The microaerophilic magnetotactic bacterium Magnetospirillum gryphiswaldense swims along magnetic field lines using a single flagellum at each cell pole. It is believed that this magnetotactic behavior enables cells to seek optimal oxygen concentrati
on with maximal efficiency. We analyse the trajectories of swimming M. gryphiswaldense cells in external magnetic fields larger than the earths field, and show that each cell can switch very rapidly (in < 0.2 s) between a fast and a slow swimming mode. Close to a glass surface, a variety of trajectories was observed, from straight swimming that systematically deviates from field lines to various helices. A model in which fast (slow) swimming is solely due to the rotation of the trailing (leading) flagellum can account for these observations. We determined the magnetic moment of this bacterium using a new method, and obtained a value of (2.0 $pm$ 0.6) $times$ $10^{-16}$ Am$^2$. This value is found to be consistent with parameters emerging from quantitative fitting of trajectories to our model.
We present a fast, high-throughput method for characterizing the motility of microorganisms in 3D based on standard imaging microscopy. Instead of tracking individual cells, we analyse the spatio-temporal fluctuations of the intensity in the sample f
rom time-lapse images and obtain the intermediate scattering function (ISF) of the system. We demonstrate our method on two different types of microorganisms: bacteria, both smooth swimming (run only) and wild type (run and tumble) Escherichia coli, and the bi-flagellate alga Chlamydomonas reinhardtii. We validate the methodology using computer simulations and particle tracking. From the ISF, we are able to extract (i) for E. coli: the swimming speed distribution, the fraction of motile cells and the diffusivity, and (ii) for C. reinhardtii: the swimming speed distribution, the amplitude and frequency of the oscillatory dynamics. In both cases, the motility parameters are averaged over approx 10^4 cells and obtained in a few minutes.
We review recent advances in imaging the flow of concentrated suspensions, focussing on the use of confocal microscopy to obtain time-resolved information on the single-particle level in these systems. After motivating the need for quantitative (conf
ocal) imaging in suspension rheology, we briefly describe the particles, sample environments, microscopy tools and analysis algorithms needed to perform this kind of experiments. The second part of the review focusses on microscopic aspects of the flow of concentrated model hard-sphere-like suspensions, and the relation to non-linear rheological phenomena such as yielding, shear localization, wall slip and shear-induced ordering. Both Brownian and non-Brownian systems will be described. We show how quantitative imaging can improve our understanding of the connection between microscopic dynamics and bulk flow.
We present recent advances in the instrumentation and analysis methods for quantitative imaging of concentrated colloidal suspensions under flow. After a brief review of colloidal imaging, we describe various flow geometries for two and and three-dim
ensional (3D) imaging, including a `confocal rheoscope. This latter combination of a confocal microscope and a rheometer permits simultaneous characterization of rheological response and 3D microstructural imaging. The main part of the paper discusses in detail how to identify and track particles from confocal images taken during flow. After analyzing the performance of the most commonly used colloid tracking algorithm by Crocker and Grier extended to flowing systems, we propose two new algorithms for reliable particle tracking in non-uniform flows to the level of accuracy already available for quiescent systems. We illustrate the methods by applying it to data collected from colloidal flows in three different geometries (channel flow, parallel plate shear and cone-plate rheometry).
We study the pressure-driven flow of concentrated colloids confined in glass micro-channels at the single particle level using fast confocal microscopy. For channel to particle size ratios $a/bar{D} lesssim 30$, the flow rate of the suspended particl
es shows fluctuations. These turn into regular oscillations for higher confinements ($a/bar{D} simeq 20$). We present evidence to link these oscillations with the relative flow of solvent and particles (permeation) and the effect of confinement on shear thickening.
