Fig. 2: Short- and long-time dynamics of the aggregates.

Short-time dynamics of the aggregates (Δt < 5 s): a, Inset: mean squared displacement for fixed bacterial concentration ρ_B = ρ* and increasing aggregate size R_G (larger symbols for larger R_G). The mean squared displacement increased linearly with time, with an effective diffusivity D_eff(ρ_B,R_G). a, Translational diffusivity D_eff for various bacterial concentrations ρ_B and aggregate sizes R_G (black is thermal and hotter colours represent higher ρ_B). Dashed lines are the Stokes–Einstein predictions D_eff = α(ρ_B)/R_G. Data symbols with error bars represent the average and standard deviation for more than five aggregates. b, Inset: mean squared angular displacement for fixed bacterial concentration ρ_B = ρ* and increasing aggregate size R_G (larger symbols for larger R_G). The mean squared angular displacement increased linearly with time, with an effective diffusivity \({D}_{{{{\rm{eff}}}}}^{\theta }({\rho }_\mathrm{B},{R}_\mathrm{G})\). b, Rotational diffusivity \({D}_{{{{\rm{eff}}}}}^{\theta }\) for various bacterial concentrations ρ_B and aggregate sizes R_G (black is thermal and hotter colours represent higher ρ_B). Dashed lines are the Stokes–Einstein predictions \({D}_{{{{\rm{eff}}}}}^{\theta }=\beta ({\rho }_\mathrm{B})/{R}_\mathrm{G}^{3}\). Data symbols with error bars represent the average and standard deviation for more than five aggregates. c, Normalized diffusivity for translations α/α₀ (circles) and rotations β/β₀ (diamonds), where subscript 0 refers to the thermal bath. The normalized translational and rotational diffusivity collapsed. The symbols are slightly offset on the horizontal axis for clarity and represent the average and standard deviation (error bars). Long-time dynamics of the aggregates: d, Persistent clockwise rotation of aggregates observed at longer times (minutes to tens of minutes) in the glass capillary. The linear dependence of the angle θ (defined as in the inset) allowed us to extract a rotation rate Ω(ρ_B,R_G). e–h, Histograms of rotation rates Ω for different bacterial concentrations ρ_B for all aggregate sizes R_G: ρ_B = ρ* (e); ρ_B = 0.5 ρ* (f); ρ_B = 0.2 ρ* (g); ρ_B = 0, thermal (h). i, Inset: rotation rates Ω for different bacterial concentrations (ρ_B ≈ 0.2 ρ* (green), ρ_B ≈ 0.5 ρ* (orange) and ρ_B ≈ ρ* (red), with ρ* = 6 x 10⁸ cells per millilitre) and aggregate sizes R_G. i, Data collapsed onto the master curve \({{\varOmega }}/({{{\mathcal{M}}}}{\rho }_\mathrm{B}) \propto 1/{R}_\mathrm{G}^{3}\), where \({{{\mathcal{M}}}}\) is the mass of an aggregate particle. Each point represents a measurement of an aggregate particle.