Exercise 4: Flea bombs

A room fogger sprays a flea-killing insecticide throughout a room at 20oC with a 3 meter high ceiling. If the droplets range in diameter from 5.0 x 10-4 cm to 2.0 x 10-3 cm, how long should one wait before re-entering the room? Assume the density of the droplets is the same as the density of water. Use the low Reynolds number model and then after solving the problem, verify that the Reynolds number is in the appropriate range.

Note: We are ignoring two factors which also effect the settling time-the interaction of the droplets, which is significant when the distances between the droplets is small, and the random Brownian motion of the particles, which is significant if the particles are very small (D < 5 10^-4 cm).

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