Diffusion coefficient determines how fast the solute molecules move a certain distance in the solvent. Smaller molecules with higher diffusion coefficient will diffuse faster than larger molecules. So the difference in diffusion coefficients can be used to separate out two molecules from a solvent.
A microfluidic diffusion based extraction device is the H-filter or H-sensor as it is sometimes called due to its geometrical shape. A buffer liquid with two solutes A and B are forced through one inlet of the H-filter. A pure buffer liquid is forced at the other inlet arm. The two fluids are forced into a T junction. The width of the channels reduces beyond the intersection. The two fluids move side by side along the primary channel. Due to the laminar nature of flow in microfluidic channels, turbulent mixing is not present and only form of mixing is through diffusion. Solute A being small has a larger diffusion constant than solute B and will start to diffuse into the pure buffer fluid. Design of the H-filter should be such that the length of the primary channel is long enough to allow sufficient time for solute A to diffuse and not solute B.
This design form can be used to estimate the minimum length of the primary channel for separation of solute A from B. At this length the solute A concentration in the outlet will not be high but the number of larger solute B molecules will be minimal. Greater the length, larger the concentration of Solute A at the outlet. The velocity of the fluid can be estimated for the microchannel based on its shape from the section under Mechanics > Fluidics > Microchannels. The diffusion coefficient can be calculated from the section under Mechanics > Fluidics > Diffusion > Diffusion coefficient.
The plot shows the concentration field of solute A and solute B molecules at the end of the primary channel in a direction transverse to the flow as shown in the figure. This concentration profile is at the estimated minimum length of channel. When properly designed, the normalized concentration of solute B molecules will be near zero beyond half the width of the channel while that of solute A will be around 0.5.