****NON-TECHNICAL ABSTRACT**** Atoms near a solid surface experience a long range attractive force. Long range forces between molecules are important because they determine macroscopic properties of matter; such as what phase (solid, liquid, gas, etc) is stable at a given temperature and the friction between sliding surfaces. Long range forces also determine the shape of large biologically important molecules such as proteins and DNA. This individual investigator award supports project designed to measure long range forces in some simple model systems using several complementary methods. One method is to expose a solid to a gas and then weigh the amount of adsorbed gas on the surface using a microbalance that can detect fractions of a single atomic layer. Atoms adsorbed on a surface can be considered to be two dimensional matter. They undergo phase transitions such as vaporization and freezing, which are very similar to the familiar three dimensional counterparts. When the adsorbed atoms are very light and weakly interacting (e.g. helium), quantum mechanical effects become important and the adsorbed 2D liquid can also become a superfluid, which means that it can flow forever without any driving force. Theory suggests that for helium-4 on substrates of alkali metals (lithium, sodium, potassium, etc) there are unusual competitions between the liquid-vapor transition and the superfluid-normal fluid transition. This competition will be investigated using both microbalances and reflection of polarized light. Another way to measure surface forces is to use an atomic force microscope (AFM). The forces are measured by monitoring the deflection of a micromachined silicon cantilever with a laser when the cantilever is brought near the surface. Making these measurements with a cantilever immersed in liquid helium is technically challenging, and is one of the goals of this project. Students and post doctoral associates involved with this project will have the opportunity to develop novel instruments, and receive training in a broad spectrum of techniques including cryogenics, high speed imaging, laser optics, and materials preparation. This training will enable them to become productive members of the research community, whether in academia, industrial or government laboratories.
This award supports an individual investigator project with several aims in the general area of fluids, predominately quantum fluids. Wetting and its relationship to superfluid phase transitions when films of helium and helium mixtures are adsorbed on weak and intermediate strength alkali metal and metal oxide substrates will be studied and compared with theory. For another study, low temperature atomic force microscope (AFM) techniques will be developed. The AFM will be optimized to enable measurement of the distance dependence of Casimir forces due to both fluctuations in the electromagnetic field and fluctuations in the order parameter near the onset of superfluidity. Finally a smaller project will investigate the pinch-off phenomena in drops and bubbles to elucidate the connections between the phenomena in conventional viscous fluids and non-Newtonian fluids. The computer code developed for the pinch-off studies will be adapted to study the stability and breakup of multi-electron drops in Helium. Furthermore the motion of superfluid droplets on non-wetting surfaces will be studied. Students and post doctoral associates involved with this project will have the opportunity to develop novel instruments, and receive training in a broad spectrum of techniques including cryogenics, high speed imaging, laser optics, and materials preparation. This training will enable them to become productive members of the research community, whether in academia, industrial or government laboratories.
The activity in my lab during the period of support involves a rather diverse set of topics in the physics of fluids. Some of the main topics include experimental investigations of flows in drop pinch-off and coalescence in Newtonian and non-newtonian fluids, flows in cilia, flows in single nanopores, and development of a low temperature AFM to detect the thermodynamic Casimir force in liquid helium. The main issue in the pinch-off work is to study universal properties of systems undergoing catastrophic qualitative changes; in the case of drops or bubbles, the change is a topological change from one entity to two. During pinch-off, the narrowest connection point shrinks to zero in a way described by a power law in time. The exponent in the power law is universal in the sense that it is independent of the initial conditions and the details of fluid properties. We developed a unique apparatus that allowed us to study bubble pinch-off over a wide range of pressures ( 0 to 100 atm). We found that the pinch-off exponents make an abrupt transition from bubble-like values to liquid like values when the density ratio of the inner and outer fluids exceeds 0.25. This is an unusual example of a change in universality class that can be tuned with a parameter. Flow through nanoscale channels is of current interest because applications to a variety of separation processes. Studies have been done with arrays of many channels, but they are confusing because of the inevitable polydispersity of the channels. We have developed a novel technique to study flow through individual nanopipes. This allows us to observe transitions in flow regimes that were obscured in previous studies and to make direct experimental checks of the flow boundary condition in nanoscale channels. We found that conventional hydrodynamic no slip boundary conditions explain all of our results even for channels down to 30 nm . The position of the cantilever in a conventional room temperature AFM is detected using optical lever technology and split photodiodes, but these devices do not function properly at low temperatures. We are continuing development of a low temperature AFM based on optical fiber interferometry with the near term goal of accomplishing two main experimental milestones: measurements of the thermodynamic Casimir force as a function of distance near the superfluid transition in 4He and measurement of the electrodynamic Casimir force across the superconducting transition in a metal. In both experiments, we will use a cantilever with a 200 micron sphere attached at the end and monitor the deflection in the cantilever as a substrate is brought into proximity. As a preliminary step, we have measured the noise power in the fundamental bending mode of the cantilever as a function of temperature. The project supported the PhD degrees of three students ( Aggelton, Van Cleve and Huisman) and research projects for two Masters students ( Rogerson and Kahlil) and three undergraduates ( Alison , Miller and Song). Our lab has been an enthusiastic participant in outreach at the high school level. One of my former students is a physics teacher at Rancho Santa Margarita high school, and my grad students and I have visited his classes and presented demonstrations to the students. I also became acquainted with Michael Towne, who teaches AP physics at Citrus Hill high school. We hosted a visit of 15 of his students to spend a day at UCI visiting labs and participating in hands-on activities. Citrus Hill is a largely Hispanic high school near Riverside. Towne is a very charismatic teacher, and he tells me that 25% of all the Hispanic kids taking AP physics in California are in his class. In these high school visits, in addition to explaining the science we do, I make a point of discussing the advantages of STEM careers and what the students need to do in high school and the first 2 years of college to prepare for them. Both I and my grad students enjoy these interactions and we plan to continue them.
