SFB TR6 Summer School on Soft Matter

"Physics of colloidal suspensions and granular media"



09:00-10:30Arrival J. BradyA. LiuK. BinderT. MullinW. Poon Free G. NägeleA. van BlaaderenM. SperlS. Egelhaaf Departure
11:00-12:30 M. FuchsD. DurianI. RehbergA. ZippeliusS. Fraden M. ShattuckMemorial sessionO. PouliquenB. Andreotti
16:00-17:30 E. WeeksFlash talksPoster Session I J. DhontExcursion T. PalbergPoster Session IIP. ChaikinB. Dünweg
17:30-19:00 R. Behringer J.L. Barrat F. SciortinoI. Aronson 
B. Andreotti Transport of particles by a fluid and application to pattern formation: ripples, dunes, chevrons and bars [Abstract]
I. Aronson Viscosity of bacterial suspensions [Abstract] [Talk:PDF]
J.-L. Barrat Quasistatic deformation of simple amorphous and granular systems [Abstract] [Talk:PDF]
R. Behringer Forces and Fluctuations and Jamming: Multiscale Properties of Granular Matter [Abstract] [Talk:PDF/PPT]
K. Binder Monte Carlo Simulation of Colloidal Systems Confined in Thin Films Between Walls [Abstract] [Talk:PDF]
J. Brady Suspensions and Granular Media: Wet vs. Dry [Abstract] [Talk:PDF]
P. Chaikin [Abstract]
J.K.G. Dhont Shear Flow [Abstract] [Talk:PDF]
B. Dünweg [Abstract] [Talk:PDF]
D. Durian Unsteadiness and dynamical heterogeneities in dense granular materials [Abstract] [Talk:PDF]
S. Egelhaaf Scattering methods and their application to non-equilibrium situations [Abstract] [Talk:PDF]
S. Fraden Colloidal and Granular Rods: Correspondences and Contrasts [Abstract] [Talk:PDF]
M. Fuchs Nonlinear rheology of dense colloidal dispersions [Abstract] [Talk:PDF]
A. Liu The jamming scenario and glasses [Abstract] [Talk:PDF]
H. Löwen Introduction   [Talk:PDF/PPT]
T. Mullin Hydrodynamic Instabilities in Closed and Open Flows [Abstract] [Talk:PDF1/ PDF2]
G. Nägele Hydrodynamic Interaction of Colloidal Particles [Abstract] [Talk:PDF]
T. Palberg Colloidal Crystals [Abstract] [Talk:PDF]
W. Poon An introduction to the physics of active colloids [Abstract] [Talk:PDF]
O. Pouliquen Granular flow in a liquid. [Abstract] [Talk:PDF]
I. Rehberg Sniffing at sandy suspensions [Abstract] [Talk:PDF]
F. Sciortino Condensation, clustering and self-assembly of patchy colloids [Abstract] [Talk:PDF/ PPT]
M. Shattuck Granular Thermodynamics: Exploring (NESS) Non-equilibrium-Steady-State [Abstract] [Talk:PDF/ PPT]
M. Sperl Granular Matter from the Glass Transition to Random-Close Packing [Abstract] [Talk:PDF]
A. van Blaaderen Manipulating the Self-Assembly of Colloids with External Fields [Abstract] [Talk:PDF]
E. Weeks Microscopy of soft jammed materials [Abstract] [Talk:PDF1/ PDF2/ PPT1/ PPT2]
A. Zippelius Granular Fluids [Abstract] [Talk:PDF]

B. Andreotti (University of Paris-Diderot)
Transport of particles by a fluid and application to pattern formation: ripples, dunes, chevrons and bars

The first part of the lecture will be devoted to sediment transport. We will identify the mechanisms limiting the transport of particles by a flow and we will compare three modes of transport: bed load, aeolian saltation and suspension. Then, we will discuss the nature and the scaling laws of the key lengthscale associated to transport, namely the saturation length. In the second part of the lecture, we will discuss the instability of a flat sand bed submitted to a turbulent flow. We will first detail the case of a semi-infinite flow, which allows to explain the emergence of subaqueous ripples as well as aeolian and martian dunes. Then, we will see how the presence of a free surface affects hydrodynamics over bedforms. This will allow to explain the formation of giant dunes, antidunes, alternate bars and chevrons. To conclude, we will review different other morphodynamical problems involving granular matter.

