摘要:

Particle dynamics have been measured by dynamic light scattering for mixtures of colloidal particles with hard sphere interactions. The diameter ratio (small:large) is 0.6. The optical properties of the suspended particles are such that the relative contrast of the two species is very sensitive to temperature, a feature we exploit to obtain the three partial intermediate scattering functions. The glass transition is identified by the onset of structural arrest, or the arrest of the alpha process, on the time scale of the experiment. This is observed in the one-component suspension at the packing fraction 0.57. Introduction of the smaller particles, at fixed packing fraction, releases the alpha process, ie, the glass melts. Increasing the fraction of smaller particles speeds up the alpha process but interestingly, increases its amplitude. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291516

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

In this paper we demonstrate the utility of density functional theories in providing unified descriptions of two problems associated with colloidal crystallization. The first problem involves crystallization in a fixed volume. The result is a time dependent pressure and, hence, density. This process is further complicated during growth by the presence of a depletion zone in advance of the growing crystal that provides for some mechanical isolation of the crystal. The second problem concerns the effect of multiple possible stacking sites on crystallization kinetics. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291517

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

How the idea of the dynamic anomaly of the self-diffusion coefficient recently proposed by the present author works on the study of equilibrium and nonequilibrium supercooled colloidal liquids is discussed for two kinds of model suspensions, neutral and charged hard-sphere suspensions. Near colloidal glass transition, the long-lived, spatially heterogeneous glassy domains are shown to be formed for intermediate times in equilibrium and nonequilibrium systems. Those spatial heterogeneities are responsible for the slow relaxation of the density fluctuations. In fact, the long-known phenomena similar to those in glass-forming materials, such as the stretching of the &agr; process and the von Schweidler law, can be explained by the existence of those spatial structure. In the equilibrium system, however, those heterogeneities must be difficult to be observed since their sizes and magnitude are quite small compared to those in the nonequilibrium system. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291518

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

A survey is given on new developments in multidimensional solid state NMR emphasizing the unique information provided by this technique on the chain dynamics above the glass transition. By multidimensional exchange NMR geometry and rate memory of slow segmental motion is studied. By double-quantum NMR chain order and translational motion on longer length scales is elucidated. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291519

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Several surfactant molecules self-assemble in solution to form long, flexible wormlike micelles which get entangled with each other, leading to viscoelastic gel phases. We discuss our recent work on the rheology of such a gel formed in the dilute aqueous solutions of a surfactant CTAT. In the linear rheology regime, the storage modulusG′(&ohgr;)and loss modulusG″(&ohgr;)have been measured over a wide frequency range. In the nonlinear regime, the shear stress &sgr; shows a plateau as a function of the shear rate&ggr;˙above a certain cutoff shear rate&ggr;˙c.Under controlled shear rate conditions in the plateau regime, the shear stress and the first normal stress difference show oscillatory time-dependence. The analysis of the measured time series of shear stress and normal stress has been done using several methods incorporating state space reconstruction by embedding of time delay vectors. The analysis shows the existence of a finite correlation dimension and a positive Lyapunov exponent, unambiguously implying that the dynamics of the observed mechanical instability can be described by that of a dynamical system with a strange attractor of dimension varying from 2.4 to 2.9. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291520

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We recently found a new type of phase separation in condensed matter, which we call “viscoelastic phase separation”, for dynamically asymmetric mixtures such as polymer solutions and polymer mixtures whose components have very differentTg.Conceptually it is rather easy to accept that viscoelastic effects play a key role in polymeric systems, since polymer itself is viscoelastic matter. Here we argue that it is the interaction network of the slow component of a mixture that is the physical origin of viscoelasticity responsible for viscoelastic phase separation and the viscoelastic nature of the component itself is not its primary cause. This idea naturally leads to the conclusion that phase separation of colloidal suspensions should also belong to viscoelastic phase separation, even though colloidal particles themselves are not viscoelastic matter. It is known that a transient gel is formed upon phase separation of colloidal suspensions and emulsions. We view this fact as a fingerprint of the formation of the interaction network of the slower component (colloidal particles) of a mixture. We discuss the basic physical origin of viscoelastic phase separation, focusing on the formation of a transient gel in the initial stage of of phase separation. The possible mechanism of the collapsing of colloidal gel under gravity is also suggested. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291521

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The relaxation behavior of a series of ether-ketone oligomers was studied by means of dielectric spectroscopy. In order to isolate chain stiffness from other intermolecular factors, we studied chemically homogeneous, strictly monodisperse, low molecular weight ether-ketone model compounds. The dynamics of the &agr; relaxation of ether-ketone model compounds as compared with that of the homologous polymer PEKK (50/50), shows up differences which can be attributed to the variation of inter and intra molecular correlations with the chain length. Model compounds exhibit a nearly similar degree of cooperativity regardless the differences inTgvalues. The PEKK(50/50) polymer exhibits stronger cooperativity than the oligomers suggesting that in poly(ether-ketone-ketone)s molecular motions aboveTgextend to more than one monomeric unit. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291522

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Phase properties of gradientABcopolymer melts which consist of chains with the specified chemical distribution ofAandBmonomers have been studied within the Landau-Ginzburg model. All the melts with the linear distribution of the monomers exhibit only a direct continuous phase transition from disordered to the lamellar phase. The hexagonal, body-centered-cubic, double-gyroid(G) and lamellar ordered structures have been found in the melts with the monotonic but nonlinear distribution of the monomers. The G structure has been also found in the gradient copolymer melts with the distribution function of monomers similar to theA−B−Atriblock copolymers. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291523

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

When dispersed particles or solute ions have high charge densities, macroscopically homogeneous systems become microscopically inhomogeneous. Examples are the two-state structure of ordered structures in disordered region without boundary and the void structures. When the charge density is increased, the reentrant phase (liquid-solid-liquid) transition is found, which is not explainable in terms of the repulsion-only assumption. Furthermore, for relatively small charge particles and at an early stage of crystallization, space-filling ordered states are first formed, disordered regions are then created inside the ordered domains, and crystal contraction thereafter takes place, causing the two-state structure. These may be explained by invoking a counterion-mediated attractive interaction between like-charged entities in addition to the widely accepted repulsion-only assumption. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291524

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The Derjaguin-Landue-Vervey-Overbeek (DLVO) theory predicts that an isolated pair of charged colloidal spheres experience purely repulsive screened Coulomb interaction at large interparticle separations. Many evidences, which suggest the existence of long-range attractive component in the interparticle interaction, have emerged contradicting the prediction. These evidences include gas-liquid, gas-solid transitions, existence of stable voids, metastable colloidal crystallites and reentrant transitions in dilute suspensions. Some of these observations have triggered several experimental and theoretical investigations to probe the pair-potentialU(r)between like-charged in very dilute suspensions. Existence of long-range attraction inU(r)has been observed only when like-charged glass walls confine like-charged particles. Though some theoretical models predicted the existence of long-range attraction in the effective pair-potential, a clear and consensus picture is still awaited. This paper briefly reviews the experiments and theoretical results that have dealt with long-range attraction in colloidal systems. The paper presents in detail the recent studies of structural ordering in a novel two dimensional (2D) system of like-charged large metal balls. Interestingly, these studies reveal the existence of long-range attraction in the pair-interaction of like-charged metal balls spread on a dielectric surface, and the occurrence of gas-liquid transition upon variation of the area fraction of the charged metal balls. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291525

出版商:AIP    

年代:1900

数据来源: AIP