Paint molecules as informants
There is a world of difference between solid materials and liquids. Not a great deal is known about this strange world of solid materials. Molecular physicist, Professor Michel Orrit, has a mission. The European Research Council has awarded him an Advanced Grant to fulfil this mission.
Image: Fluorescing molecules in glycerol which has been colled to below melting point. Colour changes indicate revolutions by the molecules.
Last week the ERC announced the winners of these Advanced Grants in the field of research of Physical Sciences and Engineering. The Advanced Grant is a new and very prestigious subsidy for leading researchers in Europe. Nine scientists at Dutch universities or research institutes were awarded a subsidy, out of a European total of 105. Two of the nine are professors in Leiden: besides physicist Orrit, astronomer Marijn Franx also received an award.
Soft materials, soft condensed matter, are complex liquids, such as glass-forming liquids like polymers, 'normal' glass or glycerol. But they are also mixtures of solid materials and liquids, such as ketchup or mayonnaise, which change in consistency as they are put under pressure. But they do not all do this in the same way.
Orrit: ‘JThere is, for example, the quicksand variant and the cornflour variant. If you step on quicksand, you sink, but if you fill a swimming pool with a poridge made of cornflour and water, you can walk on it.'
New fundamental knowledge about soft materials is of vital importance to different branches of industry, such as the food industry and the pharmaceuticals industry, which want to know how a substance feels or how well two different substances will mix. A characteristic of all these materials is that they may be very heterogenous in consistency. This heterogeneity is not yet properly understood.
The relations between the behaviour of these materials at macroscopic and microscopic level appear to be particularly interesting. Orrit works at microscopic level. He makes use of separate flourescing paint molecules which he introduces as kind of informant into complex, transparent materials, after which he examines under an optical microscope how they behave within the material and what resistance they meet and where.
His research can be classified as halfway between chemistry and physics. Orrit: ‘Physicists study the characteristics of materials on the basis of macroscopic observations, and then apply complicated statistics to them. Chemists look at what molecules do. Between the two there is a gap, and I want to bridge that gap.' He sees his method of releasing a number of molecules as a kind of spy in soft materials with a random and changing consistency as a missing link, which should provide a great deal of decisive information.
Photo: Michel Orrit
Earlier this year with his fluorescing molecules he shed light on the process of glass formation, which Nobel Prize winner Phil Anderson in 1995 referred to as ‘the deepest and most interesting unsolved problem in solid state theory’. Orrit examined two molecular glass-forming substances. One of these was glycerol, familiar in every pharmacy. With his colleagues he demonstrated unequivocally that when cooled glycerol forms a solid material at far above the so-called glass transition temperature, but then interspersed with liquid-filled cavities.
He discovered that the paint molecules were able to rotate far more easily in these liquid-filled cavities than in the solid parts of the material. This observation was quite revolutionary and contradicted the generally held belief that glass-forming materials gradually become increasingly viscous as they cool to below melting point, but that they remain homogeneous liquids until they set into a solid material at the so-called glass transition temperature.
Orrit wants to use his ERC subsidy to find out a lot more about molecular glass-formation. How big are these liquid pools, for example? In other words: what is the radius of activity of the cavities? How long do they last, and what does that depend on? How do they react to external forces? His hypothesis is that when undercooled, glycerol becomes a kind of framework of microscopically tiny embankments enclosing microscopically tiny pools.
‘If we know whether this hypothesis is correct, and have even more sophisticated mechanical and microscopic means of testing it, we can then test out the new knowledge and techniques in macroscopic materials,' he says. 'For example, on friction between the surfaces of two solid materials at room temperature.'
Two PhD research assistants and a technical expert will help him devise new techniques and apparatus. For example, a microscopically small device with which his material can be subjected to pressure. He also wants to experiment with minuscule gold rods as detectors. Paint molecules suffer under the effects of light and will eventually dissipate. Gold is a much more robust material.
Research into soft materials is an up and coming field. 'There are many new groups across the world focusing on this theme,' says Orrit. 'The ERC grant has come at exactly the right time.'
Michel Orrit has been Professor of Molecular Physics at Leiden University since 2001. He was born in Toulouse, and studied physics in Paris (Université Paris VI), obtaining his Doctorat de 3e Cycle and his Doctorat d’État at Université Bordeaux I. From 1984 to 2001 Orrit worked at the CNRS (National Centre for Scientific Research) in Paris.
Lorentz Centre Workshop 25-8 t/m 5-9: Dynamic heterogeneities in glasses, colloids, and granular media.
The Molecular Nano-Optics and Spins research group in the Leiden Institute for Physics Research (LION).
European Research Council