Session U04: Biological Fluid Dynamics: Locomotion Non-Newtonian Fluids (8:45am - 9:30am CST)

3D effects on diffusion coupled motility of Helicobacter pylori

The bacterium\textit{ H. pylori} swims through the gastric mucus gel by diffusing ammonia from its body, de-gelling the surrounding medium to make a pocket of Newtonian fluid around itself. Confinement by this pocket impacts bacterial swimming and flows, while the flows simultaneously affect the pocket geometry via advection. Previously, a simple two-dimensional model of confinement effects predicted slight increase in swimming speeds. Here, we treat a realistic three-dimensional swimming bacterial geometry, and include the effects of its 3D swimming flows on the advection-diffusion of ammonia. \textit{H. pylori} is modelled as a swimmer with a spherical head and a flagellum using the method of regularized Stokeslets. We model the mucus gel as a random spatial distribution of regularized Stokelets placed outside the Newtonian fluid pocket. To handle the large number of Stokeslets we implement a Fast Multipole Method. Advection-diffusion of ammonia is treated numerically in the frame of flagellum allowing us to solve a steady problem. We iteratively find a pocket geometry that leads to swimming flows that self-consistently produce the pocket geometry through the advection-diffusion of ammonia. We find that the bacterium swims at a slower speed than the unconfined speed at all Peclet numbers.   

A viscoelastic two fluid model for the self-propulsion of Helicobacter pylori in a mucus layer

Experiments revealed that the bacterium \textit{Helicobacter pylori} (\textit{H. pylori}) invades the mucus layer protecting the epithelial cells, and causes ulcers in the stomach. It does that by modifying the rheological properties of the mucus layer from a high-viscosity gel to a low-viscosity gel, in which the \textit{H. pylori} propels. However, the propulsion mechanism of \textit{H. Pylori} is poorly understood. S. Y. Reigh, and E. Lauga (Phys. Rev. Fluids 2, 093101) modeled the \textit{H. pylori} using squirmer model developed by Lighthill (1952), and analyzed its propulsion in a mucus layer using a two-fluid model (swimmer confined in a low viscous drop suspended in a high viscous fluid), assuming that the two fluids are Newtonian. However, the mucus layer that coats the stomach wall is viscoelastic in nature. So, in this work, using asymptotic theory with Deborah (De) number (ratio of the material relaxation time scale to the flow time scale) as a small parameter, we solve for the O(De) velocity field, the swimming speed, the power dissipated by the swimmer, and the swimming efficiency. Our analysis may explain the experimental observations related to the swimming strategy of \textit{H. pylori}.   

Swimming with Swirl at Low Weissenberg Number

Microorganisms commonly swim through complex biological fluids, such as mucus or biofilms, that readily exhibit non-Newtonian behavior. Recently, we have shown using the squirmer model that swimmers with significant azimuthal motion (created for example by a rotating tail, as in the case of \textit{E. coli}) can swim significantly faster in viscoelastic fluids, fluids that exhibit both a viscous and elastic response to deformation (Binagia et al., 2020). In that work, the fluid was modeled using the Giesekeus constitutive equation, which models polymer molecules in the fluid as Hookean dumbbells experiencing an anisotropic drag. In this talk, we revisit this problem considering now a range of polymer constitutive equations. We present analytical results proving that, unless a regularized polymer model is used, asymptotic solutions are only valid up to a relatively modest value of the Weissenberg number, the dimensionless group characterizing the degree of fluid elasticity. We further show that the radius of convergence for such series solutions is equally small for all models when the squirmer model is used. In terms of kinematics, we show that a speed increase is predicted in all cases even when the underlying mechanism (based on the net forces acting on the swimmer) differs.   

Propulsion of microparticles in nonlinearly viscoelastic fluids through symmetry breaking

Magnetic micro- and nanoparticles are being utilized in a variety of techniques including hyperthermia, drug delivery, and magnetic resonance imaging. However, the propulsion of spherical microparticles has been limited to a handful of techniques including catalytic propulsion, external magnetic gradients, and geometric functionalization. The research presented here will demonstrate microparticle propulsion through spontaneous symmetry breaking and their control using a combination of static and rotating magnetic fields. Nonlinearities in certain fluids create rod-climbing-like effects that allow rotating microparticles to experience symmetry broken propulsion along their rotation axes. This type of propulsion was demonstrated in several nonlinearly viscoelastic fluids including synthetic mucus and low concentration polyacrylamide. The propulsion direction along the rotation axes were altered by adjusting the magnitude and direction of an overlaid static magnetic field. Several studies were performed including velocity vs. rotational frequency, velocity vs. static magnetic field, particle image velocimetry, and selected 2D and 3D trajectories under both open loop and closed loop control.   

Effect of Hypersonic and Guillotine Vitrector Devices on Rheological Behavior on Vitreous Liquor.

Variations in the rheological behavior of the vitreous can occur due to procedures including vitrectomy, extraction of vitreous. Also, vitrectomy trigger the degradation or aggregation of its protein content. The aim of this study was comparing the viscoelastic properties of porcine vitreous between different vitrectomy techniques. We evaluated the results obtained from extensional rheological studies of vitreous samples and compared with shear rheological results. Specimens were collected utilizing two different guillotine vitrectomy cutters (23 and 25-gauge), and hypersonic vitrectomy using the Bausch System. Shear rheological analysis showed that the viscosity of the chopped porcine vitreous was higher using guillotine vitrectomy devices than hypersonic vitrectomy cutter, maybe caused by shortening of protein chains related to guillotine vitrector. Conducting extensional rheological studies showed the highest relaxation time using the 23-gauge hypersonic device might be due to lowest destruction of vitreous segments. The hydrodynamic interaction of the coils investigated with Zimm model, showed that hypersonic vitrector has lower influence on the vitreous structure that causes the vitreous liquid to last longer in the viscoelastic regime.   

A note on a swirling squirmer in a shear-thinning fluid

A recent study has revealed that a squirmer with swirl in a viscoelastic fluid can lead to a significant speed enhancement (Binagia \textit{et al.}, \textit{J. Fluid Mech.}, 900, A4, 2020). Here we consider a similar calculation but focus on the effect of shear-thinning viscosity, which is another common non-Newtonian rheology of biological fluids such as blood and mucus. We employ the Carreau constitutive equation to examine how the swirling flow affects the swimming of a squirmer in a shear-thinning but inelastic fluid. The results will allow us to better separate the impacts due to viscoelasticity and shear-thinning rheology, and evaluate their relative importance in the observed phenomenon.