Anyone who washes a cup after coffee dried in it has seen a “coffee ring”. Is the ring caused by the cup’s convex bottom? Several experiments get to the bottom of this phenomenon.
Introducing the Coffee Ring Home Lab Experiment
This is a simple lab experiment, both safe and interesting for anyone. More accurately, it is an open-ended set of tests that suitable for individuals or whole families.
The basic test is to let a small puddle of coffee dry on a flat surface, such as a plate. This will leave a darker ring around the outside, with a much lighter centre.
The set of experiments is to determine if all liquids behave that way, or is there something magical about coffee?
The Author’s Flirtation with Coffee Rings
Each image shows regular coffee in the upper left, tea in the upper right, soya sauce in the lower right, and instant decaffeinated coffee in the lower left.
The same source was used for each plate. The “soap” plate had just a touch of well-diluted dish soap added to each sample.
Lab Equipment for the Coffee Ring Home Experiment
The above image shows the all the equipment except for the pots of sample liquids. The spoon that I used to dribble sample liquids is resting inside the glass that holds the dilute soapy water.
The plate with soap is on the left; the soap bubbles are just visible on those liquids.
These images were taken just as the experiment started; it took another day before every sample had dried thoroughly.
The main piece of lab equipment is a plate. Having several identical plates allows for easy comparison of different conditions. The control condition should be a drop of clean water.
Use your kitchen for this home lab experiment. If you want to leave the plate elsewhere, it is easier to bring the liquids to that room, rather than carry a wet plate very far.
The experiment also requires a flat surface to leave the plate, possibly for several days, while the liquid dries. If you have pets or small children, be sure not to use poisonous or noxious substances!
What can be Tested?
What can’t be tested?
The range of water-based liquids includes beverages such as coffee, tea, milk, soft drinks or energy drinks. You could dissolve sugars (white or brown sugar, honey, or molasses) or salt. Try a suspension of baking soda or baking powder or flour. The images show that soya sauce behaves a bit differently than others.
Take an adventure into acidity with lemon juice versus vinegar; try dissolving something into those liquids, too. Waste a few drops of beer, wine or spirits; again, try them “neat” or dissolve something into them.
Finally, consider using cooking oil to dissolve these other substances. Are rings left behind?
Variations on the Theme
Once you have made some rings, you can try to avoid the effect by adding a touch of dish soap or other surfactant. See whether that makes a difference to the ring effect.
You could also try coating a plate with dish soap before adding the coffee. If you have a plain sheet of glass and a water repellant, such as for keeping rain off your car’s windshield, try that too.
Cutting Edge Science Simply Rolls Along
Recent cutting-edge research found that pencil-shaped particles which are very elongated, or uniformly round particles, can avoid the coffee-ring effect.
Does grinding the grains of instant coffee with a rolling pin make a difference? Or do you need adjust extremely fine particles at a microscopic level?
Avoid these Hazards
As noted above, please do not leave poisons out to dry; nor anything that gives off noxious vapours. This should exclude petroleum products such as gasoline, diesel fuel, or any other liquids for your automobile.
If using your good china, remember that paint will stick rather than wash off. Don’t use drain cleaner or other harsh cleaning solutions.
Expected Results and Explanations
The coffee ring effect is due to the coloured particles suspended in the water. As it evaporates, the water draws the particles towards the edge where the liquid touches the dish. This makes the outer ring darker, and leaves the centre a lighter shade.
If the liquid has a uniform dark colour without suspended particles, then there should be no coffee ring effect.
If particles emerge from solution as the liquid dries, then we expect a ring. Salt is an example where rings should result even though the crystals had dissolved completely.
With the right amount of dish soap, there should be much less surface tension. The first result is that the liquid spreads out over a wider surface. This should also diminish the coffee ring effect.
My tests seem to have been skimpy with the dish soap; I was expecting more of a difference. Only the soya really changed, and was very surprising. It had no visible ring effect without soap, but a ring effect with soap.
Practical Applications from Coffee Ring Experiments
Normally, when we spray or paint a coating onto a surface, we prefer to have a uniform finish. The coffee ring effect makes this difficult whenever there are small droplets on a surface.
Suppose the manufacturer could control the geometry of the particles, and thus make a uniform coating with fewer suspended particles. This should create a better finished effect at lower cost.
How was Your Home Lab Experiment?
Let us know what you tested and how it turned out. Did you make coffee rings? Did soap suppress the coffee rings? Did you find anything that left no rings?
Yunker, Peter J.; Still, Tim; Lohr, Matthew A.; and Yodh, A.G. “Suppression of the coffee-ring effect by shape-dependent capillary interactions“. Nature. Published online Aug. 17, 2011. Accessed Sept. 1, 2011.