Summary

Soft materials have exceptional mechanical, optical or functional properties that find applications in many industrial sectors and in society in general. Examples of such systems are colloids, emulsions, polymers, surfactants.

These are the building blocks of biological organisms, blood and other cells, DNA, proteins, etc.. This kind of materials is also found in drug delivery systems and consumer prod­ucts such as shampoo, shaving cream, paint, plastics and foodstuffs. They are called "Soft" because they flow or deform easily under external forces, which can happen because they are structured on mesoscopic length scales.

These are scales that are large compared to simple molecules, but gener­ally too small for the naked eye to see. The mesoscopic components often spontaneously organize themselves into complex structures with very striking mechanical, optical, or functional properties.
The difference with molecular systems is that specific surface interactions play an important role in the creation of the spectacularly large variation in the properties of the systems. This applies to both the mechanical properties as well as the chemical affinity.

The key question for the proposed re­search priority is then:

How can the  mechanical and (physico-) chemical properties of soft complex materials be understood from the building blocks and their interactions?

For biological activity, but also for example to understand the flow behavior of complex fluids the fi­nal material always is very different than the sum of the individual parts.

The core of the problem is to understand and describe this emergence: the spectacular collective behavior of complex systems arising from a multiplicity of simultaneous interactions between many particles or molecules.

To solve this problem is a multidisciplinary approach which connects theory, experiment and simulation. Re­searchers of IoP (both theory and experiment) and HIMS (both experiment and simulation) cooper­ate in the Soft Matter theme.

Scientific case

The unique expertise within the Faculty of Science and the combination of simulation, theory and experiment make it possible for the UvA to force a breakthrough in understanding the collective behavior of these systems. Joining our forces will eventually enable us to design such complex systems and control their properties or behavior.

By precise interventions on very small (molecular) length scales we can bring about major changes controlled by the collective properties at a macroscopic scale.

An obvious example is that small changes in the interaction potential between colloidal particles can lead to the formation of very different macroscopic systems: colloidal liquids and gases but also ‘solid’ materials such as gels and glasses.
A concrete example are gels systems, where the simulation group of HIMS (Bolhuis) and the experimental group of the IoP (Schall, Bonn and Wegdam) look at the aggregation of colloidal particles as a function of the interactions.
These allow to control the architecture on a mesoscopic scale, which in turn greatly influence the macroscopic behavior: a reversible transition from a liquid to a solid material can for instance be triggered in this way. An experiment aboard the International Space Station (ISS) set up by Wegdam and Schall (IoP) studies these processes under microgravity conditions.

To successfully attack the challenging problem of emergence, there are two key issues that are going to be scrutinized.
These are obvious choices with a high chance of success because of the unique combination of expertise available through the participating groups.
These are: Cooperative behavior in systems with complex interactions, and Collective behavior and mechanical properties of granular, frictional and glassy systems.

The understanding of these two key issues will have a major impact on our fundamental understanding of soft materials, but will also generate important applications.

The complementarity of the groups lies in the ability to control and investigate structure and interactions from the molecular to the macroscopic level using the combination of soft matter physics, computational (physical) chemistry, photonics and molecular design.

Bringing these groups even closer together than they are already makes a further cross-fertilization between theory, experiment and simulation possible which is necessary to solve these challenging problems.

The groups are exceptionally well positioned and complimentary to do this. The Soft Matter Group of the Institute of Physics has 4 publications in PRL every year and six papers in Nature or Science since 2004.  They are the top in the field of soft matter, which is illustrated by the fact that two major national and international conferences in this field: the Dutch Soft Matter and the New York-Amsterdam Soft Matter meetings are organized by this group.
This indicates that the group leads both nationally and internationally.

There are strong links with AMOLF but also with the VU. The groups of Mackintosh, Koenderink and Bonn, for example, have joint group meetings.
The groups of Koenderink, Nienhuis and Schall work together in a network project awarded in the "Complexity" program of NWO. Likewise, the Computational Chemistry and Molecular Photonics groups of HIMS are both nationally and internationally leading, with many (inter)national collaborations and an extremely good publication output.

Six publications with large impact since 2008 are mentioned below.

Publications

1. M.R. Panman, P. Bodis, D.J. Shaw, B.H. Bakker, A.C. Newton, E.R. Kay, A.M. Brouwer, W.J. Buma, D.A. Leigh, and S. Woutersen. “Operation mechanism of a molecular machine revealed using time-resolved vibra­tional spectroscopy”. Science 328, 1255 (2010).
2. W. Lechner, C. Dellago, and P. G. Bolhuis, Role of the Prestructured Surface Cloud in Crystal Nuclea­tion, Phys. Rev. Lett. 106, 085701 (2011)
3. D. Bonn, J. Eggers, J. Indekeu, J. Meunier, E. Rolley, Wetting and spreading, Rev. Mod. Phys. 81, 739 (2009)
4. D. Bonn, M. Denn, Yield stress fluids slowly yield to analysis, Science 324, 1402, (2009)
5. Mischa Bonn, Huib J. Bakker, Gianluca Rago, Frederick Pouzy, Joanna R. Siekierzycka, Albert M. Brou­wer and Daniel Bonn, Suppression of Proton Mobility by Hydrophobic Hydration J. Am. Chem. Soc. 131, 17070 (2009)
6. Daniel Bonn, Jakub Otwinowski, Stefano Sacanna, Hua Guo, Gerard Wegdam, and Peter Schall, Direct Observa­tion of Colloidal Aggregation by Critical Casimir Forces, Phys. Rev. Lett. 103, 156101 (2009)