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Register a new userChemotactic Maneuverability of Sperm
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In this fluid mechanics video, we explore the kinematics of chemotaxing sperm cells (sea urchin, textit{Arbacia punctulata}) swimming in a chemoattractant gradient. We demonstrate that the complex swimming trajectories resulting in chemotactic behavior (`turn-and-run motility) are comprised of several distinct flagellar maneuvers. These motility patterns likely play an important role optimizing chemotaxic motility and navigation, when the sperm cells are subjected external fluid flows.
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We design and simulate the motion of a new swimmer, the {it Quadroar}, with three dimensional translation and reorientation capabilities in low Reynolds number conditions. The Quadroar is composed of an $texttt{I}$-shaped frame whose body link is a simple linear actuator, and four disks that can rotate about the axes of flange links. The time symmetry is broken by a combination of disk rotations and the one-dimensional expansion/contraction of the body link. The Quadroar propels on forward and transverse straight lines and performs full three dimensional reorientation maneuvers, which enable it to swim along arbitrary trajectories. We find continuous operation modes that propel the swimmer on planar and three dimensional periodic and quasi-periodic orbits. Precessing quasi-periodic orbits consist of slow lingering phases with cardioid or multiloop turns followed by directional propulsive phases. Quasi-periodic orbits allow the swimmer to access large parts of its neighboring space without using complex control strategies. We also discuss the feasibility of fabricating a nano-scale Quadroar by photoactive molecular rotors.
The guidance of human sperm cells under confinement in quasi 2D microchambers is investigated using a purely physical method to control their distribution. Transport property measurements and simulations are performed with dilute sperm populations, for which effects of geometrical guidance and concentration are studied in detail. In particular, a trapping transition at convex angular wall features is identified and analyzed. We also show that highly efficient microratchets can be fabricated by using curved asymmetric obstacles to take advantage of the spermatozoa specific swimming strategy.
Many experiments in recent years have reported that, when exposed to their corresponding substrate, catalytic enzymes undergo enhanced diffusion as well as chemotaxis (biased motion in the direction of a substrate gradient). Among other possible mechanisms, in a number of recent works we have explored several passive mechanisms for enhanced diffusion and chemotaxis, in the sense that they require only binding and unbinding of the enzyme to the substrate rather than the catalytic reaction itself. These mechanisms rely on conformational changes of the enzyme due to binding, as well as on phoresis due to non-contact interactions between enzyme and substrate. Here, after reviewing and generalizing our previous findings, we extend them in two different ways. In the case of enhanced diffusion, we show that an exact result for the long-time diffusion coefficient of the enzyme can be obtained using generalized Taylor dispersion theory, which results in much simpler and transparent analytical expressions for the diffusion enhancement. In the case of chemotaxis, we show that the competition between phoresis and binding-induced changes in diffusion results in non-trivial steady state distributions for the enzyme, which can either accumulate in or be depleted from regions with a specific substrate concentration.
We reconsider fluid dynamics for a self-propulsive swimmer in Stokes flow. With an exact definition of deformation of a swimmer, a proof is given to Purcells scallop theorem including the body rotation. The breakdown of the theorem due to a finite Stokes number is discussed by using a perturbation expansion method and it is found that the breakdown generally occurs at the first order of the Stokes number. In addition, employing the Purcells scallop model, we show that the theorem holds up to a higher order if the strokes of the swimmer has some symmetry.
We implement a simple hydrodynamical model to study behavioural swimming tilt angle of open swimmbladder fish. For this purpose we study the stability of forces acting on a fish swimming horizontally with constant velocity. Additionally, the open swimbladder compression with the depth is modelled by Boyles law. With these, our model gives an analytical solution relating the depth with the body tilt angle and the velocity. An interesting result for steady horizontal swimming is that the body tilt decreases with velocity almost like $v^{-1}$. Moreover, we give an expression for the maximum tilt angle. Then, by introducing the assumption of constant swimming power we relate the swimming velocity with the tilting. Furthermore, we show that the hydrodynamical influence of a temperature gradient produced by a thermocline seems to be negligible for the fish tilting. These results are considerably helpful for more realistic modelling of the emph{acoustic target strength} of fish. Finally, we tested our results by comparing the hydrodynamics solutions with others obtained from acoustic observations and simulations of target strength for Argentine anchovy.
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