Ever notice how a dried coffee stain looks like a ring? You take a long sip, spill a drop, and look up a while later to see that as the liquid evaporated, the particles suspended in the coffee drop moved to the outer boundaries of the drop and deposited there.
Known as the “coffee-ring effect,” this phenomenon is not limited to table stains. From coating techniques to inkjet printing, it prohibits the even spread of particles across a surface as the particles accumulate at the edges of the liquid drop.
As it turns out, this tendency is directly linked to a particle’s shape. In an article published in the journal Nature, Yunker and colleagues found that the more spherical the particles, the more pronounced the effect. When particles are more ellipsoidal than spherical, surface tension forces them to the air-liquid interface where they assemble evenly. To make an analogy to a coffee mug, ellipsoidal particles will move towards the centre of the mug’s curved bottom.
The mechanics behind the coffee-ring effect were uncovered a few years ago. The liquid adheres to the substrate — your table — especially the edges of the drop, which may attach based on the substrate’s surface roughness. When the drop adheres to the substrate, the drop radius remains constant. During evaporation, the liquid will tend to move from the center outward (as a result of surface tension) in order to replace any lost liquid at the edges.
Yunker and colleagues targeted the surface tension that leads to the coffee-ring effect and manipulated the particle shapes so that the particles would self-assemble along the air-liquid interface. Doing this increases the interfacial viscosity so that particles won’t disperse and create a ring; this works since increasing a liquid’s viscosity makes its particle suspension bind together more like mud than water. Benjamin Franklin demonstrated this principle of increasing interfacial viscosity to stabilize flows within a bulk of liquid when he poured about a teaspoonful of oil into a London pond, instantly causing the waves and ripples to disappear.
The researchers evaluated how particle shapes, ranging from spheres to ellipsoids of varying eccentricity, influenced drop-drying patterns. Ellipsoids deform the air-water interface to create attractions between particles and become “stuck” to the interface. Even though the particles might flow to the edge of the drop, they effectively block each other’s way like a traffic jam and gradually blanket the surface of the drop. The same cannot be said for spherical particles, which clump together near the edges of the drop.
This is impressive cocktail banter and all, but what is the practical significance of the study results? Simply including a small amount of non-spherical particles (as few as 0.015 per cent) can help strengthen the interfacial flow resistance enough to suppress ring formation such that one can spread a thin film of uniform coating. Because interfacial resistance to bulk flows takes place whenever liquid is in contact with a solid, we can transfer the findings to paints, coatings, and lubricants.
The finding applies to food and consumer products, as many of them come in foams or emulsions — creams, lotions, hairstyling products, shaving foam, and so on. The popping of bubbles in the foam or drops in emulsions, caused by flow in thin films, can leave behind ring stains as well. Luckily, however, the addition of ellipsoidal particles have already displayed the ability to stabilize emulsions thanks to strong interfacial flow resistance. Manipulating particle shapes may eventually become a valuable widespread method to stop certain types of bulk flows that are caused by surface tension.
