Microtornadoes under a Nanocrystalline Igloo: Results Predicting a Worldwide Intensification of Tornadoes
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 6 1031-1034
Andrei P. Sommer* Materials DiVision, UniVersity of Ulm, 89081 Ulm, Germany ReceiVed February 22, 2007; ReVised Manuscript ReceiVed March 28, 2007
ABSTRACT: When a sessile water drop of radius R containing polystyrene nanospheres evaporates in an open Petri dish, the nanospheres eventually form a dense layer of nanoclay enclosed by a ring of radius r e R. When it evaporates in a closed Petri dish, the predominant structure is a crystalline ring with a thin nanoclay film in its interior. When it evaporates in a closed and well-sealed Petri dish, securing a constantly high vapor pressure, instead of a ring, a crystalline igloo with multiple cells emerges. Light microscopy complemented by scanning electron microscopy provides evidence for vortices in the igloo (one per cell), which could only be formed by microtornadoes. Tornadoes with intense vortices are distinctive for thunderstorm cells (supercells) in association with deep moist convection driven by pre-existing warmer humid air at the ground, overrun by a cold front. A precondition for realistic tornadogenesis studies based on experimental models is the coexistence of a humid air layer at the ground with a colder sheet aloft. This requirement is satisfied in the nanocrystalline igloo, with the evaporation-cooled roof, spatially separated from a warmer humid air layer on the substrate protected by it. Tornadogenesis studies based on local photographic documentation are a matter of luck because the lifetime of the events is usually short. The igloo model, because of its controllable parameters (vertical temperature gradient, humidity, and surface structure), suggests itself for a systematic analysis of the conditions favoring the emergence of supercell tornadoes. A mechanism for the formation of rings from sessile drops of evaporating suspensions was proposed 10 years ago.1 Since then, it has been successively improved, with the result that a new understanding of the ring formation mechanism has emerged.2-5 Of particular interest in this context are the symmetrical crystalline rings, formed in water drops containing nanospheres.6 Figure 1 is a comprehensive representation of the parameters involved in the formation of crystalline rings in drops of aqueous suspensions evaporating on substrates: (1) radial convection transporting the liquid and suspended particles to the periphery of the drop,1 (2) particle-substrate interaction,2 (3) particle-particle interaction,2 (4) an organized subaquatic water layer,7 (5) marginal liquid outflow,7 (6) vertical temperature gradient (∆T),8 (7) gravity (Fg),9 (8) the nature of the substrate,2 (9) the nature of the particles,2 and (10) evaporation time.8 Analysis of the effects of these parameters and their interplay proved instrumental in modeling processes in sessile drops of evaporating suspensions, which cover the complete architectural spectrum of possible patterns. Nanospheres contained in an evaporating water drop tend to form hexagonal crystal structures on a variety of substrates. Normally, the crystallization starts at the periphery of the drop, begins practically at the moment when the drop is placed onto the substrate, and continues until the water is consumed. The result is a more or less pronounced ring. In smaller drops evaporating in air, convective ring formation is assisted by a break-up process, specifically, the rupture and withdrawal of the internal water film. When the water vanishes before the suspended material is integrated into the ring, an incomplete ring is formed. This picture is consistent with the experience that slow evaporation rings are superior in symmetry and crystallinity to those produced by fast evaporation. Earlier, we demonstrated that on extremely smooth hydrophobic substrates, hydroxyapatite nanoparticles suspended in a water drop assembled into a crystalline layer under the influence of the contracting drop associated with its evaporation.2 Apparently, the bond between the hydrophilic nanoparticles and the carrier medium (water) was more effective than the anchoring of the nanoparticles to the substrate. A comparable interplay among the carrier medium, nanospheres, and substrate seems to come into action when the evaporation time is slowed to an extreme, even in systems recurrently found to form crystalline rings. Materials and Methods. Three water drops of 15 µL each containing 60 nm nanospheres (Duke Scientific, Palo Alto, CA) * E-mail:
[email protected].
