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NA1, Colloidal Composites, Gels & Glasses
Introduction

The many industrially and biologically important colloidal systems are almost always present in composite form, like in foods, paints, crude oil, the content of living cells and blood.
The many as yet undiscovered phenomena in these complex systems may play a role in improving existing technologies and in the development of entirely new technologies.
Very recent experimental examples of such unexpected properties of colloid composites are the bizarre microstructural arrangement of colloidal spheres in lyotropic nematics of fd-virus suspensions and banded structure formation in mixtures of colloidal rods and polymers in shear flow.


A wide variety of soft materials have been found to undergo a liquid to solid transition under conditions of increasing density and/or reduced load (or reduced temperature).
In an attempt to unify this type of behaviour a generic qualitative jamming phase diagram has been proposed, which aims at describing the different ways by which the ability of a system to flow is lost. This represents one of the foremost challenges in soft condensed matter physics as it relates to the unresolved problem of vitrification. At the same time, it has significant implications for a variety of applications ranging from the optimisation of processing conditions, the time stability of consumer products to the creation of smart mechanical and flow properties of soft materials by nanoscopic structuring as a function of external conditions.






Figure 1:Shear banding in a stationary state of sheared suspensions of fd-virus. The band structure becomes visible by an observation through crossed polarizers.


Figure :2 Universal jamming phase diagram connecting the glass transition and the jamming process into a universal picture (Nature 396, 21, 1998).




The SoftComp Scientific Activity

Scientifically SoftComp covers a broad range of scientific activities with more than 120 researchers actively involved. Therefore it is impossible to describe their important results. Instead we present a few highlights I an exemplary way.







Network Area 1 Highlights




Mesoscopic simulations

Multi-particle collision dynamics (MPC) for modeling the rheology of viscoelastic fluids.

This algorithm consists of alternating streaming and collision steps while consving mass, momentum and energy , so that hydrodynamic behaviour emerges naturally on larger length scales. We show in Fig. 1 a typical example for the loss modulus of the simplest viscoelastic fluid, a dumbbell.

Responsive Particle Dynamics (RPD).

This coarse grained model was developed in order to investigate the slow dynamics of soft matter. It has been already applied to describe the viscoelastic properties of linear polymers and synthetic resins of the core-shell type with remarkable success.

Simulations of glassy dynamics in suspensions of monodisperse hard ellipsoids.

In equilibrium, almost spherical ellipsoids show a first order transition from an isotropic phase to a rotator phase. At high volume fractions, a two-step relaxation in positional and orientational correlators and super-Arrhenius slowing down of diffusion are found.





Figure 1: Viscoelastic loss modulus G'' from MPC as function of oscillation frequency w for dumbbells with various spring constants (different colors). The dashed lines are fits to the Maxwell model. Good comparison with results from kinetic theory of dumbbells in solution is also obtained.






Nonlinear rheological phenomena

Slip and yielding in glassy suspensions.

To explore the shear melting of glassy suspensions we developed novel experimental techniques (rheo-microscopy). We found a connection between slip (slip or sticky yielding) and bulk yield stress (Fig. 2 left). The treatment of the shear surface and particles also affected these critical stresses.

Vorticity banding in rod-like colloids.

Our rheo-optical experiments on vorticity banding of fd-virus suspensions indicated that within part of the isotropic-nematic two-phase region vorticity banding occurred (Fig. 2 right). In the quasi-stationary state, regularly stacked bands along the vorticity direction were formed. Particle-tracking experi-ments revealed the internal rolling motion of the bands. We conjectured that this rolling motion is the result of normal stresses that are generated by the inhomogeneities in the initial stages of isotropic-nematic demixing







Figure 2: The formation of vorticity bands right after a shear-rate quench. The numbers refer to minutes after the shear-rate quench. Right: The band height, as obtained from a Fourier analysis of images as given on the left, as a function of time. In the first 10 minutes after the quench, inhomoge-neities are formed through demixing and stretched, after which band formation occurs.






