Anisotropic Interparticle Interactions Research Articles

Nanoparticle localization within chiral liquid crystal defect lines and nanoparticle interactions.

Self-assembly of colloidal particles into predefined structures is a promising way to design inexpensive manmade materials with advanced macroscopic properties. Doping of nematic liquid crystals (LCs) with nanoparticles has a series of advantages in addressing these grand scientific and engineering challenges. It also provides a very rich soft matter platform for the discovery of unique condensed matter phases. The LC host naturally allows the realization of diverse anisotropic interparticle interactions, enriched by the spontaneous alignment of anisotropic particles due to the boundary conditions of the LC director. Here we demonstrate theoretically and experimentally that the ability of LC media to host topological defect lines can be used as a tool to probe the behavior of individual nanoparticles as well as effective interactions between them. LC defect lines irreversibly trap nanoparticles enabling controlled particle movement along the defect line with the use of a laser tweezer. Minimization of Landau-de Gennes free energy reveals a sensitivity of the ensuing effective nanoparticle interaction to the shape of the particle, surface anchoring strength, and temperature, which determine not only the strength of the interaction but also its repulsive or attractive character. Theoretical results are supported qualitatively by experimental observations. This work may pave the way toward designing controlled linear assemblies as well as one-dimensional crystals of nanoparticles such as gold nanorods or quantum dots with tunable interparticle spacing.

Hybrid colloidal gels with tunable elasticity formed by charge-driven assembly between spherical soft nanoparticles and discotic nanosilicates

Colloidal gels based on electrostatic interparticle attractions hold unexploited potential for tailoring their microstructure and properties. Here, we demonstrate that hetero-aggregation between oppositely charged particles with different geometries is a viable strategy for controlling their properties. Specifically, we studied hybrid colloidal gels prepared by the charge-driven assembly of oppositely charged spherical gelatin nanoparticles and two-dimensional (2D) nanosilicates. We show that the asymmetry between the building blocks and the resulting anisotropic interparticle interactions produces a variety of nanostructures and hybrid colloidal gels that exhibit high elasticity at low colloidal volume fractions. Tuning the competition between different attractive interactions in the system by varying the spatial charge heterogeneity on the 2D nanosheets, composition, and ionic strength was found to alter the mechanism of gel formation and their rheological properties. Remarkably, increasing the mass ratio of 2D nanosheets to spherical nanoparticles at a constant total mass fraction affords hybrid gels that exhibit an inverse relationship between elasticity and volume fraction. However, these hybrid gels are easily fluidized and exhibit rapid structural recovery once the stress is removed. These features allow for the engineering of versatile 3D-printable hybrid colloidal gels, whose structure and viscoelastic response are governed by parameters that have not been explored before.

Anisotropic interaction driven surface modulation on spray-dried microgranules

Rapid evaporation of solvent from spray colloidal droplets induces directed self-assembly among the nanoparticles, eventually interlocking them into correlated granular structures. In this work, it is demonstrated that anisotropy in colloidal interparticle interaction plays a key role in governing the surface topology of spray-dried granules. Colloidal dispersion comprised of spherical nanosilica (NS) and cylindrical carbon nanotubes (CNT) was chosen as a model system in this regard. For identical polarities of the colloidal components, granules with prominent wrinkle-like modulations are obtained, which is in drastic contrast with the case of opposite polarities. The extent of surface modulation depends on the relative concentration of CNT with respective to NS. A plausible mechanism for the formation of surface modulation is elucidated on the basis of the evolving anisotropic interparticle interactions during assembly. Electron microscopy, small-angle scattering, Raman spectroscopic techniques have been used for quantitative characterization of these micro-granules.

Diffusion in monodisperse ferrofluids

The diffusion coefficients characterizing the translational and rotational Brownian motion of a particle in a concentrated suspension were determined in the long time diffusive regime for a ferrofluid suspension with the use of a Langevin equation approach. These dynamical properties depend on the equilibrium micro-structural information of the suspension and take into account the effect of direct anisotropic inter-particle's interactions on self-diffusion. The comparison of this theory with Brownian dynamic simulations results is made in terms of colloid density and dipole interaction strength.

