Fast-Response Pressure-Sensitive Paint
Image Courtesy of James Gregory
Any discussion of fast responding Pressure-Sensitive Paint (PSP) should include a short background on traditional PSP. The operation of fast responding PSP is identical to traditional PSP. The major differences is in the physical properties of the paint binder. Just as traditional PSP, fast PSP can be used to produce high spatial resolution measurements of barometric pressure on surfaces. The operation of the sensor is based on the quenching of luminescent dye that is sensitive to the local concentration of Oxygen. The molecule is excited by the absorption of a photon and from the excited state the molecule has several competing relaxation paths. The path of interest for PSP involves a forbidden transition to an excited triplet state from which the molecule may simply emit a photon or be quenched by oxygen. Oxygen quenching results in a system where the luminescent intensity from the molecule is a function of the partial pressure of oxygen to which the molecule is exposed.
The experimental setup for fast PSP data acquisition is identical to traditional PSP setups. The surface is coated with the paint and the surface is illuminated with excitation of the appropriate wavelength. The surface is imaged through a long pass filter using a scientific grade CCD camera. The luminescent intensity distribution is recorded and stored for conversion to pressure using a previously determined calibration. Just as with traditional PSP, the luminescent intensity distribution is not only a function of the partial pressure of oxygen. The luminescence from the painted surface varies with illumination intensity, paint layer thickness, and probe distribution. If we assume that these parameters don't vary in time they can be eliminated by taking the ratio of the image at the test condition or wind-on image to an image taken at a known reference condition or wind-off image. This wind-off to wind-on ratio is often referred to as radiometric PSP.
The intensity versus pressure relationship is determined by placing a sample of the PSP is in a calibration chamber. The sample is exposed to a series of temperatures and pressures and the luminescent intensity of the sample is recorded at each condition. Each intensity is normalized by the intensity at a reference condition and plotted versus pressure. A plot of the calibration of Fast FIB (PtTFPP in FIB) is shown here. This plot shows the same temperature sensitivity that is evident in traditional PSP. Temperature sensitivity for Fast PSP systems can theoretically be using Binary, however, temperature compensating fast binary paints are not currently available.
The temporal-response characteristics of PSP are primarily governed by the thickness of the paint formulation and the diffusion coefficient of the binder material. The response time due to diffusion (tdiff) increases with the paint thickness (h) squared and decreases with increasing diffusion coefficient (Dm). Some investigators have focused on decreasing the thickness of the paint in order to improve the response characteristics. This is the approach that is used for Fast FIB for example. This approach has the disadvantage of sacrificing luminescent output from the paint for fast response. Low signal output from the paint results in poor signal-to-noise ratio. Other investigators have focused on increasing diffusion by creating porous structures, for example Anodized Aluminum.
The difference between a conventional polymer-based PSP and a porous PSP is described schematically in the Figure. For conventional PSP, oxygen molecules in a test gas must permeate into the binder layer for oxygen quenching. The process of oxygen permeation in a polymer binder layer produces slow response times for a conventional PSP. On the other hand, the dye in a porous PSP is opened to the test gas so that the oxygen molecules are free to interact with the dye. The open binder creates a PSP that responds much more quickly to changes in oxygen concentration and, and therefore pressure. A large effective surface area due to the porous surface improves luminescence intensity; thus, a higher SNR can be achieved. The drawback of the porous PSP approach is that the dye is too accessible to the oxygen. This results in near-complete quenching of all of the dye molecules at very low pressures. These formulations are effective for supersonic tunnels where the static pressure is < 3 psia. For lower speed applications, the signal-to-noise ratio suffers.
Polymer/ceramic PSP has been developed as a hybrid paint formulation that incorporates the advantages of both traditional and porous PSP. The resulting system is a fast response paint layer with favorable SNR at higher pressure. It is also noted that the paint can be air brushed onto a model, unlike Anodized Aluminum. The polymer/ceramic formulation incorporates a high percentage of ceramic particles that provide the porous structure for rapid oxygen quenching, with a small amount of polymer to bind the paint to the surface. Gregory and Sullivan have used these polymer/ceramic PSP formulations to measure oscillating pressure fluctuations with frequencies up to 20-kHz, as shown in the Figure.
Gregory, JW, Asai, K, Kameda, M, Liu, T, and Sullivan, JP, 2008, “A Review of Pressure-Sensitive Paint for High Speed and Unsteady Aerodynamics,” Proceedings of the Institution of Mechanical Engineers, Part G, Journal of Aerospace Engineering, vol. 222, no. 2, pp. 249-290. http://dx.doi.org/10.1243/09544100JAERO243