Drag on a circular intruder traversing a shape-heterogeneous granular mixture.

The main aim of our work is to explore the effect of particle shape heterogeneity on the dynamics of an intruder moving through a two-dimensional mixture of dumbbells and disks. In spite of similar physical conditions (the mass of the dumbbell is the same as that of the disk) and the same area fraction, we noticed a significant difference in the drag experienced by the intruder as the mixture concentration varies. The propagation of stress from the intruder to the granular grains manifests in the form of force chains, and interestingly these force chains can vary significantly depending on the shape of the grains. These differences, however, appear to be suppressed in the frictionless case where the force chains cannot extend very far from the initial point of contact. Apart from particle shape, the effect of the area fraction of the system and the size of the intruder have also been explored. As the area fraction increases, the drag force on the intruder increases owing to the increase in the contact forces. Finally, we present the velocity and stress fields at different dumbbell fractions and for various intruder diameters to show the effect of the moving intruder on its surrounding particles.

Interfacial synthesis of large-area ultrathin polyimine nanofilms as molecular separation membrane.

Thin film membranes of covalent organic frameworks are promising for high-permeance molecular separation. However, their synthesis needs a high temperature or longer reaction time, unsuitable for large-scale fabrication of thin film composite membranes. The ultrathin film of porous organic polymers as a separation layer of the composite membrane could be a close alternative to COF membranes. Here we report transition metal ion-catalyzed room temperature fabrication of the ultrathin (≈12 nm) polyimine nanofilms via interfacial polymerization of melamine and triformylphloroglucinol onto hydrolyzed polyacrylonitrile support within a short reaction time. Composite membranes exhibit high water permeance (≈78 L m-2 h-1 bar-1), high rejection (99.6%) of brilliant blue R (825.9 g mol-1), low rejection of NaCl (≈1.8%) and Na2SO4 (≈17%), and enable efficient molecular separation. The role of metal ion catalysts for large-area fabrication of the ultrathin polyimine nanofilm membranes used for molecular separation is demonstrated.

What governs the nature of fouling in forward osmosis (FO) and reverse osmosis (RO)? A molecular dynamics study.

Membrane fouling is a performance hampering phenomenon, which impacts forward osmosis (FO) and reverse osmosis (RO) differently. Experiments have found that the fouling layer structure for FO and RO is very different, but the reasons are not yet very clear, and hence a mechanistic understanding of the fouling in FO and RO is indispensable. Here we used molecular dynamics (MD) simulations to characterize the nature of fouling in FO and RO. Lysozyme and layered graphene oxide (GO) were used as typical representatives of the model foulant and desalination membrane, respectively. It was found that protein-solvent and protein-ion interactions are at the core of the structural differences between RO and FO. In particular, we suggest hydration repulsion and charge screening as probable mechanisms, which lead to different fouling layer structures in FO and RO. Also, a probable mechanism of lysozyme adsorption on the GO surface is proposed, which is based on the transport of protein towards the surface due to hydraulic pressure and Coulombic interactions induced between basic residues of lysozyme (arginine, lysine) facing the surface and oxygen-rich functional groups present on the GO surface. Both hydraulic pressure and Coulombic interactions acted synergistically, which led to lysozyme adsorption on the GO surface. Furthermore, the effect of initial protein orientation on protein-membrane interaction was also explored and was found to be an important factor in determining the nature of interaction and the time scale within which an adsorption event could be observed. This study facilitates the current understanding of the fouling in FO and RO and provides a probable molecular mechanism of how fundamental forces such as hydration repulsion and electrostatic interaction make fouling structurally different in FO and RO.

Dynamics of bidensity particle suspensions in a horizontal rotating cylinder.

We report Stokesian dynamics simulations of bidensity suspensions rotating in a horizontal cylinder. We studied the phase space and radial and axial patterns in settling as well as floating systems. Each system was composed of particle mixtures of two different densities. As many as eight unique phases are identified for each system along the radial plane. The bidensity system shows similarity to the monodisperse case only when the radial distribution of the particles is completely uniform. Characteristic behavior of the bidensity systems is identical at low rotation rates and contrasting when centrifugal force dominates. Expressing the phase boundaries in terms of dimensionless parameters U_{s}/(ΩR) and g/(Ω^{2}R) gives a linear fit unifying the data in the gravity-dominated regime. At high rotation rates, the behavior is opposing for either system though linear in nature. In the axial direction, number density profiles of both systems affirm the phenomenon of band formation. Location of the axial bands remains the same for heavy and light particles in both systems. We have also reestablished that an inhomogeneous particle configuration in the radial plane induces growing instabilities in the axial plane which amplify to form particle bands similar to monodisperse suspensions.

Magnus effect in granular media.

