11942
Langmuir 2007, 23, 11942-11942
Reply to Comment on Evaporation-Induced Patterns from Droplets Containing Motile and Nonmotile Bacteria
In a letter published in Langmuir,1 we had reported the use of nutrient-free aqueous dispersions of Escherichia coli and other bacteria in experiments involving the evaporation of droplets containing bacteria. Furthermore, in chemotaxis, bacteria move toward higher concentrations of foods and nutrients for survival, as opposed to the reported studies where the bacterial droplets were completely free of those. Therefore, bacterial chemotaxis, for movement either toward the central or peripheral direction of the evaporating droplet, may not be applicable in the reported results. In addition, the situation in an evaporating sessile droplet cannot possibly be the same as in common microbiology experiments, and hence a one-to-one correlation may not be valid. In other words, the situation in a test tube full of nutrients and bacteria cannot be considered to be identical to that in a rather small evaporating droplet containing only bacteria. However, if one considers the number of bacteria (and corresponding volume of liquid) present in the periphery versus that in the apex of the droplet, it may be pertinent to consider that some of the bacteria may indeed move away from the highly concentrated (and containing small volume of liquid) edge of the droplet toward higher volume and lower bacteria containing the apex of the droplet. This could especially be true when there are no nutrients present anywhere in the droplet and the bacteria are motile. In particular, the comment2 “In other words, the bacteria assumed by the authors to migrate against the convective flow were most likely simply more sticky than the nonmotile ones, which formed a distinct ring.” goes against conventional knowledge about the bacteria. In particular, the bacteria (recombinant GFP expressing E. coli DH5-R) that have been used by us are not known to be sticky. It is difficult to fathom
that motile bacteria would stick to glass surface and nonmotile ones (of the same kind) would not. Hence, the stickiness of the motile bacteria may indeed not be the cause of disclike pattern formation. Furthermore, in the letter,1 we had written “The formation of so-called ‘coffee rings’, in systems involving hydroxyapatite nanoparticles, coffee, and polystyrene latex microspheres, has been explained using the concept of contact-line ‘pinning’ and enhanced evaporation from the edge of the droplet in comparison to its free surface.” This, however, is not any way in conflict with the observations and conclusions made by Sommer and coworkers in their articles,3,4 as has been claimed (to the contrary) in their comment.2 In other words, the formation of coffee-ring stains, which have also been observed in the aforementioned work of Sommer and co-workers, has been explained by invoking contact line pinning and enhanced evaporation from the edge of the droplet in comparison to the apex. Although Sommer and co-workers reported in their papers that contact line pinning is not essential, they did not exclude contact line pinning and enhanced evaporation from the edge as sources of ring formation. The above statement1 made no comments (and drew no conclusions) on the so-called “uniform” depositions observed in the work of Sommer and co-workers involving hydroxyapatitie particles. Thus, the sentence mentioned above need not be considered as misinterpreting the patterns observed by Sommer and co-workers.3,4 Tittu Thomas Nellimoottil,‡ Pinjala Nagaraju Rao,| Siddhartha Sankar Ghosh,*,†,‡ and Arun Chattopadhyay*,†,§
Centre for Nanotechnology, Department of Biotechnology, Department of Chemistry, and Department of Chemical Engineering, Indian Institute of Technology, Guwahati-782031, India
* Corresponding authors. E-mail:
[email protected];
[email protected]. † Centre for Nanotechnology. ‡ Department of Biotechnology. § Department of Chemistry. | Department of Chemical Engineering. (1) Nellimoottil, T. T.; Rao, P. N.; Ghosh, S. S.; Chattopadhyay, A. Langmuir 2007, 23, 8655-8658. (2) Sommer, A. P.; Zhu, D. Langmuir 2007, 23, 11941.
ReceiVed August 22, 2007 LA7026048 (3) Sommer, A. P.; Rozlosnik, N. Cryst. Growth Des. 2005, 5, 551-557. (4) Sommer, A. P.; Gheorghiu, E.; Cehreli, M.; Mester, A. R.; Whelan, H. T. Cryst. Growth Des. 2006, 6, 492-497.
10.1021/la7026048 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/13/2007
