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USING HIGH-POWERED LASER RESURFACING TO FABRICATE SUPERHYDROPHOBIC METALS
Research led by the American University of Sharjah has resulted in a method of fabricating self-cleaning and water-repellent metals that is faster and less harmful to the environment than existing techniques.
Conditioning a metal to repel water can improve the function of that metal in many industrial applications. This superhydrophobicity can help steel beams used in underwater construction resist corrosion, keep water filtration systems running longer and more efficiently, and even reduce friction on the prow of a ship as it travels.
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Typically, superhydrophobicity is imparted to metals and other surfaces through the application of chemical coatings. But these coatings are costly, time-consuming, harmful to the environment, and they erode over time. In response to the need for a better way to achieve superhydrophobicity in metal, a team led by the American University of Sharjah (AUS) Physics Department examined the wetting properties, or wettability, of the surface of various metals.
“Wettability has gained a lot of importance in recent years because it has a wide range of applications in the aerospace, marine, biomedical, and automotive industries, among others. Superhydrophobic surfaces have great potential, such as anti-icing, anti-fogging, corrosion resistance, self-cleaning, oil/ water separation, and high resistance to bacterial contamination,” explained AUS Physics Department Head and Professor Dr. Ali S. Alnaser.
THE METHOD COULD HELP THE UAE TAP INTO THE SUPERHYDROPHOBIC CHEMICAL COATINGS MARKET, WHICH IS CURRENTLY VALUED BETWEEN $8 BILLION AND $15 BILLION
He was the principal investigator on the project, working with AUS research assistant Sharjeel A. Khan, AUS Postdoctoral Researcher Ganjaboy S. Boltaev, Senior Laboratory Instructor Mazhar Iqbal, research assistant Vyacheslav V. Kim, and research fellow Dr. Rashid A. Ganeev. A paper on their research was recently published in the journal Applied Surface Science.
Wetting refers to the ability of a liquid to maintain contact with a solid surface. When a metal surface attracts, holds, or spreads water, it is referred to as being hydrophilic. When it repels water, it is hydrophobic. The wettability of a surface is characterized by its contact angle, which is a factor of the surface texture and chemistry. The rougher the surface of a material, the better it repels liquid.
Dr. Ali S. Alnaser, Physics Department Head and Professor, AUS
Recognizing the role surface structure plays in wettability, Dr. Alnaser and his collaborators decided to use a highpowered laser to create a pattern on the surface of the metal that would increase surface roughness, leading to superhydrophobicity. By etching directly on the metal surface, they intended to preclude the need for costly and toxic coatings and ensure longer performance without erosion.
The team used femtosecond lasers to achieve the desired surface roughness, followed by exposure to a high-vacuum environment. Femtosecond lasers emit ultrashort optical pulses with a duration of one quadrillionth of a second. These extremely short pulses enable powerful but brief blasts of energy to be directed onto a surface to ablate or destroy material at a minute scale. Femtosecond lasers are commonly used in Lasik surgeries to reshape the cornea and in micromachining to create lab-on-achip devices and other photonic and microfluidic devices. AUS is one of very few universities in the world to possess a high-power femtosecond laser capable of fabricating and altering the optical, chemical, and mechanical properties of almost all kinds of materials including metals, semiconductors, and insulators.
Dr. Ali S. Alnaser, Professor & Head of the Physics Department American University of Sharjah
The femtosecond laser surfacing method was to be applied on three types of metal – aluminum, copper, and galvanized steel – selected because of their ubiquity in industry.
“These metals are among the most common commercial metals used in construction and building materials, for tubing, fitting, frames, sheets, housing, and other applications. The water repellent characteristic on the surfaces of those metals would be quite beneficial in many applications, including corrosion resistance, self-cleaning, anti-fogging, oil/water separation and tribology, which is the interaction of surfaces in relative motion and includes friction, wear, and lubrication,” Dr. Alnaser said.
To test their proposed fabrication method, commercially available sheets of aluminum, copper, and galvanized steel with a thickness of 1mm were gathered. The metal sheets were subjected either to femtosecond laser ablation or to slightly slower picosecond laser ablation for comparison. Following laser ablation, some sheets were left in the open-air to age for 30 days, while others were stored in high vacuum conditions for six hours. Both aging processes were intended to enable the newly ablated surface structure to invert, transforming its characteristic from hydrophilic to superhydrophobic.
The treated metal samples were then analyzed to study their surface morphology and superhydrophobicity. Scanning electron microscopy (SEM) images revealed that the femtosecond laser treatment resulted in the removal of metal to create grid-like patterns with small valleys in the laser beam’s path. In the copper and galvanized steel, the femtosecond laser beam produced laserinduced periodic surface structures, while in the aluminum only rough random structures were observed.
Left to right: Ms. Aya Abu Baker, Dr. Ali S. Alnaser, Mr. Mazhar Iqbal, Dr. Ganjaboy S. Boltaev
Regarding the implications of the findings, Dr. Alnaser said: “We demonstrated a single-step, contactfree, high-resolution and highly versatile technique to achieve superhydrophobicity in metals that can be applied to precisely control the surface properties of almost all types of materials, regardless of their chemical composition.”
Comparing their novel femtosecond laser fabrication method to the chemical coating-based method typically used to achieve superhydrophobicity, the researchers said it was faster, easier, non-toxic, and more durable.
“Our method is completely eco-friendly, with no contamination to the surface, and is also time-efficient and effective compared to the traditional chemical methods of surface coatings, which may fade away over time. Most notably, our method is far faster. Using our laser-based surface modification method to treat an area of 2 cm2 would take 100 seconds, compared to the chemical methods, which would take hours or even days. In hightech applications, in which contamination is crucial and time and precision are required, the ultrafast laser technique could stand as the most suitable and effective approach,” Dr. Alnaser shared.
For the UAE, the superhydrophobic material fabrication method developed by the team could be of particular use in the country’s desalination and oil sectors, in the form of filtration and separation membranes. It could also help the UAE tap into the superhydrophobic chemical coatings market, which is currently valued between $8 billion and $15 billion.
Going forward, Dr. Alnaser and his collaborators will be exploring the use of their superhydrophobic material fabrication method on other materials and testing specific uses and applications, like oil/water separation, anti-corrosion, fuel cells, and implants.