I. Aronson (Argonne National Laboratory)
Viscosity of bacterial suspensions

In the first part of my lecture I'll describe recent experimental studies of rheology of "living grains": suspensions of swimming micro-organisms, such as motile bacteria or algae. Recent measurements of the shear viscosity in suspensions of swimming Bacillus subtilis have revealed that the viscosity can decrease by up to a factor of 7 compared to the viscosity of the same liquid without bacteria or with non-motile bacteria. In the second part of the lecture I'll present theoretical arguments supporting these observations.

1. Reduction of Viscosity in Suspension of Swimming Bacteria, Andrey Sokolov and Igor S. Aranson, Phys. Rev. Lett. 103, 148101 (2009)
2. Three-dimensional model for the effective viscosity of bacterial suspensions, Brian M. Haines, Andrey Sokolov, Igor S. Aranson, Leonid Berlyand, and Dmitry A. Karpeev, Phys. Rev. E 80, 041922 (2009)

J.-L. Barrat (University of Claude Bernard Lyon I and the Institut Universitaire de France)
Quasistatic deformation of simple amorphous and granular systems

I will present results from quasistatic simulations of simple amorphous systems, including ideal, frictionless granular systems in the vicinity of point J and systems of particles interacting trough continuous potentials. I will discuss first the specificity of the response associated with isostatic correlations in the vicinity of point J, and then some aspects of the more general plastic deformation scenario in amorphous materials away from point J.

1. Jamming Transition as Probed by Quasistatic Shear Flow, C. Heussinger, J-L. Barrat Phys. Rev Lett. 102, 318303 (2009)
2. Superdiffusive, heterogeneous, and collective particle motion near the fluid-solid transition in athermal disordered materials, C. Heussinger, L. Berthier, JL Barrat EPL 90 20005 (2010)
3. Plastic Response of a 2D Lennard-Jones amorphous solid: Detailed analysis of the local rearrangements at very slow strain-rate, A. Tanguy, F. Leonforte, J.-L. Barrat, European Physical Journal E 20, 355 (2006)
4. Local elasticity map and plasticity in a model Lennard-Jones glass, M. Tsamados, A. Tanguy, C. Goldenberg, J-L. Barrat Phys. Rev E., 80, 026112 (2009)

R. Behringer (Duke University)
Forces and Fluctuations and Jamming: Multiscale Properties of Granular Matter

Granular materials are ubiquitous: food grains, pharmaceuticals and soils are a few everyday examples. And, they are one group in a broader collection of systems--foams, glasses, colloids, and related--that are thought to exhibit similar behavior near jamming. Despite the fact that granular and related materials are so common, our understanding of the collective properties of grains remains a great challenge. For instance, we do not know how to start from knowledge at the smallest scales, inter-particle interactions, and arrive at a reliable predictive model for the collective behavior of a granular system. By comparison, for molecular systems, the well defined machinerty of statistical mechanics allows us to compute macroscopic properties starting from inter-molecular interactions. But, key assumptions of this process, such as energy-conserving forces, are invalid for the granular case. The transition from grain to macro-scale can only be achieved by replacing conventional approaches by alternatives that reflect the physics of macroscopic grains. This talk will focus on granular phenomena, as observed in experiment, and in particular experiments involving materials that consist of quasi-2D particles made from a photoelastic material. It is then possible to obtain all physically relevant information about such systems, starting at the level of inter-particle forces. We use these and other systems to probe a range of questions that involve the nature of fluctuations, novel mesoscopic measures, relevant length and time scales, homogenization, alternative statistical approaches, and systems near jamming. Regarding the last of these points, we find that granular systems jam below the isotropic jamming density under applied shear. These stress-anisotropic jammed states do not fit simply into the conventional jamming diagram. It is likely that friction is an essential element needed to attain these states.

