A. Neal Watkins

Portable, Low-Cost, Solid-State Luminescence-Based O2 Sensor

A novel sensor for quantifying molecular O2 based entirely on solid-state electronics is presented. The sensor is based on the luminescence quenching of tris(4,7-diphenyl-1, 10-phenanthroline)ruthenium(II) ([Ru(dpp)3]2+) by molecular O2. The sensor involves immobilizing the ruthenium complex within a porous sol-gel-processed glass film and casting this film directly onto the surface of a blue quantum-well light-emitting diode (LED). The ruthenium complex is excited by the LED, the [Ru(dpp)3]2+ emission is filtered from the excitation with a low-cost acrylic color filter, and the emission is detected with an inexpensive silicon photodiode. The sensor response to gaseous O2 and dissolved O2 in water is presented. The sensor exhibits fast response times and good reversibility, and detection limits are 0.5%, 0.02%, and 110 ppb, respectively, for O2 in the gaseous (linear Stern–Vobner and multi-site Stern–Volmer analysis) and aqueous phase. This sensor provides a cost-effective alternative to traditional electrochemical-based O2 sensing and also provides a platform for other optically based sensors.

Effects of Processing Temperature on the Oxygen Quenching Behavior of Tris(4,7′-diphenyl-1,10′-phenanthroline) Ruthenium (II) Sequestered Within Sol-Gel-Derived Xerogel Films

Sol-gel processing methods offer novel pathways for tailoring glasses. Amongst the issues that have received the least attention are the effects of the curing temperature on the behavior and photophysics of a dopant molecule sequestered within a sol-gel-derived xerogel. Of particular interest to our group are the effects of processing variables on the ability of a dopant molecule, that is sequestered within a xerogel glass, to be accessed by an analyte and the distribution of the dopant sites within the xerogel. The thermal stability of the luminophore tris(4,7′-diphenyl-1,10′-phenanthroline) ruthenium (II) ([Ru(dpp)3]2+) provides a convenient way to address these issues and develop an understanding of how one might best exploit curing temperature to construct improved chemical sensors. This paper focuses on quantifying how the film curing temperature affects the spectroscopy and O2 quenching of ([Ru(dpp)3]2+) sequestered within sol-gel-derived xerogel thin films. Our quenching data on films once they have been cured demonstrate that there is a dramatic increase in the sensitivity of the ([Ru(dpp)3]2+) molecules to O2 quenching when the films have been cured at elevated temperatures. This arises primarily because there are two main types of ([Ru(dpp)3]2+) microenvironments within the glass and higher temperature curing leads to an increase in the bimolecular quenching rate between O2 and ([Ru(dpp)3]2+). This is accomplished as follows. Below a curing temperature of 100–150°C, ∼15% of the xerogel-doped ([Ru(dpp)3]2+) molecules are not accessed to any detectable degree by the O2 molecules during the ([Ru(dpp)3]2+) excited-state luminescence lifetime. However, as the xerogel is cured at or above 150°C, residual silanol-bound waters (or other impurities) dissociate from the xerogel and those ([Ru(dpp)3]2+) molecules that were initially inaccessible become accessible to O2. The dissociation of these water molecules, plus other events, also causes the originally inaccessible ([Ru(dpp)3]2+) population to ultimately exhibit a quenching rate that is greater than the fraction of initially accessible ([Ru(dpp)3]2+) molecules that were formed under ambient curing conditions.

Hypersonic Laminar Boundary Layer Velocimetry with Discrete Roughness on a Flat Plate

Laminar boundary layer velocity measurements are made on a 10-degree half-angle wedge in a Mach 10 flow. Two types of discrete boundary layer trips were used to perturb the boundary layer gas. The first was a 2-mm tall, 4-mm diameter cylindrical trip. The second was a scaled version of the Orbiter Boundary Layer Transition (BLT) Detailed Test Objective (DTO) trip. Both 1-mm and 2.5-mm tall BLT DTO trips were tested. Additionally, side-view and plan-view axial boundary layer velocity measurements were made in the absence of these tripping devices. The free-stream unit Reynolds numbers tested for the cylindrical trips were 1.7x10 6 m -1 and 3.3x10 6 m -1 . The free-stream unit Reynolds number tested for the BLT DTO trips was 1.7x10 6 m -1 . The angle of attack was kept at approximately 5-degrees for most of the tests resulting in a Mach number of approximately 8.3. These combinations of unit Reynolds numbers and angle of attack resulted in laminar flowfields. To study the precision of the measurement technique, the angle of attack was varied during one run. Nitric-oxide (NO) molecular tagging velocimetry (MTV) was used to obtain averaged axial velocity values and associated uncertainties. These uncertainties are as low as 20 m/s. An interline, progressive scan CCD camera was used to obtain separate images of the initial reference and shifted NO molecules that had been tagged by the laser. The CCD configuration allowed for sub-microsecond sequential acquisition of both images. The maximum planar spatial resolution achieved for the side-view velocity measurements was 0.07-mm in the wall-normal direction by 1.45-mm in the streamwise direction with a spatial depth of 0.5-mm. For the plan-view measurements, the maximum planar spatial resolution in the spanwise and streamwise directions was 0.69-mm by 1.28-mm, respectively, with a

