Porous substrates are frequently used to explore fluids in confined geometries. Two examples of these which frequently come up in my field of Nuclear Magnetic Resonance (NMR) studies of helium are from the far ranges of applied science to pure physics, and they are only different aspects of the same experimental conditions: there are an abundance of studies of how fluids flow through porous media and how that pertains to removing crude oil from rocks, and there are also many studies of nearly ideal two-dimensional systems which are used to test the very basic theories of physics. In the former, the porous medium is used as a model of the complexity of a rock, while in the latter it is used to get a lot of surface area into a very small volume. Porous media play a significant role in many studies of physics, and it is important to understand them before we can understand the behavior of fluids on and in them.
There is a range of types of porous media. Most are plastics or glasses: substances which will not have significant chemical or physical effect on the various adsorbates being studied in them. The aspect most desired is a well understood geometry with varying degrees of regularity of pore shapes, sizes and orientations, depending on how complicated or simple a substrate one requires. A porous plastic with one of the simpler geometries which is frequently used is the brand Nuclepore. I have done NMR and superfluidity studies of helium on Nuclepore for my graduate work during the last six years. There are numerous other experimental research labs and theoretical groups who work on calculations of the properties of various substances (often helium) on Nuclepore. Since so much work is done on Nuclepore, it is easy to compare data and make many conclusions, but surprisingly, the Nuclepore has never been very well characterized. There is much disagreement between the surface area measurements and the strength of the binding (the van der Waals constant) between helium and the substrate itself. The characterization of a surface to obtain these numbers uses a standard technique called a BET isotherm. In this process, small amounts of a gas are admitted onto the substrate while carefully keeping the temperature constant and measuring the pressure. The isotherm is usually carried out at 77 degrees kelvin with nitrogen as the adsorbate. If the isotherm is instead done at 4 kelvin with helium as the adsorbate, it is expected that the measure will be more accurate because the ordering of the helium atoms is more regular than the nitrogen.
We are working to build an isotherm apparatus, and to make measurements of both helium and nitrogen isotherms in order to compare the two methods and to increase the accuracy of the known surface area and van der Waals constant. This is just the first in a series of low temperature experiments that can be run in this apparatus. We plan to continue studies of adsorbates and substrates in order to better characterize surfaces, question the assumptions of the BET isotherm, and to further explore interesting phenomena which are not yet understood.