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Dynamics of Active Suspensions

Final Report Summary - ACTIVEDYNAMICS (Dynamics of Active Suspensions)

The proposed research programme is about the investigation of the dynamics of active (self-propelled) particles suspensions using mainly natural motile particles (swimming bacteria). We employ a new method, ‘differential dynamic microscopy’ (DDM), which uses everyday laboratory apparatus (a microscope and a camera) to generate comprehensive information on the dynamics of suspended particles (in the form of the so-called ‘intermediate scattering function’). Compared to single-particle tracking, the standard tool in active colloids and bacterial motility research to date, DDM yields much better averages and is orders of magnitude quicker to perform. A second goal of the proposed work is to develop DDM into a versatile tool for studying active particles of all kinds, at a range of concentrations. The project is divided into three main objectives:

PART A: Methodology establishment
PART B: Dynamics of active suspensions in 3D
PART C: Dynamics of active suspensions in polymer solution in 3D

The success of investigating the collective dynamics of active suspension (dense active suspension) is first based on the development of a careful methodology (Part A) and the understanding of the dynamics of dilute active suspensions (partly Part B and C). The researcher has successfully developed a careful protocol and methodology to investigate the dynamics of suspensions of swimming bacteria as function of bacterial concentration and polymer concentration. Several key results have been obtained. First, a detailed experimental investigation of the dynamics of tracer particles (non-motile bacteria) in a bath of swimming bacteria has been done in three-dimension for the first time. In addition, a theory, based on hydrodynamic interactions considering the flow field created by swimming bacteria, has been developed that allows a quantitative description of the experimental data. Secondly, an extensive investigation of bacteria swimming in polymer solution has been performed. This has brought new insights in our understanding of microorganisms swimming in complex polymeric environment, e.g. bacteria swimming in intestinal mucus. Thirdly, the methodology developed primarily for swimming bacteria during this project has also been applied successfully to other i) microorganisms such as swimming algae, sperm cells, or synthetic self-propelled particles such as active Janus particles; and ii) systems with anisotropic dynamics, e.g. magnetic colloidal suspensions or magnetotactic bacteria. These results have brought interests from not only a broad area of academic science but also industrial research.