Colloidal Chains Research Articles

Bending dynamics of DNA-linked colloidal particle chains

Colloidal particles and assemblies have proven to be useful models for analogous atomic and molecular systems. In this article, we present a colloidal worm-like chain (WLC) model using DNA-linked colloidal particle chains as an analogue to semiflexible linear polymers. The bending rigidity, or persistence length, of these chains can be determined by monitoring the thermal fluctuations of the chains. We show that the persistence lengths of the chains can be tuned from 1 to 50 mm which corresponds to a length-to-persistence length ratio of 0.002 to 0.1, by changing the length of the DNA used to link adjacent particles from 75 to 15 bases, respectively. We also studied the bending relaxation dynamics of these chains, which match well with theoretical predictions, further supporting the validity of using these colloidal chains as models for semiflexible polymer systems in both equilibrium and dynamic studies.

Dynamics of colloids in a narrow channel driven by a nonuniform force

Using Brownian dynamics simulations, we investigate the dynamics of colloids confined in two-dimensional narrow channels driven by a nonuniform force Fdr(y) . We considered linear-gradient, parabolic, and deltalike driving-force profiles. This driving force induces melting of the colloidal solid (i.e., shear-induced melting), and the colloidal motion experiences a transition from elastic to plastic regime with increasing Fdr. For intermediate Fdr (i.e., in the transition region) the response of the system, i.e., the distribution of the velocities of the colloidal chains upsiloni(y) , in general does not coincide with the profile of the driving force Fdr(y), and depends on the magnitude of Fdr, the width of the channel, and the density of colloids. For example, we show that the onset of plasticity is first observed near the boundaries while the motion in the central region is elastic. This is explained by: (i) (in)commensurability between the chains due to the larger density of colloids near the boundaries, and (ii) the gradient in Fdr. Our study provides a deeper understanding of the dynamics of colloids in channels and could be accessed in experiments on colloids (or in dusty plasma) with, e.g., asymmetric channels or in the presence of a gradient potential field.

Nematic Braids: Modeling of Colloidal Structures

Our recent advances in modeling of colloidal superstructures in spatially confined nematic liquid crystals are briefly reviewed. The approach is phenomenological and is based on Landau- de Gennes type free energy, where the effects of confinement and external fields are also taken into account. The inter-particle couplings that result in a nematic solvent are effectively many-body interactions. The complexity of the couplings leads to numerous nematic and colloidal structures not present in simple liquids where van der Waals and electrostatic interactions are dominant. A local “heating” to the isotropic phase is performed, followed by a fast quench to the nematic phase. In the relaxation process that conserves the topological charge, a number of metastable colloidal superstructures are formed. Here we focus our attention on the entangled structures where colloidal particles are coupled by an entangled network of delocalized disclination lines (nematic braid): colloidal dimers, chains, and multiscale structures. The string-like coupling provided by such disclination lines is much more robust compared to an interaction based on an array of localized disclinations. The controlled formation of entangled colloidal structures opens new ways to the assembly of colloidal photonic crystals and hierarchical structures that could lead to metamaterials.

Universalities in the Dynamics of Suspensions of Magnetic Colloidal Chains Confined in Thin Films

A quasi two-dimensional model of magnetic colloidal chains is examined by extensive Brownian-dynamics simulations on dilute suspensions of monodisperse magnetic colloidal chains confined in thin films at different thicknesses under an external magnetic field applied perpendicular to the films. Universalities in the dynamics of chains are explored based on the mean-field theory recently proposed by Tokuyama. The long-time self-diffusion coefficient of a chain is shown to obey a singular function of a control parameter proportional to the square of the applied field strength. The value of the control parameter at the singular point, where the long-time self-diffusion coefficient of a chain becomes zero, is then shown to be inversely proportional to the square of the number of magnetic colloids forming one chain.

