Session A48: Swimming, Motility and Locomotion

Sperm motility in modulated microchannels

Sperm cells swim through the fluid by a periodic wave-like beating of their flagellum [1-3]. At low Reynolds numbers and in confinement, the directed motion of sperm and other microswimmers is strongly influenced by steric and hydrodynamic surface interactions [1]. We model sperm motility in mesoscale hydrodynamics simulations by imposing a planar traveling bending wave along the flagellum [2]. Sperm are simulated swimming in curved, straight, shallow and zigzag-shaped microchannels. Changes in the sidewall modulations and the imposed beat pattern allow the identification of a strong dependence of the surface attraction on the beat-shape envelope of the sperm cell. The simulations reveal a strong dependence of the deflection angle on the orientation of the beat plane with respect to the channel sidewall, and thus deepen the understanding of sperm navigation under strong confinement. Detachment of sperm, while swimming along curved walls, is dominated by either the emergence of a nonplanar component of the flagellar beat with increasing wavelength or the strong confinement in shallow channels.

[1] J. Elgeti et al., Rep. Prog. Phys. 78, 056601 (2015) [2] J. Elgeti et al., Biophys. J. 99, 1018 (2010) [3] G. Saggiorato et al., Nat. Commun. 8, 1415 (2017)   

Development of a Microfluidic Device to Sort Sperm based on their Swimming Potential against the Flow

The first step of in-vitro fertilization is to sort out the motile sperm from the non-motile ones. Currently, centrifugation based sperm swim-up and density gradient separation are common methods to sort sperm. However, these methods reduce sperm quality during the repetitive centrifugation steps and isolate sperm with high DNA fragmentation. In this work, we construct a microfluidic device based on the observation that motile sperm can swim against the flow within a specific range of flow rates. This sperm sorting device consists of two chambers, separated by a filter. After 45 minutes the sorted motile sperm are collected from the top retrieval chamber and is placed on a glass slide for visual inspection and data collection. We find that the most motile and functional sperm pass selectively through the micropores against the flow, showing that our device provide an efficient way to sort sperm.   

Varying pH influences the motility of Helicobacter pylori more strongly in porcine gastric mucin solutions than in broth.

Helicobacter pylori, a pathogen which inhabit the gastric mucus has to swim across the naturally occurring pH gradient in the mucus layer varying from 2-4 in the lumen to 7 on the epithelial surface. The mucus has a pH-dependent viscoelasticity forming a gel at low pH, whereas at higher pH it is solution-like. Previous studies have shown that H. pylori were immobile in porcine gastric mucin (PGM) below pH 4. How much of this effect is due to gelation of PGM and how much due to the effects of pH on the flagellar motors is unclear. To address this question we compared the translation and rotational motion of the bacteria in PGM versus broth at different pH. In broth, H. pylori swimming speed increased as pH was lowered from pH 7 to 4, followed by a drop in speed below pH4 indicating decrease of proton motive force, whereas in PGM the speed peaked at pH 5 and bacteria became immobile below pH3, indicating the increase in viscosity in PGM dominating the loss of motility. The body rotation rate is weakly dependent on pH in broth. in PGM the bacteria stuck in the low pH gel rotate much faster than the mobile bacteria at higher pH’s, indicating that bacteria can sense the environment’s mechanical properties and attempt to free themselves from being immobilized.   

Experimental Evidence of Passive Separation Control by Shortfin Mako Shark Scale Bristling

The shortfin mako has scales (on the order of 0.2 mm in size) flexible to angles in excess of 40 degrees, but only in the direction of reversing flow and strategically placed on the body, such as flank and fins, for controlling flow separation to reduce pressure drag. Various experiments have been carried out to document the separation control capability of mako skin samples, including high-speed video evidence of passive, flow-actuated scale bristling. In water tunnel studies, a flat plate boundary layer was grown to Re > 10⁵ and passed over flank skin samples whereon flow separation was induced by a controllable adverse pressure gradient produced by a rotating cylinder located above the test area. Velocity measurements, both instantaneous and time-averaged, were captured using Digital Particle Image Velocimetry (DPIV). Results confirm separation control was achieved under both laminar and tripped turbulent boundary layer conditions as quantified by backflow coefficient, or the percentage of time the flow was reversed. We hypothesize that the width of a single shark scale corresponds to the sizing of the reversing flow, which induces scale actuation, documented within the turbulent boundary layer case as occurring within a low speed streak.
  

