Physics-defying oil droplets hover, move against liquid flow in a first

Scientists at the Technical University of Darmstadt in Germany have, for the first time, observed microscopic oil droplets hovering and moving upstream against a flowing liquid, defying conventional fluid dynamics. Using the Ouzo effect—where mixing alcohol-oil solutions with water creates tiny oil droplets without surfactants—the team injected this mixture into a narrow flow channel and recorded the droplets’ behavior with high-speed cameras. They found that droplets could resist flow, remain stationary, or even move against the current due to differences in surface tension at their upper and lower ends, a phenomenon driven by Marangoni stresses in multi-component liquid mixtures. Although this effect occurs at microscopic scales under controlled conditions, the researchers suggest it may also manifest at larger scales, such as in emulsions containing numerous oil droplets, potentially leading to complex pattern formations. This discovery holds promise for applications in process engineering, microfluidics, and analytical chemistry, particularly for selectively trapping or extracting tiny droplets or bubbles from flowing liquids. The findings, funded by the