Dusty plasma as a unique object of plasma physics

The self-consistency and basic openness of dusty plasma, charge fluctuations, high dissipation and other features of dusty plasma system lead to the appearance of a number of unusual and unique properties of dusty plasma. “Anomalous” heating of dusty particles, anisotropy of temperatures and other features, parametric resonance, charge fluctuations and interaction potential are among these unique properties. Study is based on analytical approach and numerical simulation. Mechanisms of “anomalous” heating and energy transfer are proposed. Influence of charge fluctuations on the system properties is discussed. The self-consistent, many-particle, fluctuation and anisotropic interparticle interaction potential is studied for a significant range of gas temperature. These properties are interconnected and necessary for a full description of dusty plasmas physics.

On the possibility of determining the forces of anisotropic interaction between plasma microparticles

The possibility of experimental diagnostics of forces of the anisotropic interaction between dusty particles in plasma is considered for the first time. To this end, a modification of the method based on the solution of the Langevin inverse problem for systems with anisotropic interparticle interaction is presented. It is tested on the results of numerical simulation of chain structures of particles with the quasi-dipole–dipole interaction similar to the interaction appearing between dusty particles in near-electrode layers of gas discharges.

The effect of nonreciprocal interaction on the redistribution of kinetic energy in the system of particles

Dynamical properties of particles in the chain-like and layered structures are studied with the help of Langevin MD approach. We considered infinite, confined chain-like structures, and extended layered structures with vertical (chain-like) and hexagonal ordering of particles in adjacent layers over a wide range of parameters, corresponding to the experimental conditions in the laboratory dusty plasmas. The nonreciprocal anisotropic interparticle interaction leads to a growth of kinetic energy of stochastic motion of particles especially its vertical component. Efficacy of the heating depends on the thermal particle motion, the anisotropy of the considered interparticle interaction and with decreasing the friction coefficient.

Quantum kinetics of spinning neutral particles: General theory and Spin wave dispersion

Plasma physics gives an example of physical system of particles with the long-range interaction. At the small velocities of particles we can consider the plasmas approximately as the systems of particles with the Coulomb interaction. The Coulomb interaction is an isotropic interaction. Systems of spinning neutral particles are involved in the spin–spin interaction, which is a long-range anisotropic interparticle interaction. Therefore they can reveal more rich properties than the Coulomb plasmas. Furthermore, to study the systems of spinning particles we can develop the kinetic and hydrodynamic methods similar to the methods applying in the plasma physics. We derive the kinetic equations by a new method, which is the generalization of the many-particle quantum hydrodynamics. Obtained set of the kinetic equations is truncated, so we have a closed set of two equations. First of them is the kinetic equation for the quantum distribution function. The second equation is the equation for the spin-distribution function, which describes the spin kinetic evolution and contributes to the time evolution of the distribution function. Our method allows one to obtain equations for both the three dimensional systems of particles and the low dimensional systems. Hence we consider the spin waves in the three- and the two-dimensional systems of neutral spinning particles.

Solution of the inverse Langevin problem for open dissipative systems with anisotropic interparticle interaction

Solution of the inverse Langevin problem is presented for open dissipative systems with anisotropic interparticle interaction. Possibility of applying this solution for experimental determining the anisotropic interaction forces between dust particles in complex plasmas with ion flow is considered. For this purpose, we have tested the method on the results of numerical simulation of chain structures of particles with quasidipole-dipole interaction, similar to the one occurring due to effects of ion focusing in gas discharges. Influence of charge spatial inhomogeneity and fluctuations on the results of recovery is also discussed.

Fluid of Janus molecules between two walls: The solvation force

We apply a density functional theory to calculate the solvation force in the system involving Janus particles confined between two planar walls. Janus particles are modeled as spheres composed of attractive and repulsive parts and their orientation is described by the vectors representing internal degrees of freedom. We consider the cases of pores with identical walls, as well as the pores with competing walls (the so-called Janus-like pores). The density functional approach we employ combines fundamental measure theory with a mean-field approximation for the anisotropic interparticle interaction. We study how the solvation force and the orientational structure of confined particles depend on the competition between the surface field and the interactions between confined molecules and on the parameters of the model such as temperature and density. It is shown that the anisotropic interaction between the confined molecules and the character of the walls significantly influence the solvation force.