How Hummingbirds Reorient Forces During Maneuvering Flight

Hummingbirds are among the most agile of birds with the unique ability to hover in flight. While their flight kinematics have been studied extensively, their aerodynamic forces have primarily been studied using indirect methods like inverse dynamics and particle image velocimetry, which are insufficient to capture the full weight support of the bird. Here we present in vivo force recordings of maneuvering Anna’s hummingbirds feeding from a moving flower using a novel 3D aerodynamic force platform. The pressure field generated by the maneuvering bird travels to the boundaries of the flight arena, and the six instrumented plates mechanically integrate the resulting pressure and shear distribution at a high enough sample rate to record wingbeat-resolved forces. With these data, we can determine the tracking effectiveness of hummingbirds as well as the control methods they employ during feeding from moving flowers as well as inflight prey capture. Unraveling how hummingbirds manipulate aerodynamic forces with their wings to maneuver has profound applications to the study of other flying animals and the development of more maneuverable aerial robots.   

DNS of squirmers (spherical microswimmers) with rotlet

The squirmer model introduced by Lighthill and later extended by Blake allows the description of microorganisms such as algae and bacteria. It consists in a spherical particle with a prescribed tangential surface velocity, neglecting the radial component, responsible for the self-propulsion. If the microorganism repels fluid along its axis and repels it to the sides it is called pusher (like the bacterium Escherichia Coli); in the opposite case it is called puller (like the alga Chlamydomonas Reinardtii). In this study the squirmer model is incorporated into the Smoothed Profile Method, an efficient DNS scheme to simulate solid objects into a fluid taking fully into account the hydrodynamics, which has already been successfully used in the past to study collective motion and interactions of squirmers. Now the traditional squirmers (pusher and puller) are modified by introducing a rotlet term, an azimuthal component of surface velocity to give a more realistic description of the motion of microorganisms like bacteria whose flagellar and body rotate in the opposite direction.

  

From Single to Many: Swimming at Intermediate Reynolds Numbers

We propose a simple, self-propelled model swimmer which uses steady streaming flows for propulsion at intermediate Reynolds numbers (Re). Our model swimmer is composed of two unequal spheres that are tethered and oscillate in antiphase. For all Re>0, our reciprocal swimmer swims and interestingly, as Re increases, switches swimming direction from a small-sphere-leading to a large-sphere-leading regime. Varying a broad range of parameters (viscosity, amplitude, distance between the spheres, sphere radii and sphere-radii ratio), we identify a universal swimming transition at a critical Re. Flow fields are analyzed, and we determine that propulsion occurs as a result of the interfering steady streaming flows of the two spheres forced to oscillate close to one another. We also show that their bi-directional behavior is linked to their reversal in steady streaming flows. We continue by investigating interactions between multiple swimmers in both swimming regimes.
  

Propulsion of asymmetric bodies through soft lubricated tubes

The motion of tightly fitting objects through soft tubes is a scenario that frequently arises in physiological processes. An example is that of avian egg laying, where observations across avian species suggest that eggs move through the oviduct pointy-end first, even though they are usually then laid blunt-end first. We investigate the mechanistic implications of this observation by considering the motion of fore-aft asymmetric intruders moving through lubricated elastic tubes. Using asymptotic theory, we find that the thickness of the lubricating fluid layer scales inversely with the square root of the slope of the intruder surface near its nose, in the direction of motion. Consequently, the force required to drive motion scales with the square root of this slope, and also depends on the translation velocity, the viscosity of the lubricant and the elasticity of the tube walls. Our findings show that asymmetric objects are more efficiently moved pointy-end-first through lubricated soft tubes, suggesting a mechanistic rationalization for the observed orientation of eggs moving in avian oviducts.   

