology

φc Percolation
φm Packing
N
Fractal Dim. d
Aspect Ratio Af
φ* : φc calc
Log(γ̇)
Log Normal
Viscosity
Stress
High γ̇ Corr.
Shear Thicken
λ Size ratio
k Fraction Small
φmaxBimodal
E in kT
φmax

The app is based on a very full analysis of particulate systems by Campbell et al 1. Amazingly, the effects of interest here can be described via just two parameters (E and r are used for Yield Stress, see below):

1. φc, the critical percolation threshold which is the volume fraction where "percolation" sets in - i.e. the particles form coherent clusters that extend across the whole volume. The value used in the calculation changes when the aspect ratio is changed.
2. φm which is the maximum packing fraction.

Up to φc, all that happens is that the low shear viscosity (relative to the continuous phase viscosity) increases according to one of the typical laws discussed in the viscosity app which all involve φm. In this app we use a more appropriate law based on our two inputs:

η_"rel"=exp[(φ_m-φ_c)/(φ_m-φ)]-1

Above φc we start to get shear thinning because the particles behave more and more as a solid mass, which means there is no dissipation. This follows a power law depending on the shear rate, γ̇, and the exponent n which is less than 1:

η = γ̇^(n-1)

Happily, n depends, again, only on φ and our two inputs:

n=[(φ/φ_m)^-0.667-1]/[(φ_c/φ_m)^-0.667-1]

Finally, the viscosity falls to an infinite shear value, η which depends on a "dissipation ratio" ε which, fortunately, is related to n via:

n=1-4/(3ε)+1/(3ε)^4

As ε's meaning is rather obscure, the authors point out that ν=ε² is the "volume fraction of percolating clusters" which, of course, approaches 1 as φ increases. The ratio of η to the original low shear viscosity η0 is given by:

η_∞/η_0=(1-ε)^2[1-(8ε)/3+2ε^2-ε^4/3]

### Plotting the data

There are many ways to look at the results of these formulae. As most of us are interested in viscosity versus shear rate, that's the default. It needs a log plot to show the large range of properties (you can look at the non-log to confirm this). Because viscosities get super-high close to φm, the plots cover a range from φc/2, φc, then some unequal steps to 0.9φm. If you change parameters that change the effective φc or φm then you will see large changes for better or worse. An alternative system where the plots were relative to the effective φ values proved too confusing.

The η don't show because they are down in the very small range. However, if you choose to plot stress rather than shear rate, you see a sudden increase in an otherwise smooth curve when the η becomes the limit.

### φc and φm

For spherical particles above ~1μm, φc should be around 0.28 (see below for discussions on estimating this and the Percolation app). For smaller particles, Brownian motion intervenes and values might rise somewhat. For elliptical particles, φc decreases as the ellipse gets more pronounced (larger aspect ratio of the two axes). The value used in the calculation is the smaller of your input φc and φc calc derived from the aspect ratio, as discussed below.

φm is trickier. A value around 0.63 should be normal, but self-associating particles (i.e. with poor dispersants) show values closer to 0.5 and poly-disperse systems can get up to 0.8 as the smaller particles can pack into the close-packed spaces of the larger ones. An effect currently not taken into account is that for spheres at high shear, φm is ~0.71. So entering 0.71 will give you wrong values at low shear but better values at high shear.

It is well-known that multi-modal distributions can give a higher φm. Although it is possible to provide some complex algorithms, I've chosen the Qi and Tanner model based on the chosen φm, size ratio, λ, and the fraction, k, of smaller particles. Although there is brave talk about getting packing fractions >0.8, for any normal size ratio of ~10:1, the gain (automatically added to the viscosity calculations), at 33% small particles, is modest (0.63→0.70), though the impact on viscosity for high φ values can be large.

Less well known is that a log normal particle distribution with a relatively high standard deviation above ~0.5 has a φm value in the region of 0.7. If you believe that this is what you have, then select the Log Normal option. Although most of us would think that this is because a wide distribution has lots of small particles to fill the holes between the larger ones, this, apparently, is incorrect. Those keen to read why can find an explanation in G.T. Nolan and P.E. Kavanagh, The size distribution of interstices in random packings of spheres, Powder Technology, 78 (1994) 231-238.

