New insight of biophysical aggregates' geometric distributions from side and bottom views during their flocculation and settling in saline water.

Biophysical mud aggregates, consisting of clay minerals and extracellular polymeric substances (EPS), play critical roles in water quality and aquatic ecosystem health. However, the geometric properties from different side views and flocculation dynamic behavior of these aggregates with highly irregular shapes in saline water environments remain poorly understood, particularly under the combined effects of buoyancy and gravity during settling processes. This study introduces a novel dual-view approach, combining an inverted depth-of-field microscope and side camera, to capture high-resolution images of floc geometries from both side and bottom perspectives. A series of laboratory-controlled experiments were conducted to investigate floc formed from pure clay mineral and clay-EPS mixtures, analyzing their size, shape and settlings. The results reveal significant discrepancies between side-view and bottom-view measurements, demonstrating that bottom-view imaging provides a more accurate understanding of floc geometry considering buoyancy and gravity during flocs settling in saline water. Moreover, flocs formed from clay-EPS mixtures exhibited greater size and irregularity compared to those formed solely of clay minerals. EPS-mediated biological cohesion was found to promote the formation of larger, stable macroflocs (≥200 μm) capable of maintaining their structure under turbulent conditions, while pure mineral flocs remained smaller and more compact. Despite increasing floc size, EPS reduced effective density, resulting in overlapping settling velocities between macroflocs and microflocs (<200 μm). This study provides a foundational dataset and innovative methodology for analyzing mud floc geometries, offering new insights into the role of bio-cohesion in natural sediment flocculation dynamics. These findings underscore the need for flocculation models to incorporate organic influences and advocate for multi-perspective observations to improve predictions of fine-grained sediment transport in saline environments.