In a world first, scientists in Germany have successfully recorded tiny oil droplets hovering within a flowing liquid, and even drifting upstream against the current, defying typical fluid behavior. <\/p>\n
Captured under a microscope by a research team at the Technical University of Darmstadt (TU Darmstadt) in Germany, the oil droplets\u2019 striking behavior points to a previously unknown fluid phenomenon. <\/p>\n
The scientists, led by Steffen Bisswanger, a PhD candidate at TU Darmstadt and head author of the study, believe that the discovery may be particularly relevant for process engineering, analytical chemistry and microfluidic applications.<\/p>\n
\u201cThe 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,\u201d Bisswanger explained.<\/p>\n
A surprising phenomenon<\/p>\n
For the study, the researchers relied on the Ouzo effect to create oil droplets for a laboratory experiment. The Ouzo Effect is the clouding that occurs when water is added to anise-flavored spirits.<\/p>\n
The phenomenon causes essential oils<\/a> to create tiny droplets that turn the drink milky white without added surfactants. By injecting an alcohol-oil mixture into water inside a narrow flow channel, the team generated microscopic oil droplets.<\/p>\n They then used high-speed cameras to observe how the droplets behaved under flowing conditions<\/a> and found that they could resist the flow, remain stationary, or even move upstream.<\/p>\n \u201cThe force that holds the droplet in place results from a difference in surface tension at its upper and lower ends,\u201d Bisswanger elaborated in a press release<\/a>.<\/p>\n He explained that the equilibrium allowing the droplet to hover depends on its size and position, as well as the flow rate and the type of liquid<\/a> moving through the channel.<\/p>\n \u201cA general conclusion is that Marangoni stresses can reverse the motion of droplets through channels, where the surrounding liquid is a multi-component mixture,\u201d the researchers said. <\/p>\n Unexpected flow dynamics<\/p>\n Although the phenomenon can only be observed at microscopic scales and under carefully controlled experimental conditions, its impact could extend beyond lab settings. <\/p>\n According to the researchers, the effect could also become visible at larger scales, such as in emulsions, liquids in which countless tiny oil droplets are distributed in water. <\/p>\n Steffen Hardt, PhD, a professor of nano- and microfluidics<\/a> at TU Darmstadt, noted that in such systems the observed effect could take place billions of times at once in a single container, and potentially lead to the formation of complex patterns.<\/p>\n \u201cThe phenomenon we observed and described could also be used to extract tiny droplets or bubbles from a liquid for analysis,\u201d Hardt concluded. The project was funded by the German Research Foundation (DFG).<\/p>\n The discovery could also open new possibilities in analytical chemistry<\/a>. The team suggested the effect could be used to selectively trap or extract tiny droplets or bubbles from flowing liquids, a potentially useful tool for lab-on-a-chip systems, chemical analysis and diagnostics. <\/p>\n