Spies cloaked in blue noise

Brownian movement under close observation

The slightest trembling can tell us something. Tiny particles, dissolved in liquid, register what is going on in their surroundings, and react as a result. Without sophisticated measuring techniques and high precision instruments it is impossible to decode how their messages work. However, if successful, amazing discoveries await decryption experts. Based on a combination of theoretical foundations and highly complex experiments, physicists from Erlangen, Lausanne and Basel have, for the first time, been able to observe how small particles behave in solvents. At first sight, their discoveries may seem fantastical: the particles’ movement can be categorised by colour.

Thomas Franosch, professor at the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) Institute for Theoretical Physics describes the findings in more objective terms. “This has confirmed an assumption that experts have held for fifty years”, explains the first author of the study on this topic, which was published on Thursday in the specialist journal ‘Nature’1). “Until now it was not possible to directly measure the spectrum of forces that affect particles.” The research group was able to do this by using strong optical traps that can capture individual particles.

Coffee chaos

One of the pillars of modern theoretical physics is “Brownian molecular motion”. Anyone observing particles in a solvent under a microscope, say ground coffee in hot water, can see a break dance performance in miniature. Chaos rages beneath the apparently still surface. The wriggling and twitching of each individual particle cannot essentially be predicted. This movement was named after the Scottish botanist Robert Brown, who studied grains of pollen in water droplets and was first to classify their “dancing” as an indication of their vitality.

What was imagined in the 19th century has now long since been succeeded by a commonplace explanation: the water molecules continually bump, on all sides, against the larger, visible grains of pollen. This also applies to particles in other solvents. Movement increases with temperature; the forces at play here, consequently, are described as thermal. In 1905 Albert Einstein submitted his thesis. In this famous work he approximately defined an indicator proportional to temperature and this reflects the close connection between the friction of a particle dissolved in a solution and the random bumping of liquid particles.

Into the blue

A vivid representation of the processes taking place in a liquid can be achieved through comparison with two sensory perceptions: hearing and sight. Just like the overlapping of radio waves, the irregular wriggling of molecules is described as “noise”. As all types of radiation with different frequencies can also overlap each other, light waves are also described as noise. “Here, just as with sunlight, if the full spectrum comes together, it appears white. If a part of the spectrum is missing then we see a colour”, explains Prof. Franosch. Einstein arrived at an approximate description, based on the assumption that Brownian molecular motion is driven by white noise. Thanks to innovative measurement devices and further advancements in mathematics, slight changes have come to light. The German-Swiss research group has now found that the spectrum displays a shift towards blue.

To this end high precision measurement devices had to be combined with highly effective traps. A laser beam can hold a dissolved particle in place because of its optical characteristics. Detectors with a spatial resolution are applied to the captured particle, which is smaller than the nanometre scale. At the same time, measurement can make periods of time as small as micro-seconds visible. “Both distorting influences from the surroundings and errors which can result from a strong laser beam must be excluded”, project leader Sylvia Jeney highlights this as the greatest difficulty in developing such experiments. “Then the particle can deliver a report on the thermal forces at work the liquid.” But in terms of size-ratios, is that not like a swarm of tadpoles attempting to push a hippopotamus through the mud? “Particles are not so thick-skinned that they do not notice the prodding from water molecules”, Thomas Franosch assures us. The so-called hydrodynamic memory, which slows particles swimming in a solvent, comes into play here. Anyone who likes milk in their coffee will be familiar with this effect: left to its own devices the milk will only disperse within the coffee very slowly. That is why there is a spoon to stir it.

Based on the research team’s findings, hydrodynamic memory may be used for entirely innovative measurement processes. As a central part of nanomechanical sensors, particles captured by lasers could be useful in material sciences and biomedicine, for example as spies in the blood.

¹⁾ doi:10.1038/nature10498

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uni | media service | research No. 49/2011 on 6.10.2011