Diffusion of colloidal particles in swimming suspensions

Previously, we have proposed to analyse the hydrodynamic interactions in a suspension of swimmers with respect to an effective hydrodynamic diffusion coefficient, which only considers the fluctuating motion caused by the stirring of the fluid. In this work, we study the diffusion of colloidal particles immersed in a bath of swimmers. To accurately resolve the many-body hydrodynamic interactions responsible for this diffusion, we use a direct numerical simulation scheme based on the smooth profile method. We consider a squirmer model for the self-propelled swimmers, as it accurately reproduces the flow field generated by real microorganisms, such as bacteria or spermatozoa. We show that the diffusion coefficients of the colloids are comparable with the effective diffusion coefficients of the swimmers, provided that the concentration of swimmers is high enough. At low concentrations, the difference in the way colloids and swimmers react to the flow leads to a reduction in the diffusion coefficient of the colloids. This is clearly seen in the appearance of a negative-correlation region for the velocity-correlation function of the colloids, which does not exist for the swimmers.     

Dynamics in dense suspensions of charge-stabilized colloidal particles

The dynamic behavior of charge-stabilized colloidal particles in suspension was studied by photon correlation spectroscopy with coherent X-rays (XPCS). The short-time diffusion coefficient, D(Q) , was measured for volume concentrations φ ⩽ 0.18 and compared to the free particle diffusion constant D₀ and the static structure factor S(Q) . The data show that indirect, hydrodynamic interactions are relevant for the system and hydrodynamic functions were derived. The results are in striking contrast to the predictions of the PA (pairwise-additive approximation) model, but show features typical for a hard-sphere system. The observed mobility is however considerably smaller than the one of a respective hard-sphere system. The hydrodynamic functions can be modelled quantitatively if one allows for an increased effective viscosity relative to the hard-sphere case.     

Fluid suspensions of colloidal ellipsoids: direct structural measurements

A fluid of spheroids, ellipsoids of revolution, is among the simplest models of the disordered matter, where positional and rotational degrees of freedom of the constituent particles are coupled. However, while highly anisometric rods, and hard spheres, were intensively studied in the last decades, the structure of a fluid of spheroids is still unknown. We reconstruct the structure of a simple fluid of spheroids, employing direct confocal imaging of colloids, in three dimensions. The ratio t between the polar axis and the equatorial diameter for both our prolate and oblate spheroids is not far from unity, which gives rise to a delicate interplay between rotations and translations. Strikingly, the measured positional interparticle correlations are significantly stronger than theoretically predicted, indicating that further theoretical attention is required, to fully understand the coupling between translations and rotations in these fundamental fluids.     

Simulation method of colloidal suspensions with hydrodynamic interactions: fluid particle dynamics

We develop a new simulation method of colloidal suspensions, which we call a "fluid particle dynamics" (FPD) method. This FPD method, which treats a colloid as a fluid particle, removes the difficulties stemming from a solid-fluid boundary condition in the treatment of hydrodynamic interactions between the particles. The importance of interparticle hydrodynamic interactions in the aggregation process of colloidal particles is demonstrated as an example. This method can be applied to a wide range of problems in colloidal science.     

电解质对浓悬浮液中胶体颗粒扩散特性的影响

本文利用相位调制光纤低相干动态光散射装置, 研究了不同物质量浓度下电解质 (NaCl及 BaCl₂) 对浓悬浮液中聚苯乙烯胶体颗粒扩散特性的影响. 实验结果表明, 当电解质浓度低于0.01 mol/L 时, 恒温条件下浓悬浮液中聚苯乙烯胶体颗粒扩散系数随电解质离子浓度以及离子化合价的增大而增大, 实验测量得到的扩散系数与Stern模型所得到的扩散系数符合较好. 关键词： 测量 电解质 浓悬浮液 低相干动态光散射     

Low-energy modes and Debye behavior in a colloidal crystal

We study the vibrational spectrum and the low-energy modes of a three-dimensional colloidal crystal using confocal microscopy. This is done in a two-dimensional cut through a three-dimensional crystal. We find that the observed density of states is incompatible with the standard Debye form in either two or three dimensions. These results are confirmed by numerical simulations. We show that an effective theory for the projections of the modes onto the two-dimensional cut describes the experimental and simulation data in a satisfactory way.     

