The effects of particle size on the deposition of fluorescent nanoparticles in porous media: Direct observation using laser scanning cytometry

The effects of particle size on the deposition of fluorescent nanoparticles in porous media were investigated using a microfluidic flow cell in conjunction with a laser scanning cytometer (LSC). All nanoparticle sizes investigated, i.e., 57 nm, 210 nm, and 510 nm particles, were found to attach to both the upstream and downstream surfaces of the collector, indicating that diffusion is a controlling mechanism for the transport of nanoparticles in this size range. Under an unfavorable condition (3 mM NaCl, pH 7), the 510 nm particles achieved a maximum retention capacity after the injection of 40 PV, 0.0025% particle suspensions; however, the retention capacity was not yet achieved for the 57 nm particles after the injection of 300 PV, 0.0025% particle suspensions. Under a favorable condition (100 mM NaCl, pH 7), the 57 nm particles seemed to be more sensitive to changes in the water chemistry, showing more obvious mobilization and reattachment after the injection of DI water. The size dependent attachment rate and size dependent sensitivity to the surface heterogeneity are considered to contribute to the observed different behaviors of these particles.