UCalgary researchers look to nature to beat the 'coffee-ring effect'

Anyone who has a regular cup of coffee in the morning can attest to the daily cleaning regimen that comes with the delicious drink.

The omnipresent coffee ring left behind on a table by a cup of freshly consumed java can be stubborn to clean, especially on the edges where the mug once sat.

Not only is it a common issue for beverage drinkers, but it has also drawn the attention of scientists around the world because it is similar to a problem that plagues the worlds of printing, genotyping and diagnostics. The problem: how to produce a microscopically uniform coat. And the solution could even impact future cancer treatments.

A group of University of Calgary researchers, led by Schulich School of Engineering associate professor Dr. Amir Sanati Nezhad, PhD, have developed a technology that could be a game-changer for those dealing with what is called the “coffee-ring effect.” Their findings were recently published in Nature Communications.

The common conundrum

The pattern of the coffee ring, just like with red wine or ink stains, is always the same: lighter in the centre and darker on the perimeter.

The reason is that when a droplet of coffee dries, the small, solid particles of coffee are drawn to the edge where the liquid evaporates fastest, at the intersection of the liquid, the solid and the air.

This problem is prevalent in the manufacturing of high-tech materials like sensors, soft electronics and displays. As it’s invisible to the naked eye, the coating used in manufacturing is often uneven and can affect the performance of the device.

Nezhad has been investigating the issue alongside Dr. Hossein Hejazi, PhD’11, and Hossein Zargaratalebi, Eyes High scholar and PhD candidate, hoping to find a solution.

“Although there are some methodologies for rectifying the coffee-ring problem, the proposed methods are either invasive for sensitive applications or suffer from a lack of uniformity in particle disposition,” Nezhad says. “Therefore, there is a pressing need to develop a non-invasive, reliable, and repeatable method for depositing highly ordered particles, appropriate for a wide range of applications.”

Inspiration found in nature

The team turned to nature for a promising solution.

In liquids, the meniscus is the curve of its surface close to the surface of the container or another object which is caused by surface tension.

“When a salt lake evaporates, it leaves a uniform distribution of the salts on its bed because of the meniscus-free film of water, in which the liquid-free interface is free of curvatures associated with the coffee-ring effect,” Zargartalebi says.

Responding to this discovery, the researchers developed what they call the “meniscus-free, coffee-ring-free” method.

“The proposed technique enables the fabrication of a new generation of nanosensors, appropriate for clinical study due to their robust and reliable characteristics,” Nezhad says.

He adds they are also able to construct three-dimensional nanofilters that help alleviate the challenges of multi-step filtration.

A revelation for industry

Nezhad is convinced his team’s approach can easily be scaled up and applied in many applications.

He says the new protocol is also a “robust replacement” for the current processes, which not only deal with a degree of non-uniformity that is inappropriate for sensitive applications, but also are laborious and require additives or external devices.

Nezhad is also excited about what the future has in store for this area of research, as it goes beyond the world of inkjet-printed electronic devices like solar cells and transistors.

“We will now be using this technique for making repeatable electrochemical biosensors for point-of-care applications for rapid and early diagnosis of brain injuries, and diseases like cancer and infections,” he says.