Interactions and Aggregation of Colloidal Biological Particles

The separation of cells or particles from solution has traditionally been accomplished with centrifuges or by sedimentation; however, many particles have specific densities close to unity, making buoyancy-driven motion slow or negligible, but most cells and particles carry surface charges, making them ideal for electrophoretic separation. Both buoyancy-driven and electrophoretic separation may be influenced by hydrodynamic interactions and aggregation of neighboring particles. Aggregation by electrophoresis wm analyzed for two non-Brownian particles with different zeta potentials and thin double layers migrating through a viscous fluid. The results indicate that the initial rate of electrophoretically-driven aggregation may exceed that of buoyancy-driven aggregation, even under conditions in which buoyancy-driven relative motion of noninteracting particles is dominant. INTRODUCTION , Electrophoresis is a common analytical technique for separating charged biological particles and macromolecules. In an electric field, suspended particles will migrate at a velocity proportional to their surface charge and the applied field strength. Smoluchowski [l] showed that, for thin double layers, the electrophoretic velocity of an isolated particle is: VEjo= (:)E, , where e is the dielectric constant, 6 is the zeta potential of the charged par-ticle surface, E, is the applied electric field, p is the viscosity of the fluid, and the group in parenthesis is the electrophoretic mobility, p e , a physical constant of a given particle. Morrison [2] later showed that the electrophoretic mobility wm independent of the size and shape of the particle. In a heterogeneous suspension, particles with different surface charges will have different electrophoretic velocities, leading to the possibility that they will collide and aggregate [3]. Such aggregation inhibits the ability for different subpopulations of cells or other biological particles to be separated and is the subject of this article. Particle aggregation has been investigated theoretically for buoyancy-driven motion by Davis [4] and Melik and Fogler [5] who used a trajectory analysis to determine the aggregation rates. A similar analysis for electrophoretic particle aggregation is performed in this work and compared with the results for gravity-induced aggregation. THEORETICAL DEVELQ We restrict our attention to suspensions which are dilute, so that only two particles interact at one time. The particles are assumed to have negligible Brownian motion and the surrounding fluid is assumed to have negligible inertia. The zeta potentials are of moderate strength or less, and the electrical double layers are thin (&a > 1). These conditions are typically met for biological cells and other charged particles of 1-50 pm radius. To study coagulation and stability of dilute suspensions of spherical particles, a quantitative description of the relative motion between two spheres is needed. When the particles are far apart, this relative motion 379 https://ntrs.nasa.gov/search.jsp?R=19950008164 2018-04-03T21:05:08+00:00Z