Concentration of Bioparticles by Axial Migration in Laminar Flow

Dilute suspensions of Spirulina microalgae were pumped in laminar flow thru a 650‐micron diameter vertically mounted tube. Photographs of the tube bore at various downstream positions from the entrance at different Reynolds numbers indicate significant migration of the particles. Equilibrium radial position appears to be established within 30 cm from the tube entrance. A concentrated region of particles occupies a decreasing cross‐sectional area as the Reynolds number increases. Based on an order argument involving a balance between inertial and viscous forces, a relationship is established between the diameter of the concentrated particle region (dimensionless) and the tube Reynolds number, which provides a good fit of the data. The form is similar to an empirical form for spheres with one major difference: the helical Spirulina particles migrate inward with increasing Reynolds number whereas the spheres migrate outward. Since the Spirulina are individually aligned in the flow direction and since nonrotating spheres are known to migrate inwards, we believe that nonrotation might explain the inward migration. Since the particles migrate to a narrow region around the tube axis, Bioseparation potential is discussed and a comparison is made with microfiltration, which indicates significant advantages for an effective separator based on axial migration.