Ottavio Croze

Followers

Following

Co-authors

Public Views

Interests

Uploads

Papers by Ottavio Croze

Taylor-Aris dispersion of swimming algae in pipe flow: predictions and experimental tests

by Ottavio Croze, Martin Bees, and Rachel Bearon

Classical Taylor-Aris dispersion theory is extended to describe the transport of suspensions of b... more Classical Taylor-Aris dispersion theory is extended to describe the transport of suspensions of biased swimming cells in a vertical pipe flow. These suspensions differ from those of molecular or colloidal solutes: e.g. algae or bacteria in suspension respond to directional stimuli (e.g. chemotaxis, phototaxis or gyrotaxis). Gyrotactic instabilities focus bottom-heavy swimming cells into beautiful plumes. Solving for axial moments, we have derived general exact expressions for the mean drift and effective diffusivity of cells along such plumes, and apply these to predict the dispersion of a "dyed slug" of gyrotactic algae in a down-welling flow in a tube. We present predictions for the effective axial drift and diffusivity of the slug using consitutive relations from: (a) generalised Taylor dispersion theory and (b) Fokker-Planck models of dispersion. We then discuss experimental measurements to test our predictions using dyed cells and the relevance to bioreactor design.

Dispersion of biased swimming microorganisms in a fluid flowing through a tube

by Ottavio Croze and Martin Bees

Proceedings of the Royal Society a Mathematical Physical and Engineering Sciences, Jan 20, 2010

Classical Taylor-Aris dispersion theory is extended to describe the transport of suspensions of s... more Classical Taylor-Aris dispersion theory is extended to describe the transport of suspensions of self-propelled dipolar cells in a tubular flow. General expressions for the mean drift and effective diffusivity are determined exactly in terms of axial moments, and compared with an approximation a la Taylor. As in the Taylor-Aris case, the skewness of a finite distribution of biased swimming cells vanishes at long times. The general expressions can be applied to particular models of swimming microorganisms, and thus be used to predict swimming drift and diffusion in tubular bioreactors, and to elucidate competing unbounded swimming drift and diffusion descriptions. Here, specific examples are presented for gyrotactic swimming algae.

Download

Physics comes to life

Physics World, 2012

Download

Bacteria as colloids/A quick guide to working with bacteria

Download

Mathematics for streamlined biofuel production from unicellular algae

by Ottavio Croze and Martin Bees

Biofuels, 2014

ABSTRACT One of the greatest challenges of this century is to employ Nature’s resources to addres... more ABSTRACT One of the greatest challenges of this century is to employ Nature’s resources to address the World’s energy, food, water and chemical requirements without further unsettling the potentially precarious environmental balance in which we live. The recent resurgence of interest in green algae for biotechnological applications, such as bioenergy, carbon capture and pharmaceuticals, means it is vital that we understand the dynamics of suspensions of living cells. It is widely appreciated that mathematics can aid the optimization of the production of biofuels from algae. However, less obviously, mathematics can reveal mechanisms associated with the fact that many species of unicellular algae swim, and do so in preferred directions in response to environmental cues. Accumulations of cells can induce macroscale hydrodynamic instabilities due to their buoyancy, called bioconvection. There are immediate consequences for algal photobioreactor design, such as methods for cell harvesting, avoiding biofouling and understanding cellular dispersion in pipe flow.

Dispersion of biased swimming micro-organisms in a fluid flowing through a tube

by Ottavio Croze and Martin Bees

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2010

Download

Biased swimming cells do not disperse in pipes as tracers: A population model based on microscale behaviour

by Ottavio Croze, Martin Bees, and Rachel Bearon

Physics of Fluids, 2012

Download

Sheared bioconvection in a horizontal tube

by Ottavio Croze and Martin Bees

Physical Biology, 2010

The recent interest in using microorganisms for biofuels is motivation enough to study bioconvect... more The recent interest in using microorganisms for biofuels is motivation enough to study bioconvection and cell dispersion in tubes subject to imposed flow. To optimize light and nutrient uptake, many microorganisms swim in directions biased by environmental cues (e.g. phototaxis in algae and chemotaxis in bacteria). Such taxes inevitably lead to accumulations of cells, which, as many microorganisms have a density different to the fluid, can induce hydrodynamic instabilites. The large-scale fluid flow and spectacular patterns that arise are termed bioconvection. However, the extent to which bioconvection is affected or suppressed by an imposed fluid flow, and how bioconvection influences the mean flow profile and cell transport are open questions. This experimental study is the first to address these issues by quantifying the patterns due to suspensions of the gravitactic and gyrotactic green biflagellate alga Chlamydomonas in horizontal tubes subject to an imposed flow. With no flow, the dependence of the dominant pattern wavelength at pattern onset on cell concentration is established for three different tube diameters. For small imposed flows, the vertical plumes of cells are observed merely to bow in the direction of flow. For sufficiently high flow rates, the plumes progressively fragment into piecewise linear diagonal plumes, unexpectedly inclined at constant angles and translating at fixed speeds. The pattern wavelength generally grows with flow rate, with transitions at critical rates that depend on concentration. Even at high imposed flow rates, bioconvection is not wholly suppressed and perturbs the flow field.

