(167c) An Omniphobic Polymer-Based Hybrid Coating for Effective Dust-Repellency

Background: The harsh conditions of space lead to the accumulation of dust, especially lunar dust, on spacecraft surfaces, which has a negative impact on their performance and longevity. The surface of the moon is covered by lunar dust, a component of lunar regolith that comprises particle sizes less than 20 μm. Because of its fine texture, rough edges, and electrostatic charge, lunar dust tends to adhere to and coat any surface it comes into contact with easily. This poses a known risk to the proper functioning of electronic and mechanical equipment that is used on the lunar surface. Other solid debris and airborne particulates (aerosols) can penetrate, accumulate, and transport through porous and non-porous surfaces as well, posing significant operational and bodily risks to space planetary explorers. Furthermore, these dangers impact crew equipment such as rovers, space vehicles, and onboard instruments, sensors, thermal switches, solar and electronic panels. The effect is more damaging in the presence of high-speed dust and debris, especially at sub-zero temperatures or temperature gradient cycles, which can cause significant mechanical damage and considerable economic losses. According to the NASA Glenn Research Center, the accumulation of dust on thermal control surfaces can increase their temperature by up to 50 °C, leading to thermal stress and potential damage to the spacecraft. Dust accumulation on camera lenses can degrade their image quality and reduce the accuracy of scientific measurements. Protective coatings currently available in the commercial or niche market have several shortcomings, including durability, inability to repel low surface tension liquids, solid (ice/aerosols) adhesion, low breathability of coated fabrics, excess weight, and use of environmentally harmful compounds that may pose unknown exposure in the lunar and Martian atmospheres. Aside from mechanical damage, the economic loss amounts to millions of dollars in damages for every mission. These statistics highlight the importance of developing effective dust-repellent coatings for spacecraft surfaces. By reducing the accumulation of dust on spacecraft surfaces, these coatings can improve the performance, efficiency, and longevity of spacecraft, reducing the need for frequent cleaning and maintenance.

Conductive surfaces can reduce dust accumulation through electrostatic discharge. When two objects with unlike charges come into contact, an electric current can flow between them, resulting in a discharge of static electricity. In the context of dust accumulation, when a surface is charged, it can attract and hold onto dust particles, causing a buildup of dust over time. However, if the surface is conductive, it can discharge this static electricity, preventing the buildup of a charge and the resulting adhesion of dust particles. Conductive surfaces can also help to mitigate the effects of triboelectric charging, which occurs when two materials come into contact and exchange charges. This can occur when surfaces on a moving spacecraft encounter dust particles and other materials. By providing a conductive path, any charge buildup can be quickly dissipated, reducing the accumulation of dust and the risk of damage to spacecraft surfaces. It has been reported that conductive surfaces can help to reduce dust accumulation by providing a pathway for the dissipation of static charges, which is particularly important in environments where triboelectric charging is common, such as the space environment. Capillary forces also play a significant role in dust repellency by affecting the adhesion of dust particles to surfaces. They are the result of the intermolecular interactions between a liquid and a solid surface, which can cause the liquid to be drawn up or adhere to the surface. In the context of dust repellency, capillary forces can cause liquid films to form on surfaces, which can attract and hold onto dust particles. However, if the surface is engineered to repel the liquid, the capillary forces can be reduced, preventing the formation of liquid films and reducing the adhesion of dust particles. In addition to reducing the adhesion of dust particles, capillary forces can also affect the self-cleaning properties of surfaces. If the capillary forces are too strong, the liquid may not be able to easily roll off the surface, leading to the accumulation of dirt and debris. If the capillary forces are appropriately balanced, the liquid can easily roll off the surface, collecting dust particles and resulting in a self-cleaning effect.

PEDOT:PSS is a well-known polymer mixture used widely in designing conductive polymer systems and coatings. Its solubility in water and high conductivity make it a versatile material for fabricating various kinds of devices and coatings. However, its high hydrophilicity and tendency to break apart in the presence of water limit its scope in aqueous environments and make PEDOT:PSS coatings difficult to clean and reuse. In this study, we present a novel approach to developing a dust-repellant hydrophobic coating using a combination of octadecyltrichlorosilane (OTS) and PEDOT:PSS. Silanes are commonly used materials in the fabrication of (super)hydrophobic coatings by the self-assembly of monolayers or micellar formation. OTS has been used for this project owing to its long chain structure and fluorine-free composition in an attempt to minimize environmental hazards. Due to strong networks of hydrophobic-hydrophilic interactions in the OTS/PEDOT:PSS mixture, rapid gelation is achieved, aiding in the formation of a versatile coating solution that can be coated on a number of materials such as glass, metals, fabric, polymers etc. The resultant coated surface is both hydrophobic and dust-repellant. It has fair transparency and is water-resistant.

Methodology: A suspension of concentrated PEDOT:PSS in water and OTS was mixed and diluted with hexane to prepare the coating solution. The solution was then spin-coated on various surfaces, including glass, cotton fabric, polyester fabric, and metals. Substrates were also dip-coated and spray-coated to study the viability of other coating methods as well. To study the gelation of the mixture, rheology measurements have been made via an oscillatory time sweep experiment. The coated substrates were characterized by scanning electron microscopy to study the surface texture, and FTIR and EDS were performed to analyze the composition. The hydrophobicity was studied by contact angle goniometry, and both static and dynamic contact angles were recorded. A contact angle hysteresis as low as 6° was observed. To evaluate the dust-repellent performance of the coating, a custom-built dust deposition setup is being developed to simulate the space environment. Further, the conductivity of the coating is analyzed by impedance spectroscopy and four-point probe conductivity measurements. Studies demonstrating the mechanical performance of the coating as well as its chemical stability, will also be presented.

Conclusion: In conclusion, this project presents a promising environment-friendly dust-repellent coating for outer space applications that can potentially improve the performance and longevity of spacecraft or other equipment, including protective upgrades for the upcoming anticipated ARTEMIS missions. The hybrid approach used in the synthesis of the coating can be further optimized to improve its performance and compatibility with different spacecraft materials. Further, the development of a dust-repellent coating for outer space applications can have broader implications for other industries. For example, the coating can be applied to solar panels, wind turbines, and other structures and devices exposed to harsh environments, providing an economical solution for damage mitigation and maintenance needs.