Superhydrophobic Coatings as Anti-Icing Systems for Small Aircraft

Effect of Camera Parallax Angle on the Accuracy of Static Contact Angle Measurements

Measuring the contact angle at the solid/liquid/vapor triple point in sessile drop experiments is one of the most popular and simple ways to quantify the wettability of surfaces and determine the surface free energy. Despite decades of technical advancements in contact angle measurements, which allowed for improving the precision of sessile drop measurements below ±1°, an often overlooked source of experimental error in these measurements originates from the camera's parallax angle (PA) - the angle between the camera optical axis and the sample stage surface. Here, we quantified the systematic errors in the measurement of contact angles due to the acquisition of drop images at finite PA values by simulating sessile drop experiments in which synthetic drops were created using the Young-Laplace equation. The absolute contact angle error induced by imaging drops at nonzero PAs was found to increase as the true contact angle (TCA) deviates from 90° and resulted in an overestimation (underestimation) of the contact angle for drops having TCAs lower (higher) than 90°. The computed absolute contact angle error reaches values as high as -20° (+12.2°) for drops having a TCA of 175° (5°) when imaged with a PA of 10°, thus indicating the importance of considering the PA when accurately quantifying contact angles in sessile drop experiments. The shape and, by extension, volume of the sessile drop was also found to affect the magnitude of the absolute contact angle error as sessile drops with higher apex curvatures exhibited lower absolute error than those with lower curvatures at any given PA. The outcomes of this work provide guidelines for minimizing systematic errors in sessile drop measurements due to the collection of drop images at nonzero PAs.

Cyclic Voltammetry for Accurate Icing Detection on Simulated Aircraft Surfaces

Icing and ice accretion on aerodynamically critical surfaces of an aircraft increase drag, reduce lift, and raise stalling speed, which pose significant safety hazards to aircraft while in flight. Icephobic coatings have been intensively investigated by the Canadian and global aerospace industries for passive ice protection. Nevertheless, effective icephobic coatings suitable for aerospace applications are far from ideal. Ice protection of an aircraft still relies on active ice protection systems based on heating, mechanical expulsion, and deicing fluids, which are heavy-weight, power-intensive, and unfriendly to the environment. To address these challenges, rapid and accurate detection of icing is highly desirable to activate these ice protection systems only when needed. To this end, cyclic voltammetry was used for the first time to detect the initiation of icing on aircraft surfaces with or without icephobic coatings. In this study, a water droplet was sandwiched between a screen-printed electrode and a simulated aircraft surface. Cyclic voltammograms were then collected as the temperature was slowly decreased until the droplet froze to form ice. A sharp spike in faradaic current was recorded in the voltammograms during the phase transition, suggesting a switch in the mass transfer mechanism from diffusion to a surface-confined pathway. This electrochemical signal could then be used to precisely indicate the onset of icing. The developed sensing method shows potential in icing detection to manage active ice protections and ameliorate icing risks in the aerospace and aviation industries.

Dynamic Wetting Properties of Silica-Poly (Acrylic Acid) Superhydrophilic Coatings

Superhydrophilic coatings based on a hydrophilic silica nanoparticle suspension and Poly (acrylic acid) (PAA) were prepared by dip coating. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) were used to examine the morphology of the coating. The effect of surface morphology on the dynamic wetting behavior of the superhydrophilic coatings was studied by changing the silica suspension concentration from 0.5% wt. to 3.2% wt. while keeping the silica concentration in the dry coating constant. The droplet base diameter and dynamic contact angle with respect to time were measured using a high-speed camera. A power law was found to describe the relationship between the droplet diameter and time. A significantly low experimental power law index was obtained for all the coatings. Both roughness and volume loss during spreading were suggested to be responsible for the low index values. The water adsorption of the coatings was found to be the reason for the volume loss during spreading. The coatings exhibited good adherence to the substrates and retention of hydrophilic properties under mild abrasion.

Effect of the Composition of Copolymers Based on Glycidyl Methacrylate and Fluoroalkyl Methacrylates on the Free Energy and Lyophilic Properties of the Modified Surface

This study proposes to use reactive copolymers based on glycidyl methacrylate and fluoroalkyl methacrylates with a low fluorine content in the monomer unit as agents to reduce the surface free energy (SFE). This work reveals the effect of the structure and composition of copolymers on the SFE and water-repellent properties of these coatings. On a smooth surface, coatings based on copolymers of glycidyl methacrylate and fluoroalkyl methacrylates with fluorine atoms in the monomer unit ranging from three to seven are characterized by SFE values in the range from 25 to 13 mN/m, which is comparable to the values for polyhedral oligomeric silsesquioxanes and perfluoroalkyl acrylates. On textured aluminum surfaces, the obtained coatings provide time-stable superhydrophobic properties with contact angles up to 170° and sliding angles up to 2°. The possibility of using copolymers based on glycidyl methacrylate and fluoroalkyl methacrylates for the creation of self-cleaning polymer coatings is shown.

