Session Q49: Focus Session: Swimmers

Suppression of E. coli tumbling and wobbling in dilute polymeric fluids

Bacteria commonly utilize a run-and-tumble swimming behavior to navigate through complex environments, such as mucus in the lungs or digestive system. This swimming behavior has been extensively studied in water-like fluids; yet, studies on the role of particles/polymers on the run-and-tumble technique are limited. Here, we experimentally investigate the role of polymer concentration on the swimming dynamics of \textit{E. coli}. We find that small amounts of polymer drastically change the run-and-tumble behavior of \textit{E. coli} cells, significantly enhancing the translational diffusion. The average cell velocity increases with polymer concentration (and viscosity) and the mean run times are enhanced. By varying polymer molecular weight, we show that enhanced translation is a result of two mechanisms: (1) suppression of cell wobbling due to elasticity and (2) enhancement of run times due to viscosity. Our results show that the transport of chemotactic cells can be independently modified by viscosity and elasticity.   

Differential dynamic behaviors of undulatory nematodes in liquid vs. soft gel environment

Caenorhabditis elegans (C. elegans) is an undulatory nematode which exhibits two distinct locomotion types of swimming and crawling. Although in its natural habitat C. elegans lives in complex fluidic environment, our current understanding has been limited to the behavior of C. elegans in a simple Newtonian fluid. Here, we present some experimental results on the penetrating behavior of C. elegans at the interface from liquid to solid environment. Once C. elegans, which otherwise swims freely in a liquid, makes a contact to the solid gel boundary, it begins to penetrate vertically to the surface by changing its stroke motion characterized by a stiffer body shape and a slow stroke frequency. The particle image velocimetry (PIV) analysis reveals the flow streamlines produced by the stroke of worm. For the worm that crawls on a solid surface, we utilize a technique of traction force microscopy (TFM) to find that the crawling nematode forms localized force islands along the body where makes direct contacts to the gel surface.   

Directed Paramagnetic Colloidal Swimmers

A novel micoscale swimmer can be generated by placing two paramagnetic colloids of different sizes in a rotating magnetic field. For propulsion at the microscale, viscous forces dominate over inertial forces. This results in the scallop theorem, where reversible displacements does not lead to any net motion. To achieve controlled swimming at the microscale, the swimmer must be able to make a sequence of deformations that are cyclic but not time reversible. Two paramagnetic bodies in a circular eccentric rotating magnetic field influence each other and propel together in a directed manne. The motion of each body tracks a half-moon course, shown in the figure below. We will describe this method and show how Brownian motion enhances this propulsion.   

Mechanics of swimming at the small scale in complex fluids

Recent experiments with bacteria in liquid crystalline solutions have revealed that nematic order affects the swimming behavior of bacteria. Motivated by these observations, we study a simple model of low-Reynolds-number swimming in an anisotropic fluid, that of an infinitely long two-dimensional sheet deforming via propagating transverse or longitudinal waves and immersed in a hexatic or a nematic liquid crystal. The liquid crystal is categorized by the dimensionless Ericksen number Er, which compares viscous and elastic effects. Paying special attention to the anchoring strength at the interface of the liquid crystal and the swimmer, we calculate how swimming speed depends on Er for small amplitude waves. We study both the sinusoidal steady-state problem as well as the startup problem in which the swimmer starts from rest.   

Anomalous swimming behavior of bacteria in nematic liquid crystals

Flagellated bacteria stop swimming in isotropic media of viscosity higher than 0.06kgm$^{-1}$s$^{-1}$. However, Bacillus Subtilis slows down by only about 30\% in a nematic chromonic liquid crystal (CLC, 14wt\% DSCG in water), where the anisotropic viscosity can be as high as 6kgm$^{-1}$s$^{-1}$. The bacteria velocity ($V_b$) is linear with the flagella rotation frequency. The phase velocity of the flagella $V_f \approx 2V_b$ in LC, as compared to $V_f \approx 10V_b$ in water. The flow generated by the bacteria is localized along the bacterial body axis, decaying slowly over tens of micrometers along, but rapidly over a few micrometers across this axis. The concentrated flow grants the bacteria new ability to carry cargo particles in LC, ability not seen in their habitat isotropic media. We attribute these anomalous features to the anisotropy of viscosity of the CLC, namely, the viscosities of splay and twist is hundreds times higher than that of bend deformation, which provides extra boost of swimming efficiency and enables the bacteria swim at considerable speed in a viscous medium. Our findings can potentially lead to applications such as particle transportation in microfluidic devices.   

Propulsion and instability of flexible helical flagella

We consider a macroscopic analogue model for the locomotion of uni-flagellar bacteria in a viscous fluid. The rescaling from the original micron-scale onto the desktop-scale is made possible by the prominence of geometry in the deformation process. As a model for the flagellum, we fabricate elastomeric filaments with fully customizable geometric and material properties, and rotate them at low Reynolds number conditions in a glycerin bath. Using digital imaging, we analyze the dynamics of the geometrically nonlinear deformed configurations. Our precision experiments are compared against numerical simulations that employ the Discrete Elastic Rods (DER) method, with an emphasis on quantifying the generated propulsive force. A novel mechanical instability is uncovered, whereby the filament buckles above a critical rotation frequency and we quantify its dependence on the physical and control parameters of the system. A scaling analysis allows us to rationalize the underlying physical mechanism and informs the original biological system that motivated the study.   

