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Coffee Ring Effect in Bacterial Systems
Have you ever observed a drop of coffee/any colloidal solution drying? If you've observed it carefully, you'll find that it dries up in a ring form-the particles collect together at the boundary of the drop while the liquid evaporates. This is termed as coffee ring effect. Following is a typical coffee dried drop stain!
[Image: coffee.jpg]

What Causes Coffee Ring Effect?
For coffee ring effect to take place, the liquid must be having high surface tension (as is the case with most colloidal solutions). Due to high surface tension, the liquid droplet doesn't shrink at circumference, but rather pins to the surface and a capillary movement of the solute takes place towards the boundary of the liquid drop. This phenomena leads to accumulation of all the liquid solute at the periphery.
Following is an attempt to show you the movement involved:
[Image: cre.jpg]

Significance of this effect:
Scientists have been trying to counter the effect for long owing to it's industrial implications like the need of uniform spreading paints/dyes/varnishes which shouldn't exhibit coffee ring effect. Though in some instances, it has been tried to use it to advantage too-like developing very fine wires out of colloidal gold, and it's role in stretching the DNA in solution.

Bacterial broths don't exhibit Coffee Ring Effect!:
Surprisingly, most of the bacterial broths/culture don't exhibit coffee ring effect despite being fairly dense colloidal solutions. Recently, the work of Wouter Sempels et al.,(2013) established the auto-production of biosurfactants as the reason behind the ability of the bacterial systems to counter the coffee ring effect. Their model bacteria was Pseudomonas aeruginosa which is highly pathogenic in nature, and is known to cause infections in open wounds. It's ability to avoid the coffee ring effect enables it to infect on as large breeding ground as available on/in the wound, rather than just concentrating on the periphery of the wound. And, the research group of Wouter Sempels found that it was the production of biosurfactants by this bacterium, which attributed such infecting traits to it. The surfactant produced by it was Rhamnolipid

Culturing of mutant strain of P.aeruginosa lacking the gene responsible for rhamnolipid synthesis, lead to coffee ring like bacterial spots on agar plate, confirming the role of the biosurfactant in ensuring the homogeneous growth/spread.

Significance of Countering Coffee Ring Effect to Bacteria:

Production of biosurfactants tends to provide the ability to swarm (spread on the surface) to the bacteria. The swarming increases as the population density of the bacteria increases (owing to greater production of the surfactant). This in turn enables the competing population to utilize the nutrients evenly, uniformily and maximally under the developing unfavorable condition(s). A clustered colony would tend to use the nutrients unevenly, with limitation of nutrient transfer to the core of the cluster and hence early death of the bacteria. It is thus a sort of response of bacteria to the unfavorable conditions, and the intimate need to counter the undesirable coffee ring effect situation.

Application of surfactant action in non-biological world:

Taking inspiration from the role of biosurfactant in ensuring homogenous spread of bacterial culture, if synthetic surfactants like detergents are tried on the normal coffee stain, it doesn't tend to reverse the coffee ring effect! Though it's surprising, but the explanation was given by Wouter Sempels et al., by asserting that surfactants can reverse the effect only in the colloidal solutions of very fine material/particles, as is the case of bacterial systems. The reversing of the coffee ring effect on fine particles by surfactants occurs by Marangoni flow (localized vortices & reverse capillarity). So, if one wants to reverse/avoid the coffee ring effect, use the particles of very fine size in the solution!

Industrial Implication of this research:
Clearly, the research has direct implications on the paint/varnishes/dyes industry. Where use of very fine particles with surfactants can make sure that the paint quality is superior in terms of the spreading properties. Further, addition of surfactants to nano-materials can also ensure the perfect distribution in the target site. The finding is indeed new and it's biotechnical/medical applications haven't been thought yet, but considering the easy availability of surfactants in the market, their overall industrial usability is expected to boom after this research. Apart from that, use of biosurfactants (which might be more compatible to biological systems) may get triggered in nano-technological research aimed at nano-particle delivery/distribution.

Suggested reads:

Wouter Sempels, Raf De Dier, Hideaki Mizuno, Johan Hofkens & Jan Vermant. Auto-production of biosurfactants reverses the coffee ring effect in a bacterial system. The Nature (April 2013)

Deegan, R. D. et al. Contact line deposits in an evaporating drop. Phys. Rev. E 62, 756–765 (2000)

Hu, H. & Larson, R. G. Marangoni effect reverses coffee-ring depositions. J. Phys. Chem. B 110, 7090–7094 (2006)
Sunil Nagpal
MS(Research) Scholar, IIT Delhi (Alumnus)
Advisor for the Biotech Students portal (
Computational Researcher in BioSciences at a leading MNC

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