Drops, Bubbles and Wetting Phenomena in Superfluid and Normal Helium

非技术性摘要：这项研究的主要目标是使用超流和正常氦来研究液滴在由铯组成的特殊准备的弱衬底上的运动，并使用一种名为椭偏测量的光学探针技术来研究分子与固体表面之间的润湿和长程作用力。将建造一种设备，提供对基础温度约为0.35K的实验电池的光学访问。由于表面力对表面污染特别敏感，将购买一台高功率脉冲激光，用于在低温下就地清洁衬底，并使用激光烧蚀形成新的高质量衬底。在液滴研究中，将使用高速视频观察超流体液滴在这些表面上的运动。这些实验将测量超流体液滴的净力，并将确定超流体液滴是否以零摩擦运动。我们最近开发的低温椭偏仪将用于监测各种衬底上氦薄膜的生长和润湿情况，特别是那些传统的微天平方法无法获得的衬底，包括块状Rb和锂。椭偏仪还将用于监测传统坚固衬底上的氦膜厚度，这些衬底可以用原子平坦的表面制备，如解理的碱卤化物晶体、石墨、云母和金。这些实验的目的将是消除表面粗糙度对控制薄膜厚度的微细力测量的影响。在所有这些情况下，将使用激光脉冲传递的能量来清洁或沉积衬底。激光烧蚀也将被探索为一种利用单层氧化铯产生铯的方法，这是一种可用于超弱衬底的候选方法。技术摘要：本研究的主要目标是使用超流和正常氦来研究液滴在衬底表面的运动，并使用椭偏测量来研究各种衬底上的润湿和长程作用力。将建造一种装置，提供对基础温度约为0.35K的实验池的光学访问，并将购买高功率脉冲激光，用于在低温下原位清洁衬底，并使用激光烧蚀形成新的高质量衬底。在液滴研究中，目标是使铯表面具有有限的前进和后退接触角，并使用高速视频观察超流体液滴在这些表面上的运动。这些实验将测量超流液滴上的净力，并将确定超流液滴的低速运动本质上是无耗散的，还是移动的接触线产生本征耗散。我们最近开发的低温椭偏仪将用于监测各种衬底上氦薄膜的生长和润湿情况，特别是那些传统的微天平方法无法获得的衬底，包括块状Rb和锂。在这种中等强度的衬底上，超流和预润湿似乎是纠缠在一起的，标准的Kosterlitz-Thouless模型不能很好地描述超流转变。椭偏仪还将用于监测传统坚固衬底上的氦膜厚度，这些衬底可以用原子平坦的表面制备，如解理的碱卤化物晶体、石墨、云母和金。这些实验的目的将是消除表面粗糙度对测量Casimir力的影响，以及其他可能的有限尺寸对薄膜自由能的影响。在所有这些情况下，将使用激光脉冲传递的能量来清洁或沉积衬底。激光烧蚀也将被探索为一种用氧化铯单层来生产铯的方法，氧化铯是已知功函数最低的，是超弱衬底的候选材料。

NON-TECHNICAL ABSTRACT:The primary goals of this research are to use superfluid and normal helium to investigate the motion of droplets on specially prepared weak substrates made of cesium and to use an optical probe technique called ellipsometry to study wetting and long range forces between a molecule and a solid surface. An apparatus will be constructed that will provide optical access to an experimental cell with a base temperature of approximately 0.35 K. Since surface forces are particularly sensitive to surface contamination, a highpower pulsed laser will be purchased and used to clean substrates in situ at low temperature and to form novel high quality substrates using laser ablation. In the droplet studies, the motion of superfluid drops on these surfaces will be observed using high speed video. The experiments will measure the net force on a superfluid droplet and will determine whether superfluid drops move with zero friction. Our recently developed cryogenic ellipsometer will be used to monitor the growth and wetting of helium films on a variety of substrates, particularly those that are inaccessible to conventional microbalance methods, including bulk rubidium and lithium. The ellipsometer will also be used to monitor the helium film thickness on conventional strong substrates, which can be prepared with atomically flat surfaces, such as cleaved alkali halide crystals, graphite, mica and gold. The goal of these experiments will be to eliminate the effects of surfaceroughness on measurements of delicate forces which control the thickness of thin films. In all of these cases, substrates will be cleaned or deposited using energy delivered by a laser pulse. Laser ablation will also be explored as a method of producing cesium with a monolayer of cesium oxide, which is a candidate for a superweak substrate.TECHNICAL ABSTRACT:The primary goals of this research are to use superfluid and normal helium to investigate the motion of droplets on cesiated surfaces and to use ellipsometry to study wetting and long range forces on a variety of substrates. An apparatus will be constructed that will provide optical access to an experimental cell with a base temperature of approximately0.35 K. A high power pulsed laser will be purchased and used to clean substrates in situ at low temperature and to form novel high quality substrates using laser ablation. In the droplet studies, the goal will be to make cesium surfaces with finite values of both the advancing and receding contact angle and to observe the motion of superfluid drops on these surfaces using high speed video. The experiments will measure the net force on a superfluid droplet and will determine whether low velocity motion of superfluid drops is essentially dissipationless or if moving contact lines generate intrinsic dissipation. Our recently developed cryogenic ellipsometer will be used to monitor the growth and wetting of helium films on a variety of substrates, particularly those that are inaccessible to conventional microbalance methods, including bulk rubidium and lithium. On this type of intermediate strength substrate, superfluidity and prewetting appear to be entangled, and the superfluid transition is not well described by the standard Kosterlitz-Thouless model. The ellipsometer will also be used to monitor the helium film thickness on conventional strong substrates, which can be prepared with atomically flat surfaces, such as cleaved alkali halide crystals, graphite, mica and gold. The goal of these experiments will be to eliminate the effects of surface roughness on measurements of the Casimir force and other possible finite size contributions to the free energy of a thin film. In all of these cases, substrates will be cleaned or deposited using energy delivered by a laser pulse. Laser ablation will also be explored as a method of producing cesium with a monolayer of cesium oxide, which has the lowest known work function and is a candidate for a superweak substrate.