NUS researchers have created hydrodynamic moiré superlattices using vortex lattices in fluid systems, paving the way for innovative applications in energy transport, thermal management, and quantum-inspired physics.

Controlling energy transfer in fluids has been a longstanding challenge in physics and engineering. Unlike in solids, where structural periodicity allows for the precise control of energy transfer, the control of energy in fluids is uniquely complex, as they lack rigidity and experience constantly changing dynamics. Energy also dissipates unpredictably within fluid systems, often leading to inefficiencies in processes like cooling, heating, or energy harvesting. Imposing order and harnessing the fluid’s natural motion for energy transfer requires innovative approaches.

Introducing order to fluids

Moiré patterns, known for their ability to enhance energy localisation and transfer in solid-state systems, offer a promising solution for introducing ordered periodicity into fluid systems. Moiré patterns are visual effects that arise from the overlay of two single-layered periodic structures. Their formation results in larger-scale periodicities and unique optical or electronic properties.

When introduced into fluids, moiré patterns could impose structure on what is otherwise chaotic motion, to enable efficient energy transfer and minimises dissipation. Introducing such ordered periodicity into fluids is however, no small feat.

Now, a team of researchers from NUS College of Design and Engineering, led by Professor Qiu Cheng-Wei, along with collaborators from Chongqing Technology and Business University, and Nanyang Technological University, have unveiled the creation of hydrodynamic moiré superlattices. This was achieved by leveraging hydrodynamic vortices, a novel approach to manipulate energy transfer and mass transport in fluid systems.

A new phase in material physics

Hydrodynamic vortices are localised areas of rotational motion within a fluid, creating well-defined points of energy and momentum concentration. These vortices are inherently stable and can be arranged in periodic patterns when controlled by external forces, such as electromagnetic fields.

In the study, each vortex acted as an individual lattice site, analogous to atoms in a crystal lattice. Two layers of these vortex fluids, consisting of sodium chloride solution, were carefully stacked and twisted. Heat was applied along the z direction to visualise the profile of the moiré superlattice and the propagations of energy across the lattice. The resulting bilayer moiré superlattice was capable of localising and delocalising energy. This occurred when the angles between the two fluid layers was altered.

“The ability to control energy transport and localisation through geometry alone is a game-changer,” said Prof Qiu. “This discovery offers a new framework for designing more efficient systems to manage energy in fluids.”

For instance, in cooling systems or heat exchangers, these superlattices can help optimise thermal performance by directing energy flow with precision, reducing waste and improving overall efficiency.

The role of twist angles

The researchers found that angles corresponding to Pythagorean triples – sets of integers that satisfy the Pythagoras theorem – resulted in ordered, periodic structures with translational symmetry. Conversely, non-Pythagorean twist angles led to aperiodic, quasicrystalline patterns.

For example, at Pythagorean angles such as 36.87°, the system exhibited periodic distributions of pressure and velocity fields, enabling efficient energy transport. In contrast, non-Pythagorean angles disrupted this periodicity, resulting in localised energy states and quasicrystalline behaviours, such as the inhomogeneous energy distributions and localised “hot spot”. These findings highlight the versatility of moiré physics in designing fluidic systems with tailored properties.

Advancing the field

In industrial processes, hydrodynamic moiré superlattices could be used to improve fluid mixing or design advanced microfluidic devices, besides optimising heat exchange systems.

Beyond practical applications, the discovery bridges the gap between fluid dynamics and quantum physics. By mimicking quantum phenomena such as flat bands and topological states through various geometric configurations, hydrodynamic moiré superlattices enable the ability to explore, test, and visualise quantum behaviours in a classical, easily controllable setting. Unlike quantum systems, which often require ultra-low temperatures, precise instrumentation, and challenging setups to probe their properties, hydrodynamic systems operate under more accessible laboratory conditions, such as room temperature and macroscopic scales. These systems could inspire new research in wave dynamics, energy transfer, and other interdisciplinary fields.