 NSFPAR ID:
 10225647
 Date Published:
 Journal Name:
 Soft Matter
 Volume:
 16
 Issue:
 34
 ISSN:
 1744683X
 Page Range / eLocation ID:
 7982 to 8001
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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We develop a linearly scaling variant of the force coupling method [K. Yeo and M. R. Maxey, J. Fluid Mech. 649, 205–231 (2010)] for computing hydrodynamic interactions among particles confined to a doubly periodic geometry with either a single bottom wall or two walls (slit channel) in the aperiodic direction. Our spectrally accurate Stokes solver uses the fast Fourier transform in the periodic xy plane and Chebyshev polynomials in the aperiodic z direction normal to the wall(s). We decompose the problem into two problems. The first is a doubly periodic subproblem in the presence of particles (source terms) with freespace boundary conditions in the z direction, which we solve by borrowing ideas from a recent method for rapid evaluation of electrostatic interactions in doubly periodic geometries [Maxian et al., J. Chem. Phys. 154, 204107 (2021)]. The second is a correction subproblem to impose the boundary conditions on the wall(s). Instead of the traditional Gaussian kernel, we use the exponential of a semicircle kernel to model the source terms (body force) due to the presence of particles and provide optimum values for the kernel parameters that ensure a given hydrodynamic radius with at least two digits of accuracy and rotational and translational invariance. The computation time of our solver, which is implemented in graphical processing units, scales linearly with the number of particles, and allows computations with about a million particles in less than a second for a sedimented layer of colloidal microrollers. We find that in a slit channel, a driven dense suspension of microrollers maintains the same twolayer structure as above a single wall, but moves at a substantially lower collective speed due to increased confinement.more » « less

Summary A hybrid computational method coupling the lattice‐Boltzmann (LB) method and a Langevin‐dynamics (LD) method is developed to simulate nanoscale particle and polymer (NPP) suspensions in the presence of both thermal fluctuation and long‐range many‐body hydrodynamic interactions (HIs). Brownian motion of the NPP is explicitly captured by a stochastic forcing term in the LD method. The LD method is two‐way coupled to the nonfluctuating LB fluid through a discrete LB forcing source distribution to capture the long‐range HI. To ensure intrinsically linear scalability with respect to the number of particles, a Eulerian‐host algorithm for short‐distance particle neighbor search and interaction is developed and embedded to LB‐LD framework. The validity and accuracy of the LB‐LD approach are demonstrated through several sample problems. The simulation results show good agreements with theory and experiment. The LB‐LD approach can be favorably incorporated into complex multiscale computational frameworks for efficiently simulating multiscale multicomponent particulate suspension systems such as complex blood suspensions.

Hypothesis: The dip coating of suspensions made of monodisperse nonBrownian spherical particles dispersed in a Newtonian fluid leads to different coating regimes depending on the ratio of the particle diameter to the thickness of the film entrained on the substrate. In particular, dilute particles dispersed in the liquid are entrained only above a threshold value of film thickness. In the case of anisotropic particles, in particular fibers, the smallest characteristic dimension will control the entrainment of the particle. Furthermore, it is possible to control the orientation of the anisotropic particles depending on the substrate geometry. In the thick film regime, the LandauLevichDerjaguin model remains valid if one account for the change in viscosity. Experiment: To test the hypotheses, we performed dipcoating experiments with dilute suspensions of nonBrownian fibers with different lengthtodiameter aspect ratios. We characterize the number of fibers entrained on the surface of the substrate as a function of the withdrawal velocity, allowing us to estimate a threshold capillary number below which all the particles remain in the liquid bath. Besides, we measure the angular distribution of the entrained fibers for two different substrate geometries: flat plates and cylindrical rods. We then measure the film thickness for more concentrated fiber suspensions. Findings: The entrainment of the fibers on a flat plate and a cylindrical rod is primarily controlled by the smaller characteristic length of the fibers: their diameter. At first order, the entrainment threshold scales similarly to that of spherical particles. The length of the fibers only appears to have a minor influence on the entrainment threshold. No preferential alignment is observed for nonBrownian fibers on a flat plate, except for very thin films, whereas the fibers tend to align themselves along the axis of a cylindrical rod for a large enough ratio of the fiber length to the radius of the cylindrical rod. The LandauLevichDerjaguin law is recovered for more concentrated suspension by introducing an effective capillary number accounting for the change in viscosity.more » « less

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The rheological behaviour of dense suspensions of ideally conductive particles in the presence of both electric field and shear flow is studied using largescale numerical simulations. Under the action of an electric field, these particles are known to undergo dipolophoresis (DIP), which is the combination of two nonlinear electrokinetic phenomena: inducedcharge electrophoresis (ICEP) and dielectrophoresis (DEP). For ideally conductive particles, ICEP is predominant over DEP, resulting in transient pairing dynamics. The shear viscosity and first and second normal stress differences
and$N_1$ of such suspensions are examined over a range of volume fractions$N_2$ as a function of Mason number$15\,\% \leq \phi \leq 50\,\%$ , which measures the relative importance of viscous shear stress over electrokineticdriven stress. For$Mn$ or low shear rates, the DIP is shown to dominate the dynamics, resulting in a relatively lowviscosity state. The positive$Mn < 1$ and negative$N_1$ are observed at$N_2$ , which is similar to Brownian suspensions, while their signs are reversed at$\phi < 30\,\%$ . For$\phi \ge 30\,\%$ , the shear thickening starts to arise at$Mn \ge 1$ , and an almost fivefold increase in viscosity occurs at$\phi \ge 30\,\%$ . Both$\phi = 50\,\%$ and$N_1$ are negative for$N_2$ at all volume fractions considered. We illuminate the transition in rheological behaviours from DIP to shear dominance around$Mn \gg 1$ in connection to suspension microstructure and dynamics. Lastly, our findings reveal the potential use of nonlinear electrokinetics as a means of active rheology control for such suspensions.$Mn = 1$