Experimental Measurement of RCS Jet Interaction Effects on a Capsule Entry Vehicle

An investigation was made in NASA Langley Research Center s 31-Inch Mach 10 Tunnel to determine the effects of reaction-control system (RCS) jet interactions on the aft-body of a capsule entry vehicle. The test focused on demonstrating and improving advanced measurement techniques that would aid in the rapid measurement and visualization of jet interaction effects for the Orion Crew Exploration Vehicle while providing data useful for developing engineering models or validation of computational tools used to assess actual flight environments. Measurements included global surface imaging with pressure and temperature sensitive paints and three-dimensional flow visualization with a scanning planar laser induced fluorescence technique. The wind tunnel model was fabricated with interchangeable parts for two different aft-body configurations. The first, an Apollo-like configuration, was used to focus primarily on the forward facing roll and yaw jet interactions which are known to have significant aft-body heating augmentation. The second, an early Orion Crew Module configuration (4-cluster jets), was tested blowing only out of the most windward yaw jet, which was expected to have the maximum heating augmentation for that configuration. Jet chamber pressures and tunnel flow conditions were chosen to approximate early Apollo wind tunnel test conditions. Maximum heating augmentation values measured for the Apollo-like configuration (>10 for forward facing roll jet and 4 for yaw jet) using temperature sensitive paint were shown to be similar to earlier experimental results (Jones and Hunt, 1965) using a phase change paint technique, but were acquired with much higher surface resolution. Heating results for the windward yaw jet on the Orion configuration had similar augmentation levels, but affected much less surface area. Numerical modeling for the Apollo-like yaw jet configuration with laminar flow and uniform jet outflow conditions showed similar heating patterns, qualitatively, but also showed significant variation with jet exit divergence angle, with as much as 25 percent variation in heat flux intensity for a 10 degree divergence angle versus parallel outflow. These results along with the fabrication methods and advanced measurement techniques developed will be used in the next phase of testing and evaluation for the updated Orion RCS configuration.

Using Pressure- and Temperature-Sensitive Paint for Global Surface Pressure and Temperature Measurements on the Aft-Body of a Capsule Reentry Vehicle

Pressure Sensitive Paint (PSP) and Temperature Sensitive Paint (TSP) were used to visualize and quantify the surface interactions of reaction control system (RCS) jets on the aft body of capsule reentry vehicle shapes. The first model tested was an Apollo-like configuration and was used to focus primarily on the effects of the forward facing roll and yaw jets. The second model tested was an early Orion Crew Module configuration blowing only out of its forward-most yaw jet, which was expected to have the most intense aerodynamic heating augmentation on the model surface. This paper will present the results from the experiments, which show that with proper system design, both PSP and TSP are effective tools for studying these types of interaction in hypersonic testing environments.

Deployment of a Pressure Sensitive Paint System for Measuring Global Surface Pressures on Rotorcraft Blades in Simulated Forward Flight

This report will present details of a Pressure Sensitive Paint (PSP) system for measuring global surface pressures on the tips of rotorcraft blades in simulated forward flight at the 14- x 22-Foot Subsonic Tunnel. The system was designed to use a pulsed laser as an excitation source and PSP data was collected using the lifetime-based approach. With the higher intensity of the laser, this allowed PSP images to be acquired during a single laser pulse, resulting in the collection of crisp images that can be used to determine blade pressure at a specific instant in time. This is extremely important in rotorcraft applications as the blades experience dramatically different flow fields depending on their position in the rotor disk. Testing of the system was performed using the U.S. Army General Rotor Model System equipped with four identical blades. Two of the blades were instrumented with pressure transducers to allow for comparison of the results obtained from the PSP. This report will also detail possible improvements to the system.

