Accurate wind pressure measurement on aircraft surfaces is crucial for aerodynamic design and safety. Existing methods, such as pressure taps and pressure-sensitive paints, have limitations: pressure taps are invasive and difficult to implement in certain areas, while pressure-sensitive paints suffer from low resolution and environmental instability. Flexible pressure sensor-based skins offer a promising alternative, but current technologies lack the sensitivity and range required for high-pressure applications like those found in full-scale wind tunnel testing. This research addresses this gap by developing a flexible iontronic skin capable of accurately measuring both positive and negative pressures with a high resolution across a wide pressure range (-100 kPa to 600 kPa). Iontronic sensors, which utilize ionic soft materials to enhance sensitivity and range, are particularly well-suited to this task. However, previous iontronic sensors have not addressed the need for negative pressure sensing, which is crucial in many aerodynamic scenarios. The innovation presented here is a novel design and fabrication method which enables the precise measurement of both positive and negative pressure values and high sensitivity which is crucial for applications in high-speed wind tunnels. This advance is particularly significant for wind tunnel experiments to obtain high-fidelity experimental data for high-speed or large scale aircraft and wind-turbine design.

The literature review discusses existing methods for wind pressure measurement, highlighting their limitations. Pressure taps, while accurate, are invasive and challenging to implement. Pressure-sensitive paints (PSP) offer a non-invasive approach but suffer from poor pressure resolution, instability in varying weather conditions, and difficulties in monitoring hard-to-reach areas. Flexible pressure sensor-based skins have emerged as a potential solution, but current technologies using these technologies often lack the sensitivity and wide pressure range required for high-speed and high-altitude flight testing of commercial aircraft. The paper emphasizes the advantages of iontronic sensors, specifically their ability to achieve ultrahigh sensitivity and wide pressure response range due to their unique electric double layer (EDL) capacitance mechanism. However, the existing literature lacks designs and methodologies for constructing highly sensitive flexible iontronic skins that address the full pressure range (both positive and negative) required for wind tunnel applications. This study builds upon the foundation of iontronic sensor technology, addressing the significant gap in achieving both positive and negative pressure sensing over a wide pressure range while maintaining a high degree of sensitivity and resolution.

The research involved designing and fabricating two types of iontronic pressure sensors: one for negative pressure (Sn) and another for positive pressure (Sp). Both sensors utilize a multilayer structure comprising micro-structured top and bottom electrodes (gold-coated polyimide), a micro-structured ionic gel film (poly(vinylidene-fluoride-co-hexafluoropropylene) (P(VDF-HFP)), polyurethane (PUA), and an ionic liquid (IL, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imid ([EMIM][TFSI])) sandwiched between the electrodes, and a polydimethylsiloxane (PDMS) spacer. The key difference lies in the initial contact state of the ionic gel with the electrodes. Sn incorporates a pre-pressure, ensuring complete initial contact and allowing for negative pressure detection by measuring the reduction in contact area under negative pressure. Sp, conversely, begins without contact, only responding to positive pressure which increases contact area between the ionic gel and electrode. The microstructures were created using a reverse molding technique with sandpaper templates. Finite element analysis (FEA) using COMSOL Multiphysics 5.5 was employed to optimize the microstructures for sensitivity and response time, focusing on "intrafillable" designs. The double network ionic gel was characterized for its mechanical properties, such as toughness, fatigue resistance, and response to humidity and temperature variations. AC impedance measurements were performed to determine the ionic conductivity and capacitance of the gel. The sensing properties of Sn and Sp, including sensitivity, pressure resolution, response time, and stability under various conditions were characterized using custom measurement systems. Finally, wind tunnel experiments were conducted using a NACA-0012 airfoil with an array of integrated sensors. Data acquisition was achieved using a multi-channel synchronous acquisition circuit with a capacitance-to-digital converter and microcontroller unit. The experiments varied the free stream velocity and angle of attack to assess the performance of the iontronic skin under realistic conditions.

The developed iontronic skin demonstrated exceptional performance. Sn achieved a pressure resolution of -20 Pa (0.025%) in the negative pressure range (-100 kPa to -10 Pa). The sensitivity of Sn increased from 1.60 × 10⁻³ pF·kPa⁻¹ (0 to -10 kPa) to 1.69 × 10⁻³ pF·kPa⁻¹ (-20 to -10 kPa) and decreased slightly to 1.58 × 10⁻³ pF·kPa⁻¹ (-20 to -100 kPa), attributed to the initial mechanical interlocking of the microstructures. The relaxation time of Sn was 38.7 ms under -80 kPa, incorporating the response time of the valve and gas flow. The sensor demonstrated high repeatability, stability under repeated bending, and negligible drift under various humidity conditions. Sp achieved a pressure resolution of 100 Pa (0.025%) across the entire positive pressure range (0–600 kPa). The sensitivity of Sp varied across different pressure ranges due to the microstructures stiffness. Sp exhibited a rapid response time (0.9 ms) and relaxation time (1.8 ms) at 100 kPa, enabling high-frequency vibration detection up to 400 Hz. The sensor also showed high stability under long-term cyclic loading tests (10,000 cycles at 420 kPa). Wind tunnel experiments using the integrated iontronic skin on a NACA-0012 airfoil showed reliable measurement of pressure distribution along the wing chord under varying angles of attack (AOA) and free-stream velocities. The sensor array accurately captured the changes in pressure associated with different AOA and velocities, including airflow separation effects. The data obtained from wind tunnel testing is highly correlated with the simulated data from FEM simulation, demonstrating high reliability of the newly developed flexible iontronic skins.

The results demonstrate the successful development of a high-resolution, flexible iontronic skin capable of measuring both positive and negative pressures over an extremely wide pressure range relevant to aviation applications. The high pressure resolution surpasses that of existing technologies, enabling more precise aerodynamic analysis in wind tunnel tests. The ability to detect high-frequency vibrations using a single sensor simplifies data acquisition. The integration of the sensor array on a NACA-0012 airfoil and its successful application in wind tunnel experiments validates its potential for practical use. The findings address the critical need for flexible, high-resolution pressure sensing in aerospace engineering, providing a significant advancement in wind tunnel technology. These results open possibilities for enhancing aircraft design, optimizing flight performance, and improving safety. The detailed understanding of the pressure distribution on the wing under various conditions can help engineers to optimize the aerodynamic performance, reducing drag and improving lift for more efficient flight.

This research successfully developed a flexible iontronic skin with high-resolution pressure sensing capabilities for both positive and negative pressures, surpassing the limitations of existing technologies. The integration of the skin on an airfoil and successful wind tunnel experiments validated its applicability for aerodynamic analysis and flight dynamics studies. The high-resolution data obtained can significantly improve aircraft design and optimize flight performance. Future work may focus on extending the operational temperature range for applications in broader wind tunnel settings, miniaturizing sensor size for more complex and detailed pressure mapping, and exploring the integration with machine learning algorithms for real-time data analysis and predictive modeling.

While this study demonstrates a significant advancement in flexible pressure sensing, some limitations exist. The current design may exhibit slight temperature drift, although this is manageable with temperature compensation. The long-term stability of the sensor over extremely extended periods needs further investigation. The study focused on a specific airfoil shape; further testing on different geometries is needed to establish broader applicability. The fabrication process, while successful, might be optimized for larger-scale manufacturing to make it more cost-effective and easier to implement on industrial scales.