A coarse-grained explicit solvent simulation of rheology of colloidal suspensions

We use a simple extension of the dissipative particle dynamics (DPD) model to address the dynamical properties of macrosolutes immersed in complex fluid solvents. In this approach, the solvent particles are still represented as DPD particles, thereby retaining the time and length scale advantages offered by the DPD approach. In contrast, the solute particles are represented as hard particles of the appropriate size. We examine the applicability of this simulation approach to reproduce the correct hydrodynamical characteristics of the mixture. Our results focus on the equilibrium dynamics and the steady-state shear rheological behaviors for a range of volume fractions of the suspension, and demonstrate excellent agreement with many published experimental and theoretical results. Moreover, we are also able to track the glass transition of our suspension and the associated dynamical signatures in both the diffusivities and the rheological properties of our suspension. Our results suggest that the simulation approach can be used as a one-parameter model to examine quantitatively the rheological properties of colloidal suspensions in complex fluid solvents such as polymeric melts and solutions, as well as allied dynamical phenomena such as phase ordering in mixtures of block copolymers and particles.     

Nematic Homopolymers: From Segmented to Wormlike Chains

We present a density functional approach to orientational ordering in homopolymeric systems. The polymers are modeled as chains of identical rodlike segments connected via a simple generic bending potential. The segments are impenetrable to each other, and it is their mutual excluded volume that drives the transition from the orientationally disordered isotropic phase to the orientationally ordered nematic fluid. These excluded volume effects are accounted for within the so‐called Onsager approximation at the chain–chain level and in an independent pairwise overlap approximation at the segment–segment level. The Khokhlov and Semenov formalism for nematic wormlike polymers is shown to be an exact limiting case of our treatment. The ordering transition is studied analytically by using a linear stability analysis of the isotropic phase yielding the properties of the system at the isotropic‐nematic (I–N) bifurcation point. Using a numerical scheme, the equilibrium distribution functions in the nematic phase are calculated, and the location of the thermodynamic I–N transition is determined. For stiff bending potentials, chains with a relatively small number of segments are found to behave like wormlike chains, and we determine the regime of model parameters for which this identification holds.     

Dynamics simulation of disperse systems in the presence of a structure-related mechanical barrier resulting from surfactant adsorption

A computer model has been developed to refine the notions of the kinetics of structural transformations in surfactant-modified disperse systems, the mechanism of the influence of adsorption layers on viscosity, conditions of aggregation, and evolution of nonuniformities in microstructures. Classical concepts of the structure-related mechanical barrier providing disperse systems with stability to aggregation, as well as the criterion of adsorption-layer breakdown upon interparticle collisions under dynamic conditions, have been used. It has been shown that adsorption-layer breakdown under dynamic conditions may lead to the appearance of an extreme in the viscosity curve of a disperse system. The combined effect of additives of surfactants with different molecular masses successively added to the systems has been studied via taking into account the influence of surfactant adsorption layers on the dynamics of contact interactions. The simulation results make it possible to optimize the regulation of structure-related rheological properties of dispersions so as to decrease their apparent viscosity with a simultaneous increase in the uniformity of the structure.     

Spinodal decomposition in mixtures containing nematogens

A general stability criterion is presented for mixtures containing nematic liquid crystals. Conditions for absolute phase instability are found and represent a generalization of the usual spinodal concept. Physically, these conditions represent instabilities with respect to variations in the systems composition, degree of molecular ordering or some combination thereof. An example calculation corresponding to a liquid-crystalline polymer is presented. This calculation is based upon a well known lattice model which includes energetics that are dependent upon molecular orientation. Spinodal decomposition has profound effects on the morphology of a system which undergoes phase separation. Recent experimental and theoretical studies have attempted to explore these consequences in liquid-crystalline systems. None gives a rigorous definition of what is meant by spinodal in a mixture with orientational degrees of freedom; the material presented here clarifies this issue.     

Mesoscale constitutive modeling of magnetic dispersions: material functions for shear flows

We recently developed a constitutive model for magnetic dispersions by modeling the magnetic particles as rigid dumbbells dispersed in a solvent. The theory yielded a constitutive equation in which the stress tensor could be expressed as a function of the velocity gradient, an orientational order tensor, S, an average alignment vector, J, and any imposed external magnetic field, H. The constitutive equation is used here to predict material functions for steady shear flow (shear-rate dependent viscosity and first normal stress coefficient) as well as those for unsteady shear flows (stress growth upon inception of steady shear and small-amplitude oscillatory shear). The importance of effects of concentration, equilibrium nematic ordering in the dispersion, and anisotropy in the hydrodynamic drag are emphasized. Comparisons with available experimental data on viscosity for magnetic inks under steady shear flow and inception of steady shear flow show reasonably good agreement.     