K. Binder (University of Mainz)
Monte Carlo Simulation of Colloidal Systems Confined in Thin Films Between Walls

Colloid-Polymer mixture are a model system for condensed matter, where interactions are tunable via the choice of the sizes of both particles, and the polymer concentration. In bulk solution, such colloidal dispersions can display vapor-like, liquid-like, and crystal-like phases. Particularly interesting are interfaces between coexisting phases, due to the large sizes of the particles collective statistical fluctuations such as capillary waves are more easily accessed than in small molecule systems. When such systems are confined between walls, quasi-two-dimensional phase transitions such as capillary condensation/evaporation or interface unbinding transitions and wetting phenomena can be studied, at least for computer models of these systems via computer simulation. A brief introduction to the necessary simulation methodology will be given, as well as an introduction to the phenomenological theory of these phenomena.

J. Brady (Caltech)
Suspensions and Granular Media: Wet vs. Dry

Suspensions and dry granular media are found widely in nature and industry. Suspensions generally refer to small particles (from less than 1 mm to say 1 cm in size) dispersed in a viscous liquid such as water or oil. While dry granular media are particles of the same size in air (or vacuum) where it is generally assumed that the air has little effect on the flow of the grains. Common examples can be found throughout industry - pneumatic conveying, coal slurries, pharmaceutical powders, etc., and in nature - sediment transport, debris flows, blood flow, etc. Studies of suspensions and granular media have in large part proceeded independently. In both areas significant advances have been made as experiments, simulations and theories have all come together to elucidate many aspects of flow behavior. And while similarities in behavior have sometimes been noted, the two fields nevertheless remain separate. Why is this? Is the underlying physics so different that they should be separate? Or can trying to take a more unified view teach us more about each? Here we suggest that this separation is unnecessary and that suspensions and granular media - wet and dry - actually correspond to different limiting behaviors of one common system. The linking parameter is the Stokes number - the ratio of the inertial to shear forces: small Stokes numbers correspond to viscous suspensions and high Stokes numbers to dry granular media. In this talk we will discuss the microstructural physics underlying the flow of Brownian and nonBrownian suspensions and dry granular media. The connection between wet and dry will be made through computer simulation studies of the rheological and diffusive behavior of viscous suspensions and rapid granular flows.

P. Chaikin (New York University)


J.K.G. Dhont (Forschungszentrum Jülich)
Shear Flow

Various shear-flow induced phenomena colloidal systems will be discussed, both for spherical and rod-like colloids. Shear flow has a pronounced effect on structures of large spatial extent, such as inhomogeneities that are formed during spinodal decomposition and long-ranged microsttructural order near the gas-liquid critical point. We shall therefore discuss, (i) The effect of shear flow on initial spinodal decomposition kinetics. (ii) The shear-induced shift of the gas-liquid critical point and the cloud-point curve. This discussion is limited to spherical colloids. For rod-like colloids there is an additional shear-aligning effect, which affects the location of isotropic-nematic spinodals. The third topic that will be addressed is therefore, (iii) The effect of shear flow on the location of isotropic-nematic spinodals for rod-like colloids. Theoretical predictions will be compared to experiments. The basic equation that is used to describe these phenomena is the Smoluchowske equation, the derivation of which will be discussed. This is the fundamental equation of motion for the description of dynamics and non-equilibrium phenomena in general. From this fundamental equation, the Cahn-Hilliard diffusion equation is re-derived, but now including shear flow, for topic (i), the extension of the Ornstein-Zernike theory to include shear flow is discussed for topic (ii), and the Doi-Saupe theory that describes the isotropic-nematic phase transition is rederived, again including shear flow, for topic (iii).