Crystallization, structural diversity and anisotropy effects in 2D arrays of icosahedral viruses.

We investigate two-dimensional (2D) assembly of the icosahedral turnip yellow mosaic virus (TYMV) under cationic lipid monolayers at the aqueous solution–vapor interface. The 2D crystallization of TYMV has been achieved by enhancing electrostatically induced interfacial adsorption, an approach recently demonstrated for another virus. In situ X-ray scattering reveals two close-packed 2D crystalline phases of TYMV that are distinct from the previously reported hexagonal and centered square (√2 × √2) arrays of TYMV. One of the newly observed phases arises from either a dimeric double-square (2 × 1) or tetrameric square (2 × 2) unit cell. The other is a rhombic crystal with a lattice angle of 80°. The two observed crystal phases are substantially less dense (by over 10%) than a 2D lattice of TYMV could be according to its known size and shape, indicating that local anisotropic interparticle interactions play a key role in stabilizing these crystals. TYMV's anisotropy attributes and numerical analysis of 2D arrays of virus-shaped particles are used to derive a model for the rhombic crystal in which the particle orientation is consistent with the electrostatic lipid–TYMV attraction and the interparticle contacts exhibit steric complementarity. The interplay between particle anisotropy and packing is contrasted between the rhombic crystal model and the square (√2 × √2) crystal. This study highlights how the high symmetry and subtle asphericity of icosahedral particles enrich the variety and complexity of ordered 2D structures that can be generated through self-assembly.

Faceted nanoparticles in a nematic liquid crystal: defect structures and potentials of mean force

We investigate the defect structures and the potentials of mean force (PMF) that arise when faceted nanoparticles, namely cubes and triangular prisms, are immersed in a nematic liquid crystal (NLC). Using a mesoscale theory for the tensor order parameter Q of the nematic, we have determined the thermodynamic stability of different orientations of one nanoparticle with respect to the far-field director n(r). A nanocube with perpendicular anchoring of the nematic at its surfaces tends to align in such a way that none of its faces is parallel or perpendicular to n(r); the most stable defect structure consists of a distorted Saturn ring with sharp bends, which covers six of the edges of the cubic particle. In contrast, a triangular nanoprism with homeotropic anchoring of the nematic at its surfaces tends to align with its long axis perpendicular to the far-field director n(r), and with one of its rectangular faces perpendicular to n(r). For such a configuration, the defect structure consists of two large disclination regions covering the two triangular faces of the prism, and two narrow disclination regions surrounding two of the rounded edges of the prism. We also studied the thermodynamic stability of different arrays of two particles, finding that for two nanocubes that approach each other keeping their orientations fixed, the nematic forms a distorted ‘entangled hyperbolic’ defect structure around the particles, in analogy to what was observed for pairs of spherical and spherocylindrical nanoparticles in close proximity. The NLC-mediated interactions between the nanocubes in this case are of the order of − 85k B T, which are weaker than those observed for spherical nanoparticles of comparable diameter (∼ − 110k B T). For systems of two nanoprisms having their long axes perpendicular to the far-field director, we considered three particle arrays: linear (the long axes of the particles are collinear and the particles have the same orientation), parallel (the long axes of the particles are parallel and the particles have the same orientation) and inverted parallel (the long axes of the particles are parallel, and one of the prisms is inverted with respect to the other one). Our results suggest that inverted parallel arrays are thermodynamically more stable than linear arrays, which in turn are more stable than parallel arrays. The minima observed in the PMF curves for the inverted parallel (∼ − 1050k B T) and linear arrays (∼ − 525k B T) are significantly deeper than that observed for the parallel array (∼ − 150k B T). In comparison, a pair of nanospheres with a diameter comparable to the size of the triangular faces of our nanoprisms has a PMF minimum of ∼73k B T when the spheres are in close proximity. These NLC-mediated, anisotropic interparticle interactions can make the particles bind together at specific locations, and thus could be used to assemble the particles into ordered structures with different morphologies.

