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 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 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 surface roughness 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.
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 approximately 0.35 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.