Probing the Stability of Magnetically Assembled DNA-Linked Colloidal Chains

The self-assembly of colloidal particles using DNA linker molecules has led to novel colloidal materials. This article describes the development and characterization of a new class of colloidal structures based on the directed assembly of DNA-linked paramagnetic particles. A key obstacle to assembling these structures is understanding the fundamental chemistry and physics of the assembly processes. The stability of these cross-linked chain structures is the first step toward reliable assembly and thus important for its applications; however, chain stability has yet to be systematically studied. In this paper, we investigate both theoretically and experimentally, the stability of DNA-linked paramagnetic colloidal chains as a function of externally applied magnetic field strength and surface grafted DNA length and density. A total interparticle free energy potential model is developed accounting for all major forces contributing to chain stability, and a phase diagram is obtained from experiments to illustrate linked chain phases, unstable unlinked particle phases, and their transitions, which agree well with those predicted by the model. From this study, optimized parameters for successful linking and building stable linked chains are obtained.

Conjugation of Colloidal Clusters and Chains by Capillary Condensation

Capillary condensation was used to establish connections in colloidal clusters and 1D colloidal chains with high regional selectivity. This vapor-phase process produced conjugated clusters and chains with anisotropic functionality. The capillary condensation method is simple and can be applied to a wide range of materials. It can tolerate geometric variations and even permits conjugation of spatially separated particles. The selective deposition was also used to modulate the functionality on the colloid surfaces, producing tip-tethered nanosized building blocks that may be suitable for further assembly via directional interactions.

Controlled formation of colloidal structures by an alternating electric field and its mechanisms

A detailed phase diagram, revealing a variety of processes including various colloidal structures of monodisperse charged colloidal particles from the colloidal chains, vortex rings, three-dimensional aggregation to a two-dimensional crystal under different frequencies, and strengths of an alternating electric field, is obtained for the first time. The occurrence of different colloidal structures is driven by the electrohydrodynamic interaction and induced dipolar interaction near the polarized layer on the electrode. This simple colloidal system can be employed as a model system to understand the complex phase behavior of the assembly/aggregation of the nanoparticles and biomacromolecules under external perturbation. Detailed phase diagram provides vital guidance for the fabrication of desired colloidal structures with single-particle resolution, which could be employed as a sort of templates for nanolithography or imprinting. Moreover, the sensitivity of the electrohydrodynamic interaction on the particle size and the dependence of the convective flow on the frequency and strength could be utilized in microfluidic devices for manipulating nanoparticles, biomacromolecules, and vesicles.

Manipulation of Defect Structures and Colloidal Chains in Liquid Crystals by Means of Photochemical Reactions of Azobenzene Compounds

Photonic control of defect structures and colloidal chains has been performed in azobenzene-containing liquid crystals dispersed with glycerol or water droplets. When we dispersed the colloidal droplets into the host liquid crystals, we could observe, at an initial state, Saturn ring and hedgehog around the glycerol and water droplets, respectively, indicating that normal alignment of liquid crystal molecules was induced on the droplets by the adsorption of the azobenzene molecules onto the droplets. In the case of the glycerol droplets, we achieved photochemical manipulation of the defect structures between Saturn ring and boojums on irradiation with ultra-violet and visible light. For the water droplets, an inter-droplet distance of the self-organized colloidal chain could be controlled by changes of the defect size on the photoirradiation. Photoinduced structural changes in the topological defects can be explained by the modulation of surface anchoring of the droplets by means of the cis-trans photoisomerization of the adsorbed azobenzene molecules.

Two-dimensional dipolar nematic colloidal crystals

We study the interactions and directed assembly of dipolar nematic colloidal particles in planar nematic cells using laser tweezers. The binding energies for two stable configurations of a colloidal pair with homeotropic surface alignment are determined. It is shown that the orientation of the dipolar colloidal particle can efficiently be controlled and changed by locally quenching the nematic liquid crystal from the laser-induced isotropic phase. The interaction of a single colloidal particle with a single colloidal chain is determined and the interactions between pairs of colloidal chains are studied. We demonstrate that dipolar colloidal chains self-assemble into the two-dimensional (2D) dipolar nematic colloidal crystals. An odd-even effect is observed with increasing number of colloidal chains forming the 2D colloidal crystal.

