Supercool water
The tiny droplets are embedded in oil and lined up behind each other in a row. Shardt guides the entire water-in-oil emulsion into a fine plastic tube. Arranged in parallel, these tubes form a type of artificial mini cloud. The researcher can gradually lower the cloud’s temperature with a cooling unit she has also developed herself. “At this size, pure water droplets only turn into ice at around minus 35 degrees Celsius,” she explains. Although they are literally ice cold beforehand, they do not actually crystallise. The technical term for this process is “supercooling”.
In the microscope’s black field of view, Shardt can watch around 300 droplets freezing at the same time. A camera takes images continuously. “When ice crystals form, they appear as white dots,” the chemical engineer explains. The screening process is only semi-automated at present and the image quality can still be improved, but the basic experimental approach works: "The findings tally with results from previous studies,” Shardt says.
Fun experimenting
“Developing an entire apparatus from scratch was a brilliant experience,” the scientist says. “I would never have imagined I would actually put to use so much of what I learned during my chemical engineering studies.”
Shardt specialised in thermodynamics as part of her doctorate. She is particularly interested in phase transitions. Her PhD thesis focused on theoretical problems and data from the literature. Keen to expand her horizons, she then decided to familiarise herself with the experimental side of research and “develop experiments and generate data on my own”.
Relevant for climate
She is doing just that with the new apparatus. “I'm interested in exactly how different dust particles in the atmosphere affect the formation of ice in clouds." Dust particles act as so-called crystallisation nuclei that trigger the formation of ice crystals. The scientist is therefore planning experiments in which she will mix mineral dusts, such as silicates, with the water droplets – first individually, and then in clearly defined mixtures. Her results should help to improve climate models and thus allow more precise forecasting. “It may be a tiny detail,” the postdoc explains, “But still a very important one”.
In the atmosphere, the transition from water to ice on crystallisation nuclei occurs at low- to mid-altitudes when temperatures are subzero. The process is relevant for weather and climate, because: "Ice formation changes the properties of clouds. For example, how much sunlight they let through or how much thermal radiation they retain from the Earth's surface," Shardt explains. “Or their propensity to produce precipitation”.
