3. A hemispherical droplet of liquid water, lying on a flat surface, evaporates by molecular diffusion

through still air surrounding the droplet. The droplet initially has a radius R. As the liquid

water slowly evaporates, the droplet shrinks slowly with time, but the flux of the water vapor is

a nominal steady state. The temperature of the droplet and the surrounding still air are kept

constant. The air contains water vapor of fixed concentration at an infinitely long distance from

the droplet's surface.

• Draw a picture of the physical process,

• Select a coordinate system that will best describe this diffusion process,

• List at least five reasonable assumptions for the mass-transfer aspects of the water-

evaporation process,

• Simplify the general differential equation for mass-transfer in terms of the flux NA.

• Specify the simplified differential form of Fick's flux equation for water vapor (species A)

• Identify the proper boundary conditions,

• Write down the differential equation for CA-