If you frequently drink coffee, chances are you may have spilled it or left a small puddle in your cup. If you leave the puddle of coffee untouched, you'll notice a peculiar outline left behind by the spilled coffee. Instead of drying completely, an "outline" forms. Why does this happen? In this article, we'll explore the coffee-ring effect and learn more about this phenomenon.
How does the coffee-ring effect happen?
In a puddle of coffee, there is a contact line, which is the rim of the puddle that forms its perimeter (see the green dots). The free surface has a property called surface tension, which allows the puddle to maintain its round shape due to interactions between the coffee particles and the surrounding air.
When the puddle evaporates, this contact line remains fixed due to stronger interaction between the solid surface (e.g the table the puddle is on) and the liquid, compared to the surface tension at the free surface. This fixed contact line also causes liquid molecules to leave more readily, causing a higher evaporation rate at the contact line. As a result, the evaporation rate is not uniform throughout the puddle.
With a non-uniform evaporation rate, the particles that are in the centre will now move towards the contact line at the edge of the puddle in order to compensate for the higher evaporation rate. Since only the liquid would evaporate, this would leave behind the solid coffee particles, hence leading to the ring of coffee that you see when you spill it!
Is coffee the only substance that causes this effect?
Despite its name, there are actually other substances that, when evaporated, would leave a ring-like residue behind, creating the effect we see with coffee. One typical example would be colloids (or more specifically colloidal suspensions). Coffee is one type of colloidal suspension where the liquid (water) contains small round solid particles (coffee) that result in this phenomenon. Other examples would include wine and milk, both having small solid particles in liquid in order to give rise to the effect when they evaporate.
Though most colloidal suspensions would exhibit such behaviour when evaporating, there is also an important criteria to the shape of the particles: they must be round. If the particles are elongated, this effect would not be seen. Instead, while the particles do move to the contact line, they will form loose structures and be across the entire puddle instead of being stuck at the contact line as the liquid evaporates.
To watch the coffee ring effect in action and see the differences between round and elongated particles under a microscope, you can watch this video!
Conclusion
In conclusion, the coffee ring effect is a phenomenon that occurs with substances that contain round particles such as coffee when a puddle of it evaporates. The ‘outline’ formed is, in fact, a culmination of the solid particles having been moved there while the liquid evaporates.
Works Cited
Britannica, The Editors of Encyclopaedia (2024). surface tension. [online] Encyclopedia Britannica. Available at: https://www.britannica.com/science/surface-tension [Accessed 28 Apr. 2024]
Cimbala, J. (2021). TMFM: Why Do Coffee Rings Form? [online] www.youtube.com. Available at: https://www.youtube.com/watch?v=gwzHnNqBfAw [Accessed 27 Apr. 2024].
Dombrowski, C. (2011). Beating the coffee ring effect for smooth coatings. [online] Ars Technica. Available at: https://arstechnica.com/science/2011/08/ever-wonder-why-a-drop/?comments=1&comments-page=1 [Accessed 28 Apr. 2024].
The Fluid Dynamics Lab. (2020). Drop evaporation and the coffee ring effect. [online] Available at: https://ronshnapp.wordpress.com/2020/10/30/drop-evaporation-and-the-coffee-ring-effect/ [Accessed 27 Apr. 2024].
University of Pennsylvania (2011). The Coffee Ring Effect - Intriguing Microscopic Video. [online] www.youtube.com. Available at: https://youtu.be/ZaCGoSTMHyc?si=KCnUIgMCh7y_JJcP [Accessed 26 Apr. 2024].
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