Hydrodynamic interactions between artificial swimmers and obstacles

Bimetallic micro-rods swimming in hydrogen peroxide solutions are a
standard example of self-propelled particles used in a large number of
experiments. Here, we present how micro-rods can be designed to propel
like pullers, pushers or symmetric swimmers.

We combine experiments and numerical simulations to investigate the dynamics
of rods swimming around obstacles. We find that the characteristic
residence time around an obstacle is longer for symmetric swimmers
than for puller or pushers.
When the obstacles form a lattice the swimmer speed and its residence time
control the long time diffusion coefficient; for non-symmetric
obstacles the displacement bias is different for each kind of
rod. These differences suggest that microfluidic devices can be used
to sort self-propelled particles with different swimming natures.   

A Robotic Fast-Start Fish Utilizing Post-Buckling Dynamics of Slender Column Under Compression to Passively Produce Rapid Underwater Locomotion

An experimental study is conducted on a robotic fish designed to emulate the fast-start response. The fish body is constructed of 3D-printed materials and a light spring steel spine. The body is actuated using pressurized pistons. A total of two pistons are supplied with pressure through lightweight high-pressure service lines. The source of pressure is carbon dioxide with a 4.82 MPa peak operating pressure resulting in a body response that emulates a C-start maneuver in milliseconds. The motion of the fish is controlled using large bandwidth solenoids with a control signal produced by a programmable microprocessor. The buckling modes of a slender column in compression are used to produce organic movements in the body with only two sources of actuation. The interaction of the fluid with the underactuated structure results in a travelling wave in the body of the robotic fish that is kinematically similar to the live fish. The effect of the tail is considered using the model to test performance changes with various geometries.   

Rotation-translation coupling in a highly symmetric propeller at low Reynolds number

Nature has developed several strategies to generate locomotion at low Reynolds (Re) numbers. Ciliates and sperm cells beat cilia in a non-reciprocal way to overcome the scallop-theorem. Bacteria use the inherently symmetry-broken shape of a rotating helical flagellum to create translational motion. Artificial corkscrew propellers were successfully employed to mimic the rotation-translation coupling of the latter [Nanoscale, 3, 557-563 (2011)]. A long time ago Purcell stated: “Turn anything - if it isn't perfectly symmetrical, you'll swim.” [Am. J. Phys. 45, 3 (1977)]. All these examples inevitably indicate the close connection between an objects’ symmetry and its propulsion dynamics at low Re. In turn, this raises the question if all propellers have to be chiral to be propulsive? Can highly symmetrical, achiral shapes be propulsive too? We provide a rigorous symmetry analysis of an achiral, planar V-shaped object and experimentally prove that this shape can be indeed propulsive when driven by an external field. This is, to our knowledge, the first demonstration of a truly achiral micro-object that propels; showing that chirality is not prerequisite for propulsion at low Reynolds number.   

Maximum in density heterogeneities of active swimmers

Suspensions of unicellular microswimmers such as flagellated bacteria or motile algae can exhibit spontaneous density heterogeneities at large enough concentrations. We introduce a novel model for biological microswimmers that creates the flow field of the corresponding microswimmers, and takes into account the shape anisotropy of the swimmer's body and stroke-averaged flagella. By employing multiparticle collision dynamics, we directly couple the swimmer's dynamics to the fluid's. We characterize the nonequilibrium phase diagram, as the filling fraction and Péclet number are varied, and find density heterogeneities in the distribution of both pullers and pushers, due to hydrodynamic instabilities. We find a maximum degree of clustering at intermediate filling fractions and at large Péclet numbers resulting from the competition of hydrodynamic and steric interactions between swimmers. We develop an analytical theory that supports these results. This maximum might represent an optimum for the microorganisms' colonization of their environment.   

No net motion for oscillating near-spheres

We investigate the hydrodynamics of oscillating nearly-spherical particles -- defined by their radius $r=1+\epsilon f(\theta,\phi)$ -- at low, yet non-vanishing, Reynolds numbers. In contrast with the results of [1], we analytically demonstrate that no net motion can arise up to order one in Re and order one in the asphericity parameter $\epsilon$, regardless of the shape function $f$.
[1] F. Nadal and E. Lauga, Physics of Fluids 26(8) 082001 (2014)