Drops, Bubbles and Wetting Phenomena in Superfluid and Normal Helium

摘要：本研究的主要目标是利用超流体和正常氦来研究液滴在特殊制备的铯弱基板上的运动，并使用一种称为椭圆偏振的光学探针技术来研究分子和固体表面之间的润湿力和远程力。将建造一个装置，该装置将为基温约为0.35 K的实验细胞提供光学通道。由于表面力对表面污染特别敏感，因此将购买高功率脉冲激光器，用于在低温下原位清洁基材，并使用激光烧蚀形成新的高质量基材。在液滴研究中，超流体液滴在这些表面上的运动将使用高速视频进行观察。实验将测量作用在超流体液滴上的净力，并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膜的生长和润湿，特别是那些传统微平衡方法无法达到的衬底，包括大块铷和锂。该椭偏仪还将用于监测传统强光衬底上的氦膜厚度，这些衬底可以用原子平面制备，如切割碱卤化物晶体、石墨、云母和金。这些实验的目的是消除表面粗糙度对控制薄膜厚度的微妙力测量的影响。在所有这些情况下，基材将使用激光脉冲传递的能量进行清洁或沉积。激光烧蚀也将作为一种用单层氧化铯生产铯的方法进行探索，这是一种候选的超弱衬底。技术摘要：本研究的主要目的是利用超流体和正常氦来研究液滴在静止表面上的运动，并利用椭偏仪来研究各种基底上的润湿力和远程力。将建造一个装置，该装置将为基温约为0.35 K的实验细胞提供光学通道。购买高功率脉冲激光器，用于低温原位清洗基片，并利用激光烧蚀形成新型高质量基片。在液滴研究中，目标将是使铯表面具有前进和后退接触角的有限值，并使用高速视频观察超流体液滴在这些表面上的运动。实验将测量作用在超流体液滴上的净力，并确定超流体液滴的低速运动是本质上无耗散，还是运动接触线产生固有耗散。我们最近开发的低温椭偏仪将用于监测各种衬底上氦膜的生长和润湿，特别是那些传统微平衡方法无法达到的衬底，包括大块铷和锂。在这类中等强度衬底上，超流和预润湿表现为纠缠态，超流过渡不能用标准Kosterlitz-Thouless模型很好地描述。该椭偏仪还将用于监测传统强光衬底上的氦膜厚度，这些衬底可以用原子平面制备，如切割碱卤化物晶体、石墨、云母和金。这些实验的目标是消除表面粗糙度对卡西米尔力测量的影响，以及其他可能对薄膜自由能有有限贡献的有限尺寸。在所有这些情况下，基材将使用激光脉冲传递的能量进行清洁或沉积。激光烧蚀也将作为一种用单层氧化铯生产铯的方法进行探索，这种单层氧化铯具有最低的已知功函数，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.