The riddle of the icy droplets

The droplet hovers motionless within the trap. There is a camera focused on the drop while a sensor measures its infrared radiation and thus its temperature. It is minus 30 degrees Celsius here in the chamber. The droplet measuring a mere two millimeters in diameter freezes quickly in this environment and reaches a low at minus 9 degrees Celsius before crystallization starts. Then the droplet warms up a little again and changes shape.

Oliver Eppers is wearing an insulated jacket. He has just introduced a tiny droplet in the ultrasonic trap. He has already repeated the process about a hundred times today. He always uses the same water sample that contains dust particles that have made it distinctly turbid. And it is these particles that make the experiment so interesting and informative because they determine the temperature at which the droplets freeze.

"We still do not quite understand why some particles trigger freezing and others don't," explains Dr. Miklós Szakáll. "What is it that preordains whether a particle will do this or not? We think it has something to do with the molecules or the crystal structure, but what exactly it might be we do not know. All we can do is create a series of experiments to simulate the actual process and then combine the results we obtain with our cloud models."

Unique wind tunnel

The laboratory has a cold chamber and a wind tunnel, which make it one of the most extraordinary facilities on the Gutenberg Campus. Using an American design as his pattern, Professor Hans R. Pruppacher installed a wind tunnel here in the mid-1980s. At the time, the lab was headed by Dr. Subir Kumar Mitra. The original wind tunnel in the USA is now out of action, which makes the one based at Mainz University the only one of its kind still in use. The cold chamber where the droplets are frozen is similarly a one-off.

The ice lab belongs to the Institute of Atmospheric Physics at JGU. Szakáll currently runs the facility together with Professor Stephan Borrmann. Eppers is studying at the institute and, as a student assistant, performs various experiments in the cold chamber and in the wind tunnel. He is also working on his Master's thesis, which is also concerned with water droplets.

Those who suffer from claustrophobia may well feel uncomfortable when they enter the lab. It’s not huge and there are ventilation shafts on the walls and under the ceiling. They are almost omnipresent. And there are computer stations wherever space is available.

The visible portion of the wind tunnel is not very spectacular. It is just a few decimeters wide and maybe one and a half meters high. Air can be blown through at a speed of up to 40 meters per second. In his Master’s thesis, Eppers is analyzing the introduction of droplets here and the addition of various particles in order to determine when the droplets freeze in ice-cold air.

Ice Nuclei Research Unit

Steel steps lead down to the cold chamber, where space is even more constricted. A coat rack holds insulated jackets and matching pants. "At the height of summer, it’s actually quite pleasant having to work in the chamber," says Eppers. "But it hits you even harder when you come out again."

The water sample that Eppers is using droplet by droplet has been supplied by the Institute of Meteorology and Climate Research at the Karlsruhe Institute of Technology (KIT), which is one of the partners in the INUIT – Ice Nuclei Research Unit financed by the German Research Foundation (DFG). Further involved in this large-scale project are Goethe University Frankfurt am Main, Technische Universität Darmstadt, and the Max Planck Institute for Chemistry in Mainz.

Because the experiment needs to be conducted in a blinded fashion to avoid bias, the researchers in Mainz do not know what kind of particles are present in the sample they have been sent. However, Szakáll takes a look at the turbid liquid in the test tube and decides to venture a guess: "It could be desert sand."

The purpose of INUIT is to examine the effect of atmospheric ice crystals on the earth's radiation budget. The focus is primarily on mixed phase clouds, which make up the largest proportion of cloud cover. They are composed of low-temperature droplets and ice particles. It is quite apparent that the processes that occur in these clouds influence atmospheric processes and thus the weather and the climate.

A visit to the cold chamber

However, it is still unclear what exactly happens within the clouds. "We do not really know how and when ice forms in the clouds," explains Szakáll. So the INUIT partners send their samples to each other. The individual institutes then perform various experiments to try to solve the riddle of the icy droplets.

Eppers fills a needle with the fluid from Karlsruhe, puts on his insulated jacket, and enters the cold chamber. The door is several centimeters thick and even in the tiny antechamber the temperature is minus 10 degrees Celsius. It feels like an air lock. The first door needs to be closed before the door to the true chill opens. In the main chamber are the experimental equipment and a cold box. "We can perform experiments at 100 degrees below zero in here," reports the student.

Fortunately, the droplets today only need to be cooled to minus 30 degrees Celsius. Eppers needs to ensure the droplet is placed precisely within the trap otherwise the drop will not remain levitating between the ultrasonic transmitter below and the reflector above. A computer monitor can be seen through a small window in the chamber. Eppers can see an image from the camera with a grid superimposed over it. It is very important that the droplet has the correct diameter of two millimeters. Also to be seen is a graph with a line of red dots indicating the temperature.

The droplet cools down more and more until it reaches a temperature of minus 10 degrees Celsius, then it freezes, and warms up a little. The camera shows how it then deforms. It looks rather as if ice crystals are being extruded from the droplet. "The density of ice is lower than that of water so the whole thing expands," elucidates Szakáll. "Because the outside has frozen first to form a shell, the ice now has to break through this shell. No problem there – so much is obvious. What we do not understand is how it all works, where it all begins on the particle surface, and what determines when exactly the freezing process starts."

At present, science cannot yet provide the final answers. However, this single day with its series of experiments will contribute one part towards the jigsaw that, when complete, will provide a picture of the large-scale processes that occur in the atmosphere. INUIT is working on this together with the Mainz-based researchers in their cold chamber.