Context:

Researchers at the University of Queensland, Australia, have developed a microscopic “wave flume” using superfluid helium to study nonlinear wave dynamics at an unprecedented scale. Their findings, published in Science on October 23, reveal wave behaviors never before observed in classical fluids.

About superfluid helium:

When cooled a few degrees above absolute zero, helium enters a superfluid state, allowing it to flow without viscosity or friction. Ultra-thin films, just a few nanometres thick, can move freely, defying the behavior of normal fluids. This property enabled the creation of a microscopic channel on a silicon chip, effectively a nanometre-scale wave tank.

Key Observations:

1.       Backward Steepening: Unlike normal water waves, the troughs moved faster than the crests, causing waves to lean backward.

2.      Shock Fronts: Extreme waves formed near-vertical leading edges, akin to miniature tsunamis.

3.      Soliton Fission: Single waves split into trains of solitary waves (solitons), propagating as depressions rather than peaks, confirming theoretical predictions for superfluids.

Behavior of Normal Fluids:

A normal fluid is any liquid or gas that behaves according to classical fluid mechanics. Unlike superfluids, it has viscosity, resistance to flow, and obeys Newton’s laws of motion in bulk.

Key Properties:

1.       Viscosity: Normal fluids experience internal friction, which resists flow. For example, honey flows slower than water due to higher viscosity.

2.      Surface Tension: Normal fluids form a meniscus at interfaces and resist deformation, but they cannot climb surfaces indefinitely.

3.      Flow Resistance: When a normal fluid moves through narrow channels, pipes, or thin films, friction slows it down. It cannot flow endlessly in ultra-thin layers.

4.     Energy Dissipation: Waves in normal fluids spread out and gradually lose energy due to friction and turbulence. For instance, water waves eventually flatten due to viscosity and drag.

5.     Gravity Dominance: In normal fluids, macroscopic forces like gravity dominate flow and wave behavior.

Wave Behaviour in Normal Fluids:

·         Wave crests move faster than troughs, causing forward leaning before breaking.

·         Solitons exist in special conditions but are less pronounced than in superfluids.

·         Nonlinear effects are limited by viscosity and turbulence.

Conclusion:

This nanometre-scale “tank” of helium bridges quantum fluid dynamics and classical hydrodynamics, opening a new frontier in the study of nonlinear waves. By recreating complex wave phenomena in a controllable microscopic setting, it offers potential applications in fluid mechanics, energy systems, sensors, and optomechanics.