We’re delighted to share the news that we have received travel funds from the EPSRC Network Plus Emergence and Physics Far From Equilibrium to kickstart a collaboration with the groups of Dr. Giorgio Volpe (UCL, UK), Dr. Nuno Araújo (U. Lisbon, Portugal) and Dr. Idan Tuval (IMEDEA-UIB, Spain). The project, which will start later this year, focusses on understanding and controlling transport properties of binary suspensions where microscopic active particles interact with passive ones (cargoes).
Research
Microparticle entrainment à la Taylor
Close contact between microorganisms underpins fundamental interactions including infection, microbial grazing and fertilisation, but whether or not these interactions actually happen depends critically on the duration of contact. For swimming microorganisms, prolonged contact with an object should manifest as entrainment, and its extend hinge on the physics of escape from the entrained state. At present, neither the existence of entrainment nor the physical mechanisms determining its duration are well established. In our new paper, published in Physical Review Fluids, we combine experiments and theory to show that particle entrainment is indeed a generic feature of swimming microorganisms, and that its duration depends on an interplay between advection and diffusion. A Taylor-dispersion-type theory rationalises the dependence of the distribution of contact times on swimmers’ parameters, and predicts an optimal size for entrainment (~1μm), which we confirm experimentally. [ArXiv Preprint]
Chlamydomonas Phototaxis: turn and what?
Phototaxis is one of the main categories of motility regulation by microorganisms. Arguably, it is particularly important for motile micro algae, due to their photosynthetic activity. One of the organisms where it has been studied the most is our beloved micro alga Chlamydomonas reinhardtii. Currently, we have a pretty good idea of the mechanism leading the cells to reorient towards/away from the light, but not much is known about what happens after they’ve reoriented…. In our recent paper we start looking into this, with surprising results.
Phase Defects
Does a stronger interaction always make for a more stable system? Certainly not for synchronising oscillators, as we show in a paper just accepted in Physical Review Fluids. There we study the behaviour of a strip of colloidal rotors as the system is lifted from a no-slip surface. As the hydrodynamic coupling strengthens, the system develop recurring phase defects which worsen its synchronisation. Our simulations show that defects result from a competition between short-range and long-range coupling. The paper is currently accessible through the ArXiv.
Update: The paper has been published (open access) and is now available here.
Introduction to Chlamy
Our brief introduction to Chlamydomonas reinhardtii -aimed at physicists- is published in by the European Physical Journal as part of a Special Topics Issue on Microswimmers.
Colloidal Entrainment
Particle entrainment by Raphaël, just accepted on Nat Com!
What happens to passive microparticles within a suspension of microorganisms? If the particles are small, they can be entrained over large distances by the micro swimmers. These interactions are rare, but their magnitude is large and -as it turns out- they end up dominating particle dynamics, which now resembles a jump-diffusion process. This is presented and discussed in details in a new work led by Raphaël, just accepted on Nature Communications. A preprint of the article (well.. a previous version) is currently available on the Arxiv.
Update. The article is now available here.
Physics Viewpoint!
Together with Idan Tuval, I have been recently working on a Viewpoint for Physics, about an interesting recent PRL publication by Greta Quaranta, Marie-Eve Aubin Tam and Daniel Tam, from the University of Delft. They proved that flagellar synchronisation in Chlamydomonas depends on the presence of striated fibres joining the basal bodies of the two flagella. Apparenly, synchronisation of flagella from different cells or from the same cell can be based on completely different mechanisms! This is a really nice work, which opens a lot of new questions…
Scattering Microalgae
Matteo’s first paper as just been accepted in Physical Review Letters!
The paper concerns the following problem: which forces determine the motion of microorganisms through heterogeneous media (think e.g. soil or bottom sediments in lakes or coastal areas)? Current theories are divided in two groups, those that consider this to be mainly a microhydrodynamics problem, and those which do not consider fluid dynamics at all and treat it as a contact interaction problem. So: which one is right? For microorganisms pushing themselves from the back, recent work has shown that the interaction is fundamentally hydrodynamic. Matteo has now shown that for organisms with front-mounted flagella, instead, the situation is much more complex and both fluid-mediated interactions and direct contact have to be taken into account. The paper is not out yet, but you can already read a draft version in the arXiv.
This work was a collaboration with Kela Lushi (Brown), Idan Tuval (IMEDEA), and Vasily Kantsler (Warwick).
Update: the paper has been published! Check it out here!
Metachronal Wave Defects on Interface!
The Journal of the Royal Society: Interface just accepted our latest paper, which includes the first report of recurring defects in metachronal coordination of eukaryotic flagella (see also Arxiv version). These defects, which we observe in Volvox carteri, are a type of incomplete coordination which we think might originate from a variation, along the surface of Volvox, in the frequency at which cells beat their flagella. They might be related to so-called chimera states of groups of interacting oscillators…. More to come on this: we’re still working on it!
Eukaryotic flagella synchronise through hydrodynamics
Article published on eLife. See also accompanying Insight by B. Friederich and I. Riedel-Kruse.
After many years working on Chlamydomonas, we finally graduated to multiple cells… But from a different organism: Volvox carteri. This multicellular relative of Chlamy has thousands of biflagellate somatic cells on its surface, which can be easily extracted from the colony and keep on beating for several hours. We grabbed two with independent micropipettes and showed that below a critical separation, the cells synchronise their beating. Synchronisation has a purely hydrodynamic origin. At the same time, their interaction changes the waveform of their flagella. Flagellar elasticity cooperates with hydrodynamic stresses to generate synchrony as predicted a few years ago!
Polin Lab 