György
Tegze
*a,
Frigyes
Podmaniczky
a,
Ellák
Somfai
a,
Tamás
Börzsönyi
a and
László
Gránásy
ab
aInstitute for Solid State Physics and Optics, Wigner Research Center for Physics, P.O. Box 49, H-1525 Budapest, Hungary. E-mail: tegze.gyorgy@wigner.hu
bBCAST, Brunel University, Uxbridge, Middlesex UB8 PH3, UK
First published on 2nd September 2020
Suspensions of neutrally buoyant elliptic particles are modeled in 2D using fully resolved simulations that provide two-way interaction between the particle and the fluid medium. Forces due to particle collisions are represented by a diffuse interface approach that allows the investigation of dense suspensions (up to 47% packing fraction). We focus on the role inertial forces play at low and high particle Reynolds numbers termed low Reynolds number and inertial regimes, respectively. The suspensions are characterized by the orientation distribution function (ODF) that reflects shear induced rotation of the particles at low Reynolds numbers, and nearly stationary (swaying) particles at high Reynolds numbers. In both cases, orientational ordering differs qualitatively from the behavior observed in the Stokesian-regime. The ODF becomes flatter with increasing packing fraction, as opposed to the sharpening previous work predicted in the Stokesian regime. The ODF at low particle concentrations differs significantly for the low Reynolds number and inertial regimes, whereas with increasing packing fraction convergence is observed. For dense suspensions, the particle–particle interactions dominate the particle motion.
A single non-Brownian ellipsoid immersed in a simple shear flow of a Stokesian fluid rotates with modulated angular velocity resulting in a time averaged orientation parallel to the streamlines.7 The case of interacting non-Brownian rods placed into shear flow was studied by several authors mostly for fiber like objects, i.e., for aspect ratios above 10. Stover et al. investigated experimentally the orientation distribution in a semi-dilute non-Brownian fiber suspension (nL2d ≪ 1) of particle aspect ratios L/d = 17–30 in cylindrical Couette geometry and found a slight misalignment relative to the flow direction.8 Rahnama et al. calculated the effect of hydrodynamic interactions between non-Brownian fibers using slender body theory and found good agreement with Stover's results.9 Yamane et al. carried out numerical simulations to explore the semidilute regime (1/L3 < n < 1/L2d), where short range hydrodynamic interaction between the particles cannot be neglected.10 These simulations showed that entering the semidilute regime does not bring strong changes in the behaviour of the suspension. Sundararajakumar and Koch performed numerical calculations to study the effect of direct mechanical contacts in semi-concentrated suspensions (nL2d = O(1)) of non-Brownian fibers11 and found a slight misalingment for L/d = 30. We note, however, that the addition of Brownian noise results in a deviation of the time averaged orientation from the flow direction already for a single ellipsoid.12,13 Brownian suspensions or single component Brownian systems (thermotropic nematics) also show a slight misalingment in shear flow.1,3,4,12,13 In this case the orienting effect of the shear flow is competing with the Brownian noise. The dynamical behavior and rheological properties of such systems were investigated by Doi, Edwards, Marrucci, Dhont and others14–18 and it was shown that the rotational diffusion coefficient rapidly decreases with increasing concentration and elongation of the particles. This is due to a hindering effect which neighboring rods exert on the rotational motion of the particle. Shaqfeh and Fredrickson quantitatively analyzed how the surrounding particles screen the hydrodynamic disturbance induced by the particle in the large elongation limit.19,20 Despite the similar behaviour observed in the systems mentioned above, they often show distinct features, which depend on the forces (e.g., contact, hydrodynamic, viscous or inertial) that dominate the particle trajectories. An interesting comparison could be made with the flow of dry granular matter,21 where particles are also known to misalign with respect to the flow direction.
In the present work, we explore ensemble averaged alignment of non-Brownian elliptic particles of aspect ratio L/d = 3, while varying the volume fraction of the particles in a wide range. We focus on how particle–particle interactions perturb single particle dynamics. Our results will be compared to the angular distribution measured for dry granular materials,21 in which the particles’ motion is driven exclusively by contact forces owing to the absence of a fluid medium.
