While the dynamics of gases which occupy the majority of the universe are dominated by billiard ball-like collisions between particles, at temperatures which are less than a millionth of a degree above absolute zero quantum mechanics takes over. Here, a class of particles called bosons cease to behave like individual particles. Rather than bouncing off each other, the millions of bosons present enter the same quantum state, and behave as one, single, giant wave of matter. This quantum fluid has several remarkable properties, including its ability to flow without viscosity.
Quantum fluids are an active area of research, with hundreds of state-of-the-art laboratories around the world creating and studying quantum fluids. The main advantage of quantum fluids is that, due to their high degree of experimental control (experimentalists are able to precisely tune the fluid's physical properties and manipulate it in time and space), they are a "clean" way to realise a many-particle quantum system, giving rich insight into the quantum world. Quantum fluids can also be used as a testbed for complicated physical phenomena such as superconductors, black holes, and the Big Bang. They also offer access to new physical regimes of fluid dynamics, the ability to study broader topics in turbulence, and the potential to solve challenges in imaging quantum fluid flows.
Just like in classical fluid, it is possible to have turbulence in quantum fluids, although the physics behind this turbulence are subtly different. In classical fluids, a vortex can have an arbitrary size and strength - from the whirlpool created when you empty the bath, to the red spot of Jupiter. In quantum fluids, on the other hand, vortices have a fixed quanta of circulation and a fixed vortex core size, forming a point in 2D, or a vortex filament in 3D. These quantum vortices are the building blocks of turbulence in quantum fluids; in large systems which are driven out of equilibrium, turbulence manifests itself as many vortices arranged in a complex tangle.
Unfortunately it is difficult to visualise the flow of quantum fluids, since line-of-sight imaging can't be used to image a complex distribution of tangled vortices in 3D. While in classical fluids small particles can be traced by ultra-fast cameras providing snapshots of the flow, these particles are much larger than the size of a quantum vortex. As a result, the particles alter the dynamics of the fluid, suppressing turbulence, and obscuring the flow that they are attempting to visualise.
It is also possible to create a mixture of quantum fluids. This mixture may be miscible (where the two constituent fluids form a homogeneous mixture) or immiscible (where it is energetically unfavourable for the fluids to overlap), like oil and water. In the immiscible regime, if one of the fluids is heavily populated (the majority fluid) and the other is weakly populated (the minority fluid) , the minority fluid will in-fill the vortex cores of the majority fluid. This provides a potential route to tracing the vortices in the majority fluid, however changing properties of the minority fluid (such as adding more particles to this fluid) can modify the properties of the vortex, suggesting new regimes of vortex dynamics and turbulence.
The aim of this fellowship is to explore the nature of immiscible quantum fluids across a range of length scales. I will build up from the microscopic scale of one or a few vortices (dynamics of vortex pairs and vortex nucleation in 2D, vortex reconnections and Kelvin wave cascades in 3D) to macroscopic systems of many vortices (Onsager vortex formation in 2D, quantum turbulence in 2D and 3D). The relevance of these results to current state-of-the-art experiments will be enhanced by collaborations with world leading experimentalists, while predictions on potential flow visualisation applications will inform future cold atom experiments.
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