Almost a year ago Leonie Mueck posted as question of the month “Why does coffee form ring-shaped stains?“. The answer was: Coffee is a colloidal suspension in water. This means that a coffee drop on a hydrophilic surface is quite flat: It exhibits a large contact area with the surface and therefore retains a low contact angle at its edges. The drops edge on the surface is called contact line. For cases like coffee on glass this contact line is pinned, meaning the drop’s area is constant. In such a pinned drop capillary flow to the contact line occurs, causing (coffee) particle deposition near the contact line upon evaporation of the solvent. Consequently the dried drop leaves a ring-shaped stain. This is generally referred to as the ‘coffee ring effect’.
Recently a group of Belgian chemists set out to unravel why many bacterial colonies do not leave such a ring-shaped stain upon drying. This question is important for battling bacteria but also to improve any industrial application of colloidal solutions that are supposed to dry uniformly.
The existence of drops that dry differently implies that there are effects counteracting the ‘coffee ring effect’. These so called ‘Marangoni effects’ all result out of gradients of surface tension in the droplet. Thermal cooling may be the most plausible of these effects: Evaporation causes cooling of the drop’s edges which in turn results in recirculating flows due to the temperature gradient generated. The next likely candidate to cause Marangoni effects are surfactants, since they change surface tension. In fact the surfactant sodium dodecyl sulfate (SDS) generates ‘Marangoni eddies’ which result in an absolutely uniform particle deposition.
To answer the question: Some bacteria do not leave coffee ring stains because they produce non-ionic surfactants that generate strong swirling of the bacteria inside the drying drop. Due to this distribution forces the bacteria form an uniform deposit upon drying. The authors argue that uniform deposition on surfaces is advantageous to bacteria since wide-spread bacteria have better access to nutrients. With this new hypothesis, however, the question is not yet fully answered: We do now understand the actual mechanism, but we do not understand the actual advantage. Why do some kind of bacteria are content with leaving coffee rings stains and others are not? Is there really a selective advantage?
 W. Sempels, R. De Dier, H. Mizuno, J. Hofkens and J. Vermant, Nature Comm. 2013, 4, 1757, doi:10.1038/ncomms2746 Last accessed 18.05.2013.
 T. Still, P. J. Yunker, A. G. Yodh, Langmuir 2012, 28, 4984–4988.