Phase Transitions of Weakly Physisorbed Films

9705270科尔这项重点研究小组资助了一项关于弱物理吸附薄膜相变的理论-实验合作研究。研究的重点是惰性气体氦的薄膜，包括质量为4和3的同位素混合物，它们被吸附在提供最弱结合的衬底上。这种底物包括碱金属和分子氢。这种选择是为了更清楚地显示在亚单层二维薄膜中的超流开始和相分离领域的新行为，在润湿行为的变化中，以及在非常厚的薄膜的吸附方面。在所有情况下，研究都是协作的，涉及实验、理论和模拟之间的紧密耦合。实验方法包括介电常数、体积、毛细上升、扭转振荡器、热容和石英微天平。该理论将涉及薄膜动力学、统计力学、范德华力和势的计算。宏正则模拟将主要针对润湿问题。%这一重点研究小组资助了一项关于气体分子薄膜与几种类型的衬底的粘附性的合作研究，并对分子润湿表面的能力感兴趣。研究中的气体是相对惰性的惰性气体，特别是质量为三个和四个原子质量单位的氦原子，研究是在绝对零度附近进行的，在这种温度下，氦原子可能会以几种不同类型的超流体状态出现。在上述情况下，超流状态是指氦的运动完全不受其与周围容器或衬底的相互作用的阻碍。例如，如果液体超流体旋转(就像玻璃杯中旋转的水一样)，旋转运动将永远持续下去，这一效应是陀螺仪探测极慢旋转的基础。这项拟议的研究将阐明衬底对超流体行为首次发生的温度的影响，并将探测到平均不到一个原子层厚度的层。这些结果可能导致在各种情况下有用的新见解，如化学工程、聚合物科学、催化和其他润湿现象很重要的领域。这些结果还将有助于对原子和分子对表面进行单层和亚单层覆盖的情况的一般知识。除了可能在单层润湿和相变方面取得的发现外，美国的科学和工程研究基地还将从该项目的研究生培训中受益匪浅。***

9705270 Cole This Focused Research Group grant supports a collaborative theoretical-experimental study of phase transitions of weakly physisorbed films. The investigations focus on films of the noble gas helium, including mixtures of the isotopes of masses four and three, adsorbed on the substrates which offer the weakest possible binding. Such substrates include alkali metals and molecular hydrogen. This choice is made to more clearly display novel behaviors in the areas of superfluid onset and phase separation in submonolayer two dimensional films; in the varieties of wetting behavior; and in the adsorption of very thick films. In all cases the research is collaborative, involving close coupling between experiment, theory, and simulation. The experimental methods include dielectric constant, volumetric, capillary rise, torsional oscillator, heat capacity and quartz microbalance measurements. The theory will deal with film dynamics, statistical mechanics, van der Waals forces, and calculation of potentials. Grand canonical simulations will be directed primarily toward the wetting problem. %%% This Focused Research Group grant supports a collaborative study of the adherence of thin films of gas molecules to several types of substrates, with an interest in the ability of the molecules to wet the surfaces. The gases under study are relatively inert noble gases, specifically helium atoms of masses three and four atomic mass units, and the studies are carried out at temperatures near the absolute zero, where the helium atoms can appear in several different types of superfluid state. A superfluid state is one in which the motion of the helium is not at all impeded by its interaction with the surrounding container or substrate, in the case mentioned above. If, for example, the liquid superfluid is put into rotation (like swirling water in a glass) the swirl ing motion will persist forever, and this effect is the basis for a gyroscope which can detect extremely slow rotations. The proposed research will clarify the effects of the substrate on the temperature at which the superfluid behavior first occurs, and will probe these behaviors down to layers less than one atomic layer thick, on average. The results may lead to new insights useful in the variety of situations, such as chemical engineering, polymer science, catalysis, and others where wetting phenomena are important. The results will also contribute to general knowledge of situations where monolayer and submonolayer coverages of surfaces by atoms and molecules occur. The nation's science and engineering research base will benefit strongly from the training of graduate students in this program, in addition to the discoveries which may be made on monolayer wetting and phase transitions. ***