### Shape and agglomeration

If the shape starts to change its fractal dimension d from 3 (sphere) to 2 (platelets) and starts to form agglomerates of N particles, then φm gets replaced by φ* given by equations from a review by Bicerano et al2:

φ"*"=φ_mN^(1-3/d)

As soon as d becomes less than 3 and N greater than 1, φ* decreases rather rapidly so the viscosity at a given φ can increase rapidly. When you play with N and d nothing much seems to happen in the graph, but look at the shown φ=xx values - they decrease rapidly, which means that at any given φ, viscosity is increasing sharply. I tried keeping the lines at φ values fixed with respect to φm, but the graphs quickly got out of hand.

Because spheres have d=3 I was confused about how flocculating spheres (N>1) cause increases in viscosity, because nothing happens when d=3. But the fractal dimension of flocculating spheres ~2. It takes only a moment playing with mild flocculation (N=3, d=2.5) to see why we need really good dispersants if we want low viscosity!

The percolation threshold, φc also changes with shape and can be estimated from the Aspect Ratio, Af via:

φ_"c calc"=(9.875A_f+A_f^2)/(7.742+14.61A_f+12.33A_f^(1/5)+1.763A_f^2+1.658A_f^3)

### High Shear Corrections

At high shear, two things happen:

1. φm increases because we go from a random packing limit to a close packing limit.
2. Flocculated particles (i.e. a combination of N and d) become ripped apart by the flow and behave closer to unflocculated particles.

The science of both these effects is well-known, but I don't have a reliable way to add the effects. Instead I've created an ad-hoc correction (which you can turn on and off): φm increases by 0.07 (the case for spherical particles) up to a shear of 1000 which is here considered to be "infinite"; N decreases by 1 over this shear-rate range. Both changes are conservative, but the reductions in viscosity are clearly seen. If some more accurate way to simulate these effects is provided, I'll be happy to update the code.

### Shear Thickening

In a well-protected particle dispersion, shear thinning continues to an "infinite shear" plateau. However, many systems, at higher loadings, show a large increase of viscosity at high shear rates. This is a transition from "hydrodynamic flow" where the particles are like cars driving along a multi-lane highway to a state of "lubrication dynamics" where (effectively) the flow around each "car" sucks the neighbouring one into contact. This creates (effectively) a gigantic local pile-up ("hydroclusters") which inhibits the flow along the highway. The theory of this effect is complex, so the Shear Thickening option is just a visual representation of the effect.

The Particle Rheology MCT app says more about the various flow regimes.

### Yield Stress

The YODEL model for yield stress from Flatt and Bowen3 is a great combination of simplicity and power. The equation, using the nomenclature of this page is:

"Yield Stress" = (m_1φ(φ-φ_c)^2)/(φ_m(φ_m-φ)) = k(E/r)(φ(φ-φ_c)^2)/(φ_m(φ_m-φ)

The φ values have already been discussed, so all we need is m1. Unfortunately, at present I don't properly understand how to calculate m1 from typical values. However, it clearly depends on E, the size of the repulsion forces between particles, which we can estimate from the DLVO app as being a few kT, and 1/r, the radius of the particle. To provide values that are typical of those found in the original paper I have simply used a fudge factor. So the general trends are correct, the details aren't. Hopefully I will be able to update the calculations at some point. My (poor) understanding is that the value of E to be used is the overall attractive force (which is a negative value in DLVO). If you have sufficient steric or charge repulsion to avoid attraction then, I assume, there is no yield stress. At high volume fractions this seems non-intuitive.

The plot goes up to your chosen φmax. Because the assumption is that these are spherical particles, there is no yield stress from the particles below the percolation limit of 0.28 or lower (set from whichever is smaller, φc and φc calc which is calculated when you change the aspect ratio), the smallest allowed φmax is 0.3.

1Gregory A. Campbell, Michael E. Zak, Mark D. Wetzel, Newtonian, power law, and infinite shear flow characteristics of concentrated slurries using percolation theory concepts, Rheol Acta(2018) 57:197–216

2Jozef Bicerano, Jack F.Douglas, Douglas A. Brune, Model for the Viscosity of Particle Dispersions, J. Macromolecular Science, Part C, 39, 561-642, 1999

3Robert J. Flatt and Paul Bowen Yodel: A Yield Stress Model for Suspensions, J. Am. Ceram. Soc., 89, 1244–1256 (2006)