Rheological characterization of aging in a concentrated colloidal suspension

Suspensions of soft particles display a rheological transition when their concentration is increased: above a critical concentration, they become pasty and display a yield-stress. We report experiments on silica suspensions in the pasty phase that show how the response of the system to the application of mechanical perturbations depends on its history. Stress relaxation experiments performed after different waiting times clearly show aging taking place in the pasty phase, characterized by an aging exponent of order μ∼0.5–0.7.     

Optical properties of a suspension of hard spheres

The optical properties of suspensions are studied in a wide range of concentrations. An expression for the polarization operator is obtained taking into account the contributions of two-and three-particle correlations. The extinction length l and the transport length l* are calculated in terms of a model of hard spheres. A detailed comparison of the results of calculations with experimental data is performed. In calculations, the structure factor is determined in the Percus-Yevick approximation, while the form factor is taken into account in the Rayleigh-Gans approximation and in terms of the Mie theory. It is shown that taking into account the contribution of three-particle correlations improves the agreement of the theory with experiment. It is found that, in the range of high suspension concentrations, the optical parameters are more sensitive to the choice of the model for the structure factor than for the form factor.     

Superradiant properties of a suspension of composite nanoparticles

The characteristics of gold nanoparticles coated with silicon dioxide containing embedded dye molecules are experimentally investigated at room and liquid-nitrogen temperatures. The origin of the observed narrowing of the luminescence line upon cooling is discussed. Model analysis of the nanosystem under study indicates that the observed behavior cannot be related to the temperature dependence of individual parameters of the components of the medium. The effect is explained by the occurrence of nonlinear feedback that leads to the enhancement of phase synchronization between the polarizations of the active transitions in the dye molecules and plasmons. This effect is an analog of plasmon polariton superradiance.     

Rheological properties of a nematic cell oriented in a planar manner

We propose a simple model to investigate the rheological properties of a nematic cell oriented in a planar manner. The storage and loss modulus are evaluated in the case of strong and weak anchoring conditions. The contribution of the surface viscosity to the rheological parameters is also considered.     

Rheological properties of a highly confined film of a lyotropic lamellar phase

We investigated the rheological properties of a thin film of a lyotropic lamellar phase with a Dynamic Surface Forces Apparatus. The minimum thickness of the film is varied between one to several tens of layers by confining the materials between solid surfaces. The rheometric properties are measured with the application of a small harmonic compression. These properties depend clearly on the smectic order of the material. Whole mechanical properties may be easily described by taking into account interactions between membranes and motion of the dislocation line loops. In particular, it is shown that at the dynamical frequencies investigated in this study, the solvent flows between membranes which remain undeformed. Consequences and perspectives of this study will be discussed.Received: 22 April 2004, Published online: 11 August 2004PACS: 61.30.St Lyotropic phases - 61.30.Jf Defects in liquid crystals - 83.80.Qr Surfactant and micellar systems, associated polymers     

Rheological properties of dense granular flows

Recent progresses in understanding the behavior of dense granular flows are presented. After presenting a bulk rheology of granular materials, I focus on the new developments to account for non-local effects, and on ongoing research concerning the surface rheology and the evolution of mechanical properties for heterogeneous systems.     

Review: Rheological properties of biological materials

Eukaryotic cells and biological materials are described from a rheological point of view. Single cells possess typical microrheological properties which can affect cell behaviour, in close connection with their adhesion properties. Single cell properties are also important in the context of multicellular systems, i.e. in biological tissues. Results from experiments are analyzed and models proposed both at the cellular scale and the macroscopic scale. To cite this article: C. Verdier et al., C. R. Physique 10 (2009).     

Ultrasonic properties of a suspension of microspheres supporting negative group velocities

The ultrasonic attenuation coefficient, phase velocity, and group velocity spectra are reported for a suspension that supports negative group velocities. The suspension consists of plastic microspheres with an average radius of 80 microm in an aqueous medium at a volume fraction of 3%. The spectra are measured using a broadband method covering a range from 2 to 20 MHz. The suspension exhibits negative group delays over a band near 4.5 MHz, with the group velocity magnitude exceeding 4.3 x 10(8) m/s at one point. The causal consistency of these results is confirmed using Kramers-Kronig relations.     

