Christopher J. Wohl

Laser ablative patterning of copoly(imide siloxane)s generating superhydrophobic surfaces.

Low surface energy copoly(imide siloxane)s were generated via condensation polymerization reactions. The generated materials were characterized spectroscopically, thermally, mechanically, and via contact angle goniometry. The decrease in tensile modulus and opaque appearance of copoly(imide siloxane) films indicated phase segregation in the bulk. Preferential surface partitioning of the siloxane moieties was verified by X-ray photoelectron spectroscopy (XPS) and increased advancing water contact angle values (theta(A)). Pristine copoly(imide siloxane) surfaces typically exhibited theta(A) values of 111 degrees and sliding angles from 27 degrees to >60 degrees. The surface properties of these copoly(imide siloxane) films were further altered using laser ablation patterning (frequency-tripled Nd:YAG laser, 355 nm). Laser-etched square pillar arrays (25 microm pillars with 25 microm interspaces) changed theta(A) by up to 64 degrees. Theta(A) values approaching 175 degrees and sliding angles from 1 degree to 15 degrees were observed. ATR-IR spectroscopy and XPS indicated polymer chain scission reactions occurred as a result of laser ablation. Initial particle adhesion studies revealed that the copoly(imide siloxane)s outperformed the corresponding homopolyimides and that laser ablation patterning further enhanced this result.

Laser ablative surface treatment for enhanced bonding of Ti-6Al-4V alloy.

Adhesive bonding offers many advantages over mechanical fastening, but requires certification before it can be incorporated in primary structures for commercial aviation without disbond-arrestment features or redundant load paths. Surface preparation is widely recognized as the key step to producing robust and predictable adhesive bonds. Surface preparation by laser ablation provides an alternative to the expensive, hazardous, polluting, and less precise practices used currently such as chemical-dip, manual abrasion and grit blast. This report documents preliminary testing of a surface preparation technique using laser ablation as a replacement for the chemical etch and abrasive processes currently applied to Ti-6Al-4V alloy adherends. Surface roughness and surface chemical composition were characterized using interference microscopy and X-ray photoelectron spectroscopy, respectively. A technique for fluorescence visualization was developed which allowed for quantitative failure mode analysis. Wedge crack extension testing in a hot, humid environment indicated the relative effectiveness of various surface treatments. Increasing ablation duty cycle reduced crack propagation and adhesive failure. Single lap shear testing showed an increase in strength and durability as laser ablation duty cycle and power were increased. Chemical analyses showed trends for surface chemical species, which correlated with improved bond strength and durability.

Engineered Surfaces for Mitigation of Insect Residue Adhesion

Maintenance of laminar flow under operational flight conditions is being investigated under NASA s Environmentally Responsible Aviation (ERA) Program. Among the challenges with natural laminar flow is the accretion of residues from insect impacts incurred during takeoff or landing. Depending on air speed, temperature, and wing structure, the critical residue height for laminar flow disruption can be as low as 4 microns near the leading edge. In this study, engineered surfaces designed to minimize insect residue adhesion were examined. The coatings studied included chemical compositions containing functional groups typically associated with abhesive (non-stick) surfaces. To reduce surface contact by liquids and enhance abhesion, the engineered surfaces consisted of these coatings doped with particulate additives to generate random surface topography, as well as coatings applied to laser ablated surfaces having precision patterned topographies. Performance evaluation of these surfaces included contact angle goniometry of pristine coatings and profilometry of surfaces after insect impacts were incurred in laboratory scale tests, wind tunnel tests and flight tests. The results illustrate the complexity of designing antifouling surfaces for effective insect contamination mitigation under dynamic conditions and suggest that superhydrophobic surfaces may not be the most effective solution for preventing insect contamination on aircraft wing leading edges.