Non-spherical systems Compared to their spherical analogues, non-spherical colloidal systems have received less attention. Yet, due to their shape, orientation and packing behavior are important parameters which influence their phase state, rheology and response to external fields. Within NA1 a great deal of progress has been made in three fronts: synthesis of model non-spherical systems, behavior and properties in particular related to external fields, and modeling/simulation description of their performance.
The present highlight attempts at presenting representative results, and in particular it shows: New metallic nanoparticles of different shapes (Vigo), Fig. 1;
Novel gels from oppositely charged rods (Julich-Dhont), Fig. 2;
Aggregated ellipsoids at liquid interfaces (K. U. Leuven), Fig. 3;
Sheared ellipsoids (K. U. Leuven), Fig. 4.




Two types of glasses in asymmetric binary star mixtures of large and small colloidal stars exhibit a rich dynamic behavior. At constant, high large star fraction (in the glassy state: single glass of large star), adding small star (depletant) at different concentrations ( ) induces glass melting via depletion upon increasing the small/large size ratio (q), and eventually yields a re-entrant glass (double glass: both large and small stars are vitrified), yielding a U-shape kinetic diagram (see figures below, experimental-left and theoretical-right).
This offers possibilities for tuning the rheology of soft glasses and distinguishing different kinds of glasses.
Ref: E. Zaccarelli et al, PRL, 95, 268301 (2005)







Figure 1: Novel metal nanoparticles of different shapes.



Figure 2: Flocculation of oppositely charged PEG-grafted fd-viruses. At low concentration of added salt, flocs are formed (left image).



Figure 3: Self-assembled network of ellipsoids at the water/oil interface



Figure 4: Shered ellipsoids in velocity-gradient plane, forming local packing (right) in flow direction (arrow)







Binary mixtures of PMMA spheres carrying opposite surface charges have been studied with confocal microscopy by the University Utrecht team (group: A. van Blaaderen). The results obtained showed for the first time that a colloidal mixture of oppositely charged particles form equilibrium structures. Surprisingly these mixtures form a wide variety of ionic colloidal crystals, which have analogues in molecular salts. The crystals observed can be exotic, for example structures that have only been observed in metal fullerides, and even completely new structures. An example of such a structure (referred to LS8) is seen in Figure 2. Calculations showed that, contrary to ordinary salts, the stoichiometry of the crystals is not dictated by the charge numbers on the two types of particles, because the diffuse double layer compensates for any charge mismatch. This allows for many more structures, some of which could have applications in for example the fabrication of photonic crystals.







Figure 2: Fluorescently labeled PMMA particles of opposite charge (red versus green) and schematic of the crystal structure LS8.






Two types of glasses in asymmetric binary star mixtures. Mixtures of large and small colloidal stars exhibit a rich dynamic behaviour. At constant, high fraction of large stars (in the glassy state: single glass of large star), adding small star (depletant) at different concentrations ( ) induces glass melting via depletion upon increasing the small/large size ratio (q), and eventually yields a re-entrant glass (double glass: both large and small stars are vitrified), yielding a U-shape kinetic diagram (Fig.4, experimental-left and theoretical-right). In other words, maintaining constant q and increasing one goes from a glass to a liquid behaviour (depletion). Similarly, at constant and with increasing q, the system undergoes a (single) glass to liquid to re-entrant (double) glass transition. This type of U-shape kinetic diagram has been predicted and observed for added small stars of different functionality f2 (from 16 to 64). This offers possibilities for tuning the rheology of soft glasses and distinguishing different kinds of glasses (Univ. Düsseldorf/FORTH Crete).





Figure 4: Glass line for a mixed system of small and large stars as a function of composition in size ratio Q and concentration of depletents (small stars) r2s13.






Figure 4 shows a photo of a cone-plate shear cell mounted on a confocal microscope, where the two vertical towers are the two independent motors for the cone and the plate respectively. The location of the zero velocity plane can be tuned by hand by means of a joystick and automatically by imaging on a certain structure. In this way a displacement of the structure under investigation out-of the zero velocity plane can be kept in the view. The shear cell was used to study colloidal composites under shear and revealed a transition from spinodal to nucleation and growth dominated dimixing kinetics of differently shaped colloids in a nematic background.








Figure 4: Cone-plate shear cell at a confocal microscope.