Characterization in Relevant Icing Conditions of Two Superhydrophobic Coatings

The formation of ice can be very detrimental to flight safety, since the ice accumulated on the surfaces of the aircraft can alter both the aerodynamics and the weight, leading in some cases to catastrophic stall situations. To date, only active Ice Protection Systems (IPS), which require energy to work, are being employed. The use of passive coatings able to prevent, delay, or reduce ice accretion in real flight icing conditions can be viewed as a valuable instrument to reduce the environmental footprint of aircraft. The majority of work in the literature focuses on testing superhydrophobic coatings at a speed equal to or lower than 50 m/s or rather in combination with an active system. The present study was aimed at understanding the effectiveness of two superhydrophobic coatings applied on two NACA0015 wing profiles in reducing the ice formation in relevant flight icing conditions, through tests carried out in an Icing Wind Tunnel at 50 and 95 m/s and at temperatures ranging between −3 and −23 °C. Results demonstrated that at temperatures higher than −12 °C, at both 50 and 95 m/s, with exposure time ranging between 72 and 137 s, the developed coatings can be helpful in reducing the ice accretion by 12 to 100%.

Ice Accretion on Fixed-Wing Unmanned Aerial Vehicle—A Review Study

Ice accretion on commercial aircraft operating at high Reynolds numbers has been extensively studied in the literature, but a direct transformation of these results to an Unmanned Aerial Vehicle (UAV) operating at low Reynolds numbers is not straightforward. Changes in Reynolds number have a significant impact on the ice accretion physics. Previously, only a few researchers worked in this area, but it is now gaining more attention due to the increasing applications of UAVs in the modern world. As a result, an attempt is made to review existing scientific knowledge and identify the knowledge gaps in this field of research. Ice accretion can deteriorate the aerodynamic performance, structural integrity, and aircraft stability, necessitating optimal ice mitigation techniques. This paper provides a comprehensive review of ice accretion on fixed-wing UAVs. It includes various methodologies for studying and comprehending the physics of ice accretion on UAVs. The impact of various environmental and geometric factors on ice accretion physics is reviewed, and knowledge gaps are identified. The pros and cons of various ice detection and mitigation techniques developed for UAVs are also discussed.

A Review on the Current Status of Icing Physics and Mitigation in Aviation

Icing on an aircraft is the cause of numerous adverse effects on aerodynamic performance. Although the issue was recognized in the 1920s, the icing problem is still an area of ongoing research due to the complexity of the icing phenomena. This review article aims to summarize current research on aircraft icing in two fundamental topics: icing physics and icing mitigation techniques. The icing physics focuses on fixed wings, rotors, and engines severely impacted by icing. The study of engine icing has recently become focused on ice-crystal icing. Icing mitigation techniques reviewed are based on active, passive, and hybrid methods. The active mitigation techniques include those based on thermal and mechanical methods, which are currently in use on aircraft. The passive mitigation techniques discussed are based on current ongoing studies in chemical coatings. The hybrid mitigation technique is reviewed as a combination of the thermal method (active) and chemical coating (passive) to lower energy consumption.

Hydrogels as Durable Anti-Icing Coatings Inhibit and Delay Ice Nucleation

The accumulation of ice on surfaces brings dangerous and costly problems to our daily life. Thus, it would be desirable to design anti-icing coatings for various surfaces. We report a durable anti-icing coating based on mussel-inspired chemistry, which is enabled via fabricating a liquid water layer, achieved by modifying solid substrates with the highly water absorbing property of sodium alginate. Dopamine, the main component of the mussel adhesive protein, is introduced to anchor the sodium alginate in order to render the coating applicable to all types of solid surfaces. Simultaneously, it serves as the cross-linking agent for sodium alginate; thus, the cross-linking degree of the coatings could be easily varied. The non-freezable and freezable water in the coatings with different cross-link degrees all remain liquid-like at subzero conditions and synergistically fulfill the aim of decreasing the temperature of ice nucleation. These anti-icing coatings display excellent stability even under harsh conditions. Furthermore, these coatings can be applied to almost all types of solid surfaces and have great promise in practical applications.

Deep Undercooling of Aqueous Droplets on a Superhydrophobic Surface: The Specific Role of Cation Hydration

An extraordinary prolonged freezing delay was detected for the first time for deeply undercooled sessile droplets of aqueous solutions of alkali metal chlorides deposited onto a superhydrophobic surface. Accounting for the variation in the hydration energy of ions, their distribution in the vicinity of charged interfaces of solution/air and solution/superhydrophobic surface allows qualitative description of the observed ice nucleation kinetics and ionic specificity in freezing phenomena.