Review on the dynamics and micro-structure of pH-responsive nano-colloidal systems

This review presents an overview on the research on pH-responsive microgel particles in the last 10 years. Microgels are cross-linked latex particles that are swollen in a good solvent. Significant quantitative studies have been conducted to investigate the swelling behavior (microscopic) and rheological (macroscopic) properties of the pH-responsive microgel particles as a function of neutralization degree, ionic strength, and cross-linked density. Mono-dispersed, alkali-swellable microgels containing carboxylic acid lattices, whose properties display extreme pH sensitivity in water is considered in detail in terms of swelling behavior and rheological properties. Their stability in solution and ability to undergo reversible volume phase transitions in response to pH makes them ideal model systems for the development of a semi-empirical as well as theoretical approach for predicting the viscosity of dilute and concentrated hard and soft sphere systems. The review concludes with a discussion of some recent applications of pH-responsive microgel particles.     

Orientational effects of hydrogen bonding in liquid-crystalline solutions containing Schiff bases

Intermolecular hydrogen bonding in binary mixtures containing nematogenic Schiff bases as solvents and proton-donating non-mesomorphic solutes has been considered. Reasons for the anomalous concentration dependences of solute order parameters are discussed. A solution structure model of acetic acid in nematic solvents is proposed; constants of complex-dimer equilibrium and coefficients of the orientational correlation of the non-mesogenic solute are calculated on the basis of this model. Hydrogen bonded complex structure using ¹³C NMR has been studied and stability constants in isotropic solutions in chloroform have been calculated. The influence of the solvent orientational ordering on the complex stability is discussed. Data on the solvation isotopic effects in the solutions investigated, which confirm the adequacy of the model are given.     

Simulation of structure-related mechanical characteristics of disperse systems under dynamic conditions

Results of simulating the structure-related rheological properties of disperse systems under dynamic conditions are reported. Computer-simulation data on the influence of some parameters on the viscosity and structure-related characteristics of dispersions agree with analytical dependences and experimental results. Problems concerning the dynamic equilibrium of dispersions subjected to shear deformation, as well as disturbance of equilibrium, are considered. Results of computer simulating the percolation characteristics of dispersion microstructure under dynamic conditions are reported.     

Simulations of the PDF functions for dilute colloidal suspensions of molecular particles flowing in mesopores with rough surface boundaries

Simulations have been carried out to analyze the dynamics of dilute colloidal suspensions of macromolecular particles in solutions flowing in pores, subject to hydrodynamic forces, Brownian motion and stochastic collisions at rough pore boundaries in a two-dimensional spatial frame. A theoretical model is developed and intensively analyzed for the treatment of the mechanical restitution of the particles due to dynamic collisions at these boundaries. In particular we are able to calculate the Probability distribution functions for the spatial positions and the orientations of rod-like particles inside the pores. The results are presented for different widths of pore channels referenced to the size of a rod-like particle. These simulations are general in the sense that they are developed for confining and open pore channels, rough at the nano scale. The simulations also permit calculating the nematic order parameters for colloidal suspensions; the model calculation is applied for dilute colloidal suspensions of carbon nano-tubes in an aqueous single-stranded DNA solution flowing inside pores. Our calculated nematic order results for dilute suspensions of particles of known lengths flowing inside porous systems should indicate, when coupled to birefringence and dichroism experimental results, the possibility to estimate the pore widths for these systems.     

Magnetorheological properties of ferrofluids containing clustered particles

A theoretical model is proposed to describe experimental data on the magnetorheological properties of magnetic fluids containing clustered particles consisting of single-domain ferromagnetic nanoparticles distributed in a polymeric shell 80–100 nm in diameter. These fluids combine the sedimentation stability typical of nanodisperse ferrofluids with the high sensitivity of rheological parameters to magnetic fields. The developed model explains the experimentally found long-term rheological relaxation and residual stress that is retained after the medium ceases to flow.     

Analysis of the degree of nematic ordering within dense aqueous dispersions of charged anisotropic colloids by 23Na NMR spectroscopy

Aqueous dispersions of Laponite, a synthetic clay neutralized by sodium counterions, are used as a model of charged anisotropic colloids to probe the influence of the shape of the particle on their organization within a macroscopic nematic phase. Because of the large fraction of condensed sodium counterions in the vicinity of the clay particle, (23)Na NMR is a sensitive probe of the nematic ordering of the clay dispersions. We used line shape analysis of the (23)Na NMR spectra and measurements of the Hahn echo attenuation to quantify the degree of alignment of the individual clay particles along a single nematic director. As justified by simple dynamical simulations of the interplay between the sodium quadrupolar relaxation and its diffusion through the porous network limited by the surface of the clay particles, we probe the degree of ordering within these clay nematic dispersions by measuring the variation of the apparent (23)Na NMR relaxation rates as a function of the macroscopic orientation of the clay dispersion within the magnetic field.     

