Josué Sznitman

Three-Dimensional Convective Alveolar Flow Induced by Rhythmic Breathing Motion of the Pulmonary Acinus

Low Reynolds number flows (Re<1) in the human pulmonary acinus are often difficult to assess due to the submillimeter dimensions and accessibility of the region. In the present computational study, we simulated three-dimensional alveolar flows in an alveolated duct at each generation of the pulmonary acinar tree using recent morphometric data. Rhythmic lung expansion and contraction motion was modeled using moving wall boundary conditions to simulate realistic sedentary tidal breathing. The resulting alveolar flow patterns are largely time independent and governed by the ratio of the alveolar to ductal flow rates, Qa/Qd. This ratio depends uniquely on geometrical configuration such that alveolar flow patterns may be entirely determined by the location of the alveoli along the acinar tree. Although flows within alveoli travel very slowly relative to those in acinar ducts, 0.021%<or=Ua/Ud<or=9.1%, they may exhibit complex patterns linked to the three-dimensional nature of the flow and confirm findings from earlier three-dimensional simulations. Such patterns are largely determined by the interplay between recirculation in the cavity induced by ductal shear flow over the alveolar opening and radial flows induced by wall displacement. Furthermore, alveolar flow patterns under rhythmic wall motion contrast sharply with results obtained in a rigid alveolus, further confirming the importance of including inherent wall motion to understand realistic acinar flow phenomena. The present findings may give further insight into the role of convective alveolar flows in determining aerosol kinematics and deposition in the pulmonary acinus.

Propulsive force measurements and flow behavior of undulatory swimmers at low Reynolds number

The swimming behavior of the nematode Caenorhabditis elegans is investigated in aqueous solutions of increasing viscosity. Detailed flow dynamics associated with the nematode’s swimming motion as well as propulsive force and power are obtained using particle tracking and velocimetry methods. We find that C. elegans delivers propulsive thrusts on the order of a few nanonewtons. Such findings are supported by values obtained using resistive force theory; the ratio of normal to tangential drag coefficients is estimated to be approximately 1.4. Over the range of solutions investigated here, the flow properties remain largely independent of viscosity. Velocity magnitudes of the flow away from the nematode body decay rapidly within less than a body length and collapse onto a single master curve. Overall, our findings support that C. elegans is an attractive living model to study the coupling between small-scale propulsion and low Reynolds number hydrodynamics.

Respiratory microflows in the pulmonary acinus

Over the past few decades, our understanding of the fluid mechanics characterizing the pulmonary acinus of the lungs has been fundamentally revisited. In the present paper, we review the current knowledge of acinar convective airflows and their role in determining the fate of inhaled aerosols in the distal regions of the lungs. We discuss the influential body of computational and experimental efforts following the revealing bolus studies initiated by Heyder et al. (1988) that have dramatically advanced our description of acinar flow phenomena. In particular, we characterize the range of complex flow topologies that exist locally in alveolar cavities and describe the ensuing convective mechanisms known to generate kinematic irreversibility in the acinus, despite low-Reynolds-number flows. By using dimensional analysis, we shed some light on the intimate coupling that arises in the pulmonary acinus between diffusive, convective and sedimentation mechanisms for aerosol deposition. Finally, we evoke some of the critical challenges that lie ahead in predicting accurately the deposition of inhaled particles across the acinar region and give a brief outlook toward novel approaches for resolving acinar flow dynamics at the real scale.

Heat transfer variations of bicycle helmets

Abstract Bicycle helmets exhibit complex structures so as to combine impact protection with ventilation. A quantitative experimental measure of the state of the art and variations therein is a first step towards establishing principles of bicycle helmet ventilation. A thermal headform mounted in a climate-regulated wind tunnel was used to study the ventilation efficiency of 24 bicycle helmets at two wind speeds. Flow visualization in a water tunnel with a second headform demonstrated the flow patterns involved. The influence of design details such as channel length and vent placement was studied, as well as the impact of hair. Differences in heat transfer among the helmets of up to 30% (scalp) and 10% (face) were observed, with the nude headform showing the highest values. On occasion, a negative role of some vents for forced convection was demonstrated. A weak correlation was found between the projected vent cross-section and heat transfer variations when changing the head tilt angle. A simple analytical model is introduced that facilitates the understanding of forced convection phenomena. A weak correlation between exposed scalp area and heat transfer was deduced. Adding a wig reduces the heat transfer by approximately a factor of 8 in the scalp region and up to one-third for the rest of the head for a selection of the best ventilated helmets. The results suggest that there is significant optimization potential within the basic helmet structure represented in modern bicycle helmets.

Undulatory locomotion of Caenorhabditis elegans on wet surfaces.

The physical and biomechanical principles that govern undulatory movement on wet surfaces have important applications in physiology, physics, and engineering. The nematode Caenorhabditis elegans, with its highly stereotypical and functionally distinct sinusoidal locomotory gaits, is an excellent system in which to dissect these properties. Measurements of the main forces governing the C. elegans crawling gait on lubricated surfaces have been scarce, primarily due to difficulties in estimating the physical features at the nematode-gel interface. Using kinematic data and a hydrodynamic model based on lubrication theory, we calculate both the surface drag forces and the nematodes bending force while crawling on the surface of agar gels within a preexisting groove. We find that the normal and tangential surface drag coefficients during crawling are ∼222 and 22, respectively, and the drag coefficient ratio is ∼10. During crawling, the calculated internal bending force is time-periodic and spatially complex, exhibiting a phase lag with respect to the nematodes body bending curvature. This phase lag is largely due to viscous drag forces, which are higher during crawling as compared to swimming in an aqueous buffer solution. The spatial patterns of bending force generated during either swimming or crawling correlate well with previously described gait-specific features of calcium signals in muscle. Further, our analysis indicates that one may be able to control the motility gait of C. elegans by judiciously adjusting the magnitude of the surface drag coefficients.