Synthesis, Characterization, Topographical Modification, and Surface Properties of Copoly(Imide Siloxane)s

Novel copoly(imide siloxane)s were synthesized from commercially available aminopropyl terminated siloxane oligomers, aromatic dianhydrides, and diamines. This synthetic approach produced copolymers with well-defined siloxane blocks linked with imide units in a random fashion. The copoly(amide acid)s were characterized by solution viscosity and subsequently used to cast thin films followed by thermal imidization in an inert atmosphere. Thin films were characterized using contact angle goniometry, attenuated total reflection Fourier transform infrared spectroscopy, confocal and optical microscopy, and tensile testing. Adhesion of micron-sized particles was determined quantitatively using a sonication device. The polydimethylsiloxane moieties lowered the copolymer surface energy due to migration of siloxane moieties to the film’s surface, resulting in a notable reduction in particle adhesion. A further reduction in particle adhesion was achieved by introducing topographical features on a scale of several to tens of microns by a laser ablation technique.

Synthesis of Fluorophore-Doped Polystyrene Microspheres: Seed Material for Airflow Sensing

Kiton red 620 (KR620) doped polystyrene latex microspheres (PSLs) were synthesized via soap-free emulsion polymerization to be utilized as a relatively nontoxic, fluorescent seed material for airflow characterization experiments. Poly(styrene-co-styrenesulfonate) was used as the PSL matrix to promote KR620 incorporation. Additionally, a bicarbonate buffer and poly(diallyldimethylammonium chloride), polyD, cationic polymer were added to the reaction solution to stabilize the pH and potentially influence the electrostatic interactions between the PSLs and dye molecules. A design of experiments (DOE) approach was used to efficiently investigate the variation of these materials. Using a 4-factor, 2-level response surface design with a center point, a series of experiments were performed to determine the dependence of these factors on particle diameter, diameter size distribution, fluorescent emission intensity, and KR620 retention. Using statistical analysis, the factors and factor interactions that most significantly affect the outputs were identified. These particles enabled velocity measurements to be made much closer to walls and surfaces than previously. Based on these results, KR620-doped PSLs may be utilized to simultaneously measure the velocity and mixing concentration, among other airflow parameters, in complex flows.

Laser velocimetry with fluorescent dye-doped polystyrene microspheres

Simultaneous Mie scattering and laser-induced fluorescence (LIF) signals are obtained from individual polystyrene latex microspheres dispersed in an air flow. Microspheres less than 1 μm mean diameter were doped with two organic fluorescent dyes, Rhodamine B (RhB) and dichlorofluorescein (DCF), intended either to provide improved particle-based flow velocimetry in the vicinity of surfaces or to provide scalar flow information (e.g., marking one of two fluid streams). Both dyes exhibit measureable fluorescence signals that are on the order of 10(-3) to 10(-4) times weaker than the simultaneously measured Mie signals. It is determined that at the conditions measured, 95.5% of RhB LIF signals and 32.2% of DCF signals provide valid laser-Doppler velocimetry measurements compared with the Mie scattering validation rate with 6.5 W of 532 nm excitation, while RhB excited with 1.0 W incident laser power still exhibits 95.4% valid velocimetry signals from the LIF channel. The results suggest that the method is applicable to wind tunnel measurements near walls where laser flare can be a limiting factor and monodisperse particles are essential.

Characterization of Fluorescent Polystyrene Microspheres for Advanced Flow Diagnostics

** †† ‡‡ §§ *** ††† Fluorescent dye doped polystyrene latex microspheres (PSLs) are being developed for velocimetry and scalar measurements in variable property flows. Two organic dyes, Rhodamine B (RhB) and dichlorofluorescein (DCF), are examined to assess laser-induced fluorescence (LIF) properties for flow imaging applications and single-shot temperature measurements. A major interest in the current research is the application of safe dyes, thus DCF is of particular interest, while RhB is used as a benchmark. Success is demonstrated for single-point laser Doppler velocimetry (LDV) and also imaging fluorescence, excited via a continuous wave 2 W laser beam, for exposures down to 10 ms. In contrast, when exciting with a pulsed Nd:YAG laser at 200mJ/pulse, no fluorescence was detected, even when integrating tens of pulses. We show that this is due to saturation of the LIF signal at relatively low excitation intensities, 4-5 orders of magnitude lower than the pulsed laser intensity. A two-band LIF technique is applied in a heated jet, indicating that the technique effectively removes interfering inputs such as particle diameter variation. Temperature measurement uncertainties are estimated based upon the variance measured for the two-band LIF intensity ratio and the achievable dye temperature sensitivity, indicating that particles developed to date may provide about ±12.5 °C precision, while future improvements in dye temperature sensitivity and signal quality may enable single-shot temperature measurements to sub-degree precision.