Theory of the Relaxation spectra of polymer networks with included hard rodlike particles manifested in NMR

A theory of nuclear magnetic resonance relaxation of¹³C nuclei and nuclear Overhauser effect (NOE) of polymer networks with included rodlike particles is developed for the case when the length of each rod is comparable or greater than the average distance between neighboring crosslinks. The long-scale dynamics of the network is described by means of a regular cubic “coarse-grained” model. The effects of entanglements of long rods in the network are described by a quasi-elastic potential acting between rods and network fragments. The frequency dependences of 1/T _1C and NOE are calculated for the case when the internuclear vector is directed along each rodlike particle. The frequency dependences of 1/T _1C and NOE for rods included into a polymer network are shifted to high frequencies as compared with these dependences for free rods due to quasi-elastic interactions between rods and network fragments. At strongly different viscoelastic parameters of rods and network fragments, the frequency dependences of 1/T_1C and NOE may have two maxima. The high-frequency maximum corresponds to localized motions of rods at immobile network domains. The low-frequency maximum is caused by involving rods in long-scale network motions. The intensity of the low-frequency maximum increases when the degree of interactions between rods and the network increases.     

Non-equilibrium behavior of sticky colloidal particles: beads,clusters and gels

To understand the non-equilibrium behavior of colloidal particles with short-range attraction, we studied salt-induced aggregation of lysozyme. Optical microscopy revealed four regimes: bicontinuous texture, beads, large aggregates, and transient gelation. The interaction of a metastable liquid-liquid binodal and an ergodic to non-ergodic transition boundary inside the equilibrium crystallization region can explain our findings.     

Experimental observation of structural crossover in binary mixtures of colloidal hard spheres

Using confocal microscopy, we investigate the structure of binary mixtures of colloidal hard spheres with size ratio q=0.61. As a function of the packing fraction of the two particle species, we observe a marked change of the dominant wavelength in the pair-correlation function. This behavior is in excellent agreement with a recently predicted structural crossover in such mixtures. In addition, the repercussions of structural crossover on the real-space structure of a binary fluid are analyzed. We suggest a relation between crossover and the lateral extension of networks containing only equally-sized particles that are connected by nearest-neighbor bonds. This is supported by Monte Carlo simulations which are performed at different packing fractions and size ratios.     

Brownian dynamics simulation of monolayer formation by deposition of colloidal particles: A kinetic study at high bulk particle concentration

Brownian dynamics simulations (BDS) of sedimentation and irreversible adsorption of colloidal particles on a planar surface were carried out at bulk particle volume fractions (φ) in the range 0.05 to 0.25. The sedimentation and adsorption of colloidal particles were simulated as a non-sequential process that allows simultaneous settling and adsorption of particles. A kinetic model for the formation of particle monolayers based on the available surface fraction (θ ( A )) is proposed to predict simulation results. The simulations show a value of 0.625 for the maximum fractional surface coverage (θ (∞)) and a monolayer structure insensitive to φ. However, the kinetic order of the monolayer formation process has a strong dependence with φ, changing from a value close to a unit, at low φ, to a value around two at high φ. This change in the kinetic reaction order is associated to differences of particle adsorption mechanism on the surface. At low φ values, the monolayer formation is achieved by independent adsorption of single particles and the reaction order is close to 1. At high φ values, the simultaneous adsorption of two particles on the surface leads to an increase of the reaction order to values close to 2.     

Laser trapping of small colloidal particles in a nematic liquid crystal: clouds and ghosts

We show that, contrary to intuition, small (< or =1 microm) transparent particles can be trapped and manipulated in a nematic liquid crystal using an intense laser beam, although their index of refraction is lower than both refractive indices of the surrounding birefringent fluid. Two mechanisms are identified that are responsible for this anomalous trapping: (i) surface-induced distortion of the birefringent media around the particle, creating a high-index "cloud" around the colloid, and (ii) laser-induced distortion or (partial) melting of a nematic, creating a ghost colloid.     

