CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2373–2375
Communications Microtornadoes under a Nanocrystalline Igloo. 2. Results Predicting a Worldwide Intensification of Tornadoes Andrei P. Sommer* and Dan Zhu Institute of Micro and Nanomaterials, UniVersity of Ulm, Ulm 89081, Germany ReceiVed June 18, 2007; ReVised Manuscript ReceiVed September 2, 2007
ABSTRACT: We recently documented the formation of microtornadoes in a nanocrystalline igloo grown in the laboratory. The igloo had 60 nm nanospheres as building blocks. The existence of the microtornadoes was derived from a vortical arrangement of nanospheres on the Petri dish forming the igloo base. A large body of parallels between the igloo-system and tornadic clouds attracted the interest of tornado researchers, who encouraged us to provide more details on the mechanism of formation of microtornadoes. Tornado research faces a turbulent period with an apparent intensification of tornadoes (in number and violence), but a lack of models that permit their reliable prediction in time. For this we focused on strategies to scale up our miniature igloo and to build larger systems, including nanocrystalline tents with a central stick. This involved the use of both larger drops and larger building blocks (200 nm nanospheres). Larger systems promised to facilitate further insight into details. To our surprise, systems built by 200 nm nanospheres tended to collapse. Therefore, we returned to think small and produced a second generation of nanocrystalline igloos based on 60 nm nanospheres, including ones with a diameter of 4 mm, twice the size of our first igloo. A closer inspection of the igloo compartments revealed a series of previously unrecognized details. Here we report the prevalence of congruent vortices on both sides of the system generating the microtornadoes: in the evaporation cooled igloo roof and on the warmer humid substrate beneath. This is the specific signature of one violently rotating column of humid air interconnecting igloo and substrate, an analogue to real tornadoes interconnecting a wall cloud with the ground. In the first part of our work, we showed that the basic processes and boundary conditions implied in the formation of microtornadoes in the cells of a nanocrystalline igloo are equivalent to those favoring the genesis of tornadoes in supercell systems.1 This included the coincidence between vertical convection of warmer humid air, an evaporation-cooled igloo roof, humid air entrapped in the cells (compartments) of the igloo, and wind shear. Despite these parallels and the pronounced capacity of a drying igloo to store substantial amounts of water vapor in the space between the hydrophilic nanospheres, thereby recommending itself as an analogue to a wall cloud (Figure 1), the short lifetime of the microtornadoes, posing practical limits to their real-time observability, and a size ratio of 1:1 000 000 between the model and a real system, raised the question on the suitability of the model to simulate real tornadoes in the laboratory.2 Previously, we came to the conclusion that the vortical structures, identified by scanning electron microscopy (SEM) on the Petri dish after the removal of the igloo (Figure 3 in ref 1), were formed by microtornadoes. Here we raise the question on the origin of the nanospheres in the vortical structures. A closer inspection of the distinct distribution of the nanospheres in the central igloo cells shows that the question is in no way trivial: The SEM image provides evidence that except for the nanospheres constituting the vortical structures, microtornadic debris is virtually absent in the three central cells. Actually, this observation stimulated our present study. Noting the structural similarity of the three distinctive * To whom correspondence should be addressed. E-mail:
[email protected].
Figure 1. Wall cloud with supercell crossing Ulm, Germany, on May 14, 2007 (19:08). Severe wind (19:12) and hailstones reaching 3 cm in diameter followed (19:13). Tornadoes are frequently observed in the narrow space between wall clouds and the ground. As in a nanocrystalline igloo, in which the compressed humid air liberated from the space between the drying (contracting) igloo and the Petri dish produces wind, the air compressed by a propagating wall cloud forms severe horizontal winds. In the igloo, horizontal winds result necessarily from the humid air escaping with the propagation of lateral crackings (Figure 2 in ref 1).1
10.1021/cg070550+ CCC: $37.00 2007 American Chemical Society Published on Web 11/20/2007
2374 Crystal Growth & Design, Vol. 7, No. 12, 2007
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Figure 2. Optical microscopy photograph of tornadic cell at the periphery of a nanocrystalline igloo. The arrow pointing to the left reveals a 70 µm wide ring on the Petri dish. As documented (Figure 3 in ref 1), such rings are formed by 60 nm nanospheres. The arrow pointing to the right reveals a congruent ring in a fragment of the nanocrystalline igloo, originally covering the Petri dish and forming the edge of an igloo. The diameter of the ring in the fragment is practically equal to that of the nanospheres-free ring eye on the Petri dish. Color facilitates the simultaneous visibility of both structures.