Optical Density Visualization and Abel Reconstruction of Vortex Rings Using Background-Oriented Schlieren

These figures represent a sequence of visualizations of CO2-loaded vortex rings generated at the orifice opening of a piston-cylinder apparatus (diameter of orifice opening is 7 cm; Re = 36’000; ratio of the piston stroke length to diameter is 0.5 and field of view is 15 x 20 cm). Qualitative Schlieren visualizations are obtained using a Background Oriented Schlieren (BOS) technique (Fig. 1) and a vector map of the gradients of the refractive index is extracted using a PIV algorithm (Fig. 2). The projected density field (Fig. 3) is then obtained by integrating the measured gradient field. Finally, an Abel inverse transform is implemented to reconstruct the true radial vortex ring profiles for enhanced visualization of flow structures such as the recirculating spiral roll-ups and trailing wakes (Fig. 4). Fig. 1. Schlieren visualization: absolute difference between phase and reference are displayed. Fig. 2. Vector map of the refractive index gradient (scale in arbitrary units).

Motility of small nematodes in wet granular media

The motility of the worm nematode Caenorhabditis elegans is investigated in shallow, wet granular media as a function of particle size dispersity and area density (). Surprisingly, we find that the nematodes propulsion speed is enhanced by the presence of particles in a fluid and is nearly independent of area density. The undulation speed, often used to differentiate locomotion gaits, is significantly affected by the bulk material properties of wet mono- and polydisperse granular media for ≥0.55. This difference is characterized by a change in the nematodes waveform from swimming to crawling in dense polydisperse media only. This change highlights the organisms adaptability to subtle differences in local structure and response between monodisperse and polydisperse media.

Particle dynamics and deposition in true-scale pulmonary acinar models.

Particle transport phenomena in the deep alveolated airways of the lungs (i.e. pulmonary acinus) govern deposition outcomes following inhalation of hazardous or pharmaceutical aerosols. Yet, there is still a dearth of experimental tools for resolving acinar particle dynamics and validating numerical simulations. Here, we present a true-scale experimental model of acinar structures consisting of bifurcating alveolated ducts that capture breathing-like wall motion and ensuing respiratory acinar flows. We study experimentally captured trajectories of inhaled polydispersed smoke particles (0.2 to 1 μm in diameter), demonstrating how intrinsic particle motion, i.e. gravity and diffusion, is crucial in determining dispersion and deposition of aerosols through a streamline crossing mechanism, a phenomenon paramount during flow reversal and locally within alveolar cavities. A simple conceptual framework is constructed for predicting the fate of inhaled particles near an alveolus by identifying capture and escape zones and considering how streamline crossing may shift particles between them. In addition, we examine the effect of particle size on detailed deposition patterns of monodispersed microspheres between 0.1–2 μm. Our experiments underline local modifications in the deposition patterns due to gravity for particles ≥0.5 μm compared to smaller particles, and show good agreement with corresponding numerical simulations.

Microfluidic platforms for advanced risk assessments of nanomaterials

Abstract In the past few years, promising efforts to utilize microfabrication-based technologies have laid the foundation for developing advanced, and importantly more physiologically-realistic, microfluidic methods for risk assessment of engineered nanomaterials (ENMs). In the present review, we discuss the wave of recent developments using microfluidic-based in vitro models and platforms for nanotoxicological assays, such as determination of cell viability, cellular dose, oxidative stress and nuclear damage. Here, we specifically highlight the tangible advantages of microfluidic devices in providing promising tools to tackle many of the current and ongoing challenges faced with traditional toxicology assays. Most importantly, microfluidic technology not only allows to recreate physiologically-relevant in vitro models for nanotoxicity examinations, but also provides platforms that deliver an attractive strategy towards improved control over applied ENM doses. In a final step, we present examples of state-of-the-art microfluidic platforms for in vitro assessment of potential adverse ENM effects.

Visualization of respiratory flows from 3D reconstructed alveolar airspaces using X-ray tomographic microscopy

A deeper knowledge of the three-dimensional (3D) structure of the pulmonary acinus has direct applications in studies on acinar fluid dynamics and aerosol kinematics. To date, however, acinar flow simulations have been often based on geometrical models inspired by morphometrical studies; limitations in the spatial resolution of lung imaging techniques have prevented the simulation of acinar flows using 3D reconstructions of such small structures. In the present study, we use high-resolution, synchrotron radiation-based X-ray tomographic microscopy (SRXTM) images of the pulmonary acinus of a mouse to reconstruct 3D alveolar airspaces and conduct computational fluid dynamic (CFD) simulations mimicking rhythmic breathing motion. Respiratory airflows and Lagrangian (massless) particle tracking are visualized in two examples of acinar geometries with varying size and complexity, representative of terminal sacculi including their alveoli. The present CFD simulations open the path towards future acinar flow and aerosol deposition studies in complete and anatomically realistic multi-generation acinar trees using reconstructed 3D SRXTM geometries.Graphical Abstract