Isotropic-nematic phase transition of nonaqueous suspensions of natural clay rods

A novel model system for studying the behavior of hard colloidal rods is presented, consisting of sterically stabilized particles of natural sepiolite clay. Electron microscopy and scattering results confirmed that the organophilic clay particles were individual, rigid rods when dispersed in organic solvents. With a length-to-diameter ratio of approximately 27, the particles showed nematic ordering for volume fractions phi > 0.06. Polarizing microscopy revealed that the phase separation process involved nucleation, growth, and coalescence of nematic domains. The phase volumes and particle concentrations in the coexisting phases were determined. The dependence of these quantities on the total concentration of the suspension agrees well with Onsager's [Ann. N. Y. Acad. Sci. 51, 627 (1949)] isotropic-nematic phase transition theory extended to bidisperse and polydisperse rod systems, and with previous experimental results for rigid rodlike particles. Particle size distributions were obtained by analyzing transmission electron microscopy images. A significant fractionation with respect to rod length (but not diameter) was observed in the coexisting isotropic and nematic phases. The relative polydispersity of both daughter phases was distinctly smaller than that of the parent suspension. The phase behavior of these daughter fractions agrees well with the predictions for hard spherocylinders of corresponding aspect ratios. An isotropic-nematic-nematic phase equilibrium was seen to develop in phase separated samples after 1 month standing and is ascribed to the effect of polydispersity and possibly gravity. The second nematic phase appearing is dominated by very long rods.     

Anisotropic electroconducting polymer-silicate composites based on polyaniline

Approaches for the development of anisotropic electroconducting composite materials based on polyaniline and Na-montmorillonite prepared by the me thods of boundary and intercalation polymerization of aniline and mechanical blending are proposed. Parallel plane compression of solid and plasticized dispersions is shown to lead to the development of primarily planar ordering of anisometric clay particles with sorbed or intercalated polymer; as a result, nanocomposites with anisotropic electrical conductivity are formed. In the prepared polymer-silicate films, the parameter of anisotropy in electrical conductivity achieves 6 × 10³.     

Theory of Intrachain Relaxation Spectra for Polymer Networks Possessing a Short- or Long-Scale Ordering. Effects of the Nematic Ordering on the Relaxation Spectrum of a Polymer Network with Included Rods

We discuss the relaxation properties of polymer networks possessing either short-scale ordering caused by rigidity of network strands or long-scale liquid crystalline order. The main topics of the paper are the equilibrium and local dynamic properties of a polymer network ordered due to nematic-like interactions of the network segments with included rod-like particles. A simplified three chain network model is used. Lagrange multipliers in the equations of motion of hard rods are replaced by their averaged values. This approximation corresponds to modelling the rod-like particles by elastic Gaussian springs, their mean-square lengths independent of the ordering. Nematic-like interactions between network segments and rods are taken into account in terms of the Maier-Saupe mean-field approximation. Nematic ordering of rods induces ordering of the network segments. Relaxation spectrum of the ordered network splits into two main branches for the parallel and perpendicular components of the chain segments with respect to the director. We calculate the relaxation times of a polymer network as functions of the wave number. The relaxation spectrum of an isotropic network and that of the ordered network with included rods are compared.     

DNA‐Mediated Assembly of Boron Nitride Nanotubes

The dispersion of nanomaterials in solutions is of primary importance for the improvement of their processability, but it also provides a way to investigate phase behavior and to assemble nanostructures in solvents. Several methods based on different interactions have been developed to disperse carbon nanotubes, whereas little development has been made for their boron nitride nanotube (BNNT) counterparts. A direct way to obtain long‐range ordering may be through spontaneous nematic ordering in solutions at sufficiently high concentrations of the nanomaterial fraction. Lyotropic nematics have been observed in various organic and inorganic systems. In this work, the strong interactions between DNA and BNNTs were exploited to fabricate high‐concentration BNNTs aqueous solutions by a simple method, and then, for the first time, nematic ordered ensembles of BNNTs were obtained by filtration. It is proposed that a localized liquid‐crystal phase appears during filtration, as the ordering trend for the BNNTs was found to depend on the concentration of the aqueous solutions of the BNNTs. Moreover, BNNTs were successfully localized on a predefined area by using a thiol‐modified DNA–BNNT hybrid.     

