Drops are everywhere, and they are extremely diverse. Meteorologists have found specimens almost a centimeter in diameter in clouds. Even larger droplets can form on surfaces. The slowest drops fall in a laboratory in Brisbane, Australia: there, as early as 1927, a physicist filled a tar-like liquid into a funnel. Since then, the viscous mass has only “dribbled” out of the container once every few dozen years. The ninth drop fell in 2014, and now the fans of the curious spectacle are waiting for drop number ten. In 2003, the Guinness Book of Records named the spectacle the "longest-running laboratory experiment" in the world. The fastest known droplets, on the other hand, fly as fast as bullets, for example through gas turbines or engines.
Meteorologists are interested in how fast water drops fall in the air. They measure that with radar. This involves determining the amount of precipitation or the turbulence of raindrops of different sizes due to air turbulence. The smaller a drop, the slower it falls to the ground. Some water droplets grow up to five millimeters in size and then fall to the ground - in a heavy rain shower at speeds of up to 30 kilometers per hour. In a drizzle or drizzle, on the other hand, they gently sink down. The transition to fog, in which the fine droplets float, is fluent.
The smallest droplets are not only invisible even to a microscope, but sometimes form objects that only quantum physicists can understand. Tiny droplets that spread viruses are dangerous - as in the corona pandemic. But they are also useful in many everyday technical devices. Scientists are investigating how such droplets behave, for example for use in an inkjet printer, for material processing, in chemical processes, the refining of crude oil or when filtering ingredients in food production. Companies also use droplet microfiltration for pharmaceutical and cosmetic products.
Merge and Split
A major problem for "microfluidicists" is still understanding how droplets merge or split. In some applications, the goal is to induce fusion to mix substances and initiate chemical reactions. In other cases, drips should be prevented from doing just that. How surfaces are moistened and wetted is of great economic importance for the efficiency of many apparatuses.
Bernhard Weigand, Director of the Institute for Aerospace Thermodynamics at the University of Stuttgart, is someone who wants to know exactly. In the rain, you only notice how drops are falling down, explains the researcher. "But we examined this with high-speed cameras that deliver 25,000 frames per second." The super slow motion created in this way reveals, for example, how a drop of about two millimeters in size hits the ground and then bursts.
An amazing sight, because a freely falling drop is by no means in the form of a classic teardrop. At first it looks like a wobbly ball. This is due to its surface tension, which - thanks to the weak forces with which the molecules attract each other - holds the water together like a skin. The smaller the pot, the greater these forces. The structure then flattens out on the underside, while the upper side resembles a hemisphere. The reason: the airflow presses harder on the underside, while small turbulences reduce the pressure on the top. In the rain, drops often collide with others and grow as a result. But from a diameter of around four millimeters, the structures burst and break up into smaller water globules.
Water wall, splash rim and crown
Weigand's super slow motion also shows that as soon as a drop hits the wet ground, a thin ring-shaped wall of water grows around it. After a short time, the picture changes: the thin, ring-shaped wall forms thin threads of water on its upper edge – the so-called splash edge – which dissolve after just a few millimeters into strings of tiny beads. The splash rim bursts and resembles a crown for a brief moment. Dozens of water projectiles now erupt in all directions, as if fired from a shotgun.
After half a second, the wall of water, which has become thinner and thinner, tears apart and simply disappears. After that, only large and small droplets spray around. After a few more seconds, most of them have landed somewhere in the area. Only a small part of the finest water dust fogs the environment. The spook is over - with every downpour of rain a millionfold, but invisible to the human eye spectacle.
Modern supercomputers can depict such showers very realistically. All you have to do is solve the basic equations for such a pot, says Weigand. However, he and his team already use a spatial resolution of one billion grid points for a single drop. This means: "We can only resolve detailed processes." With many such processes in nature and technology, such as rain or in aircraft engines, models first have to be derived from the individual processes for application.
"The splash limit in particular is giving us a headache," says Weigand. The reason: wavy structures form at the upper edge of the water wall, which then break up into individual droplets – an instability. Because as soon as the drops detach, they are attached to an extremely thin layer that consists of only a few molecules. “Our theory fails there,” says the thermodynamics expert. This requires special mathematical treatment.
The bridge to the nano world
Steffen Hardt sees it the same way. There is still a lot to be done on the theory, says the physicist from the Nano- and Microfluidics department at the Technical University of Darmstadt. The trick is to bridge the gap between large drops of liquid and structures that are so tiny that it is necessary to look at individual molecules on the nanometer scale. It is a gigantic numerical task to calculate the processes in water - not as a liquid per se, but as a collection of individual molecules. In order to get practical results, one would have to simulate systems that are about 100 times larger than what even the best supercomputers can do - perhaps with up to a billion water molecules. According to Hardt, we are looking for approximations and models that require far less computing effort.