A system for sterilizing a contact lens before they are packaged for shipment consists of spinning the contact lens in a solution comprised of a disinfectant dissolved in water. The volume of this disinfectant solution surrounding the contact lens is 50 ml. The disinfectant material has a diffusivity in water of 7.5 × 10−6cm2s−1, the viscosity of the solution is 1 cP, and its density is 1 g cm−3. The initial concentration of the disinfectant in this solution is 7.5 mg L−1. The lens is spun at 100 RPM. How long will it take in hours for the disinfectant concentration to decrease to 1 mg L−1assuming that the disinfectant agent reacts instantaneously at the surface of the contact lens? You can treat the contact lens as a flat disk that is 14.5 mm in diameter.
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