摘要:

Colloidal suspensions have been proven to play a pivotal role of model systems in order to understand the principles of equilibrium phase transitions such as freezing and fluid‐fluid demixing. One of the main reasons for that is that real‐space studies are possible thanks to the mesoscopic length scale of the particle size. The same model character of colloidal suspensions holds in non‐equilibrium situations as e.g. represented by an external driving field (such as shear, gravity, an electric and/or magnetic field). In this paper some current examples of non‐equilibrium transitions are reviewed where recent progress has been made by theory and computer simulation. In particular, we discuss the competition between phase separation and lane formation in driven colloidal mixtures, crystal nucleation in charged suspensions under shear and chain formation of two‐dimensional superparamagnetic suspensions induced by an external magnetic field. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764052

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

In order to investigate the universal features of collective behavior due to the many‐body interactions, we perform two types of computer simulations on hard‐sphere systems, a Brownian‐dynamics simulation on polydisperse suspensions of hard spheres, where the hydrodymamic interactions between particles are neglected, and a molecular‐dynamics simulation on atomic systems of hard spheres. Thus, we show that the long‐time self‐diffusion coefficient in atomic systems has the same form as that derived theoretically by Tokuyama and Oppenheim (TO) for the monodisperse suspension by taking into account the many‐body hydrodynamic interactions, except that the singular point is now replaced by a new one. We also show that the difference between two coefficients in both systems can be well explained by the short‐time self‐diffusion coefficient derived theoretically for a wide range of volume fractions. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764053

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Recent mode coupling theory (MCT) calculations for a hard‐sphere system with a short‐range attraction show that one may observe a new type of structurally arrested state originating from clustering effect, called the “attractive glass”, as a result of the attractive interaction. This is in addition to the well‐known glass‐forming mechanism due to the cage effect in the hard sphere system, called the repulsive glass. The calculations also indicate that, if the range of attraction is sufficiently short compared to the diameter of the hard sphere, within a certain interval of the volume fraction and the effective temperature, the two glass‐forming mechanisms can compete with each other. For example, by varying, the effective temperature at appropriate volume fractions, one may observe respectively, the glass‐to‐liquid‐to‐glass re‐entrance or the glass‐to‐glass transitions. Here we present experimental evidence for both transitions, obtained from small‐angle neutron scattering (SANS) and photon correlation spectroscopy (PCS) measurements taken from denseL64 copolymer micellar solutions in heavy water. We show, by varying the temperature in the predicted volume fraction range triggers a sharp transition between the two types of glass. In particular, according to MCT, there is an end point (calledA3singularity) of this glass‐to‐glass transition line, beyond which the long‐time dynamics of the two glasses become identical. Our findings confirm this theoretical prediction. Surprisingly, although the Debye‐Waller factors (DWF), the long‐time limit of the coherent intermediate scattering functions, of these two glasses obtained from PCS measurements indeed become identical at the predicted volume fraction, they exhibit distinctly different intermediate time relaxation. Furthermore, our SANS results on the local structure obtained from volume fractions beyond the end point are characterized by the the same features as the repulsive glass obtained before the end point. A complete phase diagram giving the boundaries of the structural arrest transitions forL64 micellar system is given. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764054

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The study of our minimal statistical mechanics model for gelling systems, by means of numerical simulations, suggests a unifying picture for gelation phenomena, connecting classical gelation and recent results on colloidal systems. By varying the model parameters the slow dynamics present a crossover from the classical polymer gelation to dynamics more typical of colloidal systems, with a glassy regime that is interpreted in terms of effective clusters. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764055

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The mode coupling theory (MCT) for the glass transition is in reasonable accord with observations on attractive colloids at high densities. This paper presents a brief review of the MCT. This is followed by a more detailed discussion of two recent extensions of it. These address (i) nonlinear rheology and jamming in colloids under shear and (ii) weak gelation of colloids at low densities. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764056

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We describe experiments on binary mixtures of superparamagnetic colloidal particles confined by gravity to a flat horizontal water‐air interface. The colloids repel each other because of their magnetic dipole moments induced by a vertical external magnetic fieldB. By tuningB, the effective temperature of the system can be adjusted over several orders of magnitude. Particle coordinates are monitored by video‐microscopy over more than five decades in time. Measured radial pair‐distribution functionsg(r) and mean‐square displacements illustrate that this system is an ideal model of a two‐dimensional (2D) glass former. We find that the effects of small amounts of aggregated particles only weakly affect the averaged structure and dynamics. Locally, a small number of elementary structural elements are observed each characterized by a special triangular shape. These triangles arrange in dense mostly space‐filling arrays and account for the essential features ofg(r). The long‐time &agr;‐relaxation is related to drifts of arrays as well as erosion due to single particle and collective hopping events. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764057

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We discuss general 2‐fluid hydrodynamic equations for complex fluids, where one kind is a simple Newtonian fluid, while the other is polymeric/elastomeric, thus being applicable to polymer solutions and swollen elastomers. The procedure can easily be generalized to other complex fluid solutions. Special emphasis is laid on such nonlinearities that originate from the 2‐fluid description, like the transport part of the total time derivatives. It is shown that the proper velocities, with which the hydrodynamic quantities are convected, cannot be chosen at will, since there are subtle relations among them. Within allowed combinations the convective velocities are generally material dependent. The so‐called stress division problem, i.e. how the elastic stresses are distributed between the two fluids, is shown to depend partially on the choice of the convected velocities, but is otherwise also material dependent. A set of reasonably simplified equations is given as well as a linearized version of an effective concentration dynamics that may be used for comparison with experiments. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764058

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Applying an effective medium approximation, we theoretically investigate the recovery of binary blends of immiscible polymers after melt elongation. In our model, we consider effective values for the Hencky strain rates of the disperse and the matrix phase. We derive temporal evolution equations which allow calculation of the transient recovered stretch. Numerical solutions of this set of equations are presented and discussed. Our analysis reveals that the capillary number strongly influences the recovery process. By comparing the predictions of our model with experiments, we show that our model captures the basic features of the experimental data, i.e. the time scale of the recovery process and the equilibrium value of the recovered stretch. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764059

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We have found the finite elasticity and characteristic visco‐elastic relaxation in novel mosaic phases of the non‐symmetric chiral twin liquid crystal. X‐ray scattering experiment shows the mosaic phase possesses smectic layer structure, but its intensity rather weak in comparison with smectic phase, and appears in the higher temperature and optical purity region than TGB phase. Thus, defects of the smectic layers are three dimensionally dispersed in the mosaic phase, and interacted with each other by the spatial deformation of the layers. Correlation of defects produces the macroscopic mobility and the unique visco‐elastic properties. These relaxation phenomena can be expected to relate to the intrinsic feature of the collective hydrodynamic modes in the defect lattices systems. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764060

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Structured fluids (concentrated suspensions, emulsions, gels&cellip;.) typically exhibit an apparent yield stress. We show here that for a number of these fluids, a unique yield stress cannot be defined. Instead, when solicited above a critical stress, typical yield stress fluids (gels, clay suspensions) and soft glassy materials (colloidal glasses) start flowing abruptly and subsequently accelerate. We demonstrate that the competition between the spontaneous restructuration (aging) and the destruction of the internal structure (‘shear rejuvenation’) lead to a bifurcation in rheological behavior. For a stress smaller than a (time‐dependent) critical value, the viscosity increases in time and the material eventually stops flowing. For a slightly larger stresses the viscosity decreases continuously in time and the flow accelerates. Thus the viscosity jumps discontinuously to infinity at the critical stress. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764061

出版商:AIP    

年代:1904

数据来源: AIP