Simulating the mobility of micro-plastics and other fiber-like objects in saturated porous media using constrained random walks

This article details an approach for modeling the motion of large, flexible, fiber-like colloids, including micro-plastic fibers, through porous media. Fibers and other fiber-like colloids, such as bacteria, have unique shapes and properties that make modeling them more difficult than dissolved solutes, and necessitate unique tools. The approach uses the concept of a bead-rod chain to discretize the fibers as discrete objects that move through the flow field. Each node of the chain is moved independently via a classical random walk but we enforce the constraint that the length of the fiber cannot change over time, which effectively adds a force balance to the random walk problem. After a brief illustrative example, our main question evaluates some of the tradeoffs between pore-water velocity, fiber length, and the size of the pore spaces in a 4 cm by 12 cm synthetic model of pore-scale flow in granular material. We find that fiber length is generally correlated to retardation, relative to a passive tracer, and the magnitude of retardation tends to increase with increasing average pore-water velocity. However, the simulations also show that, in some cases, fibers shorter than the average pore-opening can pass through the experiment faster than a solute. This alludes to the complexities and sometimes counter-intuitive nature of the transport behavior of discrete, non-dissolved objects through porous media, which merit experimental study in the future to validate our results against experimental data and to refine the proposed hypothesis testing platform.