Figure 1. Self-explanatory illustration of the parameters controlling the organization of nanospheres in an evaporating sessile water drop: (1) radial convection, (2) nanosphere-substrate interaction, (3) nanosphere-nanosphere interaction, (4) subaquatic water layer, (5) marginal outflow, (6) temperature gradient (∆T), (7) gravity (Fg), (8) substrate, (9) nanospheres, and (10) evaporation time. The interplay between the independent parameters 1 and 2 determines if a ring or a uniform deposition pattern is formed. Notably, not all parameters are independent. For instance, there would be no radial convection without marginal outflow.7
were placed onto the cover of a 35 mm Petri dish integrated into the evaporation chamber shown in Figure 2 (left panel). The chamber was designed by us for the analysis of the pattern formation parameters and their effects. Under ambient conditions, the time needed for complete evaporation of the drops was 191 h - a record in drop evaporation experiments. The igloo configuration shown in Figure 2 (right panel) is representative of the deposition pattern, produced by a 15 µL drop on the hydrophobic side of a 35 mm polystyrene Petri dish. The first interesting element in the light microscopy photograph is the deviation of the pattern from the (possibly anticipated) ring. A closer examination of the photograph exposes microscopic rings within the crystalline nanosphere fields separated by crackings - one ring per compartment. This feature is surprising. In order to gain access to the unexpected microrings, we carefully separated the roof of the igloo from the substrate beneath using a surgical blade. The scanning electron microscopy (SEM) image in Figure 3 reveals that the microrings consist of numerous nanospheres, arranged in vortices on the substrate. Results and Discussion. A synoptic view of the Figures 2 and 3 leads to the following results: (1) The igloo roof is highly transparent and crystalline, allowing one to image the microrings through it. The crystallinity was confirmed by SEM. (2) The nanospheres suspended in the drop do not form one ring.
10.1021/cg070182+ CCC: $37.00 © 2007 American Chemical Society Published on Web 05/01/2007
1032 Crystal Growth & Design, Vol. 7, No. 6, 2007
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Figure 2. (Left) Evaporation chamber designed for the analysis of extended evaporation time effects in drops. The record for the evaporation of three drops of a water-based nanosuspension of 15 µL each was 191 h. (Right) Light microscopy photograph of a nanocrystalline igloo formed by 60 nm nanospheres on the substrate. The crystalline roof is transparent, exposing numerous microrings on the substrate. Crackings formed spontaneously in the final phase of the drying and reveal pre-existing compartments. Diameter of the igloo ∼ 2.1 mm.
Figure 3. Scanning electron microscopy image of the substrate under the igloo roof shown in Figure 2. The arrow points to the compartment shown in the high-resolution representation on the right. Note the one-to-one ratio between microrings and compartments.
(3) Prior to solidifying, when numerous crackings become visible in the drying igloo, which changes its appearance from opaque to glassy, microrings are formed. Their prominent order - the oneto-one ratio of the microrings to the igloo compartments - raises the question about the cause of their formation. Their characteristic distribution and vortical structure suggest that they were neither formed by radial convection (parallel to the substrate) nor by the interplay of gravity with ground variations forming stone rings in polar regions10 - excluded by the smooth and non-deformable substrate. Thus, we have here an entirely new effect. To understand it, it is instructive to establish a connection between the nanocrystalline igloo and supercell tornadoes (which occasionally form multiple-vortex funnels). In contrast to hurricanes, which are known to lose intensity when they leave the ocean, tornadoes form principally over land. Tornadoes with intense vortices are distinctive for supercell thunderstorms with vertical convection established between pre-existing warmer humid air at the ground, and a cold front aloft. This combination is fundamental for the formation of violent tornadoes. In principle, the cold front could consist
of dry air or a cloud (e.g., with glaciation at its top). The analogy with the evaporation-cooled igloo roof and the warmer humid substrate protected by it suggests itself. The prerequisite for the formation of an intense vortex is further vertical and/or horizontal wind shear: As the warmer, humidity-laden air ascends, it is forced to turn due to the wind speed and/or direction varying with altitude and/or wind speed varying in a plane. Presumably, wind shear is also active in the drying igloo. This could be justified from consideration of the asymmetric geometry of the cells as well as the asymmetric crackings through which the trapped humid air must escape in the last phase of the evaporation. As shown in our previous work, the nanoscopic (crystalline) water layers on hydrophobic substrates presented an unusual stability and did not evaporate instantly.7 This aspect, on the one hand, and the capacity of the nanocrystalline igloo roof to store humidity in the space between the nanospheres (the factor stabilizing sand castles),11 on the other hand, makes it clear that the assumption of a coincidence of the three principal tornadogenesis factors in a drying nanocrystalline igloo is probably realistic: (1) vertical temperature gradient
