Visualising Superfluid Turbulence Using an Immiscible Second Component

虽然占据宇宙大部分的气体动力学由粒子之间台球般的碰撞主导，但在绝对零度以上不到百万分之一度的温度下，量子力学接管了。在这里，一类称为玻色子的粒子不再像单个粒子那样行为。而不是相互反弹，数以百万计的玻色子进入同一个量子态，并表现为一个单一的巨大物质波。这种量子流体具有几个显著的特性，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，世界各地有数百个最先进的实验室正在创造和研究量子流体。量子流体的主要优点是，由于其高度的实验控制（实验者能够精确地调整流体的物理性质并在时间和空间中操纵它），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。量子流体也可以用作复杂物理现象的实验平台，如超导体，黑洞和大爆炸。它们还提供了对流体动力学新物理机制的访问，能够研究更广泛的湍流主题，并有可能解决成像量子流体流动的挑战。就像经典流体一样，量子流体中可能存在湍流，尽管这种湍流背后的物理原理略有不同。在经典流体中，漩涡可以有任意的大小和强度-从你清空浴缸时产生的漩涡到木星的红斑。另一方面，在量子流体中，涡旋具有固定的循环量子和固定的涡核尺寸，在2D中形成点，或在3D中形成涡丝。这些量子涡旋是量子流体中湍流的基石;在被赶出平衡的大型系统中，湍流表现为许多排列在复杂缠结中的涡旋。不幸的是，很难可视化量子流体的流动，因为视线成像不能用于在3D中成像复杂的纠缠漩涡分布。虽然在经典流体中，小颗粒可以通过提供流动快照的超快相机来追踪，但这些颗粒比量子涡旋的大小要大得多。结果，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。也有可能创造出量子流体的混合物。这种混合物可以是可混溶的（其中两种组成流体形成均匀混合物）或不可混溶的（其中流体重叠在能量上是不利的），如油和水。在不混溶状态下，如果流体中的一种是大量填充的（多数流体）而另一种是弱填充的（少数流体），则少数流体将填充多数流体的涡核。这提供了一个潜在的路线，以跟踪大多数流体中的涡流，然而，改变少数流体的性质（如添加更多的颗粒到该流体）可以修改涡流的性质，这表明新的制度的涡流动力学和湍流。该奖学金的目的是探索在一系列长度尺度上不混溶的量子流体的性质。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）到许多涡旋的宏观系统（二维Onsager涡旋形成，二维和三维量子湍流）。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来增强，而对潜在流动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.