Coatings play a pivotal role in everyday life and across various industries. They offer protection, corrosion resistance, insulation, optical improvements, aesthetics, etc. This study investigates the design, fabrication, characterization and evaluation of surface coatings in two areas: antimicrobial activity and fast evaporation.
The COVID-19 pandemic underscored the necessity for coatings that mitigate microbial transmission through surfaces, alleviating both contagion and personal fears. The first part of this study presents the design, development, and evaluation of antimicrobial coatings that efficiently inactivate 99.9% of SARS-CoV-2 virus and kill more than 99.9% of pathogenic bacteria such as Staphylococcus aureus, methicillin-resistant Staphylococcus aureus, and Pseudomonas aeruginosa within one hour. Prioritizing rapid infectivity reduction, we designed and fabricated several coatings using silver oxide (Ag2O), cupric oxide (CuO), and zinc oxide (ZnO) particles as active ingredients.
Applying small quantities of micron-sized opaque particles onto a surface yields a transparent film. Although Ag2O particles are inherently opaque, they possess potent antimicrobial properties. Consequently, incorporating small quantities of Ag2O into the coating results in the desired antimicrobial activity while maintaining transparency. Transparent antimicrobial coatings are a necessity for applications such as touchscreens, offering the benefit of reducing disease transmission while maintaining the aesthetic appeal of surfaces. We employed a variant of the Stöber process to bind Ag2O particles to the substrate using a silica matrix. To improve this coating method, we employed room-temperature spin-coating of a suspension of Ag2O/sodium silicate solution on the substrate, eliminating reactions with toxic chemicals in Stöber process and subsequent heat treatment. Two key features of the improved coating are its high robustness and its capability to kill 98.6% of Clostridioides difficile endospores in 60 minutes.
On the other hand, CuO and ZnO particles exhibit mild antimicrobial properties; thus, their activity could be enhanced by a porous coating. When an infected droplet lands on such a coating, it is imbibed into the porous structure, where diffusion distances are smaller, and there is a larger active area to inactivate the virus or kill the bacteria. Furthermore, porosity facilitates faster droplet drying, leading to the concentration of cupric and zinc ions in the droplet, which are designed to be toxic to microbes.
The second major topic of this thesis is the development, and evaluation of porous coatings for fast evaporation. At low Bond numbers, droplet evaporation is slow on an impermeable surface. We investigated whether application of a thin, porous coating leads to faster droplet evaporation. The droplet will imbibe quickly, but progress normal to the interface will be limited to the thickness of the coating. Therefore, the liquid will spread laterally into a broad disk to expose a large liquid–vapor interface for evaporation. As a result, the evaporation of a droplet is enhanced by a factor of 7–8 on the thin porous coatings. Factors such as coating thickness, pore size and distribution, and the contact angle of the coating, as well as ambient conditions like temperature and relative humidity, could affect the droplet evaporation rates by modifying the droplet's imbibition process and the evaporation driving force. While decreasing the coating thickness and increasing pore size and distribution promoted evaporation, the impact of contact angle is insignificant. Confocal microscopy observations of a coating composed of particles with varying sizes depicted liquid migration along the top of the coating and the edges of the interface. We developed and validated an equation to estimate the rate of evaporation. The rate correlated with the radius of the imbibition area, with higher temperatures and lower humidity further augmenting evaporation. / Doctor of Philosophy / Coatings serve as integral components in various industries and everyday settings, offering multifaceted benefits such as protection, aesthetic enhancement, and functional properties. This study investigates the design, fabrication, and evaluation of two types of surface coatings; coatings that reduce microbes transmission (antimicrobial coatings) and coatings that expedite evaporation.
The COVID-19 pandemic underscored the necessity for coatings that mitigate microbial transmission through surfaces, alleviating both contagion and personal fears. The first part of this study presents the design, development, and evaluation of coatings that efficiently reduce 99.9% of COVID-19 virus and kill more than 99.9% of dangerous bacteria that can be found in hospital settings. Prioritizing rapid killing of bacteria, we designed and fabricated several coatings using metal oxides. In particular, we used silver oxide (Ag2O), cupric oxide (CuO), and zinc oxide (ZnO) particles as active ingredients.
Applying small quantities of fine-sized opaque particles onto a surface yields a transparent film. Although Ag2O particles are inherently opaque, they possess potent antimicrobial properties. Consequently, incorporating small quantities of Ag2O into the coating results in the desired antimicrobial activity while maintaining transparency. Transparent antimicrobial coatings are a necessity for applications such as touchscreens, offering the benefit of reducing disease transmission while maintaining the aesthetic appeal of surfaces. A chemical reaction was used to produce a glass matrix to bind Ag2O particles to the solid, but this method required heating and toxic chemicals. So we developed a second methods that eliminated these two disadvantages.
On the other hand, CuO and ZnO particles exhibit milder antimicrobial properties; thus, their activity could be enhanced by a porous coating. These coatings function as large reservoirs of antimicrobial agents for trapping and deactivating pathogens, while facilitating rapid droplet evaporation through enhanced wicking and porous structure.
The second part of this study elucidates the mechanisms underlying accelerated droplet drying as a result of the application of thin, porous coatings. The speed of drying is slow for small droplets on flat surfaces. However, when a droplet is placed on a porous coating, it will be wicked quickly and spread through the porous coating to create a large area for evaporation. As a result, the speed of drying was increased by a factor of 7–8 on the thin porous coatings. Coating parameters such as thickness, pore size, and distribution, surface energy, as well as environmental factors like temperature and humidity could influence the droplet drying from porous surfaces. Decreasing the coating thickness and increasing pore size and variation in pore size promoted droplet evaporation, whereas the impact of surface energy was found to be insignificant. The rate of drying correlated with the radius of the wetted area, with higher temperatures and lower humidity further augmenting evaporation.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/119175 |
| Date | 29 May 2024 |
| Creators | Hosseini, Mohsen |
| Contributors | Chemical Engineering, Ducker, William A., Whittington, Abby Rebecca, Martin, Stephen Michael, Falkinham, Joseph O. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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