D. Durian (Penn University)
Unsteadiness and dynamical heterogeneities in dense granular materials

The physics of granular flow is of widespread practical and fundamental interest, and is also important in geology and astrophysics. One challenge to understanding and controlling behavior is that the mechanical response is nonlinear, with a forcing threshold below which the medium is static and above which it flows freely. Furthermore, just above threshold the response may be intermittent even though the forcing is steady. Two familiar examples, to be reviewed, are avalanches on a heap and clogging in a silo. Another example is dynamical heterogeneities for systems brought close to jamming, where intermediate-time motion is correlated in the form of intermitted string-like swirls. This will be briefly reviewed in the context of glassy liquids and colloids, and more deeply illustrated with experiments on three different granular systems. This includes air-fluidized beads, where jamming is approached by density and airspeed; granular heap flow, where jamming is approached by depth from the free surface; and dense suspensions of NIPA beads, where jamming is approached by both density and shear rate. Emphasis will be given to measurement and analysis methods for quantifying heterogeneities, as well as the scaling of the size of heterogeneities with distance to jamming.

S. Egelhaaf (University of Düsseldorf)
Scattering methods and their application to non-equilibrium situation

Static and dynamic light scattering have evolved into powerful methods to investigate colloidal samples. Furthermore, new developments expand the range of systems which can be investigated to concentrated, turbid and/or non-ergodic samples. These techniques can provide detailed quantitative information on the shape and dynamics of the particles as well as their arrangement and relative motions. I will introduce the basic concepts of static and dynamic light scattering and illustrate the possibilities they offer with a selection of examples which range from gels and glasses to fluidized beds of granular particles and non-equilibrium situations like samples under shear.

S. Fraden (Brandeis University)
Colloidal and Granular Rods: Correspondences and Contrasts

In Nature, granular particles are never frictionless nor spherical, yet the majority of granular materials studies - theory and experiment and simulation - have been done on frictionless, spherical particles. Here we review studies of granular and colloidal rods. Colloidal hard rods possess much richer phase behavior than hard spheres exhibiting liquid crystalline phases such as nematic and smectic phases, in addition to the crystal phase, as well as non-equilibrium jammed or gel phases. Granular rod systems do not have liquid crystalline phases in 3D, but instead either exhibit interesting dynamic phases and jammed phases.

1. The coordination number of granular cylinders, J. Blouwolff and S. Fraden, Europhys. Lett., 76, 1095-1101 (2006).
2. Ordered phases of filamentous viruses. Z. Dogic and S. Fraden, Current Opinion in Colloid & Interface Science 11, 47 (2006).

M. Fuchs (University of Konstanz)
Nonlinear rheology of dense colloidal dispersions

Colloidal dispersions exhibit fluid and solid states whose nonlinear response to external fields arises from the competition between external perturbation and thermal motion ruled by the internal particle interactions. Mechanical deformation is an especially interesting probe in the vicinity of a solidification transition, as solid and fluid respond qualitatively differently. While fluids flow in response to any applied stress, solids may exhibit a yield stress threshold, which separates elastic from flowing states. I will give an overview of theoretical descriptions of colloidal fluids and glasses close to an idealized glass transition, where large effects arise because of the high Newtonian viscosity of the quiescent suspensions.

B. Dünweg (Max-Planck-Institute of Mainz)


A. Liu (Penn University)
The jamming scenario and glasses.

The jamming transition of frictionless soft spheres is singular because it coincides with the threshold for mechanical stability. This lends the marginally-jammed solid just above the transition special properties, including excess low-frequency modes that extend all the way down to zero frequency at the transition. I will review the jamming scenario, including properties of the jamming transition and of the marginally-jammed solid as well as extensions beyond the basic model and their implications for real systems. In addition I will discuss recent work showing the connection of the glass transition for soft spheres, as temperature is decreased or pressure is increased, to the glass transition of hard spheres.

