Coffee is in the news again, or should I say ‘still?’ This time it’s the coffee droplet that was studied… by physicists. Indeed, physicists have discovered that a few tweaks to a coffee droplet will totally change the pattern of the ‘coffee ring effect.’ Stay with me; it gets better….
Spill a drop of coffee and let it evaporate without interference. What you will find is that when the coffee spot has dried, there will be a dark ring around it, the color gravitating towards the edge of the spill. This happens with many liquids because their droplets shift concentration from the center to the periphery as they evaporate – going from areas of greater pressure to lesser pressure.
Coffee spills evaporate showing its stains at the outer surface of the spill: image via sciencegeekgirl.wordpress.com
Surface tension, caused by the interaction of the droplet particles with the surrounding air, also plays a part in how liquids behave. Particles with high surface tension tend to collect in the center, like water droplets often do, and those with low surface tension congregate at the edge, like coffee or tea droplets.
Arjun Yodh and colleagues at the University of Pennsylvania Department of Physics and Astronomy have discovered that the shape of liquid particles also determines how they behave in response to surrounding tension – be it air tension or liquid tension.
Dried droplet with round particles form toward the edge (left); while those with elongated particles are spread more evenly throughout the area of the droplet: image via nsf.gov
Coffee particles are round. But the physicists at Penn experimented by elongating the shape of some of the molecules, and what they found was that the round particles still gravitated to the outside of the stain, but the elliptical molecules formed other patterns at the center.
What impact will this finding have? When applied to the physical make-up of inks, colors can become more consistent throughout, say, a printed letter or shape, making them easier to recognize. Manipulating the particles in paint and can also result in more uniformity creating more vibrant colors.
A spur-of-the-moment experiment, cracking the secret of the ‘coffee-ring effect,” captured the imaginations of sophisticated scientists… and voila! They discovered a new technique for making inks and paints that will positively impact these technologies, enhance public appreciation of the techniques, and bring them prestige and income too!
sources: RDMag, Nature, National Science Foundation, NPR
