01/08/2026
'Hovering' drops
Tiny drops of oil can resist a fluid flow and hover at a constant position. Researchers at TU Darmstadt have now documented and investigated this phenomenon for the first time. They made use of the so-called Ouzo effect.
Whether it's Greek ouzo, French pastis or Turkish raki: when these spirits are diluted with water, the mixture becomes cloudy. The reason for this is that the aniseed oils contained in the spirit dissolve well in alcohol but not in water. The clear Ouzo from the bottle has a high alcohol content at which the oil is fully soluble.
However, when water is added, the aniseed oils can no longer dissolve completely in the significantly reduced alcohol content. As a result, small droplets disperse finely in the drink, creating a milky appearance. Researchers at TU Darmstadt have now used this so-called Ouzo effect to create oil droplets for a laboratory experiment. This led to a new discovery: such a droplet can resist a fluid flow and remain in place or even move upstream.
'The force that holds the droplet in place results from a difference in surface tension at its upper and lower ends,' explains Steffen Bisswanger, doctoral candidate in the field of Nano- and Microfluidics at the Department of Mechanical Engineering at TU Darmstadt and leading author of the study. The equilibrium that appears to cause the droplet to 'hover' depends on its size and position, as well as the flow rate and type of liquid in the flow channel. 'The phenomenon of droplets being held in place or even moving upstream was previously unknown and has now been documented and explained for the first time,' emphasises Bisswanger.
Basic research for process engineering and analytical chemistry
The findings of this basic research are particularly relevant for the fields of process engineering and analytical chemistry. Although the effect can only be observed under a microscope with a high-speed camera, it could also be noticeable on a large scale - for example, in emulsions, i.e. liquids in which countless tiny oil droplets are distributed in water.
'In such a system, the effect we observed could occur billions of times simultaneously in a single container, resulting in the formation of patterns, for example,' explains Steffen Hardt, Professor of Nano- and Microfluidics at TU Darmstadt and co-author. 'The phenomenon we observed and described could also be used to extract tiny droplets or bubbles from a liquid for analysis.'
The project was led by researchers at TU Darmstadt, who carried out the experiments and mathematical modelling. The University of Twente in the Netherlands was also involved with numerical simulations. The project was funded by the German Research Foundation (DFG), among others.
Source: TU Darmstadt