Download

Nonadditivity of Polymeric and Charged Surface Interactions: Consequences for Doped Lamellar Phases

Langmuir, 2005

We explore theoretically the modifications to the interactions between charged surfaces across an... more We explore theoretically the modifications to the interactions between charged surfaces across an ionic solution caused by the presence of dielectric polymers. Although the chains are neutral, the polymer physics and the electrostatics are coupled; the intra-surface electric fields polarise any low permittivity species (e.g., polymer) dissolved in a high permittivity solvent (e.g., water). This coupling enhances the polymer depletion from the surfaces and increases the screening of electrostatic interactions, with respect to a model which treats polymeric and electrostatic effects as independent. As a result, the range of the ionic contribution to the osmotic interaction between surfaces is decreased, while that of the polymeric contribution is increased. These changes modify the total interaction in a nonadditive manner. Building on the results for parallel surfaces, we investigate the effect of this coupling on the phase behaviour of polymerdoped smectics.

Download

Dispersion of swimming algae in laminar and turbulent channel flows: consequences for photobioreactors

by Ottavio Croze and Martin Bees

Journal of The Royal Society Interface, 2013

Shear flow significantly affects the transport of swimming algae in suspension. For example, visc... more Shear flow significantly affects the transport of swimming algae in suspension. For example, viscous and gravitational torques bias bottom-heavy cells to swim towards regions of downwelling fluid (gyrotaxis). It is necessary to understand how such biases affect algal dispersion in natural and industrial flows, especially in view of growing interest in algal photobioreactors. Motivated by this, we here study the dispersion of gyrotactic algae in laminar and turbulent channel flows using direct numerical simulation (DNS) and the analytical swimming dispersion theory of Bees & Croze [1]. Time-resolved dispersion measures are evaluated as functions of the Péclet and Reynolds numbers in upwelling and downwelling flows. For laminar flows, DNS results are compared with theory using competing descriptions of biased swimming cells in shear flow. Excellent agreement is found for predictions that employ generalized-Taylor-dispersion. The results highlight peculiarities of gyrotactic swimmer dispersion relative to passive tracers. In laminar downwelling flow the cell distribution drifts in excess of the mean flow, increasing in magnitude with Peclet number. The cell effective axial diffusivity increases and decreases with Peclet number (for tracers it merely increases). In turbulent flows, gyrotactic effects are weaker, but discernable and manifested as non-zero drift. These results should significantly impact photobioreactor design.

Download

Migration of Chemotactic Bacteria in Soft Agar: Role of Gel Concentration

Biophysical Journal, 2011

We study the migration of chemotactic wild type Escherichia coli populations in semi-solid ('soft... more We study the migration of chemotactic wild type Escherichia coli populations in semi-solid ('soft') agar in the concentration range C = 0.15 − 0.5%(w/v). For C 0.35%, expanding bacterial colonies display characteristic chemotactic 'rings'. At C = 0.35%, however, we observe broadband migration rather than rings. These are growth/diffusion waves arising because of suppression of chemotaxis by the agar and have not been previously reported experimentally. For C = 0.4 − 0.5% expanding colonies do not span the depth of the agar and develop pronounced front instabilities. The rate of increase of the colony radius (front speed) v F , is weakly dependent of agar concentration below C = 0.25%, but decreases sharply above this value. We discuss these observations in terms of an extended Keller-Segel model for which we derived novel expressions for transport parameters accounting for perturbations of the chemotactic response by collisions with the agar. The observed front speed decay could be well fitted with the model in the range C = 0.15 − 0.35%, and its solutions qualitatively reproduce the observed transition from chemotactic to growth/diffusion bands. We discuss the implications of our results for microbiological chemotaxis assays and the study of bacteria in porous media.

Download

Taylor-Aris dispersion of swimming algae in pipe flow: predictions and experimental tests

by Ottavio Croze and Martin Bees

Bulletin of the American Physical Society, 2010

Classical Taylor-Aris dispersion theory is extended to describe the transport of suspensions of b... more

Differential dynamic microscopy: A high-throughput method for characterizing the motility of microorganisms

by Martin Bees, Vincent Martinez, Ottavio Croze, and rut besseling

We present a fast, high-throughput method for characterizing the motility of microorganisms in 3D... more 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 from 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 ∼ 10 4 cells and obtained in a few minutes.

Download

Ottavio Croze

Uploads

Papers by Ottavio Croze

Log In