Does \textit{Helicobacter pylori} exhibit corkscrew motion while swimming?

\textit{Helicobacter pylori} is a spiral shaped bacterium associated with ulcers, gastric cancer, gastritis among other diseases. In order to colonize the harsh acidic environment of the stomach \textit{H. pylori} has to go across the viscoelastic mucus layer of the stomach. Many studies have been conducted on the swimming of \textit{H. pylori} in viscous media however none have taken into account the influence of cell-body shape on the trajectory. We present an experimental study of the effects of body shape in the swimming trajectory of \textit{H. pylori} in viscous media by a quantitative analysis of the bacterium rotation and translation in gels using phase contrast microscopy and particle tracking techniques. Preliminary microscopic tracking measurements show very well defined helical trajectories in the spiral-shaped wild type \textit{H. pylori}. These helical trajectories are not seen in rod-shaped mutants which sometimes display whirling motion about one end acting as a hinge. We will present an analysis of the different trajectories for bacteria swimming in media with different viscoelastic parameters.   

How does \textit{Vorticella} utilize its stalk contraction-relaxation cycle?

\textit{Vorticella} is a sessile ciliate living in water, and it coils its slender stalk to pull the cell body (zooid) towards the substrate at a maximum speed of $\sim$ 1 cm/s. After stalk contraction is completed, the stalk slowly relaxes to its extended state. Although this ultrafast stalk contraction has been studied in terms of cell motility, it is poorly understood how \textit{Vorticella} utilizes its stalk contraction. Here we propose a hypothesis that \textit{Vorticella} can augment transport of particles near the substrate relying on water flow induced by the stalk contraction-relaxation cycle. We investigated our hypothesis using a computational fluid dynamics (CFD) model which models \textit{Vorticella} as a solid sphere moving normal to a solid surface in water. Having simulated water flow caused by \textit{Vorticella}, we calculated motions of particles near \textit{Vorticella}, and then quantified the transport effect of \textit{Vorticella}'s stalk contraction using microfluidic mixing indices.   

A steering mechanism for phototaxis in Chlamydomonas

{\it Chlamydomonas} shows both positive and negative phototaxis. It has a single eyespot near its equator and as the cell rotates during forward motion the light signal received by the eyespot varies. We use a simple mechanical model of {\it Chlamydomonas} that couples the flagellar beat pattern to the light intensity at the eyespot to demonstrate a mechanism for phototactic steering that is consistent with observations. The direction of phototaxis is controlled by a parameter in our model and the steering mechanism is robust to noise. In the dark, our model shows emergent run-and-tumble behavior and we see switching between directed phototaxis and run-and-tumble when we switch the light on and off.   

Sorting choanoflagellates

In freshwater environments, as well as in oceans, environmental conditions are in constant fluctuation. Some heterotrophic plankton must adapt their swimming behavior in order to survive under these conditions. In the case of the choanoflagellate, the closest animal ancestor, the ability to forage for food is given not only by its single flagellum, but also by its differentiation between fast and slow swimmers. The understanding of how these cells with different strategies to swim search for food can give us a better insight into how eukaryotes respond to different stimuli. In this work, we have designed a microfluidic device that sorts choanoflagellates by their speed. The optimal geometry was found by a numerical model using the experimentally determined motilities of each swimmer type.   

Swimming of \textit{Vorticella} in two-dimensional confinements

\textit{Vorticella }is a ciliate observed in the stalked sessile form (trophont), which consists of an inverted bell-shaped cell body (zooid) and a slender stalk attaching the zooid to a substrate. Having circular cilia bands around the oral part, the stalkless zooid of \textit{Vorticella} can serve as a model system for microorganism swimming. Here we present how the stalkess trophont zooid of \textit{Vorticella} swims in two-dimensional confined geometries which are similar to the Hele-Shaw cell. Having harvested stalkless \textit{Vorticella} zooids, we observed their swimming in water between two glass surfaces using video microscopy. Based on measured swimming trajectories and distributions of zooid orientation and swimming velocity, we analyzed how \textit{Vorticella}'s swimming mobility was influenced by the geometry constraints.   

Water droplets also swim!

Recently there has been a surge of interest in producing artificial swimmers. One possible path is to produce self-propelling droplets in a liquid phase. The self-propulsion often relies on complex mechanisms at the droplet interface, involving chemical reactions and the adsorption-desorption kinetics of the surfactant. Here, we report the spontaneous swimming of droplets in a very simple system: water droplets immersed in an oil-surfactant medium. The swimmers consist of pure water, with no additional chemical species inside: water droplets also swim! The swimming is very robust: the droplets are able to transport cargo such as large colloids, salt crystals, and even cells. In this talk we discuss the origin of the spontaneous motion. Water from the droplet is solubilized by the reverse micellar solution, creating a concentration gradient of swollen reverse micelles around each droplet. By generalizing a recently proposed instability mechanism, we explain how spontaneous motion emerges in this system at sufficiently large P\'eclet number. Our water droplets in an oil-surfactant medium constitute the first experimental realization of spontaneous motion of isotropic particles driven by this instability mechanism [1]. [1] Z. Izri et al., PRL, accepted (2014), arXiv:1406.5950