Water-wave tweezers steer tiny 'surfers' without touching them

Water-wave tweezers steer tiny 'surfers' without touching them Stephanie Baum Scientific Editor Robert Egan Associate Editor Summer brings with it the sight of surfers moving seamlessly across wave crests, with ocean waters carrying them along coastlines. A team of scientists has now created a similar phenomenon—with small objects rather than surfers—that can be controlled by humans rather than by nature. Through a series of experiments on a replicated mini-beach, NYU researchers show how water waves can be used to move floating objects or hold them firmly in place—all without direct touch or contact. "Our study shows how beaming water waves at a floating object can cause it to move sideways or be 'tweezed' and held precisely in place," explains Leif Ristroph, a professor at New York University's Courant Institute School of Mathematics, Computing, and Data Science and the senior author of the study, which appears in the journal Physical Review Fluids. "These surprising effects could be used to manipulate particles and structures, controlling their motions and positions." The technique, while early in development, offers "proof of concept" pathways for creating innovations in engineering, ocean monitoring, and drug design. Previous work has shown the functionality of water waves—specifically, that objects can be carried in the same direction of wave propagation. But the new Physical Review Fluids research reveals how objects can also be carried across waves—perpendicular to the direction of wave propagation and akin to how a surfer moves along wave crests—and how to manipulate them. To achieve this, the researchers conducted a series of experiments in NYU's Applied Mathematics Laboratory. In replicating a coastal environment, the researchers used a 40 cm x 40 cm tank holding roughly one-half cm of water. The tank also included vibrating dipping "wavemaker" bars that generated ripples, mimicking ocean waves that the scientists could control in order to shape the waves' movements. It also included foam "beaches." They then placed small objects—each a few inches in size—in the tank and precisely monitored their interactions with the waves using strobe lighting. "We were particularly interested in the problem of moving objects sideways or perpendicularly to the direction that the waves travel, which is more subtle and challenging than a direct push in the same direction as the waves," explains Ristroph, who directs the Applied Mathematics Laboratory. In transforming the water waves into "tweezers," the researchers altered the intensity of the vibrations, which—combined with the shape of the wavemaker bars—allowed them to move and hold both triangular and circular objects. "We now know that certain objects can move perpendicularly, somewhat like a sailboat cutting across the wind, but—more importantly—that they can be held in place at desired locations by controlling the shape of the wave field," observes Ristroph. "A dream application would be to create a water surface that is like a factory floor where objects can be moved, positioned, and assembled. The objects could even be floating droplets whose chemical or biological contents could be mixed for testing reactions or pharmaceuticals." Publication details Ahmed Sherif et al, Transverse transport and trapping of submerged structures due to water wave refraction, Physical Review Fluids (2026). DOI: 10.1103/6884-kzv4 Journal information: Physical Review Fluids Provided by New York University