According to a recent water safety investigation, 63 million Americans have been exposed to contaminated drinking water in the past decade alone. Testing tap water or well water at home can be done with relatively affordable, off-the-shelf test strips that check for one or more contaminants, or a comprehensive analysis can be performed by a professional for a couple hundred dollars. But researchers at Michigan State University may have found an "app for that" method to test drinking water.

When a droplet of water or any other liquid evaporates, the particulates it contains will make their way to the outside edge, leaving a ring of residue in what is known as the "coffee ring effect," due to its similarity to the stain a mug of coffee might leave on the table. The so-called coffee rings left by tap water are different for each water supply. The unique stain left by an evaporated droplet can tell us what solids (for example, lead) are dissolved in the water, the alkalinity of the water, and the hardness of the water (how much calcium and magnesium the water contains). As MSU Professor Rebecca Lahr, Ph.D., who has been studying the coffee ring effect for years, puts it, "There is a stunning amount of information there to analyze. The residue patterns for tap water are like fingerprints that can be used to identify what's in a sample."

What makes Lahr and her team's approach to testing water so unique is the method they're using to collect and compare these samples: A smartphone camera. In Lahr's lab at Michigan State, researchers let drops of water evaporate on low-cost aluminum substrates—wafer-thin plates of metal—then take cell phone photos of the leftover residue through an inexpensive jeweler's loupe.