Self-organizing Dimer Formation of Silver Nanocubes through Face Selective Functionalization and Surface Plasmon Coupling at Its Fractal Junction

AbstractWe have demonstrated the dimerization of single-crystalline Ag nanocubes with reasonably high yields through stepwise integration by following three steps: the preparation of a single layer of densely packed Ag nanocubes on a substrate by modified convective assembly, the selective functionalization of the upper face of the Ag nanocubes with a hydrophobic DT-SAM using the μCP approach, and the spontaneous dimerization in a mixture of ethanol and water driven by enhanced anisotropic hydrophobic interparticle interactions. Face-selective functionalization using hydrophobic DT-SAM gave the nanocubes directionality with respect to their anisotropic interparticle interactions under an external hydrophilic environment. We conclude that the driving force that reduced the surface area of the hydrophobic faces is sufficient large to form an ordered assembly of nanosized building blocks in an aqueous solution. Both experimental and theoretical studies revealed that the 50-nm-diameter Ag nanocubes dimers with a ca. 3.3 nm gap at their junction exhibited two plasmon peaks centered at 446 nm and 600 nm, which contributed to transverse and longitudinal plasmon resonances, respectively. Elctromagnetic calculations based on the FDTD method clearly showed that a greater enhancement of the local field occurred, with an average amplitude of the electric field of 1.0×1015, at the fractal space between the aggregated Ag nanocubes when the dimer was illuminated under longitudinally polarized light.

Polymerization transitions in two-dimensional systems of dipolar spheres

We investigate the self-organization of dipolar spheres into polymer chains as a fundamental model of the self-assembly of particles having anisotropic interparticle interactions. Our study involves a combination of modeling with vertically vibrated magnetic beads simulating a quasi-two-dimensional fluid at equilibrium and corresponding Monte Carlo simulations of hard spheres with embedded extended dipoles. We find a transition from a gas-like phase to a polymerized phase upon cooling, in accord with the analytic theory of equilibrium polymerization.

A numerical simulation of suspension flow using a constitutive model based on anisotropic interparticle interactions

We report a Brownian configuration field implementation of a recent constitutive equation for suspensions, reported by Phan-Thien et al. 1999. The numerical method is a hybrid technique, which combines a modification of the Brownian configuration field method described by Hulsen et al. 1997 and the adaptive viscosity split stress formulation proposed by Sun et al. 1996. The implementation is used to examine the flow past a sphere in a tube. The relative viscosity derived from the drag force/sedimentation velocity agrees well with a well-known empiricism. In addition, the ratio of the pressure force to the drag on the sphere seems to be weakly dependent on the volume fraction, and is somewhat higher than Brenner's results of 1962, which were derived for Newtonian fluids.

Particle simulation of two dimensional dust crystal formation in a mesothermal plasma flow

A two-dimensional simulation is used to study the crystallization of strongly coupled dusty plasmas. The dust grains are simulated as particles, while the electrons and ion solutions are obtained from fluid equations. We model gas discharges with dust particles confined in the sheath and at the sheath edge. In these regions the ions flow past the dust grains at a velocity v0 that typically is mesothermal, vTi≪v0≪vTe. A negative charged dust particle will under these conditions focus the ions, creating a local maximum in the plasma potential on the downstream side of the particle. This maximum means that there is an anisotropic interparticle interaction, which can be manifested as an attractive interparticle force along the axis of the plasma flow. Our simulation shows that the equilibrium configuration of a dust crystal structure can be profoundly influenced by this asymmetry in the ion flow and plasma potential for certain charge to mass ratios for the dust particles. We initially distribute dust particles randomly in space with zero velocity and integrate their equations of motion, tracking their orbits as they settle into equilibrium positions. We show that there are several stable final equilibria, although they do not all have the same potential energy or probability of occurring. The most likely crystal configuration depends on the charge-to-mass ratio and the horizontal particle spacing of the grains. The vertically aligned columns of particles that have been observed in plasma crystal experiments are shown to be most likely for large dust charge-to-mass ratios.