The particles are coffee. The edge to the right is the coffee ring. It gets
darker.
If you are reading this you may be drinking a cup of coffee. Or if you are not drinking some coffee you have likely experienced this in your lifetime. The coffee ring effect is the so named effect of how coffee or other materials move to the edge of the ring (see above video). This effect occurs when a puddle with particles (it does not matter if they are coffee or not) evaporates leaving a characteristic ring around the perimeter. This should be renamed to the red wine effect as it is more commonly seen after spilling wine (likely wine is more spilled because of the alcohol). This effect is also seen with other materials like paint and adhesives.
Evaporation induces a Marangoni flow occurs inside the droplet. If the Marangoni flow is strong can do the opposite redistributing the particles back to the centre instead of to the edges. For the coffee ring (or red wine ring) to take place it is suggested that the Marangoni flow must be weak. Water which composes both wine and coffee has a weak Marangoni flow. In order to disrupt this flow surfactants can be added which lower the surface tension and stop the Marangoni flow likely leaving a even surface. But what kinds of surfactants?
Recently researchers from KU Leuven found a bacterium that produces natural surfactants that counteracts the Marangoni flow and the spreads out the material causing the coffee ring effect more evenly. The bacterium does this naturally...
Pseudomonas aeruginosa is a dangerous bacterium that can cause infections in open wounds. "A Pseudomonas aeruginosabacteria colony wants to find as large a breeding ground as possible. To avoid overconcentration on the edges of a wound when spreading itself during the drying-out process, the bacterium produces substances that counteract the coffee ring effect."
Interestingly the researchers took one extra step and genetically modified the bacterium so they would not produce surfactants. The coffee ring effect was visible with these genetically modified bacterium.
Better surfactants are needed in all kinds of projects from inks, paints, adhesives to cleaning the surface of your table. So the next time you are drinking coffee or wine and produce the a unsatisfactory visible ring think about the Marangoni effect and the bacterium that can fight it.
Making surfactant formulations from this natural surfactants from this bacterium can be made better with the only High Throughput Device, The Kibron Delta-8. In only 12 minutes you will have an answer which Surfactant formula is better to get rid of the coffee stains.
