Transport of rodlike colloids through packed beds

The effect of colloid shape on filtration rates in porous media was examined by constructing particles with different aspect ratios and measuring their retention in packed beds. Spherical polystyrene latex microspheres (1.0-μm diameter) were heated, stretched to the desired aspect ratio (2:1 and 3:1, with a 1:1 control), and quickly cooled. These particles were injected into minicolumns containing glass beads (40-μm diameter) in solutions at two different ionic strengths (IS = 1 and 100 mM). The measured retentions increased with aspect ratio in both IS solutions. The ζ-potentials for all three aspect ratios we re indistinguishable, and no charge nonuniformity was measured for any of the samples. Thus, the data support that changes in retention resulted from the different aspect ratios rather than from different surface chemistries. Interpretation of the retention data in terms of a collision efficiency (a) showed an increase with aspect ratio in both IS solutions, and for 1 mM the α increased from 0.011 (1:1) to 0.095 (2:1) to 0.26 (3: 1). These results demonstrate for the first time the direct impact of particle shape on retention in porous media. Our findings have important implications for the transport of particles with high aspect ratios, such as rod-shaped bacteria, and for the modeling of such transport.