Coffee Ring Science: What’s in Your Water?

As the Flint water crisis so vividly highlights, those of us lucky enough to live in places with clean tap water often take it for granted. The problem is that when something goes awry, it’s usually discovered after people have already started getting sick. Even without criminal negligence and cover-ups, like those in the Flint crisis, contaminated water can have devastating consequences.

At the 254th National Meeting & Exposition of the American Chemical Society (ACS) this week in Washington, DC, Rebecca Lahr from Michigan State University is presenting work that could give people a powerful tool for examining their own tap water. “Tap water testing is expensive and usually requires specialized equipment, says Lahr. “We are striving to provide low cost tools that can be used by anyone, anywhere in the world to monitor tap water composition.”

The tool is based on the coffee ring effect, a phenomenon familiar to coffee lovers and those who share homes or offices with them. Dried coffee stains aren’t a uniform color—they are darker around the edges. Although a cup of coffee may have seemingly magical properties, the dark liquid is really just tiny particles of coffee beans suspended in water. As a drop of spilled coffee evaporates from a smooth surface, the suspended particles are carried to the edge of the spill where they congregate. The dark ring is caused by the high concentration of coffee bean particles around a spill’s outer edge. (For more on the physics of how this happens, check out the 2016 post The Forces in Spilled Coffee Awaken.)

Coffee Stains Texture 08
Image Credit: Jacob Gube (CC BY-SA 2.0).

As a graduate student, Lahr worked with Dr. Peter Vikesland at Virginia Tech on a way to detect microcystin, a toxin produced by bacteria commonly known as blue-green algae. Their experimental work involved placing droplets of a water-based solution containing the toxin on a solid surface. On drying, thanks to the coffee ring effect, the toxin would concentrate on the edge of the drop where its chemical properties could be analyzed.

Working with her own students now, Lahr is applying the coffee ring effect in a different way. Tap water from different sources has varying levels of mineral content, pH, sodium, chloride, sulfate, dissolved solids, iron, copper, and even lead. When a droplet of tap water evaporates, suspended particles along with ions of elements like sodium are carried toward the outside edge. As the droplet evaporates, the ions form different kinds of particles that are squished into place, sorted by size. The result is a residue pattern that, like a fingerprint, is unique to the sample.

Lahr and her team, consisting of graduate student Xiaoyan Li and undergraduates Selett Allen and Alyssa Sanderson, placed droplets of tap water taken from different southern Michigan communities on inexpensive aluminum substrates and let them dry. Using a cell phone and a cheap jeweler’s loupe—a lens used to magnify objects—they photographed the resulting patterns. These intricate patterns were reproducible and distinguishable. This was also the case for water samples with different properties that were mixed in the lab.

Tap water droplets from two buildings on the Michigan State University campus leave behind different coffee-ring patterns; “hard” water is shown on the left, and water treated with a softener is shown on the right.
Image Credit: Xiaoyan Li.

One of the end goals of this work is to create a large library of these patterns. Then, like consulting an automated fingerprint database, anyone could run a photograph of his or her own tap water through the library to look for a match.

Sure, there are much more sophisticated water analysis techniques in use, but these tests are expensive, time consuming, and require special equipment. You also need to choose a test based on what you’re looking for—lead, for example. In contrast, a system like this could be used by practically anyone, anywhere to collect a lot of data quickly and inexpensively. Ideally, residents and school children could use such a system to flag possible water contamination sites and alert relevant agencies that further testing is needed.

There’s a lot that needs to happen before such a database could be widely used, not just in terms of collecting patterns, but also things like programming the automation part and correlating specific pattern features to different concentrations of ions. However, results so far suggest that the technique is a user-friendly method for quickly and easily examining water quality that could have far reaching benefits.

For more on this project, check out this short video Coffee-ring effect used for low-cost analysis of tap water – Headline Science by the American Chemical Society.

Kendra Redmond

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