23 research outputs found
Hydrodynamics of Micro-swimmers in Films
One of the principal mechanisms by which surfaces and interfaces affect
microbial life is by perturbing the hydrodynamic flows generated by swimming.
By summing a recursive series of image systems we derive a numerically
tractable approximation to the three-dimensional flow fields of a Stokeslet
(point force) within a viscous film between a parallel no-slip surface and
no-shear interface and, from this Green's function, we compute the flows
produced by a force- and torque-free micro-swimmer. We also extend the exact
solution of Liron & Mochon (1976) to the film geometry, which demonstrates that
the image series gives a satisfactory approximation to the swimmer flow fields
if the film is sufficiently thick compared to the swimmer size, and we derive
the swimmer flows in the thin-film limit. Concentrating on the thick film case,
we find that the dipole moment induces a bias towards swimmer accumulation at
the no-slip wall rather than the water-air interface, but that higher-order
multipole moments can oppose this. Based on the analytic predictions we propose
an experimental method to find the multipole coefficient that induces circular
swimming trajectories, allowing one to analytically determine the swimmer's
three-dimensional position under a microscope.Comment: 35 pages, 11 figures, 5 table
Upstream Swimming in Microbiological Flows
Interactions between microorganisms and their complex flowing environments are essential in many biological systems. We develop a model for microswimmer dynamics in non-Newtonian Poiseuille flows. We predict that swimmers in shear-thickening (-thinning) fluids migrate upstream more (less) quickly than in Newtonian fluids and demonstrate that viscoelastic normal stress differences reorient swimmers causing them to migrate upstream at the centreline, in contrast to well-known boundary accumulation in quiescent Newtonian fluids. Based on these observations, we suggest a sorting mechanism to select microbes by swimming speed
Hotspots of boundary accumulation: dynamics and statistics of micro-swimmers in flowing films
Biological flows over surfaces and interfaces can result in accumulation hotspots or depleted voids of microorganisms in natural environments. Apprehending the mechanisms that lead to such distributions is essential for understanding biofilm initiation. Using a systematic framework we resolve the dynamics and statistics of swimming microbes within flowing films, considering the impact of confinement through steric and hydrodynamic interactions, flow, and motility, along with Brownian and run-tumble fluctuations. Micro-swimmers can be peeled o↵ the solid wall above a critical flow strength. However, the interplay of flow and fluctuations causes organisms to migrate back towards the wall above a secondary critical value. Hence, faster flows may not always be the most e"cacious strategy to discourage biofilm initiation. Moreover, we find run-tumble dynamics commonly used by flagellated microbes to be an intrinsically more successful strategy to escape from boundaries than equivalent levels of enhanced Brownian noise in ciliated organisms
Membrane penetration and trapping of an active particle
The interaction between nano- or micro-sized particles and cell membranes is
of crucial importance in many biological and biomedical applications such as
drug and gene delivery to cells and tissues. During their cellular uptake, the
particles can pass through cell membranes via passive endocytosis or by active
penetration to reach a target cellular compartment or organelle. In this
manuscript, we develop a simple model to describe the interaction of a
self-driven spherical particle (moving through an effective constant active
force) with a minimal membrane system, allowing for both penetration and
trapping. We numerically calculate the state diagram of this system, the
membrane shape, and its dynamics. In this context, we show that the active
particle may either get trapped near the membrane or penetrates through it,
where the membrane can either be permanently destroyed or recover its initial
shape by self-healing. Additionally, we systematically derive a continuum
description allowing to accurately predict most of our results analytically.
This analytical theory helps identifying the generic aspects of our model,
suggesting that most of its ingredients should apply to a broad range of
membranes, from simple model systems composed of magnetic microparticles to
lipid bilayers. Our results might be useful to predict mechanical properties of
synthetic minimal membranes.Comment: 16 pages, 6 figures. Revised manuscript resubmitted to J. Chem. Phy
Towards an analytical description of active microswimmers in clean and in surfactant-covered drops
Geometric confinements are frequently encountered in the biological world and
strongly affect the stability, topology, and transport properties of active
suspensions in viscous flow. Based on a far-field analytical model, the
low-Reynolds-number locomotion of a self-propelled microswimmer moving inside a
clean viscous drop or a drop covered with a homogeneously distributed
surfactant, is theoretically examined. The interfacial viscous stresses induced
by the surfactant are described by the well-established Boussinesq-Scriven
constitutive rheological model. Moreover, the active agent is represented by a
force dipole and the resulting fluid-mediated hydrodynamic couplings between
the swimmer and the confining drop are investigated. We find that the presence
of the surfactant significantly alters the dynamics of the encapsulated swimmer
by enhancing its reorientation. Exact solutions for the velocity images for the
Stokeslet and dipolar flow singularities inside the drop are introduced and
expressed in terms of infinite series of harmonic components. Our results offer
useful insights into guiding principles for the control of confined active
matter systems and support the objective of utilizing synthetic microswimmers
to drive drops for targeted drug delivery applications.Comment: 19 pages, 7 figures. Regular article contributed to the Topical Issue
of the European Physical Journal E entitled "Physics of Motile Active Matter"
edited by Gerhard Gompper, Clemens Bechinger, Holger Stark, and Roland G.
Winkle
Lattice-Boltzmann hydrodynamics of anisotropic active matter
A plethora of active matter models exist that describe the behavior of
self-propelled particles (or swimmers), both with and without hydrodynamics.
However, there are few studies that consider shape-anisotropic swimmers and
include hydrodynamic interactions. Here, we introduce a simple method to
simulate self-propelled colloids interacting hydrodynamically in a viscous
medium using the lattice-Boltzmann technique. Our model is based on
raspberry-type viscous coupling and a force/counter-force formalism which
ensures that the system is force free. We consider several anisotropic shapes
and characterize their hydrodynamic multipolar flow field. We demonstrate that
shape-anisotropy can lead to the presence of a strong quadrupole and octupole
moments, in addition to the principle dipole moment. The ability to simulate
and characterize these higher-order moments will prove crucial for
understanding the behavior of model swimmers in confining geometries.Comment: 11 pages, 3 figures, 3 table
Interfacial activity dynamics of confined active droplets
Active emulsions can exhibit a complex hydrodynamic mode spectrum driven by
chemical advection-diffusion instabilities. We study such an active emulsion
consisting of oil droplets that dynamically solubilize in a supramicellar
aqueous surfactant solution. It has been predicted that the interaction with
self-generated chemical fields leads to multistable higher-mode flow fields and
chemorepulsive phenomena. To investigate such chemodynamic effects, we have
studied cylindrical droplets pinned between the top and bottom surfaces of a
microfluidic reservoir, such that they only produce pumping flows, while we
quantified the chemical concentration field and the hydrodynamic velocity field
at the same time. With increasing droplet radius we observe the following: (i)
vortical structures generated by the droplet migrating around the interface,
(ii) bistability between a dipolar and quadrupolar flow mode, and, eventually,
(iii) a transition to multipolar modes. We further measured flow fields by
particle image velocimetry and compared them to a hydrodynamic model based on a
Brinkman squirmer. A simultaneous quantification of the flow fields and
oil-filled micelle distribution suggests that a local buildup of chemical
products leads to a saturation of the surface, which affects the propulsion
mechanism. The complex multistable dynamics can be explained by the competing
time scales of slow micellar diffusion governing the chemical buildup and
faster molecular diffusion powering the underlying advection-diffusion
mechanism