| dc.description.abstract | Bacterial suspension is a fundamental nonequilibrium system. At high cell concentration, the strong interplay of bacterial self-propelling force and the nonlinear bacterial couplings from the anisotropic excluded volume, chemical, and hydrodynamic interactions causes self-organized bacterial clusters. The coherent motion of clusters induces multiscaled flows with fluctuating vortices, which is so called {it bacterial turbulence} (BT). Opposite to inertia-dominated hydrodynamic turbulence (HT) and wave turbulence (WT), the energy in BT is injected from the cell level and transported to collective large scales. Whether BT exhibits similar dynamic behaviors to HT and WT is an important and open issue. In this thesis, using {it Escherichia coli} suspension at high cell concentration as a platform, for the first time, we experimentally address two important issues well studied in HT and WT: multifractal dynamics and turbulence reduction by passive additives.
The multifractal dynamics is investigated in thin liquid film at two different cell concentrations. The bacterial flow has fluctuating vortices with a broad range of scales and intensities through the nonlinear interaction of the swimming bacteria. Increasing cell concentration increases the total propelling power and the nonlinear interaction. It causes the generation of vortices with larger scale, lower frequency, and higher intensity. It also widens the histograms of the flow velocity and the velocity increment ν_r between two points separated by a distance r. The q-order structure functions S_q(r) of ν_r can be fitted by a power law function S_q(r)~r^{ζ_q}. Stronger intercell interaction at higher cell concentration can extend the power law relation toward larger r, indicating that the self-propelled energy can cascade to the larger scale. The nonlinear relation between the scaling exponent ζ_q and q are found for both cell concentrations, which manifests the multifractal dynamics. The multifractality can be enhanced by increasing cell concentration. It is also found that the extended self-similarity (ESS) exists in BT for both runs.
We also experimentally demonstrate BT reduction by passive magnetic chain additives. The micron-sized paramagnetic particles are added into bacterial suspensions. Applying an external magnetic field induces magnetic dipoles and causes the formation of chain bundles of magnetized particles. The larger effective drag from connected particles along chains, the anisotropic chain shape, and the chain alignment along the magnetic field reduce chain motion. Chains in turn form obstacles to slow down BT. The criticality feature due to the strong network of intercell interaction causes quick information propagation of local flow retardation. It causes the interruption of the upward energy flow from individual self-propelling bacteria to the larger scale in BT with multiscaled coherent flow, leading to more severe suppression in the low frequency (wave number) regimes of the power spectra. The study provides a new convenient method of quickly controlling BT for the various possible applications, through quickly turning on/off the $B$ field. | en_US |