Linear and nonlinear properties for a dilute suspension of coated ellipsoids

The effects of geometric structures of coated particles on the dielectric properties of composites are derived. For a dilute suspension of coated ellipsoids with dielectric core and metallic shell embedded in a linear dielectric host, we find that the optical absorption peak of maximum wavelength shifts due to the core-shell structure, and has a significant red shift from that of pure metallic particles, especially for thin metallic coating. Meanwhile, the shape of coated ellipsoid can tune the absorption peak in a wide frequency range by properly choosing the depolarization factor. When the composite is made of linear materials, we obtain the condition of partial resonance for coated ellipsoids, which is relative to the particle shape. Under such a condition, the property of the inner core can be extended to the outer shell. When the inner core is a weakly nonlinear material, the nonlinearity of the composite can be greatly enhanced at the linear partial resonance. To achieve the condition of partial resonance, a metallic shell may be suitable to reach the case if the frequency is far away from the plasmon frequency. It indicates that the nonlinearity can also be enhanced in terms of the geometric structure of materials as well as the properties of themselves.     

Rheological microscopy: local mechanical properties from microrheology

We demonstrate how tracer microrheology methods can be extended to study submicron scale variations in the viscoelastic response of soft materials; in particular, a semidilute solution of lambda-DNA. The polymer concentration is depleted near the surfaces of the tracer particles, within a distance comparable to the polymer correlation length. The rheology of this microscopic layer alters the tracers' motion and can be precisely quantified using one- and two-point microrheology. Interestingly, we found this mechanically distinct layer to be twice as thick as the layer of depleted concentration, likely due to solvent drainage through the locally perturbed polymer structure.     

Rheological properties of water-based magnetic fluid and its emulsion

Field-induced rheological properties were studied for pure magnetic fluids (MFs) and 5% MF–cetyltrimethylammonium bromide (CTABr) emulsion for varying CTABr concentration (0–0.1 M) at different temperatures. The zero-field rheological study for 5% MF–CTABr emulsion shows maximum viscosity for 0.01 M CTABr concentrations. In-field viscosity results for 0.01 M CTABr–MF emulsion shows nearly 10 times more change in viscosity than the pure fluid, indicating the interaction between micelles and magnetic particles.     

聚苯乙烯/纳米铝复合材料的流变特性及纺丝研究

为有效制得Z箍缩氘代聚苯乙烯/纳米铝(DPS/AlNPs)导电丝阵材料,采用PS中掺入AlNPs制备PS/AlNPs复合材料纤维进行模拟研究。研究了温度及剪切速率等因素对PS/AlNPs复合材料流变性能的影响、复合材料熔体的结构变化及流动状态与可纺性能的关系,以及PS/AlNPs纤维的形貌、热稳定性能和力学性能。结果表明：PS/AlNPs熔体属于典型剪切变稀型非牛顿流体,熔体的表观粘度与温度呈现负相关,240~260 ℃时复合材料的非牛顿指数介于0.462~0.546,结构黏度系数介于1.8~2.1,黏流活化能介于77.2~104.6 kJ·mol^-1,具有良好的可纺性。PS/AlNPs纤维表面光滑,对AlNPs粒子包覆良好且对其抗氧化非常有利,其中当AlNPs质量分数为1%时纤维的断裂伸长率突出、掺量为5%时其断裂强度较高。     

Budding and fission of a multiphase vesicle

We present a model of bi-phasic vesicles in the limit of large surface tension. In this regime, the vesicle is completely stretched and well described by two spherical caps with a fold, which concentrates the membrane stress. The conservation laws and geometric constraints restrict the space of possible shapes to a pair of solutions labeled by a parameter given by line tension/pressure. For a given value of , the two solutions differ by the length of the interface between domains. For a critical value, , the two vesicle shapes become identical and no connected solution exists above this critical value. This model sheds new light on two proposed mechanisms (osmotic shocks and molecule absorption) to explain the budding and the fission in recent experiments.     

