Session Y02: Biological Fluid Dynamics: Pumping Phenomena (11:30am - 12:15pm CST)

Spontaneous phoretic flows in symmetric and chemically-uniform microchannels

Autophoresis, i.e. the ability to drive fluid flows from self-generated physico-chemical gradients, has received much attention recently to design artificial microswimmers. These self-propel by exploiting gradients in chemical solutes in their immediate vicinity, that result from their surface activity. In such applications, the moving particles have chemically-active boundaries, and self-propulsion may be achieved through chemical and geometric design asymmetries, or an instability-based spontaneous symmetry-breaking when solutes are slowly diffusing. These ideas can be extended to pumping and mixing in microfluidic channels with fixed chemically-active walls. Geometric or chemical asymmetry of the channels is required in order to create a pump (i.e. a net flow through the channel). Yet, we analyse here how uniform and symmetric channels, which do not possess such asymmetric design, nevertheless produce spontaneous mixing flows, when solute advection is not negligible. This presentation will characterise this instability as a result of the coupling of the viscous flow dynamics to the solute transport, and analyse the resulting cellular flows within the phoretic channel.   

Fluid-structure interactions shape animal-microbe associations

The surfaces of most, if not all, animals are colonized by characteristic microbial populations that often confer important benefits to the animal host including protection from pathogens. Yet, we understand little how animals gain and maintain their specific microbial partners. Here, we hypothesize that fluid-structure interactions play a major role in this relationship. We explore this concept using the model animal system Hydra, which harbors a diverse, stable, and beneficial bacterial population on its outer skin. We show that spontaneous muscular contractions of the Hydra body cause shedding of the laminar boundary layer that facilitates transfer of nutrients and waste products from and to the microbe-colonized surfaces. Our results suggest that the Hydra could actively influence the microbial growth conditions on its surface to selectively benefit desired microbial partners. Further, we find that the contractions aid in establishing distinct fluid-mechanical microhabitats along the length of Hydra's body, which may drive the observed spatial distribution of bacteria partners. Together, our results indicate a new role of spontaneous muscle contractions in general, including the peristalsis of the human gut, in shaping animal-microbe associations via fluid mechanical processes.   

Fluid Flow of Pulsing Soft Corals

Soft corals of the family Xeniidae significantly enhance the photosynthetic capacity of their symbiotic algae by actively pulsing to generate flow in their surrounding fluid. Fluid drawn towards the coral mixes at a sufficiently slow rate to allow for the removal of photosynthetic waste and then the fluid is transported away from the coral polyp. This active motion combined with the fact that these soft corals exist in a fluid regime where both inertial and viscous forces influence the flow makes them a unique model organism for understanding fluid mixing. The fully-coupled three-dimensional fluid-structure interaction problem of a pulsing coral and its generated flow was solved using the Immersed Boundary Finite Elements (IBFE) method, a version of the immersed boundary method which uses a finite differences method to solve the Navier-Stokes equations and a finite elements method to solve the elasticity equations. We present a study of the resulting fluid flow and mixing patterns as we vary parameters of the problem. We analyze how the characteristic vertical and horizontal velocities of the generated flow change as we vary the Reynolds number and the length of the resting time period between pulses.   

Quantifying Mixing around Pulsing Soft Corals

The pulsing behavior of soft corals in the family Xeniidae is unique in sessile marine animals. It is hypothesized through experimental procedures that the pulsing facilitates the photosynthesis and photorespiration of their symbiotic algae which in turn provide the coral with most of its energy. This hypothesis is investigated through mathematical modeling and numerical simulations. The immersed boundary method is used to solve the fluid-structure interaction of the pulsing tentacles coupled with the surrounding fluid with varying Reynolds number. The fluid flow is translated into a Poincar\'{e} Map in order to use a dynamical systems approach to quantify the chaotic advection of the fluid flow. Further, the flow is coupled with the advection and diffusion of oxygen, the waste product of photosynthesis. The Pecl\'{e}t number is varied along with the Reynolds number to gain insight into the role of diffusion in different Reynolds number regimes. We will present the results quantifying the mixing, production, and transport of oxygen in these different regimes of varying Pecl\'{e}t and Reynolds numbers.   