Nematic-isotropic interfaces under shear: a molecular-dynamics simulation

We present a large-scale molecular-dynamics study of nematic-paranematic interfaces under shear. We use a model of soft repulsive ellipsoidal particles with well-known equilibrium properties, and consider interfaces which are oriented normal to the direction of the shear gradient (common stress case). The director at the interface is oriented parallel to the interface (planar). A fixed average shear rate is imposed with moving periodic boundary conditions, and the heat is dissipated with a profile-unbiased thermostat. First, we study the properties of the interface at one particular shear rate in detail. The local interfacial profiles and the capillary wave fluctuations of the interfaces are calculated and compared with those of the corresponding equilibrium interface. Under shear, the interfacial width broadens and the capillary wave amplitudes at large wavelengths increase. The strain is distributed inhomogeneously in the system (shear banding), the local shear rate in the nematic region being distinctly higher than in the paranematic region. Surprisingly, we also observe (symmetry-breaking) flow in the vorticity direction, with opposite direction in the nematic and the paranematic state. Finally, we investigate the stability of the interface for other shear rates and construct a nonequilibrium phase diagram.     

On the Theory of the Dynamic Properties of Dense Magnetic Fluids: Polydisperse Media

The theoretical model was proposed for determining functions of the dynamic response of moderately concentrated ferrofluids on the external magnetic field and their rheological properties. Ferrofluids are considered to be polydisperse colloidal systems with the interacting (albeit individual) particles. The model is based on the regular approximation of virial expansion in powers of the particle concentration and on the well-known effective field method. The effect of system polydispersity and the magnetodipole and hydrodynamic interactions between particles on the macroscopic and dynamic properties of ferrofluids was estimated. Calculations demonstrated that the interparticle interaction results in an increase in the dynamic functions of uniform ferrocolloids up to several tens of percents.     

Flow-alignment and viscosity rules for single-phase binary mesomorphic mixtures

A macroscopic model for incompressible homogeneous (single phase) binary nematic mixtures, under isothermal conditions is given. The rheological model is a generalization of the standard Ericksen's nematorheological model for single component uniaxial rod-like nematic liquid crystals. Its special cases include single component orthorhombic biaxial nematics and single component uniaxial nematics. The theory is used to formulate rules for the rotational viscosity and the reactive parameter of nematic mixtures in the presence of weak flows. The predicted mixture rules for the reactive parameter and rotational viscosity are analysed as a function of concentration and rotational viscosity ratio for various monomeric and polymeric mixtures, and for rod-rod, disc-disc, and rod-disc nematic mixtures. The mixture rules are used to compute alignment phase diagrams and alignment transition (orientational instability) thresholds.     

Design of calamitic self-assembling reactive mesogenic units: mesomorphic behaviour and rheological characterisation

Several calamitic reactive mesogens containing only two benzene rings in the molecular core and with or without lateral substitution by the methyl/methoxy groups have been designed and their mesomorphic behaviour was characterised. Depending on the molecular structure, some of the materials exhibit the nematic and the orthogonal smectic mesophases. The reactive mesogens are aimed for further design of the macromolecular materials like polysiloxane-based polymers and elastomers. Beyond the mesomorphic and structural properties, the electrorheological properties within the temperature range of the nematic and smectic A mesophases were studied with and without applied electric field for the selected reactive mesogen. The increase of viscosity was found not to be higher than three times under applied electric field strength of 2 kV/mm. The mesomorphic, structural and rheological properties of the newly designed reactive mesogens are discussed in order to contribute to better understanding of the molecular architecture–nano-organisation properties relationship of such mesogenic materials.     

Phase separations in liquid crystal-colloid mixtures

We present a mean-field theory to describe phase separations in mixtures of a nematic liquid crystal and a colloidal particle. The theory takes into account an orientational ordering of liquid crystals and a crystalline ordering of colloidal particles. We calculate phase diagrams on the temperature-concentration plane, depending on interactions between a liquid crystal and a colloidal surface and a coupling between nematic and crystalline ordering. We find various phase separation processes, such as a nematic-crystal phase separation and nematic-isotropic-crystal triple point. Inside binodal curves, we find new unstable and metastable regions which are important in phase ordering dynamics. We also find a stable nematic-crystalline (NC) phase, where colloidal particles dispersed in a nematic phase can form a crystalline structure. The coexistence between two NC phases with different concentrations can be appear though the coupling between nematic and crystalline ordering.