Ligand−Receptor Interactions in Chains of Colloids: When Reactions Are Limited by Rotational Diffusion

We discuss the theory of ligand receptor reactions between two freely rotating colloids in close proximity to one other. Such reactions, limited by rotational diffusion, arise in magnetic bead suspensions where the beads are driven into close contact by an applied magnetic field as they align in chainlike structures. By a combination of reaction-diffusion theory, numerical simulations, and heuristic arguments, we compute the time required for a reaction to occur in a number of experimentally relevant situations. We find in all cases that the time required for a reaction to occur is larger than the characteristic rotation time of the diffusion motion tau(rot). When the colloids carry one ligand only and a number n of receptors, we find that the reaction time is, in units of tau(rot), a function simply of n and of the relative surface alpha occupied by one reaction patch alpha = pirC2/(4pir2), where rC is the ligand receptor capture radius and r is the radius of the colloid.

Phase Behavior in Highly Concentrated Assemblies of Microgels with Soft Repulsive Interaction Potentials

Microgel particles with a soft repulsive interaction potential are investigated with particle tracking methods to study the phase behavior of soft-sphere systems. The use of poly(N-isopropylacrylamide) particles allows the effective volume fraction of a sample to be tuned via thermal modulation without altering the particle number density. This allows for investigation of the phase behavior of an assembly as a function of its initial packing density. In particular, we have elucidated the influence of soft colloid "overpacking" on the freezing effective volume fraction (phieff,f). These studies thereby illustrate the interplay between energetics/packing forces occurring at the colloidal and polymer chain length scales.

Phase coexistence in polydisperse mixture of hard-sphere colloidal and flexible chain particles

A theoretical scheme for the calculation of the full phase diagram (including cloud and shadow curves, binodals and distribution functions of the coexisting phases) for colloid–polymer mixtures with polymer chain length polydispersity and hard-sphere colloidal and polymeric monomer sizes polydispersity is proposed. The scheme combines the thermodynamic perturbation theory for associating fluids and the recently developed method used to determine the phase diagram of polydisperse spherical shape colloidal fluids [Bellier-Castella et al. J. Chem. Phys. 113 (2000) 8337]. By way of illustration we present and discuss the full phase diagram for the mixture with polydispersity in the size of the hard-sphere colloidal particles.

Preparation and Properties of Monodisperse Latex Spheres with Controlled Magnetic Moment for Field-Induced Colloidal Crystallization and (Dipolar) Chain Formation

We report the preparation and properties of monodisperse magnetic poly(methyl methacrylate) latex spheres that exhibit field-induced colloidal crystallization to exotic morphologies controlled by the geometry of the gradient. The magnetic moment of the novel magnetic spheres is due to an inner core of magnetite particles. These particles, obtained from a conventional ferrofluid, first form a monodisperse emulsion with a silane coupling agent, after which they are directly incorporated in PMMA latex synthesized by standard emulsion polymerization. Scattering from the latex shell dominates over light absorption by the magnetic cores such that visible Bragg reflections of the magnetic crystals can be clearly observed. Concentrated nearly white latex fluids may exhibit near a magnet the warped equilibrium menisci known from the usually dark magnetite ferrofluids. Of the many possible applications, we briefly discuss the subsequent growth and melting of crystals by a slowly oscillating gradient, the formation of radial lattices, and the formation of ordered magnetic dots in PMMA latex films.

Two-Dimensional Nematic Colloidal Crystals Self-Assembled by Topological Defects

The ability to generate regular spatial arrangements of particles is an important technological and fundamental aspect of colloidal science. We showed that colloidal particles confined to a few-micrometer-thick layer of a nematic liquid crystal form two-dimensional crystal structures that are bound by topological defects. Two basic crystalline structures were observed, depending on the ordering of the liquid crystal around the particle. Colloids inducing quadrupolar order crystallize into weakly bound two-dimensional ordered structure, where the particle interaction is mediated by the sharing of localized topological defects. Colloids inducing dipolar order are strongly bound into antiferroelectric-like two-dimensional crystallites of dipolar colloidal chains. Self-assembly by topological defects could be applied to other systems with similar symmetry.

