They usually only exist for milliseconds – droplets floating on top of a liquid surface. The liquid must have a certain viscosity for this. This phenomenon of non-coalescence, ‘staying apart’, of droplets and base fluid is well known and is an important area of aerosol research.
In order to keep a droplet suspended against gravity for a longer period of time, a continuous positive pressure must be created in the very narrow gap between the droplet and the liquid surface. There are actually different ways to do this.
If the system is made to swing, the drops can bounce off the surface like a trampoline. Due to the vibrations, the air again and again enters the gap between the liquid and the droplets, so that the droplets “float”. The same effect can be achieved with the flow of air into the gap created by the rolling of droplets on the surface or the evaporation of liquid or droplets. The latter is the so-called Leidenfrost effect, which can also be observed when water droplets dance on a hot stove.
Some methods use magnetic fields or sound waves. But they all have one thing in common: “Either we’re doing an external action on the system, or we’re creating an imbalance in which the droplets can float as long as they are there,” explains Denis Klyov of Tyumen University, Russia. Together with his classmate Natalia Ivanova, Klyuev has now discovered something amazing. They observed droplets of butyl alcohol, placed on the surface of a liquid silicone with a needle syringe, and hovering for tens of minutes. Thus, the millimeter-sized drops are floating, self-supporting and without external forces. One of these drops can be seen in the image here.
This phenomenon can be explained by concentration-related convection: molecules continually evaporate at the surface of the droplet. These vapor particles are mostly inhomogeneously distributed over the surface of the liquid. This lowers the surface tension in some places and causes so-called capillary compensatory currents – air currents along which droplets can float.
The researchers were already able to observe this phenomenon with different droplet materials and see a potential application in microbiology or biochemistry. The study of aerosols and the viruses and microorganisms they contain is important for understanding the spread of airborne diseases. The new method makes it possible to lift droplets without physical contact and thus study them without the risk of contamination with unwanted microorganisms.
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