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Characterizing floating particle clustering in free surface turbulence using LED-based PIV
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When microplastics float on the water surface, turbulence causes them to cluster together rather than spreading evenly — a behavior that influences how they accumulate and where they concentrate in aquatic environments. This laboratory study used LED-based particle imaging to precisely track how floating particles form clusters under different turbulence levels, finding that stronger turbulence causes faster clustering and that clustering happens in two distinct phases. Understanding these clustering dynamics is important for predicting how microplastics accumulate at ocean surfaces and designing more effective collection strategies.
Abstract Dispersed two-phase flows at air–water interfaces are ubiquitous in environmentally relevant flows such as in the dispersion of floating microplastics or transport processes across the air–sea interface. In the current study, we propose a method to study such flows through the study of a relatively flat turbulent free surface laden with spherical floating particles (“floaters”). The free surface is perturbed by a relatively low-mean nearly homogeneous subsurface turbulent flow that is produced in a turbulence box actuated by a 10 $$ imes$$ × 10 synthetic jet array. The free surface flow field is characterized using planar particle image velocimetry (PIV) simultaneously with Lagrangian tracking of floaters allowing insight into the floater dynamics and the surface flow coupling. This is enabled by a relatively simple setup of LED panels and a single camera. Distinction between the continuous (flow tracers) and the dispersed (floaters) phase is carried out by exploiting their size disparity and number density. The proposed method is employed to characterize the single-phase flow field and the clustering statistics of floaters for different turbulence levels, the latter achieved by varying the distance of the free surface from the jet array. Specifically, we study the effect of different turbulence levels on the floater clustering behavior. We observe that the time required for floaters to reach a clustered quasi-steady state decreases with increasing vorticity and surface divergence amplitude. In addition, the growth rate of the mean cluster size is observed to increase with increasing vorticity and surface divergence amplitude, with its temporal evolution exhibiting two distinct phases: an agglomeration phase and an equilibrium phase. In contrast, in the absence of a subsurface flow, floaters are observed to cluster at a relatively slower rate characterized by a prolonged agglomeration phase. Finally, to highlight the potential of this technique in studying floater-laden turbulent free surfaces, preliminary results of flow–floater interactions are discussed.
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