Non-Invasive Measure of Stenosis Severity Through Spectral Analysis

This research~focused~on spectral analysis of the sound signal produced by valve stenosis. The physical system mimicked~the measurement of~a human pulse using a stethoscope. Contact microphones attached to the outside of a pipe collected sound signals produced~when the cross-sectional area of the pipe~was reduced~to 20{\%} and 30{\%} of the~initial~cross-sectional area. Water~was pumped~at a frequency of 1Hz and a flow~velocity~of 0.45m/s to~maintain~a Reynolds number that approximates blood flow of the heart, which is about 6400. Power spectra for the 20{\%} and 30{\%} restrictions showed an increase in the energy content of the sound signal across all frequencies. In addition to the overall increase, the 30{\%} restriction power spectrum was made up of 3 peaks centered around 33Hz, 43Hz and 100Hz while the 20{\%} restriction power spectrum had four distinct peaks centered around 20Hz,48Hz, 84Hz and 165Hz. This clear difference between the three power spectra indicates a positive relationship between stenosis severity and frequency produced, and forms the basis for further study of the relationship.   

Pumping for life in the ocean - sealless sponges

Sponges in the ocean are suspension feeders that filter vast amounts of water. Pumping is carried out by flagellated chambers that are connected to an inhalant and exhalant canal system. In "leucon" sponges with relatively high-pressure resistance due to a complex canal system, pumping is only possible owing to the presence of a sealing, gasket-like solid structure (forming a canopy above the collar filters) that also forces the inflow through the collar filter ensuring efficient filtration of prey particles. Here we combine numerical and experimental work, and demonstrate how sponges that lack such sealing elements, e.g., calcareous sponges, are able to efficiently pump and force water through their collar filter, thanks to the formation of a "hydrodynamic gasket" above the collar. The position of this hydrodynamic gasket is determined by hydrodynamic interactions between the part of the flagellum confined inside the filter and the part extending beyond and the pressure resistance of the canal system. Our findings link the architecture of flagellated chambers to that of canal system, and lend support to the current view that the sponge aquiferous system evolved from an open-type filtration system, and that the first metazoans were filter feeders.   

Electrokinetic Micropumping Flow Model using Membrane Contraction

An electrokinetic micropumping flow model is developed to study the transient electroosmotic flow in a microchannel/capillary. A theoretical analysis based on the lubrication theory and the electrokinetic phenomena is derived to govern the flow motion. The pumping mechanism is generated by the rhythmic double membrane contractions on upper and lower walls of the microchannel/capillary. Symmetric and asymmetric membrane contraction with compression and expansion phases are considered to enhance the pumping efficiency. To implement the lubrication theory, ratio of tube radius to the tube length is assumed to be less than unity. Poisson-Boltzmann equations are employed to describe the electric potential function. The effect of parameters such as the electric double layer (EDL) thickness, electric field, membrane geometry on the pressure distribution, flow field characteristics, wall shear stress, and pumping flow rate are investigated. The results show that the induced pumping flow rate can be improved by incorporating the electro-osmosis mechanisms. This novel pump paradigm can be easily fabricated and customized for the use of micro transport of small volume liquid which can be utilized in many biomedical applications.   

Emergent circulation in the loopy network of bird lungs

The airflow in our lungs oscillates as we breathe in and out, but not so for birds. While mammalian lungs are branched and tree-like, bird lungs have loopy airways that display directed flows throughout the breathing cycle. How the air is pumped and directed without valves remains an open problem. Using lab experiments and simulations, we show that these unusual flow patterns naturally emerge within networks containing loops. Oscillatory flow imposed in one segment of a multiloop network is transformed into directed flows along other segments. Network topology and complex flows at junctions play subtle roles in this new form of flow rectification, AC-to-DC conversion or valveless pumping.   

PTV visualization of currents generated by upside-down jellyfish

\textit{Cassiopea} medusae, commonly known as upside-down jellyfish, are found in shallow, protected marine environments. These animals have a mostly sessile lifestyle, resting their bell on the substrate and pointing their oral arms upward facing the sunlight. \textit{Cassiopea} medusae periodically pulsate their bell to generate currents for suspension feeding, exchange and excretion processes. 2D PIV studies have shown the formation of a starting vortex during bell contraction that is pushed upward through the oral arms during bell relaxation. A continuous upward jet is generated above the oral arms during both bell contraction and relaxation. We use 3D PTV (shake-the-box) technique to examine the currents generated by pulsing \textit{Cassiopea} medusae with and without oral arms. Our results show that it is possible to generate unidirectional flow in the vertical direction even without oral arms. The importance of considering three-dimensionality in complementary modelling efforts will be discussed.