Register for an account


Enter your name and email address below.

Your email address is used to log in and will not be shared or sold. Read our privacy policy.


Website access code

Enter your access code into the form field below.

If you are a Zinio, Nook, Kindle, Apple, or Google Play subscriber, you can enter your website access code to gain subscriber access. Your website access code is located in the upper right corner of the Table of Contents page of your digital edition.

The Sciences

NCBI ROFL: The fluid mechanics of coffee rings.

DiscoblogBy ncbi roflAugust 30, 2012 4:00 AM


Sign up for our email newsletter for the latest science news


Formation of coffee stains on porous surfaces. "During the drying of drops of nanoparticle suspensions, segregation can occur by internal fluid flows toward the contact line, if the contact line is pinned. This leads to a characteristic ring deposit or coffee stain. On solid substrates coffee staining can be eliminated through the use of solvent mixtures that promote Marangoni flows to oppose these drying-induced flows. Here it is shown that a suspension, optimized to eliminate the formation of coffee stains on a range of solid surfaces, shows coffee staining on a number of porous surfaces. This behavior is shown to be consistent with a mechanism of fluid removal through capillary flow (draining) of the solvent into the porous substrate, combined with filtration of the particles by the small pore size, in addition to the flow from solvent evaporation. The extent of capillary driven coffee staining is a function of substrate pore size: if the pore size is small, capillary flow is slow, reducing the observed coffee staining. However, if the pore size is too large, the nanoparticles are absorbed into the material along with the draining solute and no deposition of particles is observed."


Photo: flickr/SixRevisions

Related content: Discoblog: NCBI ROFL: When your sweat turns red, maybe it’s time to eat less food coloring.

Discoblog: NCBI ROFL: Dirty liberals! Reminders of physical cleanliness influence moral and political attitudes.

Discoblog: NCBI ROFL: Free-range maggots better for cleaning putrid wounds.

NCBI ROFL. Real articles. Funny subjects. Read our FAQ


    3 Free Articles Left

    Want it all? Get unlimited access when you subscribe.


    Already a subscriber? Register or Log In

    Want unlimited access?

    Subscribe today and save 70%


    Already a subscriber? Register or Log In