Grain Boundary Dynamics in Colloidal Crystals and Active Turbulence in Living Fluids Driven by Motile E. coli

Abstract

This thesis presents an experimental investigation of collective phenomena in passive and active soft matter systems, focusing on crystallisation dynamics in two–dimensional colloidal suspensions and turbulence in dense bacterial fluids. These systems provide accessible experimental platforms for studying nonequilibrium Macroscale behaviour arising from microscopic interactions. The first part focuses on crystallisation in two–dimensional colloidal systems, which is used to validate the experimental setup and analysis methods later applied to active suspensions. Passive colloidal particles undergoing Brownian motion were imaged using Bright-Field microscopy, and particle trajectories were obtained through tracking algorithms. Structural properties were characterised using radial distribution functions and bond–orientational correlations. The steady configurations show pronounced long-range order, while orientational correlations decay algebraically, consistent with the hexatic phase expected in two dimensions. The growth of orientational order is quantified through a characteristic length scale that evolves in time in a manner consistent with curvature driven coarsening. Domain growth proceeds via grain boundary rearrangement and defect elimination, and dynamic scaling behaviour is observed in both correlation functions and structure factors. The second part investigates the collective motion of dense suspensions of motile E. coli bacteria. Confocal microscopy combined with particle image velocimetry was used to obtain coarse-grained velocity fields from fluorescence recordings. These velocity fields display disordered and highly dynamic flow patterns characteristic of active turbulence, with typical speeds of approximately 7.88 µm/s. Analysis in Fourier space shows that the kinetic energy spectrum exhibits a power-law decay at large wavenumbers. Overall, this work establishes experimental and computational approaches for probing ordering and flow in active fluids and provides insight into the physical mechanisms underlying collective dynamics in systems driven far from equilibrium.