Pressure-sensitive paint (PSP) has gained much attention as a non-intrusive pressure measurement technique for fluid mechanics. This methodology has been used for wind tunnel testing, surface pressure measurement on rotating objects, low-density gas flow measurements, oxygen concentration distribution measurements, and micro-scale gas flow measurements.
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The pressure distribution on a surface to which a PSP coating is applied can be measured by detecting the variation of the luminescence intensity emitted from the pressure-sensitive dyes in the PSP coating. A PSP coating cannot be applied to a plastic surface without resistance to organic solvents. Thus, a sticker-like PSP coating is useful because it does not require the application of an organic solvent and a high level of skill.
In recent years, there have been many studies on the fabrication methods of nano-sheets for wearable sensors and in vitro cellular studies. In general, nano-sheets have unique properties including high transparency, high flexibility, noncovalent adhesion, and excellent electrical and thermal properties due to a large size-aspect ratio. Properties like high flexibility and noncovalent adhesion also make them suitable for PSP.
This article discusses the fabrication of freestanding pressure-sensitive nano-sheet (PSNS) using a sacrificial layer process. The fabricated PSNS can be peeled off from the substrate and attached to another surface.
The basic properties of PSNF fabricated by a spin-coating method were analyzed before mass-production by a roll-to-roll process. Pressure- and temperature-sensitivity, luminescence lifetime, and the quantum yield of the fabricated PSNS have also been investigated.
Two kinds of PSNSs with different polymers: poly(1-trimethylsilyl-propyne) (PTMSP) and poly(L-lactic acid) (PLLA) were prepared. PTMSP is known as a glassy polymer with high gas permeability and is usually used as a binder of PSP. However, PTMSP is not used for the fabrication of nano-sheets.
PLLA is one of the polymer materials used to fabricate a nano-sheet by a spin-coating method and a roll-to-roll process, but it is not used as a binder of PSP. As a pressure-sensitive dye, Pt(II) meso-tetra(pentafluorophenyl)porphine (PtTFPP) was employed for both PSNS samples. PtTFPP is widely used as a pressure-sensitive dye due to high pressure sensitivity and optical stability.
Figure 1 shows the PSNS fabrication process used in this study.
Figure 1. Schematic of PSNS fabrication process based on a sacrificial layer process using a spin-coating method. (a) PVA solution is spin-coated on Si wafer. (b) PSNS is spin-coated on the PVA film. (c) fabricated PSNS and PVA film. (d) Obtained film is immersed in water. (e) PVA film is dissolved and freestanding PSNS is obtained. (f) PSNS is transferred to another substrate. Image Credit: Matsuda et al., 2021.
Figure 2 shows the typical example of the PLLA-PSNS transferred from a silicon wafer to another one. The images were taken by iPhone 8.
Figure 2. Typical example of PLLA-PSNS transferred on silicon wafer. (a) PLLA-PSNS image without illumination. (b) PLLA-PSNS image with illumination. Image Credit: Matsuda et al., 2021.
The pressure and temperature sensitivity of the fabricated PSNSs were investigated using a calibration chamber. The PSNS samples were placed in the calibration chamber, and the pressure in it was monitored and controlled by a pressure controller. The temperature in the chamber was measured by a thermistor; a Peltier device and a temperature controller were also utilized.
For the pressure-sensitivity test, the pressure in the chamber was controlled in the range of 50 to 110 kPa, and the temperature was maintained at 25 °C. For the temperature-sensitivity test, the temperature in the chamber was controlled in the range from 20 to 45 °C, and the pressure was maintained at atmospheric pressure.
The PSNS was illuminated by an LED device, the central wavelength of which was 395 nm. A CCD camera captured the emission from the PSNS with a band-pass filter of 630 ± 30 nm.
An absolute quantum yield spectrometer measured the quantum yields of the fabricated PSNSs. A lifetime spectrometer measured the lifetimes of the phosphorescence emitted from the PSNS samples. In the lifetime spectrometer, the lifetime is obtained by fitting the data with triple exponential functions.
The results of the pressure- and the temperature-sensitivity test are shown in Figures 3 and 4, respectively. The error bars indicate the standard deviation of the intensity ratio on the sample coupons.
Figure 3. Stern–Volmer plots for PTMSP-PSNS and PLLA-PSNS samples. Image Credit: Matsuda et al., 2021.
Figure 4. Result of temperature calibration tests for PTMSP-PSNS and PLLA-PSNS samples. Image Credit: Matsuda et al., 2021.
The quantum yields of PTMSP-PSNS and PTMSP-PSNS on a Teflon and an aluminum plate were measured at atmospheric pressure and the temperature of 24 °C. The results are shown in Table 1.
Table 1. Quantum yields of PTMSP-PSNS and PLLA-PSNS.
In contrast to quantum efficiency, lifetime differences between the PSNS on the Teflon and on the aluminum was minor (within 10%). Figure 5 shows the phosphorescence decay curves of the PTMSP-PSNS and the PLLA-PSNS samples on the Teflon plate at atmospheric pressure and the temperature of 24 °C measured by the lifetime spectrometer.
Figure 5. Phosphorescence decay curves for PTMSP-PSNS and PLLA-PSNS. Image Credit: Matsuda et al., 2021.
Two kinds of pressure-sensitive nano-sheet (PSNS) were fabricated based on PTMSP and PLLA as a polymer binder and called them PTMSP-PSNS and PLLA-PSNS, respectively. PtTFPP was used as a pressure-sensitive dye. The fabricated PSNS can be peeled off from the substrate by dissolving a sacrificial film PVA and can be transferred to another substrate.
Although both PTMSP-PSNS and PLLA-PSNS were fabricated by a spin-coater with the same rotational speed, the thicknesses of PTMSP-PSNS and PLLA-PSNS were 22 nm and 91 nm, respectively. This difference in the thickness was considered to be due to the viscosity between the PTMSP and the PLLA organic solution.
The pressure sensitivity of both PSNSs was examined, finding that of PTMSP-PSNS (0.59%/kPa) to be higher than that of PLLA-PSNS (0.45%/kPa). The pressure sensitivity of the fabricated PSNS is similar to those of conventional PSPs.
The quantum yield of PLLA-PSNS is much higher than that of PTMSP-PSNS and the lifetime of PLLA-PSNS is much longer than that of PTMSP-PSNS. These results show that the oxygen quenching in PLLA-PSNS is prevented by the low oxygen permeability of PLLA. Furthermore, it is validated that a white substrate enhances the quantum yield of PSNS.
This study investigated the fundamental properties of PSNS. In the future, the spatial uniformity of PSNS should be investigated for micro-scale flow applications, and a roll-to-roll fabrication method providing larger PSNS for the application of wind tunnel testing should be analyzed.
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Matsuda, Y., Orimo, R., Abe, Y., Hiraiwa, Y., Okamura, Y., Sunami, Y. (2021) Pressure-Sensitive Nano-Sheet for Optical Pressure Measurement. Sensors. Available at: https://doi.org/10.3390/s21217168.
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