Eliminating the ‘coffee-ring effect’

Dr. Neil Canter, Contributing Editor | TLT Tech Beat November 2011

Changing the shape of the particles enables a uniform coating to form after water evaporates. 

KEY CONCEPTS
• The coffee-ring effect involves the movement of particles dispersed in water to the edge of a drop as evaporation is taking place.
• Changing the shape of the particles from spherical to ellipsoidal stops the coffee-ring effect and enables a uniform coating of particles to form after water evaporates.
• Only a small fraction of dispersed particles need to be ellipsoidal to prevent the coffee-ring effect.

Let’s face it, many of us need our cup of coffee in the morning to get the day started. What we may not realize is the significance of the ring left after coffee is accidentally spilled.

Peter Yunker, graduate student at the University of Pennsylvania in Philadelphia, says, “The ‘coffee-ring effect’ takes place when a drop of water on a solid surface evaporates. Water on the edge gets pinned and cannot recede to the center of the drop. As a result, water flows from the middle of the drop to the edge to replenish what has evaporated. Any particles suspended in the water also move toward the edge.”

The result is the formation of a ring of particles around where the drop was placed on the surface. In the case of coffee, the characteristic brown ring is observed.

The question might be asked about what the coffee-ring effect has to do with lubrication. In preparation of an effective lubricant between two metal surfaces, an important objective is to form a uniform layer in order to maximize the reduction of friction and wear. For aqueous-based lubricants, the coffee-ring effect could be a major impediment in achieving this goal.

In a previous TLT article, research was discussed about how aqueous suspensions dry in order to determine why coatings fail (1). In working with drying aqueous suspensions of 11 nanometer-silica-particles onto a silicon elastomer, the researchers discovered that the stresses produced can cause a crack to form at the interface between the coating and the substrate. This crack directly causes the coating to fail.

A better understanding of how to eliminate the coffee-ring effect could lead to better methods for forming uniform coatings. Research has now determined how to overcome this phenomenon.

ELLIPSOIDAL PARTICLES
Yunker, in collaboration with Arjun Yodh, director of the Laboratory for Research on the Structure of Matter and the James M. Skinner Professor in the department of physics and astronomy, has determined how the coffee-ring effect can be stopped. He says, “We found that the shape of the suspended particles can critically affect their movement as the drop evaporates. The spherical shape of most suspended particles facilitates their movement to the edge of the drop. But if the shape of the particles is made ellipsoidal, this flow to the edge is stopped and a uniform coating can be developed once the water has evaporated.”

Micron-sized polystyrene particles were used to confirm the coffee-ring effect and show how it can be eliminated. First, the researchers prepared a suspension of spherical particles in water and then applied drops onto glass slides. After evaporation of water, the familiar coffee-ring effect was observed.

This experiment was then repeated with a series of ellipsoids of different aspect ratios. Yunker says, “An ellipsoid has one long axis and two (identical) short axes. The aspect ratio is calculated by dividing the length of the long axis by the length of the short axis.”

Ellipsoids with aspect ratios ranging from 1.2 to 3.5 were evaluated in this study. Preparation of the ellipsoids was accomplished by suspending them in a polyvinyl alcohol gel. Yunker explains, “We heated the mixture at a temperature of 120 C, which is above the glass transition temperature of polystyrene but below the glass transition temperature of polyvinyl alcohol. This facilitates the melting of the polystyrene and the softening of the polyvinyl alcohol. We were then able to stretch the polyvinyl alcohol, creating ellipsoidal cavities which, upon cooling, led to the formation of the polystyrene ellipsoidal particles.”

The same force that moves the spherical particles to the edge of the drop move the ellipsoidal particles in the same manner. But the outcome is different. Yunker says, “Ellipsoidal particles are pushed to the edge of the drop as it is evaporating, but they never make it to the edge. When ellipsoids reach the air-water interface, they experience long-ranged attractions toward other ellipsoids, which create the formation of loosely-packed jammed structures at the interface and prevent them from continuing to the drop edge.”

Once the water is evaporated, the result is the formation of a uniform coating on the surface of the glass slides, which is in sharp contrast to the coffee-ring effect. Figure 2 shows the contrast between how the particles are dispersed on the slide. The image on the left shows the coffee-ring effect, while a uniform coating is formed when the particles are made ellipsoidal. Yunker indicates that two methods of microscopy were used to evaluate this phenomenon. They are bright field microscopy and confocal microscopy.


Figure 2. The left image shows the coffee-ring effect which pushes dispersed particles to the edge of an evaporating water drop. Switching from spherical to ellipsoidal particles stops the coffee-ring effect and leads to the uniform coating shown on the right. (Courtesy of the University of Pennsylvania)

The researchers then introduced sodium dodecyl sulfate into the drop at a concentration of 0.2% to assess its effect on the ability of the ellipsoidal particles to form the loosely-packed jammed structures. Yunker says, “The surfactant lowers the surface tension at the air-water interface, which interrupts the long-range force that attracts the ellipsoidal particles to each other. As a result, the coffee-ring effect is reestablished because the ellipsoidal particles will migrate with the water to the edge of the drop.”

One interesting aspect of this work is to create a uniform coating that is only a small fraction of the particles needed to be ellipsoidal in nature. Yunker says, “We found that the use of ellipsoids, with an aspect ratio of 3.5 with spherical particles that exhibit a diameter between 5 and 10 microns, causes the spherical particles to be caught in the network of ellipsoids on the surface of the evaporating drop. This prevents the spherical particle from reaching the edge of the drop.”

In contrast, the coffee-ring effect is not stopped when smaller spherical particles with diameters below one micron because these particles are too small to get caught in the ellipsoidal particle network.

Future work will involve evaluating the effect other surfactants have in influencing the type of coating formed during the evaporation process. Yunker mentioned that one surfactant being evaluated is an ethylene oxide, propylene oxide block copolymer.

Additional information on this research can be found in a recent article (2) or by contacting Yunker at pyunker@sas.upenn.edu. 

REFERENCES
1. Canter, N. (2011), “Drying of Coatings,” TLT, 67 (4), pp. 8-9.
2. Yunker, P., Still, T., Lohr, M. and Yodh, A. (2011), “Shape Dependent Capillary Interactions Undo the Coffee Ring Effect,” Nature, 476 (7360), pp 308-311.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.