T. Mullin (University of Manchester)
Hydrodynamic Instabilities in Closed and Open Flows.

Taylor-Couette flow between concentric rotating cylinders has been extensively studied in the last thirty years as an example of a fluid mechanical systems which exhibits bifurcation sequences to low-dimensional chaos. Experimental findings together with results from numerical calculations of the Navier Stokes equations will be used to highlight the impact of low-dimensional dynamics on the understanding of the emergence of disordered motion in fluid flows. The central argument which will be put forward is that the underpinning bifurcation structure of the solution set is the key to understanding the observations. The question will then be addressed whether this approach has anything to offer in understanding the problem of transition to turbulence in a pipe. In this case, the flow is linearly stable and hence there is no sequence of instabilities. The problem has puzzled researchers for more than a century since Reynolds original investigations, but recent results suggest that ideas based in dynamical systems theory may lead to progress.

G. Nägele (Forschungszentrum Jülich)
Hydrodynamic Interaction of Colloidal Particles

Colloidal particles dispersed in a low-molecular-weight fluid such as water are dynamically interacting by the intervening solvent. On the length and time scales of the particles, this so-called hydrodynamic interaction (HI) is long-ranged, quasi-instantaneous and in general non-pairwise additive. It has an essential influence on colloidal transport including diffusion and rheology, and dynamics in external fields. In this lecture, I will give an introduction into the theory of HI. A few examples are discussed illustrating the importance of HI in colloid science. The following points will be addressed: Properties of the creeping flow equation and fundamental solutions; stress tensor and Faxén laws; friction coefficient of a sphere and thin rod; many-particle mobility and friction problems; lubrication effects; some applications to colloidal short-time dynamics; Brinkman equation of porous media with applications.

T. Palberg (University of Mainz)
Colloidal Crystals

After a short introduction of colloids as (experimental) models for crystallizing matter in general the talk will address the following points concerning colloidal crystals (CC), their use and their making from the melt: Why do colloids crystallize? (energetic and entropic arguments); What are the basic properties of colloidal crystals? (Structure and phase behaviour, elasticity of CC, diffusion in CC); How do colloids crystallize? (classical theories of bulk nucleation and growth, experiments to measure growth velocities and nuclation rate densities). The discussion will return to the role of CC as models for atomic matter and identify some interesting open challenges.

W. Poon (University of Edinburgh)
An introduction to the physics of active colloids


O. Poliquen (University of Marseille)
granular flow in a liquid.

Many geophysical situations like landslides, debris flows or industrial processes involve the flow of a mixture of grains and liquid in a very concentrated regime. In this regime, the grains interact both by contact interactions and by hydrodynamic interactions. The material then shares similarities with both granular media and dense suspensions. In this talk, we will first discuss the bulk rheology of immersed granular media, trying to make connection between the approaches developed for dry granular media and the models developed for semi- dilute suspensions. In a second part, we will see that in many situations like submarine avalanches, sediment transport, extrusion of paste, the mixture of grains and liquid can no longer be considered as a single phase. A two-phase flow description is then necessary to capture the dynamics of the flow. The framework of two-phase flow equations will be introduced, which will serve to analyze simple examples of immersed granular flows.

I. Rehberg (University of Bayreuth)

Click here (pdf) for the abstract.