Multi-island single-electron devices from self-assembled colloidal nanocrystal chains

We report the fabrication of multi-island single-electron devices made by lithographic contacting of self-assembled alkanethiol-coated gold nanocrystals. The advantages of this method, which bridges the dimensional gap between lithographic and NC sizes, are (1) the fact that all tunnel junctions are defined by self-assembly rather than lithography and (2) the high ratio of gate capacitance to total capacitance. The rich electronic behavior of a double-island device, measured at 4.2K, is predicted by combining finite element and Monte Carlo simulations, and it can be fully explained by the standard theory of Coulomb blockade with very few adjustable parameters.

Extended organization of colloidal microparticles by surface plasmon polariton excitation

In this paper we demonstrate the first use of enhanced optical forces and optically induced thermophoretic and convective forces produced by surface plasmon polariton excitation for large-scale ordering and trapping of colloidal aggregations. We identify regimes in which each of these forces dominate the colloidal dynamics and lead to different collective equilibrium states. These include accumulation and organization into hexagonal close-packed colloidal crystals, arrangement into linear colloidal particle chains, and the formation of colloids in a ring formation about the excited region.

Branching of Colloidal Chains in Capillary-Confined Nematics

We report on the observation of colloidal chain assembly and branching inside capillaries filled with a nematic liquid crystal. Because of the homeotropic anchoring of liquid crystalline molecules on the capillary and colloidal droplet surfaces, the assembly of droplets along the capillary axis is expected, producing a transformation of the nematic director field from an escape-radial to quasiradial configuration. However, the subsequent over time branching of the straight colloidal chains is counterintuitive. By numerical simulations, we demonstrate that chain branching can occur by overcoming an energy barrier and can at least dwell as a metastable configuration. Moreover, manipulation of colloidal chains by electric fields and their gradients demonstrates various regimes of chain behavior in electric fields.

Shear thickening in a model colloidal suspension

We study the rheology of model colloidal suspensions using molecular-dynamics simulations. We relate the onset of shear thickening to the transition from a low-viscosity regime, in which the solvent facilitates the flow of colloids, to a high-viscosity regime associated with jamming of the colloids and the formation of chains of colloids. In the low-viscosity regime, the colloidal particles are, on average, surrounded by two layers of solvent particles. On the contrary, in the high-viscosity regime, the solvent is expelled from the interstice between the jammed colloids. The thickening in suspensions is shown to obey the same criterion as in simple fluids. This demonstrates that jamming, even without the divergence of lubrication interactions, is sufficient to observe shear thickening.

SIMULATION OF COLLOIDAL CHAIN MOVEMENTS UNDER A MAGNETIC FIELD

Short colloidal chains are simulated by the slithering-snake-algorithm on a simple cubic lattice. The dipole character of the colloidal particles leads to a long range dipole–dipole interaction. The solvent is simulated by the nearest neighbor Ising model. The aligning of the dipoles under a magnetic field gives rise to the chains to align on their part with the field direction.

Colloidal ordering from phase separation in a liquid-crystalline continuous phase

Some binary mixtures exist as a single phase at high temperatures and as two phases at lower temperatures; rapid cooling therefore induces phase separation that proceeds through the initial formation of small particles and subsequent growth and coarsening. In solid and liquid media, this process leads to growing particles with a range of sizes, which eventually separate to form a macroscopically distinct phase. Such behaviour is of particular interest in systems composed of an isotropic fluid and a liquid crystal, where the random distribution of liquid-crystal droplets in an isotropic polymer matrix may give rise to interesting electro-optical properties. Here we report that a binary mixture consisting of an isotropic fluid and a liquid crystal forming the continuous phase does not fully separate into two phases, but self-organizes into highly ordered arrays of monodisperse colloidal droplet chains. We find that the size and spatial organization of the droplets are controlled by the orientational elasticity of the liquid-crystal phase and the defects caused by droplets exceeding a critical size. We expect that our approach to forming monodisperse, spatially ordered droplets in liquid crystals will allow the controlled design of ordered composites that may have useful rheological and optical properties.