Moving gas bubbles underwater on a surface could help scientists and engineers in the lab and in industry. But it’s not an easy task. Previous methods have relied on slippery silicone oil coatings, but bubbles move slowly on these surfaces and only in one direction. Now, researchers at the University of Science and Technology of China and the Chinese Academy of Sciences have developed a ferrofluid-based surface that lets them move bubbles quickly and in any direction with the help of a magnet (Nano Lett. 2020, DOI: 10.1021/acs.nanolett.0c02091). Using this so-called FLAM surface, the researchers were able to drag bubbles down tilted surfaces and even tow objects more than 500 times their own mass. The authors envision their method being useful for applications such as electrochemical reactions and wastewater treatment.
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The following is the script for the video.
Tien Nguyen: Submerged underwater, this air bubble sits on a special surface that lets researchers guide along gas bubbles using a magnet. Methods for manipulating bubbles underwater could help scientists in the lab and in industry. For example, scientists could direct bubbles of gas generated by electrochemical reactions away from an electrode to speed up the reactions. Or they could use bubbles to push aside debris during wastewater treatment.
In previous bubble-maneuvering strategies, bubbles slide along slippery tracks coated with silicone oil. But their movement is slow and limited to one direction. Now, researchers at the University of Science and Technology of China and the Chinese Academy of Sciences have developed a magnet-controlled approach that lets bubbles move on-demand and in any direction on these surfaces. This surface is made of a nickel-titanium alloy that’s been cut with a laser to create tiny crevices. These crevices are filled with a magnetic ferrofluid made of iron oxide and octane, which doesn’t mix with water.
The researchers deposit air bubbles on the surface by sticking a syringe underwater. By controlling a magnet placed under the surface, the scientists move the ferrofluid, which in turn pulls the gas bubble along with it.
Using their magnetic FLAM surface, the team showed that they could move bubbles at speeds greater than 25 mm/s—more than twice as fast as on previously reported silicone surfaces. In another demonstration, researchers dragged bubbles down increasingly tilted surfaces, which is challenging for the naturally buoyant bubbles. Finally, the team attached the bubbles to objects like flower petals or even a live ant to steer them around the water’s surface. A bubble could even drag around this plastic fish, which is more than 500 times the bubble’s own mass.
Next, the researchers plan to test multiphasic materials, like mixtures of bubbles and liquid droplets, to explore what else they can manipulate on their unique FLAM surface.