Session K46: Swimming, Motility and Locomotion

Anomalous swimming of a ciliary microorganism adjacent to a wall

Numerous swimming microorganisms exist in the Earth’s environment and impact on maintaining aquatic ecosystems. Some ciliates, which live in ponds, lakes, etc., are known to accumulate on the solid-fluid interfaces in nature, where they can obtain enough foods and stable environment. However, the mechanism of this accumulation had been unclear. The swimming of ciliates in bulk water is described by “neutral swimmer” in squirmer model, which is standard hydrodynamic model of active microswimmer. The neutral swimmer is repelled from a wall by hydrodynamic interaction. Where does this contradiction come from? Here, we tried to elucidate the mechanism of the accumulation through comparing the difference between actual ciliate and neutral swimmer. Experimental observation on the ciliate, Tetrahymena pyriformis, swimming near a wall without cell adhesion confirmed that the cell slid adjacent to the wall, in which the novel mechano-sensing response of cilia was observed. Hydrodynamic simulation proved that not only the ciliary response above mentioned but also the anisotropic shape of the cell was crucial for the adjacent swimming.   

Influence of pH on Bacterial Motility and the Chemotactic Response of Helicobacter pylori in Presence of a pH Gradient*

Helicobacter pylori is a bacterium that colonizes the human stomach and can cause gastric ulcers and cancer. While extensive studies have been published on the motility of H. pylori in various media, the majority of the studies were carried out in homogeneous environments. The gastric mucosa where H. pylori lives exhibits a pH gradient varying from 2 to 7 across the mucus layer. We present a live cell tracking study of the motility of H. pylori in Brucella broth and porcine gastric mucin solutions at homogeneous pH levels as well as with a pH gradient created using a microfluidic channel. We found no significant changes in the distribution of swimming speeds in homogeneous environments over pH 3 to 7 and observed that the bacteria either became immotile or died in a pH below 3. We also noted the bacteria body shape became more coccoidal as the pH decreased below pH 4. In response to the presence of a pH gradient, H. pylori travelled in directed trajectories. They can detect the boundary between the regions of neutral and low pH, and were observed to reverse and move away from the low pH region. Analysis of the rotation of the cell body is underway to examine the influence of acidity on the rotational speed of bacteria.   

Selection of Functional Human Sperm with Microfluidic Sorting Device

The first step of in-vitro fertilization is to sort out the motile sperms from the non-motile ones. In this work, we construct a sorting device based on microfluidic principles and explore its efficiency in selecting highly motile sperm. The sperm sorting device consists of two chambers (top and bottom), separated by a polycarbonate filter with pores of a particular size (5 - 12 μm). The top chamber is aligned and attached using a double sided adhesive to the bottom chamber, where there is a small injection point. A sperm sample is prepared by mixing raw semen sample with human tubal fluid plus 1% bovine serum albumin.The sperm sample is then injected into the bottom chamber, and after 45 minutes, a sample of sperm suspension collected from the top chamber is placed on a glass slide for visual observation at 200x magnification. We observe that (1) the larger the pore size of the filter reduces the DNA fragmentation in the sperms, which effectively leads to a higher outcome of the motility. (2) Our data show that within a range of sperm density, the average motility can reach 90-95 %. Taken together, the presented research may provide an efficient method to increase the selection of highly motile sperm, without centrifugation.   

Force-free manipulation of microorganisms in a 3D microfluidic channel

Our capabilities of micromanipulation have evolved with advances in contact-free trapping techniques under various disciplines, such as optical, magnetic, and microfluidic traps. In these techniques, a microscale particle is held in place under compression due to electromagnetic or hydrodynamic forces. In this work, we present a trap-free design of a microfluidic "treadmill", realized by a uniform flow along arbitrary directions in a 3D microfluidic device. The treadmill comprises a central chamber and pairs of x- and y-channels at different elevations that enable the 3D flow generation. We constructed such a 3D microfluidic device using laser lithography, which is controlled by an array of independent syringe pumps. To demonstrate the capability of 3D micromanipulation, we used the 3D flow to drive a suspension of microscale polystyrene particles and compared their movements with the desired flow pattern. Additionally, we have extended this force-free manipulation to control a collection of motile bacteria, e.g. bacterial swarms.   

Transitions in swimming behavior at intermediate Reynolds numbers of a reciprocal two-sphere swimmer

We used the immersed boundary method to investigate an internally-powered reciprocal swimmer composed of two unequal sized spheres that oscillate with respect to each other. We investigated its swimming behavior over a range of Reynolds numbers ranging from Stokes flow (Re=0) to intermediate Reynolds numbers (Re=150). Because the swimmer has a reciprocal stroke, it does not swim in the Stokes regime but it will swim at intermediate Reynolds numbers. We were surprised to find that the direction of swimming depends on the Reynolds number. We identified two motility regimes: one where the swimmer moves in the direction of the large sphere and one where the swimmer moves in the direction of the small sphere. We also found that the average flow around the swimmer is different in the two regimes and qualitatively similar to pushers and pullers.
  

