Abstract
Many particulate pollutants in the environment exist in non-spherical shape, but the influences of particle shape on pollutant migration and removal in groundwater systems are not well-understood. In this work, we simulated the three-dimensional translational and rotational motions of rod-shaped colloids in simple flow channels characterizing groundwater flow paths, with an aim to elucidate the underlying mechanisms for rod retention. Through an investigation of the interplay of multiple factors (e.g., aspect ratio, particle size/density, flow shear, channel dimension, and orientation relative to gravity), we determined under what conditions particle shape has the most pronounced impact on transport and retention under favorable deposition conditions (i.e., lacking repulsive energy barriers). Our results showed that in many cases, medium sized rods of ~0.4–2 μm in equivalent volume diameter exhibited much improved retention compared to equal-volume spheres, since for that size range, particle rotation from shape-induced fluid hydrodynamics and rotational diffusion were both important, which caused rods to drift considerably across flow streamlines to intercept collector surfaces. Particle rotation also allowed rods to travel farther downstream along flow channels for retention compared to spheres. The differences in retention between rods and spheres were more evident at relatively high fluid velocity, narrow flow channel, or when flow direction aligned with gravity. Our findings demonstrated that the effect of particle shape on pollutant transport and migration in groundwater systems was essential and provided important guidelines in optimizing parameter designs to utilize particle shape effect for better pollutant removal.
Highlights
Ellipsoidal particles have been shown to display distinct motion from their homogeneous spherical counterparts of equal volume in viscous fluid flows (Jeffrey, 1922; Kuzhir et al, 2011; Zhao and Van Wachem, 2013)
Understanding the influences of particle shape along with factors mentioned above on particle transport and deposition is important to a variety of processes, including contaminant removal from groundwater flow (Chen et al, 2017; Ta et al, 2018), tracking contaminant fate (Mendez et al, 2018), design of microfluidic devices (Gervais and Jensen, 2006; Unni and Chun, 2009; Sen Gupta, 2016), in view of the fact that the majority of natural or engineered particles encountered in these processes, such as clay particles, heavy metals, agglomerates, bacteria, or polymer molecules are often non-spherical in shape (Salerno et al, 2006; Wang and Keller, 2009; Doshi et al, 2010)
In this work, using a simple geometrical model where fluid flow pattern can be maintained as laminar flow, we systematically investigated the rotation and retention dynamics of rod-shaped colloids under the coupled influences of many factors including particle size and density, aspect ratio, fluid velocity, flow direction relative to gravity, channel dimension, and size
Summary
Ellipsoidal particles have been shown to display distinct motion from their homogeneous spherical counterparts of equal volume in viscous fluid flows (Jeffrey, 1922; Kuzhir et al, 2011; Zhao and Van Wachem, 2013). In many cases, each research group had their own customized experimental setup (e.g., specific channel dimension or orientation relative to the direction of gravity, tailored channel surface properties), which rendered difficult to compare findings among different research groups (Nelson and Ginn, 2005; Seymour et al, 2013; Thompson et al, 2013) Another reason might be due to the varying surface properties arising from the manufacturing processes (e.g., stretching, molding, synthesizing) of non-spherical shape particles (Caldorera-Moore et al, 2010; Seymour et al, 2013; Ma et al, 2020)
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