10 research outputs found
Effective viscosity of non-gravitactic Chlamydomonas Reinhardtii microswimmer suspensions
Active microswimmers are known to affect the macroscopic viscosity of
suspensions in a more complex manner than passive particles. For puller-like
microswimmers an increase in the viscosity has been observed. It has been
suggested that the persistence of the orientation of the microswimmers hinders
the rotation that is normally caused by the vorticity. It was previously shown
that some sorts of algaes are bottom-heavy swimmers, i.e. their centre of mass
is not located in the centre of the body. In this way, the algae affects the
vorticity of the flow when it is perpendicular oriented to the axis of gravity.
This orientation of gravity to vorticity is given in a rheometer that is
equipped with a cone-plate geometry. Here we present measurements of the
viscosity both in a cone-plate and a Taylor-Couette cell. The two set-ups
yielded the same increase in viscosity although the axis of gravitation in the
Taylor-Couette cell is parallel to the direction of vorticity. In a
complementary experiment we tested the orientation of the direction of swimming
through microscopic observation of single \textit{Chlamydomonas reinhardtii}
and could not identify a preferred orientation, i. e. our specific strain of
\textit{Chlamydomonas reinhardtii} are not bottom-heavy swimmers. We thus
conclude that bottom heaviness is not a prerequisite for the increase of
viscosity and that the effect of gravity on the rheology of our strain of
\textit{Chlamydomonas reinhardtii} is negligible. This finding reopens the
question of whether origin of persistence in the orientation of cells is
actually responsible for the increased viscosity of the suspension
Photofocusing: Light and flow of phototactic microswimmer suspension
We explore in this paper the phenomenon of photofocusing: a coupling between
flow vorticity and biased swimming of microalgae toward a light source that
produces a focusing of the microswimmer suspension. We combine experiments that
investigate the stationary state of this phenomenon as well as the transition
regime with analytical and numerical modeling. We show that the experimentally
observed scalings on the width of the focalized region and the establishment
length as a function of the flow velocity are well described by a simple
theoretical model
Extreme congestion of microswimmers at a bottleneck constriction
When attracted by a stimulus (e. g. light), microswimmers can build up very
densely at a constriction and thus cause clogging. The micro-alga
\textit{Chlamydomonas Reinhardtii} is used here as a model system to study this
phenomenon. Its negative phototaxis makes the algae swim away from a light
source and go through a microfabricated bottleneck-shaped constriction.
Successive clogging events interspersed with bursts of algae are observed. A
power law decrease is found to describe well the distribution of time lapses of
blockages. Moreover, the evacuation time is found to increase when increasing
the swimming velocity. These results might be related to the phenomenology of
crowd dynamics and in particular what has been called the Faster is Slower
effect in the dedicated literature. It also raises the question of the presence
of tangential solid friction between motile cells densely packed that may
accompany arches formation. Using the framework of crowd dynamics we analyze
the microswimmers behavior and in particular question the role of
hydrodynamics
Statistics of Colloidal Suspensions Stirred by Microswimmers
We present a statistical analysis of the experimental trajectories of colloids in a dilute suspension of the green algae Chlamydomonas reinhardtii. The measured probability density function (pdf) of the displacements of colloids covers 7 orders of magnitude. The pdfs are characterized by non-Gaussian tails for intermediate time intervals, but nevertheless they collapse when scaled with their standard deviation. This diffusive scaling breaks down for longer time intervals and the pdf becomes Gaussian. However, the mean squared displacements of tracer positions are linear over the complete measurement time interval. Experiments are performed for various tracer diameters, swimmer concentrations, and mean swimmer velocities. This allows a rigorous comparison with several theoretical models. We can exclude a description based on an effective temperature and other mean field approaches that describe the irregular motion as a sum of the fluctuating far field of many microswimmers. The data are best described by the microscopic model by J.-L. Thiffeault, Distribution of particle displacements due to swimming microorganisms, Phys. Rev. E 92, 023023 (2015)
Chaotic Swimming of Phoretic Particles
The swimming of a rigid phoretic particle in an isotropic fluid is studied numerically as a function of the dimensionless solute emission rate (or PĂ©clet number Pe). The particle sets into motion at a critical Pe. Whereas the particle trajectory is straight at a small enough Pe, it is found that it loses its stability at a critical Pe in favor of a meandering motion. When Pe is increased further, the particle meanders at a short scale but its trajectory wraps into a circle at a larger scale. Increasing even further, Pe causes the swimmer to escape momentarily the circular trajectory in favor of chaotic motion, which lasts for a certain time, before regaining a circular trajectory, and so on. The chaotic bursts become more and more frequent as Pe increases, until the trajectory becomes fully chaotic, via the intermittency scenario. The statistics of the trajectory is found to be of the run-and-tumble-like nature at a short enough time and of diffusive nature at a long time without any source of noise
Pattern formation by dewetting and evaporating sedimenting suspensions
Pattern formation from drying droplets containing sedimenting particles and dewetting of thin films of such suspensions was studied. The dewetting causes the formation of finger-like patterns near the contact line which leave behind a deposit of branches. We find that the strikingly low speed of dewetting is due to the high particle concentration in the contact line region, leading to a strongly enhanced viscosity. For pattern formation from drying droplets (containing particles), evaporation also causes dewetting. In both cases, we find a similar relationship between the size of the patterns and the dewetting speed. The coefficient of this relationship gives us the effective viscosity at the contact line. We present a simple model that accounts for this, and that shows that the size of the particles is the relevant length scale in both problems.</p