Instabilities and pattern formation in active particle suspensions: kinetic theory and continuum simulations

We use kinetic theory and nonlinear continuum simulations to study the collective dynamics in suspensions of self-propelled particles. The stability of aligned suspensions is first analyzed, and we demonstrate that such suspensions are always unstable to fluctuations, a result that generalizes previous predictions by Simha and Ramaswamy. Isotropic suspensions are also considered, and it is shown that an instability for the particle stress occurs in that case. Using simulations, nonlinear effects are investigated, and the long-time behavior of the suspensions is observed to be characterized by the formation of strong density fluctuations, resulting in efficient fluid mixing.     

Mesoscale hydrodynamics simulations of particle suspensions under shear flow: From hard to ultrasoft colloids

We discuss the nonequilibrium properties of rodlike and ultrasoft, star-polymer like colloidal particles in shear flow. Conformational, dynamical, as well as rheological aspects are addressed for a broad range of concentrations. For concentrated solutions of rodlike colloids, we study the nonequilibrium properties of a phase separated system, where a disordered phase coexists with a nematic phase. For starlike polymers we consider systems of various functionality, starting from linear polymers. The individual rods, polymers, or stars exhibit an intriguing dynamical behavior, which determines their macroscopic rheological properties. Despite the diversity on the colloidal level, the various systems exhibit a qualitatively similar macroscopic behavior, e.g., shear thinning, yet with some quantitative differences.     

钢中脆硬粒子裂纹形成机理

钢中的脆硬粒子对钢的解理脆断有直接的影响，解理断裂源大都发生在脆硬粒子上.根据微裂纹形成的热力学模型，利用钢中脆硬粒子开裂时所释放的弹性应变能、位错塞积弹性能，所产生的表面能，对脆硬粒子裂纹形成机理进行分析.模型计算表明，正应力和位错塞积力都是脆硬粒子开裂的必要条件，这与实验事实相符；同时给出脆硬粒子开裂的临界条件计算方法，计算发现，脆硬粒子临界开裂应力不仅取决于脆硬粒子尺寸及表面能，而且与晶粒直径有一定的相关关系，当晶粒直径较小时，这种关系与实验测定的材料解理断裂应力与晶粒尺寸的关系一致，说明整体失稳解 关键词： 解理断裂 裂纹形核 脆硬粒子     

Measurement of elastic forces between iron colloidal particles in a nematic liquid crystal

The forces that arise between two iron particles in a nematic liquid crystal with a strong homeotropic anchoring were studied. For the first time, the short range repulsive force resulting from the presence of a hedgehog defect between two particles was precisely determined thanks to application of a small magnetic field and observation of the equilibrium position resulting from the balance between the elastic and magnetic forces. Above a given threshold force, the particles stuck together whereas the hedgehog defect was expelled and transformed into a Saturn ring located between the particles. The attractive part of the interparticle force was determined with the same method on the entire range of separation distances; we found that the equilibrium distance between two particles was r = 1.19 +/- 0.05 ( was the average diameter of the pair of particles).     

Phase diagram of dipolar hard and soft spheres: manipulation of colloidal crystal structures by an external field

Phase diagrams of hard and soft spheres with a fixed dipole moment are determined by calculating the Helmholtz free energy using simulations. The pair potential is given by a dipole-dipole interaction plus a hard-core and a repulsive Yukawa potential for soft spheres. Our system models colloids in an external electric or magnetic field, with hard spheres corresponding to uncharged and soft spheres to charged colloids. The phase diagram of dipolar hard spheres shows fluid, face-centered-cubic (fcc), hexagonal-close-packed (hcp), and body-centered-tetragonal (bct) phases. The phase diagram of dipolar soft spheres exhibits, in addition to the above mentioned phases, a body-centered-orthorhombic (bco) phase, and it agrees well with the experimental phase diagram [Nature (London) 421, 513 (2003)]. Our results show that bulk hcp, bct, and bco crystals can be realized experimentally by applying an external field.     

Self-diffusion in sheared colloidal suspensions: violation of fluctuation-dissipation relation

Using memory-function formalism we show that in sheared colloidal suspensions the fluctuation-dissipation theorem for self-diffusion, i.e., Einstein's relation between self-diffusion and mobility tensors, is violated and propose a new way to measure this violation in Brownian dynamics simulations. We derive mode-coupling expressions for the tagged particle friction tensor and for an effective, shear-rate dependent temperature.