Fake plastic atoms
18 December 2018
Our world is made of atoms and molecules, but even with the most powerful microscopes we can only see snapshots, never how they move and interact with each other. To model how atoms behave, researchers have been using plastic particles in weightlessness and the latest batch of results are returning to Earth with ESA astronaut Alexander Gerst in a Soyuz spacecraft on Thursday.
The Plasma Kristall experiments have been slow-cooking since 1998 using the same recipe: mix electrically charged gas in a sealed container with particles so small they would pass through a coffee filter. Perform the experiment in ‘weightlessness’ to keep the particles suspended. Apply electrical current in a plasma-filled tube to coax the particles to behave like atoms and form three-dimensional crystal structures.
This ‘recipe’ comes from a European-Russian collaboration that started running experiments on parabolic flights, then ventured farther into space on sounding rockets followed by experiments on the Mir space station, which have been continued on the International Space Station since 2001.
“Doing this research on Earth is not possible – Plasma Kristall models atomic interactions on a large scale, making their motion visible to us,” explains Hubertus Thomas, lead scientist of this experiment at the German Aerospace Centre, DLR.
Recently, a problem with the valve that regulates gas flow forced an 18-month pause in the experiment. A refurbished valve has allowed Plasma Kristall-4 (or PK-4) to resume operations.
Proxy atoms back to science
A plasma is an electrically charged gas, somewhat like lightning, that rarely occurs on Earth. It is considered to be the fourth state of matter, distinct from gas, liquids and solids. Plasma for the PK-4 experiment is created with neon or argon gas in tubes that make particles electrically charged.
“We are back up and running with the fifth campaign for PK-4 and we have already started exciting the particles with electrical fields, a laser and changes in temperature to get them to move in the plasma,” says Hubertus.
These manipulations cause the proxy atoms to interact strongly, leading to organised structures – plasma crystals. The plastic particles in PK-4 bond or repulse each other just as atoms do on Earth in fluid state.
“By adjusting the voltage across the experiment chamber we can tailor their interactions, and observe each particle as if in slow motion,” explains Hubertus. Using PK-4, researchers across the world can follow how an object melts, how waves spread in fluids and how currents change at the atomic level.
The latest data returning to Earth with Alexander covers phase transitions, microscopic motions and shear forces. Shear forces are a hot topic in fundamental physics as they define air pressure along an aircraft wing for example.
The future of plasma
This research is offering theoretical knowledge for future scientists and engineers, and its implications will be evident in future generations. “If you had asked Einstein what his theory of relativity was for, he would never have replied that it was to build a navigation system for your mobile phone,” points out Hubertus.
Meanwhile a team of scientists has already made use of the knowhow gained from developing the experiment, to build plasma devices that disinfect wounds at room temperature. This revolution in healthcare has many practical applications, from food hygiene to treatment of skin diseases, water purification and even neutralising bad odours.