SHORT-TIME TRANSPORT PROPERTIES OF BIDISPERSE COLLOIDAL SUSPENSIONS AND POROUS MEDIA
2.7 Results for porous media Permeability (mean drag coefficient)Permeability (mean drag coefficient)
The permeability, presented in terms of the mean particle drag coefficient hFi in Eq. (2.40), is shown in Fig. 2.21 for bidisperse porous media of λ = 2 and 4 and y1= 0.5, as well as for monodisperse media. The monodisperse results of Ladd [81]
and van der Hoefet al.[25] are also shown in the figure. Note that near close packing, hFi does not diverge as the fluid can pass through the interstitial spaces between particles. The SD results agree with earlier studies forφ <0.25, and underestimate hFiat higher φ. At φ = 0.6, the drag coefficient from SD is only 40% of the LB computations of van der Hoefet al.[25] in Fig.2.21. This is becausehFiis strongly affected by the many-body HIs, and the lubrication interactions only play a limited role. As a result, the computation ofhFirelies on the accurate estimation of the grand mobility tensor. The multipole expansion to the mean-field quadrupole level used in SD is insufficient to capture the HIs between stationary particles, similar to the errors associated with the sedimentation velocityUs,α in Sec.2.6.
For bidisperse suspensions, SD remains valid for φ < 0.25, and at higher φ it is expected to capture the qualitative aspect of the particle size effects. Since each stationary particle in a porous medium acts as a force monopole, the particle size plays a relatively minor role. This is confirmed in Fig.2.21, where the bidisperse hFi closely follows the monodisperse data. At low φ, the mean drag coefficient increases slightly with the size ratio λ. The behavior for φ > 0.25 arises from the complex interplay between the HIs and the particles configurations.
The semi-empirical expressions for the drag coefficient, Eq. (2.67) and (2.68), are also plotted in Fig. 2.21. For monodisperse porous media, Eq. (2.67) accurately captures earlier simulation results [25, 81] even in the dense limit. For bidisperse porous media, comparing to the SD results at lowφ, the empirical expressions work well forλ= 2, but underestimate the size effects forλ =4. This may be because in constructing Eq. (2.68), van der Hoefet al.[25] did not consider the case ofλ = 4 at low to moderateφin their simulations.
The effects of composition y1 on the drag coefficient ratiohFi/F(φ), whereF(φ) is the monodisperse drag coefficient, for bidisperse mixtures atλ= 2, are presented in Fig.2.22. The empirical expressions Eq. (2.67) and (2.68) are not shown because they do not recover to the correct limit when y1 → 0 or 1. Over the wide range of φ presented, except when φ > 0.62, the mean drag coefficient hFi for the mixture differs from the monodisperse results by at most 10%. Introducing a second
0 0.2 0.4 0.6 0.8 1 y1
0.9 1 1.1 1.2 1.3 1.4
<F>/F(φ)
0.5 0.6 0.62 0.635
0 0.2 0.4 0.6 0.8 1 y1
0.9 1 1.1
<F>/F(φ) 0.06
0.25 0.4
Figure 2.22: The normalized mean drag coefficient hFi/F(φ) as a function of y1
at different φ for bidisperse porous media with λ = 2. The monodisperse drag coefficient at the correspondingφisF(φ).
species of a different size to a monodisperse porous medium first increases the mean drag coefficient for φ < 0.4, while at higher volume fractions, the second species reduces hFi for φ < 0.6 and then increases the mean drag coefficient again near the monodisperse close packing. Atφ =0.635, hFiis merely 21% higher than the monodisperse drag coefficientF(φ). The relative insensitivity ofhFitoy1suggests that the particle size plays a minor role in the permeability of porous media. Fig.2.21 and2.22 show that SD remains a useful tool [101] to assess qualitative aspects of polydisperse porous media.
Translational hindered diffusivity
Fig. 2.23 presents the translational hindered diffusivity, dtHD,α, as a function of the volume fraction φ for bidisperse porous media with y1 = 0.5 and λ = 2 and 4, as well as for monodisperse porous media. The self-consistent expression of Eq. (2.69) [90], also presented in the figure, agrees with the SD computation for φ < 0.05 and underestimate the results at higherφ. Note that the hindered diffusive properties describe particle relative motions in a stationary matrix, and therefore the lubrication effects are important.
Compared to the suspension short-time translational self-diffusivitydts,αin Sec.2.6, the hindered diffusivitydtHD,αexhibits a strongerφandλdependence due to stronger HIs in porous media. In particular,dtHD,α decreases quickly withφwith an initial∼
0 0.1 0.2 0.3 0.4 0.5 φ
0 0.2 0.4 0.6 0.8 1
dt HD,α/dt 0,α
λ=1 λ=2, α=1 λ=2, α=2 λ=4, α=1 λ=4, α=2 F & M (1978)
0.5 0.55 0.6 0.65 φ 0.01
0.1
dt HD,α/dt 0,α
Figure 2.23: (Color online) The translational hindered diffusivity dtHD,α, withα ∈ {1,2}for both species, as a function of φfor bidisperse porous media withy1 =0.5 andλ = 1, 2, and 4. The result of Freed & Muthukumar [90], Eq. (2.69), is shown in dashed line. The inset shows the results at highφ.
√φreduction. The hindered diffusivity for small particles,dHD,1t , exhibits moderate enhancement relative to the monodisperse systems similar to dts,1. Moreover, at a fixed φ, the large particle hindered diffusivity dtHD,2 reduces appreciably with increasing λ, in contrast to the λ-insensitive dts,2 in suspensions. The increased sensitivity is simply because the fixed particle matrix exerts much stronger HIs on a mobile particle inside. For very dense systems shown in Fig. 2.23 inset, the hindered diffusivities for both species display dramatic reductions atφ >0.6 as the nearby stationary particles get closer, and the reduction is most pronounced near the close packing volume fraction. Moreover, the large particle dHD,2t approaches the monodisperse value atφ ≈ 0.63, suggesting an enhancement of dHD,2t due to more efficient particle packing in bidisperse systems.
