SLOW DYNAMICS IN COMPLEX SYSTEMS: 3rd International Symposium on Slow Dynamics in Complex Systems

Colloidal suspensions driven by external fields
View Description Hide DescriptionColloidal 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.

Universal Features of Collective Interactions in Hard‐Sphere Systems at Higher Volume Fractions
View Description Hide DescriptionIn 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.

Neutron and Light Scattering Studies of the Liquid‐to‐Glass and Glass‐to‐Glass Transitions in a Copolymer Micellar System
View Description Hide DescriptionRecent 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 dense L64 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 (called A _{3} singularity) 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 for L64 micellar system is given.

Glassy dynamics in gelling systems: From chemical gels to colloidal glasses
View Description Hide DescriptionThe 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.

Mode Coupling Theories for Jamming and Gelation
View Description Hide DescriptionThe 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.

Glass transition in a two‐dimensional system of magnetic colloids
View Description Hide DescriptionWe 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 field B. By tuning B, 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 functions g(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 of g(r). The long‐time α‐relaxation is related to drifts of arrays as well as erosion due to single particle and collective hopping events.

General Nonlinear 2‐Fluid Hydrodynamics of Complex Fluids and Soft Matter
View Description Hide DescriptionWe 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.

Recovery of polymer blends after melt elongation: Analysis of a model for small and large capillary numbers
View Description Hide DescriptionApplying 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.

Visco‐elastic Relaxation in Novel Mosaic Phase of Non‐Symmetric Chiral Twin Liquid Crystals
View Description Hide DescriptionWe 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.

Aging and shear rejuvenation of soft glassy materials
View Description Hide DescriptionStructured fluids (concentrated suspensions, emulsions, gels⋯.) 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.

Scattering Probes of Complex Fluids and Solids
View Description Hide DescriptionWe 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.

Temporal heterogeneity of the slow dynamics of a colloidal paste
View Description Hide DescriptionWe 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, t_{w} , the dynamics smoothly slows down, as revealed by a nearly linear increase of the characteristic relaxation time, τ_{ s }, measured by DWS. At longer ages, the dynamics is quasi stationary, but τ_{ s } exhibit 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.

Dynamics around the sol‐gel transition in thermoreversible atactic polystyrene gels
View Description Hide DescriptionWe 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 α‐ and β‐relaxations related to solvent dynamics and a process related to the dynamics of the polymer network. While both the α‐relaxation and the network processes are slowed down with increasing polymer content, the β‐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.

Local particle rearrangements in a two‐dimensional binary colloidal glass former
View Description Hide DescriptionIn 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.

What is the excluded volume for an excluded volume chain?
View Description Hide DescriptionSolvated 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 υ, 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.

Flow instabilities in complex fluids: Nonlinear rheology and slow relaxations
View Description Hide DescriptionWe 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.

Molecular Dynamics Simulation of Dendrimers: Structural Formation and Internal Charge Distribution
View Description Hide DescriptionWe 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.

Droplet density dependences of the static and dynamic structures in a ternary microemulsion system
View Description Hide DescriptionA 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, φ, 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 φ = 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 φ, 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.

Anomalous Change in Lamellar Spacing by Shear Flow in Nonionic Surfactant/Water System
View Description Hide DescriptionSmall‐angle neutron scattering (SANS) is measured on a lamellar phase in C_{16}E_{7} (hepta(oxyethylene glycol)‐n‐hexadecylether) /water system (40 – 55 wt% of C_{16}E_{7}) 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, d increases slightly after taking a minimum (d ^{*}). These results are obtained for all the principal orientations of lamellae. While d at rest increases from 6.5 nm to 8.5 nm with decreasing concentration from 55 to 40 wt%, 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.

Texture Observed in a Simple Shear Flow and after Cessation of the Flow for Liquid Crystalline Polymers
View Description Hide DescriptionThe 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.