F. Sciortino (University of Rome)
Condensation, clustering and self-assembly of patchy colloids

Recent advances in the chemical synthesis and fabrication of nanometer- to-micrometer sized particles have produced a wide variety of new designs. Moving away from the isotropic case opens up several directions, the so-called anisotropy axis envisioned in the Glotzer and Solomon interesting recent review [Glotzer and Solomon2007]. These new particles are poised to become the 'atoms' and 'molecules' of tomorrow's materials if they can be successfully assembled into useful structures. One challenge is to organize them into structures for functional materials and devices. A promising approach is self-assembly, which is the spontaneous organization of matter into ordered arrangements. However, fundamental understanding of the basic principles of self-assembly is lacking, and a systematic study of the phase behavior, vitrification and gelation is needed. The goal of the scientific community is to tailor their behaviour at the macroscopic level, through control of the interactions and of the self-assembly process [Yethira and van Blaaderen 2003, van Blaaderen 2006, Manoharan et al. 2003, Kraft et al. 2009]. This goal requires a deep understanding of the modification to the phase diagram induced by the presence of patchy or specific interactions as well as effect induced by the anisotropy in shape and particle surface composition. Among the newly synthesized particles, a relevant role is expected to be played by patchy colloids, a new generation of colloidal particles the surfaces of which are patterned so that they attract each other via discrete sites of tunable number, size and strength. Patchy colloids are a suitable model to investigate the interplay between condensation and clustering, and provide a route to designing ideal gels, as well as robust control of a wide range of equilibrium self-assembled structures. In a recent line of work, [Bianchi et al. 2006, Foffi and Sciortino 2007, Bianchi et al. 2007] the phase diagram and percolation threshold of patchy colloids have been investigated. Extensive theoretical and numerical studies established that the number of bonding sites per particle (its valence) is the key parameter controlling the location of the liquid-vapour critical point: In the limit of average valence approaching two, the phase-separation region shrinks to zero and it becomes possible to reach low temperatures without encountering phase separation and the colloidal system can freeze in place the empty configuration to give a glassy state of arbitrarily low density: an ideal (reversible) gel [Bianchi et al. 2006, Foffi and Sciortino 2007, Bianchi et al. 2007]. In the lecture, I will discuss some of the most recent attempts to engineer non-isotropic colloidal particles, moving beyond the case of hard-sphere colloids, short-range attractive colloids and symmetrically charged colloids. I will then discuss the effect of the non-sphericity on the phase diagram of the system, showing how dynamical arrest can arise from bonding (differently from the case of glasses where packing plays a major role). Finally, I will show how unconventional phase diagrams can arise in extreme cases of colloids interacting with a significant non-isotropic potential (Janus particles) [Sciortino et al. 2009] or dipolar-like interactions.

M. Shattuck (City College of New York)
Granular Thermodynamics: Exploring (NESS) Non-equilibrium-Steady-State

Thermodynamics is generally not applicable to systems with energy input and dissipation present, and identifying relevant tools for understanding these far-from-equilibrium systems poses a serious challenge. Excited granular materials have become a canonical system to explore such ideas since they are inherently dissipative due to inter-particle frictional contacts and inelastic collisions. Granular materials also have far reaching practical importance in a number of industries, but accumulated ad-hoc knowledge is often the only design tool. An important feature of driven granular systems is that the energy input and dissipation mechanisms can be balanced such that a Non-Equilibrium Steady-State (NESS) is achieved. This NESS shares many properties of systems in thermodynamic equilibrium. In particular, the structure and dynamics of the NESS are almost identical to equilibrium systems. Further, we present strong experimental evidence for a NESS first-order phase transition in a vibrated two-dimensional granular fluid. The phase transition between a gas and a crystal is characterized by a discontinuous change in both density and temperature and exhibits rate dependent hysteresis. Finally, we measure a "free energy"-like function for the system, whose minimum determines the state of the system.

M. Sperl (Deutsches Luft- und Raumfahrtzentrum Cologne)
Granular Matter from the Glass Transition to Random-Close Packing

In the first part, evidence from simulation and experiments shall be used to establish the connection between random-close packing (rcp) in granular matter and the glass transition in colloidal suspensions. To follow the route from equilibrium to rcp, we study the glassy dynamics of a granular system. In such a granular system the dissipation of energy needs to be balanced by a suitable energy input in order to achieve a steady state. The resulting non-equilibrium steady state is discussed within a recent mode-coupling theory for granular matter.
In the second part, it shall be discussed how the region between the granular glass transition and rcp can be explored experimentally, especially in 3 dimensions. To this end we will look in detail at the method of stress-birefringence in three dimensions and experiments under microgravity.

