摘要:

We have studied the dynamical and structural properties of destabilized nanoparticle suspensions by a combination of small‐angle neutron scattering (SANS) and diffusing wave spectroscopy (DWS). SANS and DWS provide structural and dynamic information about opaque samples online during the gelation process. With our new experimental setup we access a broad range of length and time scales perfectly suited for the (non‐invasive) investigation of dense nano‐ and mesostructured complex fluids and solids. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764062

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We investigate the slow dynamics of a soft glass, a concentrated suspension of polydisperse colloidal particles, by using multispeckle Diffusing Wave Spectroscopy (DWS). Two distinct regimes are observed: for small sample age,tw, the dynamics smoothly slows down, as revealed by a nearly linear increase of the characteristic relaxation time, &tgr;s, measured by DWS. At longer ages, the dynamics is quasi stationary, but &tgr;sexhibit anomalously large fluctuations in time. The time scale of the dynamical fluctuations is found to be slightly shorter than the average relaxation time. The variance of the intensity correlation function is maximum for time delays comparable to the average relaxation time, in striking analogy with recent simulations of glass‐forming liquids. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764063

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We present a dielectric relaxation spectroscopy study of the dynamics in the thermoreversible polymer gel system atactic polystyrene/toluene. We observe three distinct relaxational process, the &agr;‐ and &bgr;‐relaxations related to solvent dynamics and a process related to the dynamics of the polymer network. While both the &agr;‐relaxation and the network processes are slowed down with increasing polymer content, the &bgr;‐relaxation remains practically unchanged. The behaviour of the relaxational process related to the polymer network, shows no distinct change at gelation, suggesting that there are no major changes taking place in the polymer network on the length scale of the observed relaxation. Furthermore, it is possible to construct a master curve of the temperature dependence of the relaxation time of the matrix relaxation by a simple temperature scaling. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764064

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

In a two‐dimensional (2D) glass former composed of two kinds of different sized and repulsively interacting colloidal particles, the time‐dependent particle positions were observed by video‐microscopy. Analyzing the local particle arrangements by 3‐point correlation functions, we find four different local density‐optimized configurations of nearest neighboring particles, which we call elementary triangles (ET), one for each three‐particle combination of small and big particles. These four ET form a random tiling in the 2D monolayer, which is not space filling. Therefore, a heterogeneous local particle packing does not have long‐range order and shows structural frustrations. Furthermore, an analysis of structural relaxations, using triangles of nearest neighboring particles (TNNP) in the monolayer, suggests that hopping processes are the reason for the rearrangements of the particles. In the outlook, we propose a concept of local density‐optimized crystallite‐clusters to describe the glass transition as a percolation of stable local density‐optimized triangles. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764065

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Solvated polymers are generally expanded due to the effective interaction between the monomers. In Flory’s lattice theory for polymers this “excluded volume” is the same for a polymer solvated by its own monomers and a polymer in the absence of solvent. The magnitude of the effect is determined by a parameter &ugr;, that is directly related to the lattice spacing. Computer simulations using a simple off‐lattice model (dissipative particle dynamics) show that this is not true. Only an off lattice analogue of Flory’s theory that takes into account the compressibility of the solvent quantitatively predicts the degree of expansion as a function of solvent density. We find that for the solvated model chains the distribution of mass around the centre of mass is to a good approximation Gaussian. This means that the diffusion coefficient can be calculated by simply treating the chain as an ideal chain with a Kuhn length that is modified to take into account the expansion. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764066

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We here present two simplified models aimed at describing the long‐term, irregular behaviours observed in the rheological response of certain complex fluids, such as periodic oscillations or chaotic‐like variations. Both models exploit the idea of having a (non‐linear) rheological equation, controlling the temporal evolution of the stress, where one of the participating variables (a “structural” variable) is subject to a distinct dynamics with a different relaxation time. The coupling between the two dynamics is a source of instability. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764067

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

We study the structural formation of dendrimers by stochastic molecular dynamics simulations. The density profile under different pH condition is clarified numerically. We find that the structural change of dendrimers strongly depends on the solvent quality. We also investigate the effective interaction between nanosized dendrimer molecules. It is demonstrated that the effective interaction between like‐charged dendrimers becomes attractive when the electrostatic coupling is strong. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764068

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

A neutron spin echo (NSE) experiment was performed in order to clarify the droplet density dependence of the dynamic structures in a ternary microemulsion system consisting of AOT, water and decane. At the droplet concentration, &fgr;, of 0.05, the dynamic mode due to the peanuts‐like deformation of water droplets and the translational diffusion of them were analyzed, and the estimated decay time for the droplet deformation was about 10 ns. At &fgr; = 0.3, another dynamical mode due to the packing properties of water droplet is expected to be observed. Assuming that the deformation motion of the water droplets was independent of &fgr;, the dynamical mode due to the structure factor was estimated. The order of the decay time was one order of magnitude larger than that of the droplet deformation, and was smaller than that of the translational diffusion. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764069

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

Small‐angle neutron scattering (SANS) is measured on a lamellar phase in C16E7(hepta(oxyethylene glycol)‐n‐hexadecylether) /water system (40 – 55 wt&percent; of C16E7) at 70 °C under shear flow. As the shear rate increases from 0.3 to 1 s−1, the repeat distance (d) is reduced suddenly and discontinuously. With the further increase in the shear rate,dincreases slightly after taking a minimum (d*). These results are obtained for all the principal orientations of lamellae. Whiledat rest increases from 6.5 nm to 8.5 nm with decreasing concentration from 55 to 40 wt&percent;,d*is almost independent of concentration and nearly equal to the thickness of bilayers (∼ 5 nm) obtained from the line shape analysis of small angle X‐ray scattering at rest. These results suggest segregation into surfactant‐rich and water‐rich regions. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764070

出版商:AIP    

年代:1904

数据来源: AIP

摘要:

The texture development of liquid crystalline solution of hydroxypropylcellulose (HPC‐L) after cessation of shear flow was studied. The wide stripe texture (WST) was observed after elimination of the band texture. The WST was dependent on shear rate, sample thickness and temperature. © 2004 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1764071

出版商:AIP    

年代:1904

数据来源: AIP