Figure 2

Floc reshaping by turbulence. (a) Time-evolution of mean radius of gyration (\(\overline{{R}_{g,0}}\); see Method) of flocs consisting of 20 and 40 primary particles for case S1. Floc reshaping only takes place with smaller flocs as indicated by the decrease of \(\overline{{R}_{g,0}}\) with time. Four representative flocs with \({n}_{f}=20\) at different reshaping stages are plotted to shows how the mean floc structure changes with time. (b) The same for S2 of flocs consisting of 31–40 and 71–80 primary particles. Floc reshaping also influences larger floc size in S2. Four representative flocs with \({n}_{f}=35\) at different stages are plotted. (c, d) Two floc reshaping mechanisms, namely breakage-regrowth and restructuring by hydrodynamic drag, respectively. A floc of \({n}_{f}=25\) in case S1 becomes more compact with increased contact points after several breakage-regrowth sequences in (c). A large floc of \({n}_{f}=102\) in case S2 goes through restructuring by hydrodynamic drag while all the particles stay connected in (d). The gap between two sub-flocs is closed by local converging flow indicated by the black arrows (local fluid velocity) at the particle center.