Focus on research: Physicist Peter Schall

Peter Schall

Photo: Bob Bronshoff

Physicist Peter Schall of the Van der Waals-Zeeman Institute designs and studies particles roughly a thousand times larger than atoms. Like atoms, these particles can order in regular crystal structures and they can be used to produce a range of materials. There is ‘a whole zoo' of new substances out there just waiting to be discovered, according to Schall. In order to learn more about the process behind the formation of perfectly regular crystals, undistorted by gravity, Schall will soon be sending his particles into space, to the ISS space station.

‘The entire field of chemistry revolves around atoms. You can imagine our particles as enormous atoms. They represent the next big challenge in my area of study', explains Schall. After earning his PhD in Physics in his native Germany, Schall was engaged as a researcher for three years at Harvard. In 2005 he came to the Van der Waals-Zeeman Institute to investigate the physical properties of soft substances such as gels. The scientist taps the lid of an empty metal peppermint tin. ‘This material is made up of atoms, and we are fairly familiar with their properties. But what we're researching now are particles that are much larger - in the range of nanometres to micrometres.'

Looking at nanoparticles

Three-dimensional reconstruction of the transition from a crystal to its melt. The red particles have an ordered structure within the crystal; the blue particles represent liquid. (Blue particles are drawn smaller to make the transition easier to see.)

The major advantage of particles of this size is their visibility, says Schall. ‘You can easily see micrometre-size particles under a microscope. When I came here in 2005, I bought the best confocal microscope then on the market. With it, we can make three-dimensional images of our nanoparticles and even trace their movements. Atoms are too small for that; rendering individual atoms in three dimensions that way is simply impossible.'

According to Schall, the resulting data also yields new insights into atomic matter. ‘It's amazing in how many ways nanoparticles are similar to atoms.' Both have two key ingredients, he explains. ‘To start with, there are the thermodynamics: particles wobble in place, also in solid materials. The rate depends on the temperature. The other ingredient is the attractive force exerted by each particle, which holds them together.'

The movement of nanoparticles is referred to as Brownian motion, which is a term referring to random motion directed by temperature. ‘This allows us to study the thermodynamics and draw parallels with atomic particles', Schall explains. However, the attractive forces between nanoparticles are fundamentally different from those between atoms. ‘At the atomic level, you're dealing with quantum mechanics; but the case is different for our nanoparticles.' Our nanoparticles do, however, exhibit an effect somewhat reminiscent of a known quantum effect: the Casimir effect. The variant for particles in a solution is called the ‘critical Casimir effect'. ‘Both concern the attractive force of particles', says Schall, ‘and there are some major analogies. In liquids, we use the critical Casimir effect to regulate the attractive force of our particles. By changing the temperature, we can cause the particles to aggregate and, ultimately, build nanostructures.'

Peter Schall

Photo: Bob Bronshoff

Creating crystals

One type of material that Schall hopes to produce in this manner is photonic crystal, which has the ability to change the properties of light. ‘Think of these crystals as a sort of transistor for light; a switch. Light has a wavelength on a scale of hundreds of nanometres. For a material to influence light, it needs to consist of particles of the same order of magnitude. Our nanoparticles satisfy that criterion.'

To create a crystal of any substantial volume easily requires billions of particles, says Schall. ‘Moreover, we sometimes aim for a structure that is totally perfect, with no irregularities or impurities. Because here on earth gravity is working against us in this sense, we would like to study that building process in space. ‘We build structures by floating the particles in a liquid, which has the same effect as, for example, tiny bits of fat floating in milk: a suspension. Subsequently, we cause the particles to bond by increasing their attractive force by changing the temperature. But the more particles you get to cluster together, the heavier the structure becomes, and of course then it sinks to the bottom. After that, you can forget about orderly aggregation.'

There are several tricks for getting around this problem on earth. For instance, it is possible to adjust the relative density of the liquid to that of the particles in suspension. While the structure is actually growing, that relative density has to be perfectly calibrated, Schall explains. But the temperature changes that scientists use to regulate the attractive force between particles pose a technical difficulty. ‘Another consequence of this is that materials change in volume, but the nanostructure does that differently than the liquid. Adjusting those changing relative densities is a real challenge.'

Experimenting in space

Nanostructure formation on earth will therefore always suffer from the disturbing effects of gravity. It was for this reason that, seven years ago, researchers from Amsterdam and Milan submitted a proposal to conduct their experiments on the ISS space station, far away from the earth's gravitational field. ‘Room onboard is limited, and this type of research is expensive, so we're thrilled that we were ultimately accepted. In space we'll be able to let the growing process run its course unimpeded, and study it.'

Initially, astronauts were slated to take the experiments into orbit in May, but a technical problem has forced a delay in the launch, now scheduled for the autumn. Even now, the suspensions and equipment are all set to go. Schall: ‘The astronauts will set the experiments in motion but - uniquely - we will be able to control them ourselves, too, via an Internet connection. Imaging equipment being carried on board will make it possible to document the growth process. The images will be sent directly to earth, where we will use them to decide on the temperature adjustments needed to achieve the desired interactions. We can freeze the nanoparticle structures, and then melt them again; we can turn everything around. Basically, we can play with it, and that is totally new.'

The result of these efforts - the hoped-for perfect crystals - will never see a return flight to earth, however. But for Schall, that is beside the point. ‘I should really emphasise here that, for us, this is primarily about the fundamental insights this experiment will yield.' This experiment will, he hopes, begin to pave the way towards being able to create more, and more complex, substances in future. ‘Once you know all the ins and outs and can control this production process, you've got it made.'

Published by  Faculty of Science

21 August 2012