Wesley P. Hoffman

Utilization of Surface Tension and Wettability in the Design and Operation of Microsensors

Abstract The behavior of fluid droplets contained within shaped capillaries and voids can be utilized to convert forces acting upon the droplets to observable displacements which accurately and reproducibly quantify these forces. The position of droplets within such micro-sensors is governed by surface tension, wettability, and the geometric configuration of the confining walls. These micro-sensors have no mechanical moving parts to wear out and are inherently immune to many orders-of-magnitude over-actuation. If non-wetting fluids are employed, the micro-sensors can also be made to operate as micro-valves, micro-switches, optical micro-shutters and irises, as well as other devices. Both circular and non-circular confining structures can be employed.

Surface characterization of a graphitized rayon fabric

Abstract The surface characteristics of an unsized graphitized rayon fabric, WCA, were studied. The “true” and apparent active surface areas, ASA, were determined by volumetric oxygen chemisorption at 573 K and starting oxygen pressures of 5, 50, or 200 Torr. The apparent ASA of the fabric is substantially dependent on exposure time, sample size, reactor pressure, and the time selected to evacuate the sample after the chemisorption step. The normal procedure developed for measuring the ASA of graphitized carbon blacks is not applicable to the fabric; the procedure substantially underestimates the ASA of the fabric. From the present and previous work, it is concluded that for each carbon fiber or fabric, the experimental parameters for measuring ASA have to be tailored differently to yield a true value of ASA. The adsorption of Kr at 76.6 K on three fabric samples was performed to determine the surface area and elucidate the effect of heat treatment and length of oxidation on the nature of the surface and on developing porosity. Evacuating the surface of the as-received fabric successively at 373, 473, 573, and 673 K slightly increased the surface area but substantially changed the surface characteristic from nongraphitic to graphitic. Oxidation of the fabric at 573 K for extended periods of time removed a surface layer, developed some porosity, and revealed a less graphitic structure. Evacuation of the sample from 373 to 773 K in 100 K increments opened the porosity. Further evacuation of the sample from 773 K to 1273 K lowered the surface area, closed the porosity, and started changing the surface from nongraphitic to graphitic.

Hydraulic Actuation Based on Flow of Non-wetting Fluids in Micro-channels

The behavior of non-wetting fluids in micro-channels can be utilized to create an unusual form of micro-hydraulic technology that enables fabrication of various kinds of micro-actuators and micro-bearings. In addition, this same technology can be used to construct micro-pumps capable of generating flows of wetting fluids in micro-channels and to manipulate and control these flows.

Material Science of Carbon

Carbon is a ubiquitous material that is essential for the functioning of modern society. Because carbon can exist in a multitude of forms, it can be tailored to possess practically any property that might be required for a specific application. The list of applications is very extensive and includes: aircraft brakes, electrodes, high temperature molds, rocket nozzles and exit cones, tires, ink, nuclear reactors and fuel particles, filters, prosthetics, batteries and fuel cells, airplanes, and sporting equipment.

Processing of a High Temperature Imide Copolymer into Hollow Fibers

Abstract Hollow fibers spun from synuietic fiber forming polymers have been of interest, especially for producing high-bulk, low-density fabrics. Such fibers produced from a high temperature resistant polymer provide a number of advantages in certain applications. Ultem, a polyetherimide resin marketed by General Electric company, is a copolymer with ether molecules between imide groups and is thermoplastic. Being an amorphous thermoplastic polyimide, the Ultem resin combines the high performance associated with exotic specialty polymers and the good processability of typical engineering plastics. Equipment was built at the Air Force Research Laboratory of Edwards AFB, CA to spin hollow fibers. The spinneret designed has a hollow tube supported in the center of the orifice, and an inert gas is injected through the needle to maintain the tubular shape until the solidification of the fiber. Using that set-up, fibers were spun from Ultem under several different conditions. Effect of some of the processing conditions on the evolution of structure and properties during hollow fiber formation is discussed.

Microtubes— An Enabling Technology for Numerous Fields

Abstract : The intent of this article was to acquaint the reader with some of the basic capabilities of microtube technology and was not intended to be application specific. We are currently working with industrial and academic partners to develop application specific microtube technology through cooperative research and development agreements (GRDA). We welcome the opportunity to develop additional technology with future partners.