Rheological properties of polydisperse magnetic fluids. Effect of chain aggregates

A theoretical model of medium-density polydisperse magnetic fluids is proposed. The model takes into account that the major fraction of particles in typical ferrofluids is characterized by a magnetic core diameter of about 10 nm. In addition, there is a certain proportion of large particles with a core diameter of about 16 nm. As a result of the magnetic dipole interaction, the large particles form chain aggregates. Small particles, for which the magnetic dipole interaction energy (both with each other and with large particles) is smaller than the thermal energy, remain in the individual nonaggregated state. The distribution of chains with respect to the number of (large) particles and some rheological characteristics of the ferrofluids are determined. The proposed model is capable of explaining, in principle, the giant magnetoviscosity effect and a strong dependence of the rheological properties of ferrofluids on the shear rate observed in some recent experiments.     

Static light scattering properties of a ZnO nanosphere aqueous suspension at visible and near-infrared wavelengths

The scattering properties of ZnO nanospheres with four different particle diameters of 10, 50, 100, and 200 nm suspended in water are investigated theoretical and experimentally in the spectral range of the entire visible range and part of the near-infrared region. The scattering properties of ZnO nanospheres suspended in water are described by employing three main parameters: the angular distribution of the scattering intensity I, the scattering extinction coefficient αscat, and the scattering cross section σscat. The results indicate that(i) at a certain wavelength, the angular distribution of the scattering intensity appears as an obviously forwardpropagating feature, and the forward-scattering intensity is dominant gradually when the particle diameter increases from 10 to 200 nm, and(ii) the scattering extinction coefficient and cross section can be determined by using the measured transmittance changes of a pure water sample and a given ZnO sample; they all are shown to be dependent on the particle size and incident wavelength. The experimental results of four different scattering samples agree well with the theoretical predictions within the given wavelength range.     

Dynamics of a compound vesicle in shear flow

The dynamics of a compound vesicle (a lipid bilayer membrane enclosing a fluid with a suspended particle) in shear flow is investigated by using both numerical simulations and theoretical analysis. We find that the nonlinear hydrodynamic interaction between the inclusion and the confining membrane gives rise to new features of the vesicle dynamics: The transition from tank treading to tumbling can occur in the absence of any viscosity mismatch, and a vesicle can swing if the enclosed particle is nonspherical. Our results highlight the complex effects of internal cellular structures have on cell dynamics in microcirculatory flows. For example, parasites in malaria-infected erythrocytes increase cytoplasmic viscosity, which leads to increase in blood viscosity.     

Shrinkage dynamics of a vesicle in surfactant solutions

We develop a theory for shrinkage dynamics of a vesicle interacting with surfactant molecules. A stepwise shrinkage is formulated in such a way that it consists of two processes. One is a nucleation process of a pore under increasing of the membrane tension. The other is a closure process of the pore due to the line tension of the pore edge after leakage of the inner fluid. We carry out numerical simulations and show that the results agree with experiments semi-quantitatively. An analytical study is also carried out to understand the periodic shrinkage.     

Convection of a stratified colloidal suspension

The convection of a colloidal suspension, which is a binary mixture of a carrier medium with an admixture of nanoparticles having a large positive thermal diffusion parameter, has been studied for the case of the heating of a horizontal cell from below and periodic conditions at the vertical boundaries corresponding to the experimental situation of ring channels. Bifurcation diagrams have been constructed for vibrational and monotonic regimes of the convection of the colloidal mixture. The time dependences of the maximum stream function and the stream function at a fixed point of the cell, as well as the spatial distributions of the concentration field of the colloid admixture, have been obtained. It has been shown that a stable regime of traveling waves exists in a certain region of the parameters of the problem (Boltzmann and Rayleigh numbers characterizing the gravitational stratification and intensity of the thermal effect, respectively).     

Design of a suspension of a training sulky

A design study based on a modelling approach was used to optimise the characteristics of a new concept of suspension for training sulkies. The numerical model of sulky including the suspension and, in a second stage, a mechanical model of driver, allowed the definition of technical specifications. They were used to manufacture a prototype of suspension mounted on a current production model of sulky. The prototype of suspension was then tested in the lab and in a racecourse in real conditions of use.The resulting vibration exposure was assessed from vibration measurements. It was slightly lower than the limit value enacted in the European Vibration directive (1.15 m s²), but drastically reduced in comparison with the exposure measured on the original sulky (2.56 m s²). The prototype and moreover the design procedure is currently in a transfer process towards sulky manufacturers.