Imagine a world where the most advanced concepts in physics are not just confined to high-tech labs or theoretical equations, but can be found in something as ordinary and familiar—water—the source of human life. This is not a fantasy, but a fascinating reality that has recently been unveiled by groundbreaking research, which was recently published on Nature [https://doi.org/10.1038/s41586-024-08384-y]. The concept of topology, topological structure or topological physics, once considered an esoteric branch of mathematics and physics, is now making waves—literally—in our everyday lives.
Topology, a fascinating modern branch of mathematics that studies properties preserved under continuous deformations, has revolutionized modern physics. The 2016 Nobel Prize in Physics [https://www.nobelprize.org/prizes/physics/2016/summary/] was awarded for theoretical discoveries of topological phase transitions and topological phases of matter, highlighting the profound impact of topology on our understanding of physical systems. Especially, the observation of novel topological structures in diverse real-space physical fields, for instance, Möbius strips in light fields [ Science 347, 964 (2015) ], vortices with topological charges in electron, neutron, and atomic waves [ Nature 467, 301 (2010) , Science 331, 192 (2011) , Nature 525, 504 (2015) , Science 373, 1105 (2021) ], skyrmion and meron quasiparticle topologies discovered in magnets [ Nat Rev Phys 2, 492–498 (2020) , Nat Rev Mater 2, 17031 (2017) ], liquid crystals [ Science 365,1449-1453 (2019) ], and optical fields [ Science 361, 993 (2018) , Nature 588, 616 (2020) , Nat. Photon. 18, 15–25 (2024) ], to name a few. These discoveries have not only deepened our knowledge of both quantum and classical phenomena, such as the topological Hall effect, but also paved the way for potential applications in topologically protected information processing and other advanced technologies.
Furthermore, there is no need to prove the enormous importance of optical trapping and manipulation of particles [ Nature 424, 810 (2003) ] resulted in The 2018 Nobel Prize in Physics [https://www.nobelprize.org/prizes/physics/2018/summary/] for particle trapping, as well as of acoustic manipulations involved in numerous biomedical applications [ Nat. Methods 15, 1021 (2018) , Nat. Rev. Phys. 2, 480 (2020) , Nat. Rev. Phys. 6, 231 (2024) ].
Surprisingly! Now, the very substance that sustains life on Earth—water—has now emerged as a new frontier for exploring diverse topological structures, profound topological physics and wave-matter interaction. An internationally collaborated group, including Nanyang Technological University (NTU) Nanyang Assistant Professor Yijie Shen in Singapore, internationally renowned theoretical physicist Konstantin Bliokh, now an Ikerbasque Professor in Donostia International Physics Center (DIPC) in Spain, and an experimental physics group led by Prof Lei Shi in Fudan University in China, not only, for the first time, experimentally generated of various important topological structures in water waves (skyrmions, merons, Möbius and vortices with different topological charges), but also, observed manipulation of floating particles using topological water waves and described how the spin-orbital motion of particle is related to topologies of waves (Fig.1).
These findings were published in the journal Nature [https://doi.org/10.1038/s41586-024-08384-y] in Feb 2025, which not only open a new chapter in the field of general wave mechanics but also holds the potential to revolutionize microfluidics, biomedical engineering, and beyond.
The Hidden World of Topological Water Waves
When you think of water waves, you might picture gentle ripples on a pond or the crashing waves of the ocean. But beneath this familiar surface lies a world of complex patterns and structures. The researchers have shown that by carefully interfering plane waves on a water surface, one can create intricate patterns that exhibit nontrivial topological characteristics. For example, they demonstrate that an infinite set of plane waves, with their oscillations delayed in a sequential manner, can interfere to form a phase vortex—a point of zero amplitude where the phase of the wave field wraps around in a manner described by an integer topological charge (Fig. 2).
From Mysterious Topologies to Magical Wave Manipulation
Picture a rippling water surface where scientists, through precise control of interfering water waves, create complex patterns that function like microscopic "traps." These patterns, generated by carefully orchestrating the interaction of multiple plane water waves, form structures such as wave vortices, skyrmions, and Möbius strips. These structures are more than just beautiful patterns; they are the keys to manipulating particles with incredible precision.
In their experiments, the researchers used a hexagonal array to generate wave vortices that could trap particles at their centers. By further manipulating the wave patterns with circular wave source, they also created Bessel-type vortices with different topological charges.
Moreover, they achieved even more sophisticated control over the particles, guiding them along specific trajectories and inducing both orbital and spinning motions. This level of control over microscopic particles using water waves is unprecedented and opens up a world of possibilities. We have known that various topologies can be constructed in various physical wave fields, but we have never studied how such topological wave fields interact with matter. Furthermore, both optical and acoustic structured waves are crucial for manipulating small objects, from atoms to macroscopic biological objects. Now, the efficient manipulation of subwavelength and wavelength-order floating particles with topologically structured water waves was demonstrated. This includes trapping of different kinds of particles, from small foam particles to ping-pong balls in the high-intensity field zones of topological water waves. An existing property of water waves is that their topology can be controlled and transferred to matter, i.e., controllable orbital and spinning due to the orbital and spin angular momenta of water waves. These results reveal the water-wave counterpart of optical and acoustic fields, especially generalizing light matter interaction to general wave matter interactions, which paves the avenue for applications in hydrodynamics, microfluidics, and environmental sustainability [Fig. 3].
A New Frontier in Physics: Never Stopping the Quest for Discovery
"Our work shows that all of these phenomena are naturally implemented in water-surface (gravity and capillary) waves. This provides a novel framework for smart control and use of structured water waves. Most importantly, while observations of topological wave forms in optics and acoustics was considered as an important achievement per se, without any noticeable applications, water-wave manipulations of floating particles demonstrated in our work open the avenue for numerous applications in hydrodynamics and microfluidics." —— Konstantin Bliokh
Indeed, while optical manipulations work with wavelengths of the order of micron, acoustic manipulations occupy the range from the tens of microns to millimeters, water waves can efficiently work in the next range from millimeters to centimeters (as in our experiments) and further to colossal ocean waves. The birth of topological water waves is in line with rapidly growing interest in smart control of linear water waves inspired by the achievements in advanced optical and acoustic systems.
"Yet, to the best of our knowledge, the controllable manipulation of particles with topologically-structured waves was not discussed theoretically nor observed experimentally. We did the first step in water waves, while, our results open up a new platform for general topological wave-matter interactions, especially, the advanced optical tweezers and particle trapping by structured light. The diverse topologies of light waves will play important roles in robust control of multi-degree-of-freedom motion of trapped small object for revolutionizing both fundamental and applied science of light-matter interaction." —— Yijie Shen
"I believe that the law of spin-orbital motion of floating particles controlled by topologies of water waves is inextricably linked to the spin states of elementary particles in fundamental physics. More in-depth research will continue to explain more general laws, how, exactly, the topologies of waves coupled to the spin and orbit of particle behavior, which is likely to inspire the discovery of new physical fields, new particles in fundamental physics, even the grand unified theory of the universe." —— Lei Shi
This groundbreaking research published in Nature is a testament to the relentless pursuit of scientific discovery. The researchers have not only made the seemingly impossible a reality but have also set the stage for future innovations. As one of the lead researchers stated, "We have only taken the first step; there are countless possibilities yet to be explored."
This achievement in fluid mechanics is more than just a scientific breakthrough—it is a spark that ignites the imagination and fuels the drive for progress. As we look to the future, the potential applications of this technology are as boundless as our curiosity. Let us eagerly anticipate the next wave of discoveries that will undoubtedly transform our world.
