Particles experiment on space trip
Experiment with spherical colloids leaves on Sojuz rocket for the International Space Station
Researchers from the Van der Waals-Zeemann Institute at the University of Amsterdam and from the University of Milan have devised a new system − based on the critical Casimir effect − with which established theories about the spherical particles (colloids) can be tested in practice. The ideal place to carry out the experiment is a location where there are no gravity or temperature changes. Therefore on 10 September 2010, a Soyuz rocket left for the International Space Station (ISS) carrying the researchers´ experiment. The aim of the experiment is to investigate which phase behaviour is exhibited by weak attractive spherical colloids under microgravitational conditions. By altering the potential interaction between particles, more insight can be gained into the formation of perfect macroscopic crystals. The research has partly been funded by FOM and NWO.
Casimir effect explained
In the Casimir effect, an interaction develops between two conducting plates in a vacuum as a consequence of fluctuations in the electromagnetic field. If these plates are brought closer together then only a limited number of frequencies of the field are permitted, as a result of which an effective pressure arises on the outside of the plates. The critical Casimir effect is based on the same underlying principles but instead of fluctuations in electromagnetic fields, this is induced by fluctuations in the order parameter (for example density) in fluids as they reach their critical point. The critical Casimir effect occurs, for example, in binary mixtures close to the disassociation temperature. Colloids in these mixtures with an affinity for one of the two components experience an attractive force due to this effect. The strength and the distance of this attractive force depends on the size of the concentration fluctuations in the mixture. These fluctuations are temperature dependent and are therefore easily controlled. By altering these fluctuations more insight can be obtained into structure formation and crystallisation.
From theory to practice
The formation of structures of weak attractive spherical colloids, both in equilibrium and non-equilibrium, has been investigated a lot in theory. The literature contains many studies about the phase behaviour of such systems. However experiments in this area are scarce. That is why Prof Gerard Wegdam and Dr Peter Schall have devised a system with which they can test theories in practice.
Experiment in space
On earth, the crystallisation of the colloids used will always be influenced by gravity. Large aggregates sediment before macroscopic crystals can be formed. Moreover, temperature differences in the solvent can give rise to currents that influence the formation of aggregates. Therefore to prevent these effects and to study pure crystallisation due to diffusion, the samples have been sent to the International Space Station where the experiment will be initiated on 14 September 2010.
On the ISS, various mixtures shall be examined with a new diffraction technique called near- field diffraction developed by the University of Milan. Thanks to this technique, Dr Marco Potenza and Dr Matteo Alaimo can follow structural developments in aggregate formation in real-time. The research team from the University of Amsterdam further consists of Prof. Gerard Wegdam, Dr Sandra Veen and Dr Peter Schall. Besides the experiment onboard ISS supported by Stefano Mazzoni from ESA (European Space Agency) several additional experiments will be carried out on earth. For example, FOM PhD researcher Yasser Rahmani will perform rheology measurements as part of the FOM programme 'Jamming and Rheology'. He will examine glasses that are formed by allowing this system to cool down quickly.
Reference
Direct observation of colloidal aggregation by critical Casimir forces; D. Bonn, J. Otwinowski, S. Sacanna, H. Guo, G. Wegdam, P. Schall; Physical Review Letters 103, page 156101, 2009.
Contact
For further information please contact:
Dr. Peter Schall, University of Amsterdam, telephone +31 (0)20 525 63 14.
Peter Schall has also contributed to the Scientific Block Calendar 2010. A news item about this has also been placed on the FOM website. For further information please see:
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