microtornadic patterns in the center of the igloo to those at its periphery (Figure 3 in ref 1), we turned our attention to an analysis of the numerous smaller cells forming satellite vortices at the periphery of the igloo. We placed 35 mm Petri dishes carrying dried igloos on an optically reflecting substrate (silicon wafer) and used an optical microscope operating in the reflection mode for imaging. Using this method, we checked a considerable number of igloos. The technique to produce nanocrystalline igloos, and the model employed to interpret the results, were communicated in the first part of our work. Some of the peripheral igloo cells were close to their original position, and others were missing. Figures 2 and 3 are representative of such scenarios. Both images reveal nanocrystalline igloo fragments (intact cells) and expose subtle, but clearly visible, ring- and correlated vortical structures. Earlier we concluded that microtornadoes form prior to the solidification of the igloo, presumably while the side facing the Petri dish (wet side) is soft compared to the external roof. The vortical imprint discovered in a nanocrystalline igloo fragment shown in Figure 4 supports this view. Indeed, it indicates an unexpected violence of the microtornadoes, which seem capable of printing their characteristic signature into the drying igloo. The precondition for such violence is intense wind shear. The specific distribution of the vortical structures in the three large central cells on the Petri dish, with a lack of nanospheres in the field external to the vortical structures, and the nanospheresfree microtornadic eyes (Figure 3 in ref 1), leave room for the possibility that the microtornadoes were involved in both processes: the liberation and the transport of the nanoprecipitate. An inescapable conclusion following from completing the puzzle, that is, the congruence of the vortical patterns in a nanocrystalline igloo and
Figure 3. Optical microscopy photograph of tornadic cell at the periphery of a nanocrystalline igloo. The arrow points from a tornadic ring on the Petri dish to the associated igloo fragment. As in Figure 2, the diameter of the nanospheres-free ring eye on the Petri dish is practically equal to that of the congruent ring in the associated cell.
on the substrate beneath, is that they result from microtornadic activity. Their independence on the horizontal extension of the igloo cells indicates that the cell size is less important for the formation of microtornadoes than other factors, that is, deep moist convection and wind shear. Size and vorticity of the patterns in the central and peripheral fields on the Petri dish (Figures 2 and 3 in ref 1)
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Crystal Growth & Design, Vol. 7, No. 12, 2007 2375 indicate, however, that the intensity of the microtornadoes increases with cell size. Whereas it is clear that more data are needed on both sides, tornado research and simulations in the laboratory, the nanocrystalline igloo system seems to provide interesting insights into the relative importance of the boundary conditions known to play a role in the genesis of tornadoes. Despite a prominent dimensional gap between igloo cells and supercells, the model promises to become a valid tool to simulate tornadoes in the laboratory. It is worth noting that one model has now been fruitfully applied in disciplines as different as meteorology and biomedical engineering, where it inspired potent strategies to reverse processes of biological aging.
References (1) Sommer, A. P. Microtornadoes under a Nanocrystalline Igloo: Results
Figure 4. Optical microscopy photograph shows isolated nanocrystalline fragments grown at the periphery of an igloo. The piece in the middle carries as an imprint the signature of tornadic activity. Inset shows the backside of the central fragment in reverse color representation. The translucency of the crystalline igloo complicates a precise in depth localization of the vortex.
Predicting a Worldwide Intensification of Tornadoes. Cryst. Growth Des. 2007, 7, 1031–1034. (2) Dumé, B. Lab-made ‘microtornadoes’ mimic the real thing. New Sci. 2007, May 14.
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