Reevaluating the role of history effects on microplastic particles

Plastic pollution is wreaking havoc on the world’s oceans. One of the more pernicious forms is microplastics, which are small particles less than a few millimeters in diameter. Because these particles are so small, they can permeate every cubic inch of the ocean, and estimating their number and spread can be challenging.

Most researchers modeling the movement of microparticles in the ocean avoid solving complicated equations of motion. Even in studies using equations of motion, the nonlinear history effects are tricky to model, so most approaches assume these effects are negligible and can be safely ignored. Mary Eby and Cathal Cummins set out to quantify the scale of these history effects and determine when they should be included.

“As a particle accelerates [in a fluid], there’s this growing boundary layer around it,” said Cummins. “The force it experiences is not just the instantaneous force but also involves the entire history of the particle as its boundary layer grew from rest until its current velocity.”

To evaluate the impact of history effects, the authors employed a numerical simulation using a third-order quadrature scheme. With this method, they could compute the ratio of the magnitude of the history force to the Stokes drag over a range of particle sizes. They found that history effects become non-negligible above a diameter of about 280 µm for polystyrene particles in typical wave conditions, much lower than is commonly assumed.

For microplastic particles in this range, accounting for history effects means particles reach their settling velocities algebraically rather than exponentially, a potentially significant difference that could alter model results.

“If you’re modeling in the non-negligible regime, you really want to check our paper to make sure that you can get an estimate for the history effects,” said Cummins.

Source: “Memory effects in wave-induced microplastic transport,” by Mary Eby and Cathal Cummins, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0335447 .