Communications (external roof ) cold side), (2) relatively warm humid air at the ground, and (3) wind shear. If indeed processes in real tornadoes can be described in a nanocrystalline igloo, it is challenging to apply the model to the analysis of processes in supercell tornadoes. At present, tornadogenesis analysis is largely restricted to the interpretation of photographic documentation. The material that exists is meager because the lifetime of tornadoes is frequently short and their photographic capture is a matter of chance. An important debate has focused on the question of whether supercell tornadoes descend from the cloud base or build upward from the boundary layer.12 Such questions could be answered on the basis of the model presented in this work. The approach is so simple that it could be technically realized by the use of translucent igloos built from nanospheres or microspheres, or other materials forming a nanoporous or microporous roof, with the capacity to temporarily store water. By simultaneously wetting the roof of such an igloo, if necessary, and injecting minimal amounts of water containing nanospheres into it, it should be possible to mimic basic processes in tornadoes experimentally and to explore the impact of relevant boundary conditions including terrain roughness and cloud cover stability. Numeric models used in meteorology subdivide extended territories into areas of several square kilometers. On this scale, tornadoes are virtually invisible. Consequently, meteorologists cannot do more than observe the conditions favoring the formation of tornadoes. Because of ample evidence for a worldwide intensification of tornadoes, laboratory models facilitating a better understanding of their mechanism should be welcomed. Previously, we concluded that by acting as transient cloud condensation nuclei (TCCN), pumping energy into the system cloud, nanoaerosols are likely to play a key role in the intensification of tropical cyclones.7 Interestingly, an equivalent intensification can be predicted from the igloo model for supercell tornadoes. The coincidence probability can be formulated as the product of two lifetimes - that of a warm humid air reservoir at the ground and that of a cold front aloft. The increase of one factor is sufficient to increase the probability for the formation of a tornado from the ascending humid air. For cold fronts associated with clouds, an extension of the lifetime of the cold fronts follows from the potential of nanoaerosols - a fraction of the urban, industrial, and natural air pollution - to act as TCCN.7 Urban air pollution is known to suppress precipitation.13 In principle, this suppressive effect is inversely proportional to the size of the cloud condensation nuclei (CCN) and proportional to their concentration and capacity to bind water vapor, thereby favoring nanoaerosols, particularly when hydrophilic. In the presence of a substantial number of nanoaerosols in a cumulus cloud, a column of ascending humid air would, instead of triggering precipitation, invigorate convection and/or extend the lifetime of the cloud, thereby increasing the probability for the aforementioned coincidence of events. In the absence of a substantial number of nanoaerosols, even a few larger CCN could collect, in a sufficiently short time, enough water vapor to form drops, which could reach the size and the terminal fall velocity necessary to trigger precipitation. Reciprocally, clouds that live longer are automatically better scavengers for nanoaerosols, which could enter a cloud from all sides, including the stratosphere. This aspect deserves our attention because the nanocrystalline igloo is a suitable platform for the analysis of the impact of anthropogenic effects on microphysical phenomena in clouds. Phenomena include the effects of crystalline water layers in general, and processes of nucleation, crystallization, and electrification in particular,14 whereby it is crucial to discriminate between processes over land and those over oceans: Normally, clouds over land are more exposed to sources of solid nanoaerosols than those over oceans. Clouds with a tendency to live longer, in particular over land, have been discussed tentatively.15 A lack of reliable satellite data, reflected by the recent debate on cloudiness,16 suggests that new methods for the measurement of the concentration and size of nanoaerosols in the atmosphere are essential. A practical method, which consists of the analysis of nanoaerosols collected in precipitation and
Crystal Growth & Design, Vol. 7, No. 6, 2007 1033 monitoring of the processes of electrification and crystallization in clouds by satellites, was proposed by us earlier.14 From the perspective of the coincidence of events, it is now probable that sulfate nanoparticles placed into the dry stratosphere to cool the planet17 could extend the lifetime of clouds upon their return to the earth - by binding the water available in the cloud and by enhancing the water binding capacity of less hydrophilic compounds.18 Such complex processes, and their possible effect on tornadogenesis, merit concern in predictive models and could be explored by varying the polarity of the materials used in the nanocrystalline igloo model. More research is now needed to better understand climatic effects of nanoaerosols. This implies the development of methods to collect and identify nanoaerosols in clouds. Conclusions. Nanocrystalline igloos represent a new organization pattern in drops of evaporating suspensions. The conditions causing the formation of multiple vortices in the igloos have been identified as the necessary conditions for the formation of supercell tornadoes. In both systems, the central element is the coincidence of a vertical temperature gradient with relatively warm humid air at the ground and wind shear. The one-to-one relation between the microtornadoes and the igloo compartments is a strong indication that supercell tornadoes form automatically when the conditions are favorable. From the perspective that a substantial amount of nanoaerosols extend the lifetime of convective clouds and/or invigorate convection in convective systems,7 thereby increasing the probability of a coincidence of events favoring tornadogenesis (triple coincidence), we also obtain a simple explanation for the irregular emergence of violent hurricanes that are likely to be triggered by a similar coincidence of events, instead of being caused solely by ongoing ocean warming.7 Under these circumstances, pursuing plans to use sulfate nanoaerosols to cool the planet appears to be irresponsible. The possibility to reproduce this triple coincidence in a reducedscale system should facilitate a systematic analysis of the impact of the parameters implicated in the evolution of supercell tornadoes - an important advance in meteorological modeling. A second application of the model is to evaluate effects of additives (e.g., toxins) on cell attachment using relevant substrates and living cells suspended in slowly evaporating drops of culture medium. A third is to evaluate and fine-tune the attachment of differently functionalized nanoparticles to biomaterial surfaces, including the design of nanocrystalline rings with apertures precisely adjusted to the size of cells - a biomimetic method expected to open new routes to produce cell selective surfaces.8
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