Visualising Superfluid Turbulence Using an Immiscible Second Component

虽然占据宇宙大部分的气体动力学是由粒子之间类似台球的碰撞主导的，但在绝对零度以上不到百万分之一度的温度下，量子力学就会起作用。在这里，一类叫做玻色子的粒子不再像单个粒子那样运动。数以百万计的玻色子不是相互碰撞，而是进入相同的量子态，表现得像一个单一的巨大物质波。这种量子流体有几个显著的特性，包括其无粘性流动的能力。量子流体是一个活跃的研究领域，世界上有数百个最先进的实验室在创造和研究量子流体。量子流体的主要优势在于，由于其高度的实验控制（实验学家能够精确地调整流体的物理性质并在时间和空间上操纵它），它们是实现多粒子量子系统的“干净”方式，为量子世界提供了丰富的见解。量子流体还可以作为超导体、黑洞和宇宙大爆炸等复杂物理现象的试验台。它们还提供了流体动力学的新物理机制，研究湍流中更广泛主题的能力，以及解决成像量子流体流动挑战的潜力。就像在经典流体中一样，量子流体中也可能存在湍流，尽管这种湍流背后的物理原理略有不同。在经典的流体中，漩涡可以有任意的大小和强度——从清空浴缸时产生的漩涡，到木星上的红斑。另一方面，在量子流体中，漩涡具有固定的循环量子和固定的涡核大小，在二维中形成一个点，在三维中形成一个涡丝。这些量子漩涡是量子流体中湍流的基石；在失去平衡的大型系统中，湍流表现为许多旋涡以复杂的纠缠形式排列。不幸的是，很难将量子流体的流动可视化，因为视线成像不能用于在3D中对纠缠漩涡的复杂分布进行成像。虽然在经典流体中，小颗粒可以通过提供流动快照的超快速相机来追踪，但这些颗粒的大小远远大于量子漩涡的大小。结果，这些粒子改变了流体的动力学，抑制了湍流，并模糊了它们试图可视化的流动。制造量子流体的混合物也是可能的。这种混合物可以是混相的（两种组成流体形成均匀的混合物）或不混相的（在能量上不利于流体重叠），如油和水。在非混相状态下，如果其中一种流体密度大（多数流体），而另一种流体密度小（少数流体），则少数流体将填充多数流体的涡旋核。这为追踪大多数流体中的涡流提供了一条潜在的途径，然而，改变少数流体的特性（例如向该流体中添加更多的颗粒）可以改变涡流的特性，从而提出涡流动力学和湍流的新体制。该奖学金的目的是探索跨一系列长度尺度的非混相量子流体的性质。我将从一个或几个涡旋的微观尺度（二维涡旋对和涡旋成核的动力学，三维涡旋重连和开尔文波级联）建立到许多涡旋的宏观系统（二维的Onsager涡旋形成，二维和三维的量子湍流）。这些结果与当前最先进的实验的相关性将通过与世界领先的实验学家的合作而增强，而对势流可视化应用的预测将为未来的冷原子实验提供信息。

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.