A Piezoelectric Shear Stress Sensor

In this paper, a piezoelectric sensor with a floating element was developed for shear stress measurement. The piezoelectric sensor was designed to detect the pure shear stress, suppressing effects of normal stress components, by applying opposite poling vectors to the piezoelectric elements. The sensor was first calibrated in the lab by applying shear forces where it demonstrated high sensitivity to shear stress (91.3 ± 2.1 pC/Pa) due to the high piezoelectric coefficients of 0.67Pb(Mg1∕3Nb2∕3)O3-0.33PbTiO3 (PMN-33%PT, d31=-1330 pC/N). The sensor also exhibited negligible sensitivity to normal stress (less than 1.2 pC/Pa) because of the electromechanical symmetry of the device. The usable frequency range of the sensor is up to 800 Hz.

Particle Image Velocimetry Applications Using Fluorescent Dye-Doped Particles

Polystyrene latex sphere particles are widely used to seed flows for velocimetry techniques such as Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV). These particles may be doped with fluorescent dyes such that signals spectrally shifted from the incident laser wavelength may be detected via Laser Induced Fluorescence (LIF). An attractive application of the LIF signal is achieving velocimetry in the presence of strong interference from laser scatter, opening up new research possibilities very near solid surfaces or at liquid/gas interfaces. Additionally, LIF signals can be used to tag different fluid streams to study mixing. While fluorescence-based PIV has been performed by many researchers for particles dispersed in water flows, the current work is among the first in applying the technique to micron-scale particles dispersed in a gas. A key requirement for such an application is addressing potential health hazards from fluorescent dyes; successful doping of Kiton Red 620 (KR620) has enabled the use of this relatively safe dye for fluorescence PIV for the first time. In this paper, basic applications proving the concept of PIV using the LIF signal from KR620-doped particles are exhibited for a free jet and a two-phase flow apparatus. Results indicate that while the fluorescence PIV techniques produce a signal roughly 3 orders of magnitude weaker than Mie scattering, they provide a viable method for obtaining data in flow regions previously inaccessible via standard PIV. These techniques have the potential to also complement Mie scattering signals, for example in multi-stream and/or multi-phase experiments.

Improvements in laser flare removal for particle image velocimetry using fluorescent dye-doped particles

Laser flare, or scattering of laser light from a surface, can often be a major issue in particle image velocimetry (PIV) involving solid boundaries in the flow or a gas–liquid interface. The use of fluorescent light from dye-doped particles has been demonstrated in water applications, but reproducing the technique in an airflow is more difficult due to particle size constraints and safety concerns. The following work presents fluorescent Kiton Red 620 (KR620)-doped polystyrene latex microspheres as a solution to this issue. The particles are small and narrowly distributed, with a mean diameter of 0.87 and diameter distribution standard deviation of 0.30 . Furthermore, the KR620 dye exhibits much lower toxicity than other common fluorescent dyes, and would be safe to use in large flow facilities. The fluorescent signal from the particles is measured on average to be 320 ± 10 times weaker than the Mie scattering signal from the particles. This reduction in signal is counterbalanced by greatly enhanced contrast via optical rejection of the incident laser wavelength. Fluorescent PIV with these particles is shown to eliminate laser flare near surfaces, allowing for velocity measurements as close as 100 to the surface. In one case, fluorescent PIV led to velocity vector validation rates more than 20 times that of the Mie scattering results in the boundary layer region of an angled surface.