The first theoretical description of a rotating ellipsoidal particle in sheared fluid medium was developed by Jeffery.7 The calculation assumes no-slip boundary conditions and was made in the Stokes limit, where the inertial effects are negligible. His findings were confirmed experimentally,26 and the description was extended to more general shapes.27,28 According to Jeffery's formula, the particle undergoes a periodic rotation with modulated angular velocity that shows maxima, when the major axis is perpendicular to the streamlines, and minima when it is parallel with them. We note, that due to the time reversal symmetry in overdamped dynamics, the alignment angle, defined as the angle between the average orientation of the particle major axis and the streamlines, is zero for an orbital period. Therefore the misalignment found in various experiments cannot be attributed to viscous stresses.
There is no first principles theory that accounts for the inertial effect on particle orientation, although there are a number of fully resolved simulations that address the motion of ellipsoidal29,30 or elliptic particles31,32 in a Newtonian fluid. On the particle scale the ratio of inertial and viscous forces can be characterized by the particle Reynolds number Rep = d2η, where
is the macroscopic shear rate of the colloid suspension, d is the largest diameter of the particle (i.e., the major axis for an ellipse) and η is the kinematic viscosity. It was found that with increasing Rep the orbital period of a single isolated particle increases, while at a critical value Recrp of the particle rotation, there is a transition from time periodic rotation to a steady motion through a saddle-node bifurcation.29 The fluid flow pattern around the particle is also different for the two cases: while a rotating particle involves crossing streamlines outside the particle and the flow pattern is characterized by two stagnation points,33 in the steady, torque free case the streamlines are separated.34 In general, the torque-free orientation of the particle is not aligned with the streamlines, the orientation angle increases with Rep − Recrp.32 These works provide evidence that inertial forces can cause nonzero averaged orientation angle, which is observed experimentally as well.35
The above discussion is valid for very dilute suspensions, in which the particle–particle interaction is negligible. In this work, we analyze how the hydrodynamic interaction and contact forces in suspensions modify this picture while varying the packing fraction. Herein, we investigate both the Rep < Recrp domain, termed here “low Rep regime”, and Recrp < Rep, termed the “inertial regime”. We control the average strength of the particle interaction by varying the particle volume fraction,† changing thus the average distance between the rigid particles. We explore a relatively wide range of volume fractions 0 < ϕ < 0.48, which is below the critical volume fraction reported by Cuesta et al. corresponding to the isotropic–nematic transition in a system of ellipses interacting with hard core interaction in the presence of thermal noise in the absence of shear. This critical volume fraction was found to be around 0.6 for 2D ellipses with aspect ratio L/d = 3.0.36
It is characteristic to all Stokesian or Maxey–Riley-type models22 that the characteristic length scale of fluid flow must be larger than the particle size. Therefore, the model is not applicable for high density suspensions, where complex flow patterns evolve due to the hydrodynamic interaction of particles. In such cases, a more detailed Eulerian–Eulerian approach is needed, which resolves fluid flow on the particle scale.
The equations governing fluid flow incorporate the Navier–Stokes eqn (1) along with the incompressibilty constraint (2), both of which were solved in the whole computational domain, and the rigidity constraint (3) that was applied in the volume of the particles (Γp):
![]() | (1) |
0 = −∇·v, | (2) |
![]() | (3) |
In the model the i-th particle is represented by a continuum field (ϕi), that is constructed to give zero outside the particle and one inside. The boundary of the particle is represented by a diffuse interface, with a thickness larger than the actual grid spacing, typically 3–4 grid spacing in thickness. Following Wang's DIFA method44 we apply a steric repulsion force density (4). This results in a force and torque on the particles; for the i-th particle these are given by eqn (5) and (6) respectively.
![]() | (4) |
![]() | (5) |
![]() | (6) |
Contact forces generated by the above formulae affect both particle trajectories, and the fluid's velocity field thus representing two-way particle–fluid coupling.