The effects of porous media composition y1on the diffusivity ratio dtHD,α/dtHD are shown in Fig.2.24. The translational hindered diffusivity for monodisperse porous media at the same φ is dtHD. At any φ, the diffusivities dtHD,α for both species decreases monotonically with increasing y1, towards the monodisperse results for the smaller particles and away from it for the larger particles. When presented in trace amount at a fixed φ, dtHD,1/dtHDreaches a maximum for small particles while dtHD,2/dtHD reaches a minimum for large particles. Compared to the suspension dts,1/dts, the maximum of dHD,1t /dtHD is significantly higher due to stronger HIs.
0 0.2 0.4 0.6 0.8 1 y1
1 2 3 4 5 6 7
dt HD,1/dt HD(φ)
0.5 0.6 0.62 0.635
0 0.2 0.4 0.6 0.8 1
y1 1
1.2 1.4 1.6 1.8
dt HD,1/dt HD(φ) 0.06
0.1 0.25
(a)
0 0.2 0.4 0.6 0.8 1
y1 0.2
0.4 0.6 0.8 1 1.2 1.4 1.6
dt HD,2/dt HD(φ)
0.5 0.6 0.62 0.635
0 0.2 0.4 0.6 0.8 1
y1 0.4
0.6 0.8 1
dt HD,2/dt HD(φ) 0.06
0.1 0.25
(b)
Figure 2.24: The normalized translational hindered diffusivity(a): dHD,1t /dtHDand (b): dtHD,2/dtHD as a function of y1 at different φ for bidisperse porous media of λ =2. The monodisperse translational hindered diffusivity at the corresponding φ isdHDt .
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 φ
0.2 0.4 0.6 0.8 1
dr HD,α/dr 0,α
λ=1 λ=2, α=1 λ=2, α=2 λ=4, α=1 λ=4, α=2 1-1.08φ
Figure 2.25: (Color online) The rotational hindered diffusivitydrHD,α, withα ∈ {1,2}
for both species, as a function ofφ for bidisperse porous media with y1 = 0.5 and λ=1, 2, and 4. The linear fit of Eq. (2.72) is presented in dashed line.
Moreover, the increase of dtHD,1/dtHD with decreasing y1 is clearly stronger than linear when y1 → 0. For the larger particles, at low to moderate φ, as shown in the inset of Fig. 2.24b, introducing the smaller particles to the system reduces its hindered diffusivity, and the reduction enhances with increasing φ. However, for dense porous medium, particularly whenφ >0.5, increasingφat fixed y1increases dtHD,2. For φ > 0.6, the hindered diffusivity for the larger particles dtHD,2becomes extremely sensitive to the small particles. In Fig.2.24batφ=0.635, the maximum ofdtHD,2/dtHD occurs aty1 0.1. In contrast, the suspension ratiodts,2/dts exhibits less sensitivity. Note that only at φ = 0.635, the presence of the smaller particles enhances the hindered diffusivities of both species in the porous medium.
Rotational hindered diffusivity
Finally, theφdependence of the rotational hindered diffusivitiesdrHD,αfor bidisperse porous media with y1 = 0.5 at λ = 2 and 4 and for monodisperse porous media is shown in Fig.2.25. The monodisperse rotational hindered diffusivity drHD agrees with the earlier study [35] and decreases much slower with φ compared to its translational counterpart dtHD. The SD results up to φ = 0.5 can be satisfactorily described by a linear fit,
drHD
d0r =1−1.08φ, (2.72)
also shown in Fig. 2.25. This is a stronger dependence on φ compared to the suspension short-time rotational self-diffusivity drs in Sec. 2.6. Approaching the close packing volume fraction, the diffusivity dHDr decreases but largely remains finite, as the nearby stationary particles can only weakly affect the rotation of the mobile particle.
In bidisperse porous media, drHD,α for both species is highly sensitive to the size ratio λ. The bidisperse drHD,α differs significantly from the monodisperse results, and no longer displays the almost linear relation with φ. For the smaller particles, the diffusivitydrHD,1is higher than the monodisperse results, while the for the larger particlesdrHD,2is always lower. The deviation from the monodisperse results grows with increasing particle size ratioλ, and is more significant for the larger particles.
This is because the average number of neighboring particles, which produces the most significant HI to the mobile particle, scales asλ3for the larger particles.
The effects of the medium composition y1on the ratiodHD,αr /drHDforλ =2, where drHDis the monodisperse data at the sameφ, are shown in Fig.2.26. The results are qualitatively similar to drs,α/drs in Fig.2.6. Quantitatively, the effect of y1at fixed φ ondrHD,α is slightly stronger. At low to moderateφ, dHD,αr /drHD for both species decreases monotonically with increasing y1. At a fixedφ, a trace amount of small particles yields the maximum ofdrHD,1/drHD, while a trace amount of large particles leads to the minimum of dHD,2r /drHD. At very high φ, the most notable feature is the mutual enhancement ofdrHD,1anddHD,2r with a small amount of small particles, e.g., at y1 = 0.1 andφ = 0.635. The extent of the enhancement, however, is much weaker than the translational counterpart dHD,αt , but is similar to the suspension counterpart drs,α. The similarity between drHD,α and drs,α suggests that the HIs of rotational motions are weak but sensitive to the environment throughφandλ.