Particle flocs in homogeneous isotropic turbulence: formation, destruction and lifetime

Flocculation of suspended cohesive particles in turbulence plays a crucial role in various natural and industrial processes. In this study, we carry out direct numerical simulations with a cohesive discrete element method to investigate the evolution of cohesive particle flocs in homogeneous isotropic turbulence. We focus on how particle flocs are formed and destructed in turbulence, and the influences of cohesive strength, particle inertia, particle volume fraction and turbulence intensity. The statistics show that the formation and destruction of cohesive particle flocs vary across different flocculation stages and conditions. At the early developing stage, flocs primarily form through coagulation, where individual primary particles collide and adhere together. As the system approaches equilibrium, floc formation may also proceed via separation, where small flocs detach from larger ones under the conditions of strong cohesive strength, low particle inertia or weak turbulent shear. Under these same conditions, floc destruction occurs predominantly through absorption across all stages, in which smaller flocs are incorporated into larger ones. In contrast, under the opposite conditions, floc destruction shifts to disintegration, where a floc breaks apart into primary particles or two-particle flocs. The probability distribution of the floc lifetime follows an exponential decay. Furthermore, flocs exhibit shorter lifetimes under stronger turbulent shear, larger particle inertia or weaker cohesion, as these conditions promote more frequent destruction. A higher particle volume fraction leads to more frequent floc formation and destruction, thus producing shorter lifetimes. Meanwhile, cohesion between particles markedly extends the floc lifetime compared to lifetimes of non-cohesive particle clusters. Moreover, floc compactness is positively correlated with its lifetime. These findings advance our understanding of the flocculation dynamics of cohesive particles in turbulent flow.