![]() | ||
Fig. 1 Schematic view of the setup, with the definition of the orientation angle θ and rotation angular velocity ω. According to our conventions typically 〈ω〉 ≤ 0. |
To model the behaviour of elongated particles in 2D in a uniform shear flow, we set fixed velocity boundary conditions on the top and bottom boundaries, while at the left and right boundaries periodic boundary condition was prescribed (Fig. 1). The dimensions of the computational domain were 2160 grid points in transverse and 3840 grid points in flow direction. The particles immersed in the fluid were neutrally buoyant, had an elliptical shape with major and minor axis of 30 and 10 grid points, respectively; i.e., their aspect ratio was L/d = 3.0. We examined their behaviour at two Reynolds numbers Re = 103 and Re = 104, which correspond to particle Reynolds numbers of Rep = 0.772 and Rep = 7.72, respectively. We expect different behaviors in these two regimes: at the smaller shear rate (in the low Rep regime) a single particle rotates similarly to the Jeffery orbits, though with small deviations due to the nonzero shear rate. The larger shear rate falls above Recrp, the system is in the inertial regime, in which isolated particles moves with a constant orientation angle that varies with Rep.
![]() | ||
Fig. 3 Alignment of suspended particles in shear flow. (a) and (b) Probability density of the particle orientation for various volume fractions ϕ in the investigated regimes: (a) low Reynolds number at Rep = 0.772 and (b) inertial at Rep = 7.72. Panel (c) shows the average orientation angle for both shear rates as a function of volume fraction. For comparison, in panels (a) and (b), experimental results21 for dry rice particles (of aspect ratio 3.4) are also shown (gray shaded areas). (d) Nematic order parameter as a function of the packing fraction. |
Since the macroscopic fluid flow conditions (i.e., the global shear rate and fluid viscosity) do not change, the variations in the angular distribution can be attributed exclusively to hydrodynamic and contact forces amongst the particles. Even in the dilute case (5.6% volume fraction) doublets and clusters form, and these inter-particle interactions cannot be neglected. The same conclusion was drawn for the Stokesian limit in a previous work.47 We note, however, that the effect of ordering on the angular distribution differs considerably at low Rp and in the overdamped limit of the Stokesian regime: in the latter case the peak of the angular distribution becomes narrower with increasing volume fraction.47 In turn, the shift in the peak position seems to be generic for both for nonzero Reynolds numbers and in the Stokesian regime. Our data is based on high parfticle number simulations, thus the distributions are smooth even for the lowest volume fraction case. We also note that, as shown in Fig. 3(a), the ODF does not just flatten with increasing volume fraction, but it's skewness is positive and increases.
In the inertial regime (Rep = 7.72 > Recrp), where an isolated particle would attain a steady orientation, the change of the ODF with volume fraction is more pronounced, see Fig. 3(b). This is not surprising since a standalone particle would yield a delta function ODF, while hydrodynamic interaction can either cause swaying or rotating motion of the particles, which result in a stronger widening of the ODFs, when increasing the volume fraction. This results in a strong decrease of the nematic order parameter (S) as a function of ϕ up to volume fractions of about 0.25 [Fig. 3(d)]. For larger volume fractions S increases slightly due to the suppression of flipping events by particle–particle interactions. The probability of finding flipping particles is also non-monotonic with ϕ [see inset of Fig. 3(b)]. At low ϕ it is increasing due to the disturbance of the steady orientation, while it is decreasing in dense systems due to the hindering effect of near neighbours. Interestingly, the change of the mean alignment angle is non-monotonic, at low volume fractions both the mean and the mode of the distribution is shifting towards lower angles, while above ϕ = 0.15 the mean alignment angle is increasing [see Fig. 3(c)].‡ Similarly to the low Rep regime the orientation distributions are strongly skewed: the orientation distributions show more pronounced right tails in all cases. For the sake of comparison, in Fig. 3 we also present data for quasistatic shear flows of dry granular particles taken from experiments21 with rice grains of aspect ratio 3.4. This dataset can be regarded as the high volume fraction limit (with ϕ = 0.59 in 3D), where contact forces drive the evolution of the orientation distribution. At high volume fractions, our simulations predict higher average alignment angles then observed in the experiments. The difference can be mostly attributed to the skewness of the computed distributions, which may stem from the reduced degrees of freedom of the particles in 2D.