A spinning ball moving through air experiences a net lift due to the "Magnus effect" resulting from the pressure difference across its two sides. In this work, with the help of numerical simulations, we present a similar phenomenon on a circular intruder undergoing both translational and rotational motion in a two-dimensional granular medium. The direction of this "Magnus" lift in a granular medium, irrespective of its area fraction, is opposite to the general direction in viscous fluids in the range of velocities studied. We relate this effect to the switching in the direction of tangential forces, uneven shearing of the force chains, and uneven distribution in the number of contacts across the rotating intruder. Moreover, this is observed for area fractions Φ as low as 0.35 to as high as 0.82, which lies just below the jamming point. Distribution of the contact force around the intruder surface is also studied with respect to the nondimensionalized rotation speed of the intruder. A saturation in the lift to drag ratio is achieved at very high rotation speed, and the ratio is seen to be weakly dependent on the area fraction of the granular medium. The change in local flow fields of velocity, area fraction, and granular temperature around the intruder for several spin ratios is also discussed. The downstream wake of the intruder also deflects in the opposite direction when compared to the case in viscous fluids.

Coarse-grained simulation study of sequence effects on DNA hybridization in a concentrated environment.

A novel coarse-grained model is developed to elucidate thermodynamics and kinetic mechanisms of DNA self-assembly. It accounts for sequence and solvent conditions to capture key experimental results such as sequence-dependent thermal property and salt-dependent persistence length of ssDNA and dsDNA. Moreover, constant-temperature simulations on two single strands of a homogeneous sequence show two main mechanisms of hybridization: a slow slithering mechanism and a one-order faster zippering mechanism. Furthermore, large-scale simulations at a high DNA strand concentration demonstrate that DNA self-assembly is a robust and enthalpically driven process in which the formation of double helices is deciphered to occur via multiple self-assembly pathways including the strand displacement mechanism. However, sequence plays an important role in shifting the majority of one pathway over the others and controlling size distribution of self-assembled aggregates. This study yields a complex picture on the role of sequence on programmable self-assembly and demonstrates a promising simulation tool that is suitable for studies in DNA nanotechnology.

Nucleation and crystal growth in sheared granular sphere packings.

We investigate the nucleation of ordered phases, their symmetries, and distributions in dense frictional hard sphere packings as a function of particle volume fraction ϕ, by imposing cyclic shear and constant applied pressure conditions. We show, with internal imaging, that the nucleating crystallites in the bulk consist of 10-60 spheres with hexagonal close packed (hcp) order and nonspherical shape, that are oriented preferentially along the shear axis. Above ϕ=0.62±0.005, crystallites with face centered cubic (fcc) order are observed with increasing probability, and ordered domains grow rapidly. A polycrystalline phase with domains of fcc and hcp order is observed after hundreds of thousands of shear cycles.

Orientational ordering in sheared inelastic dumbbells.

Using even driven simulations, we show that homogeneously sheared inelastic dumbbells in two dimensions are randomly orientated in the limit of low density. As the packing fraction is increased, particles first tend to orient along the extensional axis, and then as the packing fraction is further increased, the alignment shifts closer to the flow axis. The orientational order parameter displays a continuous increase with packing fraction and does not appear to exhibit a universal scaling with elongation. Except at the highest packing fractions, the orientational distribution function can be reconstructed with only the first coefficient of the Fourier expansion.

Structure and dynamics of two-dimensional sheared granular flows.

The structure and dynamics of the two-dimensional linear shear flow of inelastic disks at high area fractions are analyzed. The event-driven simulation technique is used in the hard-particle limit, where the particles interact through instantaneous collisions. The structure (relative arrangement of particles) is analyzed using the bond-orientational order parameter. It is found that the shear flow reduces the order in the system, and the order parameter in a shear flow is lower than that in a collection of elastic hard disks at equilibrium. The distribution of relative velocities between colliding particles is analyzed. The relative velocity distribution undergoes a transition from a Gaussian distribution for nearly elastic particles, to an exponential distribution at low coefficients of restitution. However, the single-particle distribution function is close to a Gaussian in the dense limit, indicating that correlations between colliding particles have a strong influence on the relative velocity distribution. This results in a much lower dissipation rate than that predicted using the molecular chaos assumption, where the velocities of colliding particles are considered to be uncorrelated.

Applicability of constitutive relations from kinetic theory for dense granular flows.

The applicability of constitutive models based on kinetic theory for dense granular flows is examined. First, we calculate the average coordination number of a particle in a dense flow down an inclined plane using discrete element simulations that employ a linear spring-dashpot model for particle interactions. It is found that the average coordination number decreases as the spring constant increases at constant coefficient of restitution, and is less than 1 for the values of spring constant corresponding to materials such as sand and glass beads. The Bagnold coefficients, which are the ratios of the different components of the stress and the square of the strain rate, are calculated using both discrete element (DE) simulations and event driven (ED) simulations; collisions are considered to be instantaneous in the latter simulations. It is found that the theoretical predictions of the Bagnold coefficients are in quantitative agreement with both DE and ED simulations provided the pair distribution function obtained from the simulations is inserted into the theory. However, it is found that the pair distribution function in a sheared granular flow is significantly larger than that in an equilibrium fluid of elastic particles.