## Filters

H-Filters function analogously to hemocoels. H-Filters continuously extract specific molecules from mixed solutions that may contain interfering particles (cells, debris). Like hemocoels, H-filters lack membranous filters that must be cleaned periodically. Because the Reynolds numbers of H-filters are less than one, and flows are slow, convective mixing does not occur. Diffusion alone transports molecules transversely across the channel of an H-filter. The root mean square distance a molecule travels in an interval is the square root of 2Dt (the Einstein equation) where D is the molecular diffusion coefficient. D scales roughly with a molecule's size, so if we ignore charge, small molecules flow faster than larger ones. The time to traverse a channel within an H-filter is proportional the channel's length, so molecules having different diffusion coefficients separate themselves along the channel. As with a five-yard dash compared with a mile race, however, slow runners distinguish themselves from faster runners only when the course is long enough to permit their separation.

Workable reproducible assays utilize the profiles of velocity for molecules in channels having different aspect ratios. Higher aspect ratios permit quantitative studies. Because the relative velocities of two fluids moving in a channel can determine the width of the stream flowing across the width of an H-filter, we can use this idea to create 3-D models. If two fluids have the same viscosity, each occupies half the channel. But if a fluid is paired with a fluid of a higher viscosity, the fluid of higher viscosity flows slower and comes to occupy a greater portion of the channel (Ref: Yagerfaculty).

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