To assess the role of the particle interactions, we varied the volume fraction from 5.6% to 47.3%. It was found that below about −60° the angular velocity varies non-monotonically with volume fraction, whereas for θ > −60° the angular velocity decreases with increasing volume fraction. This decrease is particularly prominent for 20° < θ < 70°, while the curves become flat close to zero. This behavior is reflected in the enhanced skewness of the respective ODFs [Fig. 3(a)].
In order to identify the origin of this behavior, we quantify the effect of collisions with nearby particles via computing a spatial correlation function (SCF), which is defined as the average volume fraction of nearby particles (∑ϕi) as a function of the position relative to the centre of a reference particle, whose orientation falls in the neighbourhood of a given angle θ (i.e., between θ − 1° and θ + 1°). Besides providing a visual representation of the average local structure around the particles, the SCF allows the computation of the average net torque acting on the particle at a given angle due to particle collisions.
As can be seen in Fig. 4(b), the average torque is large between 20° ≲ θ ≲ 70°, especially for high volume fractions. It can be, therefore, concluded that the particle interactions significantly block the rotation due to shear flow and reduce the average angular velocity of the particle at these angles. One may also conclude that the pronounced tail on the right side of the ODF originates from the particle collisions in this orientation regime. In contrast, for θ ≲ −20° the torque attributed to particle collisions is negative, which enhances the rotation of the particles. As a result, the angular velocity curves intersect each other at negative angles.
Visual analysis of the SDF's (Fig. 5) reveals, that the torque characteristic to a given angle originates from the local arrangement of the particles. For particles, whose orientation falls close to the average orientation, the rotation is hindered/blocked by neighbors represented by well defined peaks in the SDF.
Adding more particles (increasing the particle volume fraction) leads to larger angular velocities outside the 0° ≲ θ ≲ 70° range. The abrupt change in the angular velocity indicates a complex particle–particle interaction: at higher concentrations nearby particles pass by each other at smaller distances, which causes disturbances that may lead to eventual overturn (flipping) of some of the particles. The presence of stable orientation of particles is visible even at the highest volume fractions we used: the angular velocity is very small in the angular range 0° ≲ θ ≲ 50°.
Since single particle hydrodynamic effects are significant at these Reynolds numbers, the pair correlation function, and consequently the particle hindrance is less relevant in this regime.
We have shown that in the low Reynolds number regime, the motion of the particles resembles the predictions of the analytically solvable reference case by Jeffery, however, with the following essential differences:
(i) At nonzero Reynolds numbers, due to strong inertial effects in the surrounding fluid flow, the rotation of an isolated particle is the slowest when it is somewhat misaligned relative to the streamlines. A narrower ODF is observed, which is no longer symmetric around zero angle.
(ii) The other type of deviation is due to the particle–particle interaction. As the volume fraction of the particles increases, it becomes progressively prevalent that a nearby particle hinders the rotation of a given reference particle, especially when it is oriented at a small positive angle from the streamlines. This is clearly visible in the pair correlation plots, and is well captured by the average steric torque computed from spatial correlation functions. The particle–particle interaction leads to an asymmetric pronounced tail of the ODF, and further reduced angular velocities for the respective angles.
We note that in the inertial regime, an isolated particle attains a constant, stable orientation instead of rotating with an angular velocity depending on the momentary orientation. In this regime, the particles are slightly tilted from the streamlines by an angle that depends on the Reynolds number. When the volume fraction of the particles is increased to a level that is still so low that the particles pass by each other at a distance, the disturbance due to the hydrodynamic interaction makes them sway back and forth. This results in a broadened ODF; however, the average angular velocity stays zero. Increasing further the volume fraction, the disturbances reach a point, where some of the particles start to flip over. This results in a dramatic broadening of the ODF, and a sharp increase in the angular velocity contrasting the originally stable alignment relative to the streamlines.
Footnotes |
† We use the widely accepted term “volume fraction,” even though in our 2D simulation “area fraction” would be more precise. |
‡ A continuous transition is expected between the monotonic/non-monotonic behaviors observed at low and high Reynolds numbers. Further work is, however, needed to clarify whether this transition happens at the same critical Reynolds number as in the single particle case. |
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