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August 2021

What we can learn from a coffee ring

When a small amount of coffee is spilled or otherwise deposited on a surface - for example, from a mug – we observe small particulates migrate to the outside edge during the evaporation process. This behavior is the result of capillary flow which is further accelerated during the evaporation process. Another force, called the Marangoni flow, strives to draw the particulates back to the center of the drop during evaporation. However, since water has a very weak Marangoni flow, the capillary flow forces overpower it and the particulates end up highly concentrated on the outside edge. This behavior is called the coffee ring effect.

Most people give little thought to the coffee ring effect, but there are some commercial applications where the coffee ring behavior can be problematic. In most printing applications, for example, it is desirable that the particulates disperse in a uniform manner on the surface during evaporation - rather than congregate on the edges. There are a number of tools available to control the behavior of the ring effect during the evaporation process.

In some cases, it has been shown that adding elongated particles to the liquid can suppress the ring effect.1 Heat can also be used to modify the behavior of the liquid during evaporation. By modifying the wetting properties of a substrate, the ring effect can also be suppressed. For example, by making a surface more hydrophobic, the chances of pinning the drop at the contact line is decreased. Electrowetting with alternating current has been used to reverse capillary forces that produce the ring effect. A surfactant-like polymer (Polyethylene glycol, PEG) has been used to cancel out the effect out of the coffee ring behavior resulting in a more uniform deposition of nanoparticles on the surface.2

Recently, researchers at Monash University have discovered that the contact angle formed by the droplet is an important variable in determining the likelihood of the coffee ring appearing.3 In fact, according to their models, they claim that you can determine the likelihood of the coffee ring based on only two variables: the contact angle formed by the droplet and the percentage of solids or particles in the suspension. In general, as the contact angle decreases, the likelihood of the ring effect developing during evaporation increases. According to the researchers, these models are the first ever that have been developed that predict the ring effect based on the contact angle of the droplet. It is hoped that this research will introduce useful new tools that material scientists can use to improve manufacturing techniques and methods.

Now for you drinkers of fine American whiskeys, fasten your seat belts. Believe it or not, but researchers are working on a method for identifying counterfeit whiskeys by studying the webs that develop in the "coffee ring" during the evaporation process.4 The researchers have determined that different brands of whiskey can form different patterns or what they call “whiskey webs”, a type of fingerprint that can identify a particular whiskey with accuracies as high as 90%. We call that the whiskey ring effect.

1  Suppression of the coffee-ring effect by shape-dependent capillary interactions https://www.nature.com/articles/nature10344
2 Altering the coffee-ring effect by adding a surfactant-like viscous polymer solution https://doi.org/10.1038/s41598-017-00497-x
Predicting coffee ring formation upon drying in droplets of particle suspensions https://doi.org/10.1016/j.jcis.2021.01.092
4 Multiscale Self-Assembly of Distinctive Weblike Structures from Evaporated Drops of Dilute American Whiskeys https://doi.org/10.1021/acsnano.9b08984

And the winner is...
Here at ramé-hart instrument co. we currently offer ten different instrument models. The reason for the wide variety is based on our desire to offer the optimal tool for your application and at a price that is affordable and highly competitive. Despite the variety, not all models are equally popular. Based on sales over the past 12 months, it turns out that our most popular instrument is Model 250. You may be wondering what makes Model 250 such a popular product. Out of the box, Model 250 has excellent capabilities - including our U4 Series SuperSpeed camera which operates at 520 frames per second.

ramé-hart Model 250 (p/n 250-U4)

Additionally, Model 250 can measure contact angle, surface energy, interfacial and surface tension. And with DROPimage Advanced, it’s well-suited for dynamics and time-dependent studies. But what makes Model 250 so attractive to so many of our customers is the wide variety of powerful accessories that can be added. The most popular are the Automated Tilting Base, the Automated Dispensing System, the Heated Environmental Cell, our Oscillator, and the Environmental Fixture. The proper accessory can greatly expand the capabilities of Model 250. To take a closer look, please visit this page. If you have any questions or would like a quote, feel free to contact us.


Carl Clegg
Director of Sales
Phone 973-448-0305
Contact us


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