PARTICLE CLUSTERING AND PATTERN-FORMATION DURING ELECTROPHORETIC DEPOSITION - A HYDRODYNAMIC MODEL

Clustering of latex particles 4-10 mu m in diameter during and after e lectrophoretic deposition of the particles onto flat electrodes has be en reported by Bohmer (Langmuir 1996, 12, 5747). The particles interac ted over length scales comparable to their size in the formative stage s of the clusters. Combinations of two or more clusters already deposi ted approached each other to form larger agglomerates. A model based o n electroosmotic flow about charged particles near surfaces is develop ed here to explain these observations. A charged, nonconducting partic le near or on a flat conducting surface creates flow in the adjacent f luid due to electroosmosis about the particle's surface. Fluid is draw n laterally toward the particle near the electrode and pushed outward from the particle farther away from the electrode above the particle. Another particle near the electrode will be drawn toward the central p article by this convection. We first solve for the flow field about a single particle and then compute the rearrangement of neighboring part icles in response to the flows. The clustering times for different ini tial configurations of sets of particles (e.g., regular versus irregul ar spacing) are calculated. The average clustering times for irregular configurations are greater than those for regular arrays. The qualita tive and quantitative features of the experimental observations are ca ptured by this model if the hindrance effect of the solid wall is take n into account. For example, the model correctly predicts the observed declustering (separation) of particles when the polarity of the elect ric field is reversed as well as the observed cluster-to-cluster motio n.