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Bacterial Aerosol Neutralization by Shock Waves Using a Novel Impactor: Design, Computation and Experiments

Patrick R. Sislian1, Lutz Madler2, David Pham1, Mingheng Li3, Xinyu Zhangh1, and Panagiotis D. Christofides4. (1) Chemical and Biomolecular Engineering, University of California, Los Angeles, 450 Hilgard Ave, Los Angeles, CA 90024, (2) Production Engineering, IWT Foundation Institute of Materials Science, University of Bremen, Badgasteiner Str. 3, 28359 Bremen, Germany, (3) Department of Chemical and Materials Engineering, California State Polytechnic University, 3801 West Temple Ave., Pomona, CA 91768, (4) Department of Chemical and Biomolecular Engineering, Department of Electrical Engineering, University of California, Los Angeles, Los Angeles, CA 90095

Neutralization of bacterial aerosol releases is critical in countering bio-terrorism. As a possible method that avoids the use of chemicals, we investigate the mechanical instabilities of the cell envelope in air as the bacteria pass through aerodynamic shocks. A novel impactor (the device consists of a converging nozzle through which aerosol flows perpendicular to the collection surface) is designed to simultaneously create a controlled and measured shock and to collect the bacteria. Both experimental measurements and computational fluid dynamics simulations are used to characterize the shock wave created. Using the impactor dimensions corresponding to our experimental system we could achieve a shock Mach number of 2.36. The shock presents a discontinuity in the fluid properties (i.e. velocity, pressure, temperature). Due to its inertia, the bacterial particle can not follow the changes in velocity, pressure and temperature. The bacterium therefore experiences strong deceleration as it passes through the shock. Rayleigh-Taylor instabilities are created due to this deceleration on the bacterium surface. The bacterial envelope becomes unstable at a critical deceleration which depends on the surface tension, density and diameter of the bacterium. Spores have a higher surface tension than vegetative cells, and hence, they are more difficult to damage mechanically. The particle motion through the impactor is calculated based on the gas flow simulations. The data indicates that the bacterium (assumed to be spherical of dp=1 micron) experiences decelerations of up to 4.4x109 m/s2 in the novel impactor system. E. coli, a vegetative gram-negative bacterium, requires an acceleration of 3.0x108m/s2 and we therefore predict that the novel impactor is capable of neutralizing this aerosol. On the other hand M. hugatei, an archaea, which we assume has properties close to B. atropheus, a spore forming gram-positive bacterium, requires an acceleration of 1.0x1012 m/s2 which can not be achieved by the current setup. We therefore predict that the impactor as currently designed will not be able to neutralize spores. The computational predictions are corroborated by experimental studies. Specifically,the experimental results indicate that spores of B. atropheus retain their viability for shocks with a Mach number of 2.36. Vegetative cells of B. atropheus, however, do not retain their viability after passing through the system both under shock and non-shock conditions. The nebulization of both types of cells is tested with the same dye and we conclude that this step has no effect on the viability of the cells.