The Development and Implementation of a Cryogenic Pressure Sensitive Paint System in the National Transonic Facility

The Pressure Sensitive Paint (PSP) method was used to measure global surface pressures on a model at full-scale flight Reynolds numbers. In order to achieve these conditions, the test was carried out at the National Transonic Facility (NTF) operating under cryogenic conditions in a nitrogen environment. The upper surface of a wing on a full-span 0.027 scale commercial transport was painted with a porous PSP formulation and tested at 120K. Data was acquired at Mach 0.8 with a total pressure of 200 kPa, resulting in a Reynolds number of 65 x 106/m. Oxygen, which is required for PSP operation, was injected using dry air so that the oxygen concentration in the flow was approximately 1535 ppm. Results show qualitative agreement with expected results. This preliminary test is the first time that PSP has been successfully deployed to measure global surface pressures at cryogenic condition in the NTF. This paper will describe the system as installed, the results obtained from the test, as well as proposed upgrades and future tests.

Flow Visualization at Cryogenic Conditions Using a Modified Pressure Sensitive Paint Approach

A modification to the Pressure Sensitive Paint (PSP) method was used to visualize streamlines on a Blended Wing Body (BWB) model at full-scale flight Reynolds numbers. In order to achieve these conditions, the tests were carried out in the National Transonic Facility operating under cryogenic conditions in a nitrogen environment. Oxygen is required for conventional PSP measurements, and several tests have been successfully completed in nitrogen environments by injecting small amounts (typically < 3000 ppm) of oxygen into the flow. A similar technique was employed here, except that air was purged through pressure tap orifices already existent on the model surface, resulting in changes in the PSP wherever oxygen was present. The results agree quite well with predicted results obtained through computational fluid dynamics analysis (CFD), which show this to be a viable technique for visualizing flows without resorting to more invasive procedures such as oil flow or minitufts.

Tracking Nanosecond and Subnanosecond Protein Dynamics On-the-Fly Using Frequency-Domain Fluorescence

Fluorescence anisotropy and intensity decay experiments on proteins can provide detailed information on biomolecule dynamics and function. However, experiments of this sort are normally performed while the biomolecule is at or near equilibrium. Although information on protein dynamics under equilibrium conditions is extremely important, details about the protein behavior while it is actually undergoing change can provide significantly more insight into the overall protein behavior. Multiharmonic Fourier frequency-domain fluorescence provides a means to acquire fluorescence anisotropy and intensity decay information on a reasonably rapid time scale. As a result, one can potentially track protein nanosecond and subnanosecond dynamical processes on-the-fly as they undergo change(s) during, for example, protein–ligand binding, enzymatic reactions, or antigen/hapten–antibody association. To illustrate the potential of the frequency-domain on-the-fly methodology, we report here on the behavior of a model protein, bovine serum albumin, that has been labeled site-selectively with the fluorescent probe acrylodan (BSA-Ac). Conformational changes in the BSA-Ac are effected by using trypsin or β-mercaptoethanol (BME). BME is a disulfide interchange reagent, and trypsin cleaves and excises from the entire BSA molecule a 21 amino acid peptide segment that contains the covalently attached Ac residue. This paper focuses on the time course of the fluorescence anisotropy and intensity decay kinetics of BSA-Ac as it reacts with trypsin or BME.

Effects of Fluorescent Reporter Group Structure on the Dynamics Surrounding Cysteine-26 in Spinach Calmodulin: A Model Biorecognition Element:

Conformational changes associated with the Ca2+-dependent activation of spinach calmodulin (CaM) have been assessed in aqueous solution by using steady-state and frequency-domain fluorescence spectroscopy of acrylodan-, fluorescein-, and tetramethylrhodamine-labeled CaM. Spinach CaM was site-selectively labeled at cysteine-26 so we could study the dynamics at a well-defined location within the protein. By using multiple fluorophores attached to the same site, we determined the Ca2+-dependent changes in the CaM global rotational dynamics, and also determined how the local fluorophore dynamics were affected by the fluorophore polarity, size, and charge. Upon binding Ca2+ at pH 7.00, spinach CaM changes its conformation by exposing the acrylodan fluorescent reporter group to a more dipolar environment. There is also a concomitant increase in the fluorescein pKa when Ca2+ binds to CaM. The global motions of spinach CaM are described by rotational reorientation times of 8.4 and 10.5 ns for the apo- and Ca2+ -saturated CaM at 23 °C. In light of all the available data in the literature on CaM, these results are consistent with a small expansion of the CaM globular domains, a bending and/or rotation of the central peptide chain that connects the globular domains that host the four Ca2+ binding sites (two per domain) in such a way that residues 27 and 139 are brought closer to one another, and/or a difference in the degree of hydration between the apo- and Ca2+ -saturated CaM.