Cooperative Grains: From Granular Matter to

Nano Materials

 

October 6 – 17, 2003

 

 

The technological importance of granular matter as well as the new perspectives offered by nano-science have led to unique and lively research efforts. This workshop at the Lorentz Center at the University of Leiden brought both communities (granular and nano) together with the aim to stimulate their interaction. In fact it turned out that the hot topics as well as many basic concepts pertaining to both fields have a lot in common. Much of the excitement of the workshop was due to the challenge of the current views on the mechanical behaviour of nanopowders and colloids by the ones developed for granular media and vice versa. This pertains in particular to the connection between the description on the particle scale and the macroscopic phenomenological behaviour. The workshop was a great success in triggering communication between these two research fields. Specific examples will be given below.

 

The following key issues were covered:

 

Mechanical properties of aggregates of small particles

(Friction, cohesion, ...)

 

1.      Contact- and force networks

2.      Shear bands, yield stresses

3.      Consolidation, sintering

4.      Superplasticity of nanomaterials

 

Aggregation dynamics with long range interactions

(Hydrodynamics, electro-magnetic forces, ...)

 

1.      Stability of suspensions

2.      Instabilities in fluidized beds

3.      Aggregation, coating

4.      Particle production and detection

 

One challenge for current research in physics as well as engineering is to understand the force fluctuations during the flow of powders and their implications e.g. for silo design. Beyond others, the talks of Behringer and Clément shone new light on the intriguing effects in dense packed system. Besides force chain structures, the concept of stress propagation and especially the quest for a correct mathematical description of the stress state in granular and nano materials was discussed by Goldhirsch and many others. Recent results indicate that (elliptic) elasticity theory (anisotropic and possibly polar) suffices to describe static granular packings, while the modern concept of isostatic packings and a related hyperbolic set of equations are rather attributed to singular special cases.

 

The interest in this field was also reflected by the fact that several junior researchers from different groups are working on force propagation and force chain structures in granular packings. The relevance for nano materials was discussed too, for example in the talks by Goldhirsch and Wolf. In this context additional important aspects, such as cohesion and Brownian motion were discussed. Memorable was the demonstration experiment, where a sugar and a flour sample were both compressed, the former being practically incompressible, whereas the latter could be compressed due to the cohesion that allows for much lower initial densities. More specific, cohesion prevents that a powder collapses under its own weight, so that there remain pores, which influence the mechanical behaviour.

 

Tejchman showed that shear bands, for example, are nicely described (and quantitatively verified by experiments) via Finite Element Models (FEM) based on micropolar hypoplastic theory. This caused stimulating discussions and a layout/startup for future collaboration in order to learn about the physical foundations of (hypoplastic) continuum models from discrete element simulations (DEM).

 

Also other important phenomena, which still pose many open questions, were discussed. These are related to long range interactions due to electric charging or magnetic moments, as relevant e.g. for toner particles. They also determine the aggregation dynamics of particles and the stability of nano-suspensions. A bunch of fascinating lectures and interesting results in this field were presented by speakers including Wirth, Flekkoy, Hänel, Kudrolli, Holm, Kruis and Hinrichsen. New contacts were established this way, and also possible future collaborations between the participants from the two communities. Especially nice and didactic was the introductory lecture by Loewen, who described the world of colloids to the audience and also showed new results and open problems in this field. In particular it became clear in the discussion, that many questions in colloid science can be answered by studying equilibrium ensembles, whereas the dynamics far from equilibrium requires additional efforts to understand e.g. hydrodynamic interactions.

 

A few further examples of interactions which were triggered by this workshop:

·         Lohse - van Hecke - Wolf: The question arose during Detlef Lohse's inspiring opening talk, whether a jet would also form after the impact of an object on cohesive powders in contrast to the random loose noncohesive packings used so far. The answer (yes!) was obtained in Martin van Hecke's lab during the workshop.

·         Kudrolli and Rellinghaus discovered common research interests by realizing the structural analogies in vibrated magnetic grains and magnetic nanoparticles agitated by Brownian motion.

·         Kertesz - Pöschel - Goldhirsch - Herrmann - van Hecke - Wolf: Are rigid particle models too singular a limit to tell us something about the generic behaviour of micro- and nano-powders?

·         Luding - Tejchman: The importance of the micro-macro transition (discrete element simulations to hypoplastic continuum equations) was underlined and a collaboration started.

·         Blaauw and Brendel discovered that their experiments on compaction of limestone powder are complementary to each other (studied by neutrons respectively classical oedometer tests).

·         Holm - Wirth - Hinrichsen: What is the relevance of overcharging in suspensions of nano-particles?

·         Hou - Luding - Pitchumani: Fascinating experiments from China (so far widely unknown in the community) search for explanations (density waves and quasistatic deformations).

 

The length of the workshop was two weeks as planned. There were one to two broader talks of one hour length per day as well as a few half-hour talks, mostly by junior researchers. The longer talks were given by the invited speakers, while every young researchers working actively in one of the workshop fields had a chance to present ongoing work.

 

Topical workshops on particle-fluid interactions, granular gases, packings and colloids offered workshop participants to contribute talks about their recent research. In the future we would organize this differently: The time given to each speaker should be much shorter (ca. 10 min.), so that the topical workshop is not perceived as another session.

 

The overall participation was ranging from 30-35 persons throughout the whole workshop with still plenty time for discussion between and after the talks.  Especially the nice gathering events as the traditional wine-and-cheese party and the boat-trip as well as the conference dinner should be mentioned here as wonderful possibilities to discuss in the nice atmosphere of the Lorentz Center.

 

We thank the Lorentz Center and the SFB 445 for financial and logistic support and especially the local staff for their competent help.

 

D. Wolf (University Duisburg, Germany)

S. Luding (TU Delft, The Netherlands)