Selectable Optical Diagnostic Instrument (SODI)

Selectable Optical Diagnostics Instrument - Diffusion and Soret Coefficient (SODI-DSC) experiment hardware set up in the Microgravity Science Glovebox on the International Space Station's Destiny laboratory.

The Selectable Optical Diagnostic Instrument (SODI) instrument is an interferometer that shines a laser beam through liquid mixtures and compares it to another beam that does not pass through the sample.By comparing the time it takes to pass through the sample and air, researchers can record the mixing process itself.

The liquids of interest are mixtures of different liquids and colloids, liquids with minute particles suspended in them. Most liquids can be considered colloids, from milk to oil. Never at rest, the components move continuously, influenced by movement (such as shaking and mixing), gravity and changes in temperature.

Researchers want to understand how these factors destabilise the mixtures but investigating the process on Earth is hampered by the pull of gravity. Running an experiment on the International Space Station removes gravity from the mix and allows us to focus on a single factor in the equation: temperature.

A number of liquids have been investigated over the years with the SODI hardware, all placed inside the European-built microgravity science glovebox on the Space Station. Once setup by an astronaut and containers holding the mixtures are inserted, the experiments are controlled from Earth.

SODI has now investigated around 30 liquids in space and a next generation version is in the works that will investigate giant fluctuations of three different mixtures, an investigation that is not possible on Earth because these fluctuations are dampened by the gravity field.