$($Welty $25\mathrm{.}3,66^{th}$ edition$)$ 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 at 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. After drawing 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, and

simplify the general differential equation for mass transfer in terms of the flux $N_A.$ Finally,

specify the simplified differential form of Fick's flux equation for water vapor $($species A$),$ and

propose reasonable boundary conditions.