A. van Blaaderen (Utrecht University)
Manipulating the Self-Assembly of Colloids with External Fields

Colloids have been successfully used as physics condensed matter model systems for over a century, because of their well-defined thermodynamic temperature as expressed through Brownian motion. It is increasingly becoming clear that in the next hundred years colloidal model systems will be used to address fundamental questions also from fields such as materials science, chemistry and biology and in addition will enable new ways to study systems far out of equilibrium. In these lectures we will present our first steps towards how the self assembly of more complex colloids can be manipulated through the use of external fields and the modification of the inter particle interactions. Mostly, we will address systems that use spherical particles as a starting point. We will focus on electric fields (including optical tweezers), but will also show some results involving gravity and shear. Gravity is always present and can be used to increase volume fractions; we will show that shear is a powerful way to both induce order and decrease order. In the first step to more complex potentials we will show that the realization of colloidal systems with charges small enough that oppositely charged systems can form equilibrium phases has in the last few years more than doubled the number of experimentally realized colloidal crystal symmetries. Colloidal dumbbells, perhaps the simplest deviation from a spherical form, can be realized by controlling aggregation, in some cases even with a yield above 50%. These systems can be purified by centrifugation and can be crystallized under application of an electric field. We show first examples of what might be a colloidal plastic crystal phase. We will further show how individual spheres can be turned into bead-chains by a combination of chemistry and electric fields. By changing the flexibility of these chains we can tune the model system from a stiff biopolymer into a variant closer to a 'chemical' polymer. By using time varying fields and two electrode combinations we can generate inverse dipolar interactions which can lead to 2D sheets of colloids. Homogeneous fields can also drive opposite particles into pattern forming systems far out of equilibrium. Optical tweezers provide field gradients on the level of individual particles allowing one to manipulate hundreds of them even inside concentrated dispersions. Finally, the concentration and thus the phase behavior of small quantities of complex colloids can be conveniently manipulated by using much larger electric field gradients than in the case of optical tweezers ('electric bottle' concept).

E. Weeks (Emory University)
Microscopy of soft jammed materials

The mechanical properties and ability to flow are the defining features of soft materials (such as colloids, granular media, emulsions, foams, and gels). We would like to be able to relate the mesoscopic structure of a complex material to its macroscopic properties (such as its viscoelastic modulus). It might be expected that the relations would depend strongly on the details of each material, and that the study of such systems would be the study of many special cases. However, recently the analogy of jamming suggests the possibility of universal behavior of complex fluids under stress, and in particular, linking colloidal and granular experiments. This talk introduces a variety of soft materials, describing both their microscopic structure and their macroscopic flow properties. Features of soft jammed materials are discussed. A particular emphasis will be given to the utility of microscopy to study these sorts of materials; strengths and weaknesses of the technique will be given.

D. Weitz (Harvard University)


A. Zippelius (University of Göttingen)
Granular Fluids

Granular particles in agitated motion have numerous applications in nature and industry and, at the same time, are of fundamental interest as a model system of nonequilibrium statistical physics. The dynamics of the grains is dominated by inelastic collisions, so that energy is continuously dissipated with important consequences for the statistical properties. In contrast to molecular fluids equipartition does not hold, the spatially homogeneous state is unstable, the velocities of the grains do not follow the Maxwell-Boltzmann distibution and the translational and rotational motion of the grains are correlated. We first discuss free cooling of a granular fluid, that is the continuous decrease of the kinetic energy in an undriven fluid. Particular emphasis will be on cohesive granular fluids, which cool and form structures distinctly different from dry granular media. Subsequently our focus will be on the stationary state of a driven fluid, where energy dissipation due to collisions is balanced by energy input due to driving by external forces. We will discuss hydrodynamic correlation functions, long-time tails and the glass transition, for which a mode-coupling theory has been developed.