Collective rheotaxis of particles in a channel flow

We use computational modeling to investigate the collective rheotaxis of chemically-fueled, autonomously motile particles confined in a microfluidic channel. Rheotaxising particles rotate to move against an imposed flow. Each particle in our model is partially coated with catalysts, which convert the reagent in solution and thereby produce a local chemical gradient. The particle moves autonomously in response to this self-generated gradient. The interplay among the hydrodynamic interactions due to autonomous motion, externally imposed flow in a channel and motion in response to the chemical gradient generated by neighboring particles leads to a rich and complex behavior. To study this behavior, we use an approach that combines the immersed boundary and lattice Boltzmann techniques. The reagents are modeled as scalar fields that undergo diffusion, are advected with the background fluid flow, and are converted to product in the presence of the catalyst. We isolate the parameter space favorable for rheotaxis and quantify the collective behavior of the particles. Our findings are crucial to controlling the complex collective movement of autonomously motile particles in microfluidic devices.   

Passive transport of a particle attached to a flexible filament in a simple shear flow: Spiders’ ballooning flight as an energy extracting system

Some spiders disperse aerially using their fine silks as a sail. Some of these spiders can travel hundreds of miles and reach as high as 15,000 feet above sea level. The passive motion of a flexible filament whose one end is constrained by the weight of a particle is not well studied. Here we introduce a bead-spring model to investigate the passive behavior of this ballooning structure (spider body + silk) in a simple shear flow, which is a major part of the flow structure in the atmospheric boundary layer. Our experimental simulation shows that the terminal speed of this structure is reduced, as the shear rate of the fluid increases. Interestingly, in sufficient shear rates, this terminal speed becomes smaller than that of maximum drag-producing configuration of a filament in a non-moving viscous fluid. This means that the integrated anisotropic drag of a flexible filament, which is laid in different flow velocity fields, produces fluid-dynamic lift forces opposite to the gravity force, which results in an additional increase of the resistance coefficient; and this in turn reduces the terminal speed of the ballooning structure. With this energy extracting mechanism, spiders can stay longer time in the air for their long aerial dispersal.   

Curving to fly; synthetic evolution of helicopter seeds finds sweet-spot in wing curvature for optimal lift

Nature has invented ingenious aerodynamic design solutions, some of which are critical for plants as wind dispersal of seeds/fruits is coupled to their flight performance. This formulates into an optimization problem for plants: large seed wings can lead to increased lift and more efficient dispersion, but are costly for the tree to build and can more easily be trapped in the canopy. Double winged seeds/fruits separate from their tree when a specific level of dessication is reached, and autorotate as they descend to the ground. This leads to the question: how is the change in the wing curvature of seeds/fruits as they dessicate linked to flight performance ? To answer this, we develop a theoretical model that suggests the existence of an optimal wing curvature that yields maximal lift. To further understand the interplay between flow optimality and geometry, we perform a synthetic seed evolution by deploying 3D printing of seeds that we use in flight experiments, where we span the design space of wing curvature and seed/fruit weight. Our results confirm that there is a sweet-spot in curvature for lift similar to observations in nature, which highlights the importance of not curving too much or too little for helicopter seeds to fly.   

Performance of flexible auto-rotating wings

Seed dispersal relies on the aerodynamical fitness of seedpods that, unlike many experimental surrogates, are found with a variety of mass distribution patterns. We investigate experimentally the effect of various mass distribution on the descent motion of thin rectangular auto-rotating wings. Under carefully controlled condition, we use high-speed photography to extract the flight characteristics such as flight duration, descent angle, and flight range, and connect them to their relative effect on the dispersion of multiple flyers. We found that both span-wise modifications, primarily affecting flexibility, and chord-wise modifications, affecting the wing's moment of inertia, can lead to improvement of aerodynamic performance. In particular, we found that while whole-wing flexibility lead to a drop in flight range in agreement with previous studies, moderate tip flexibility is beneficial.   

Low-dimensional behavior and chaotic mixing by swimming starfish larvae

Physical constraints from environmental forces strongly constrained the evolution of behavior in early animals. We investigate this phenomenon in starfish larvae, a model system for cilia-based locomotion in basal organisms. We find that starfish larvae exhibit a unique behavior in which the animal surrounds its body with a time-varying number of vortices as it adjusts its feeding rate and swimming speed. We show that the animal’s entire repertoire of behaviors is readily decomposed into a finite set of time-varying parameters, which appear within an analytical solution for the motion of fluids at low Reynolds numbers. We observe that this low-dimensional behavior creates fluid dynamical patterns reminiscent of mixing patterns found in chaotic dynamical systems, and that the appearance of these patterns changes in response to local food density and other stimuli. Our work suggests that the simple nervous systems developed by early animals allowed them to adapt surprisingly complex behaviors, even under strongly-constrained conditions such as viscous environments.