Fick's Law Animation Video Lecture | Mass Transfer - Chemical Engineering

when molecules or small particles are
suspended in a liquid two opposing
forces can dictate the overall
composition of the mixture the force of
gravity depends on the density of the
particles while the buoyant force is
determined by the density of the liquid
if one force dominates the other
particles in the suspension will either
cream or sediment density is denoted by
the Greek letter Rho if the density of
the particles is less than the density
of the liquid they are suspended in the
particles rise or creep if the density
of the liquid is less than the density
of the particles then the particles sink
or sediment if the two densities are
almost equal or if the net force is too
small to direct the motion of the
particles the behavior of the suspension
is dictated by random Brownian motion if
you were to track these diffusing
particles on average there is no net
motion despite this we know that
molecules can diffuse into and out of
cells motion with a defined direction so
how is this possible
consider a collection of molecules near
the membrane of a cell assuming the
molecules can freely cross the membrane
some of the molecules will diffuse into
and out of the cell since diffusion is a
random process the number of molecules
leaving the cell will be some percentage
of the total number of the molecules
inside the cell the same is true of the
molecules on the outside of the cell
over time a fraction of these molecules
will randomly pass through the membrane
into the cell so despite the random
motion of each molecule a net inward
flow will happen if the concentration
outside the cell is greater than inside
we can now define a quantity known as
flux which is represented by the letter
J the flux is defined is the rate of
flow DN DT per unit area a across the
membrane the flux will depend on the
difference in concentration Delta C the
separation between the two regions Delta
X and the mobility of the molecules is
given by the diffusion coefficient D if
we can determine the flux of molecules
and we know the total area of the cell
that we can find the total rate of flow
of molecules into the cell
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