arXiv subjects

Manouk Abkarian

Publications and source records attributed to Manouk Abkarian.

11 recordsLinked to original sources

arXiv2024

How a table modulates the risk of airborne transmission between facing individuals

Airborne transmission has been recognized as an important route of transmission for SARS-CoV-2, the virus responsible for the COVID-19 pandemic. While coughing and sneezing are spectacular sources of production of infected aerosols with far-reaching airflows, the prevalence of asymptomatic transmissions highlighted the importance of social activities. Gathering around a table, a common scenario of human interactions, not only fixes a typical distance between static interlocutors, but influences airborne transmission, by serving both as a flow diverter and a surface for droplet deposition. Here, we use high-fidelity large-eddy simulations to characterize short-range airborne transmission when two people face each other at a typical table. We show that compared to the natural distance travelled by free buoyant puffs and jets, the distance between the table and the emission constitutes a new length scale that modifies downward exhaled flows, common during nose breathing, speech, and laughter. When the table is close to the emitter, its main effect is to restrict the forward spread of emitted particles. However, if the distance between individuals is too short, particles reaching the recipient become more concentrated, rising transmission risks. Additionally, simulations of forceful exhalations, like laughter, demonstrate that the table acts as a filter that collects medium-sized particles that would have remained in the free jet otherwise, but can in that case be involved in the fomite transmission route. The table introduces a cut-off size for particles that depends on the inertia of the exhaled material, thereby modifying the size distribution of particles suspended in the air.

physics.flu-dyn↗

arXiv2021

Dynamic stiffening of the flagellar hook

Many bacteria are motile by means of one or more rotating rigid helical flagella, making them the only known organism to use rotation as a means of propulsion. The rotation is supplied by the bacterial flagellar motor, a particularly powerful rotary molecular machine. At the base of each flagellum is the hook, a soft helical polymer that acts as a universal joint, coupling rotation of the rigid membrane-spanning rotor to rotation of the rigid extra-cellular flagellum. In multi-flagellated bacterial species, where thrust is provided by a hydrodynamically coordinated bundle of flagella, the flexibility of the hook is particularly crucial, as many of the flagella within the bundle rotate significantly off-axis from their motor. But, consequently, the thrust produced by a single rotating flagellum applies a significant bending moment to the hook. So, the hook needs to simultaneously provide the compliance necessary for off-axis bundle formation and the rigidity necessary to withstand the large hydrodynamical forces of swimming. To elucidate how the hook can fulfill this double functionality, measurements of the mechanical behavior of individual hooks under dynamical conditions are needed. Here, via new high-resolution measurements and a novel analysis of hook fluctuations during in vivo motor rotation in bead assays, we resolve the elastic response of single hooks under increasing torsional stress, revealing a clear dynamic increase in their bending stiffness. Accordingly, the persistence length of the hook increases by more than one order of magnitude with applied torque. Such strain-stiffening allows the system to be flexible when needed yet reduce deformation under high loads, allowing cellular motility at high speed.

physics.bio-ph↗

arXiv2020

Speech can produce jet-like transport relevant to asymptomatic spreading of virus

Many scientific reports document that asymptomatic and presymptomatic individuals contribute to the spread of COVID-19, probably during conversations in social interactions. Droplet emission occurs during speech, yet few studies document the flow to provide the transport mechanism. This lack of understanding prevents informed public health guidance for risk reduction and mitigation strategies, e.g. the "six-foot rule". Here we analyze flows during breathing and speaking, including phonetic features, using order-of-magnitudes estimates, numerical simulations, and laboratory experiments. We document the spatio-temporal structure of the expelled air flow. Phonetic characteristics of plosive sounds like 'P' lead to enhanced directed transport, including jet-like flows that entrain the surrounding air. We highlight three distinct temporal scaling laws for the transport distance of exhaled material including (i) transport over a short distance ($<$ 0.5 m) in a fraction of a second, with large angular variations due to the complexity of speech, (ii) a longer distance, approximately 1 m, where directed transport is driven by individual vortical puffs corresponding to plosive sounds, and (iii) a distance out to about 2 m, or even further, where sequential plosives in a sentence, corresponding effectively to a train of puffs, create conical, jet-like flows. The latter dictates the long-time transport in a conversation. We believe that this work will inform thinking about the role of ventilation, aerosol transport in disease transmission for humans and other animals, and yield a better understanding of linguistic aerodynamics, i.e., aerophonetics.

physics.flu-dyn↗

arXiv2020

High resolution photonic force microscopy based on sharp nano-fabricated tips

Sub-nm resolution images can be achieved by Atomic Force Microscopy (AFM) on samples that are deposited on hard substrates. However, it is still extremely challenging to image soft interfaces, such as biological membranes, due to the deformations induced by the tip. Photonic Force Microscopy (PhFM), based on optical tweezers (OT), represents an interesting alternative for soft scanning-probe microscopy. Using light instead of a physical cantilever to hold the scanning probe results in a stiffness ($k_{OT}\sim0.1-0.001$ pN/nm) which can be 2-3 orders of magnitude lower than that of standard cantilevers ($k_{AFM}\sim 10$ pN/nm). Combined with nm resolution of displacement measurements of the trapped probe, this allows for imaging soft materials without force-induced artefacts. However, the size of the optically trapped probe, often chosen as a $\sim μ$m-size sphere, has so far limited the resolution of PhFM. Here we show a novel and simple nanofabrication protocol to massively produce optically trappable quartz particles which mimic the sharp tips of AFM. We demonstrate and quantify the stable trapping of particles with tips as sharp as 35 nm, the smallest used in PhFM to date. Raster scan images of rigid nanostructures with features smaller than 80 nm obtained with our tips compare well with AFM images of the same samples. Imaging the membrane of living malaria-infected red blood cells produces no visible artefacts and reveals the sub-micron structural features termed knobs, related to the parasite activity within the cell. The use of nano-engineered particles in PhFM opens the way to imaging soft and biological samples at high resolution.

physics.ins-det↗

arXiv2016

Cluster of red blood cells in microcapillary flow: hydrodynamic versus macromolecule induced interaction

We present experiments on RBCs that flow through microcapillaries under physiological conditions. We show that the RBC clusters form as a subtle imbrication between hydrodynamics interaction and adhesion forces because of plasma proteins. Clusters form along the capillaries and macromolecule-induced adhesion contribute to their stability. However, at high yet physiological flow velocities, shear stresses overcome part of the adhesion forces, and cluster stabilization due to hydrodynamics becomes stronger. For the case of pure hydrodynamic interaction, cell-to-cell distances have a pronounced bimodal distribution. Our 2D-numerical simulations on vesicles captures the transition between adhesive and non-adhesive clusters at different flow velocities.

physics.bio-ph↗

arXiv2016

A new look at blood shear-thinning

Blood viscosity decreases with shear stress, a property essential for an efficient perfusion of the vascular tree. Shear-thinning is intimately related to the dynamics and mutual interactions of red blood cells (RBCs), the major constituents of blood. Our work explores RBCs dynamics under physiologically relevant conditions of flow strength, outer fluid viscosity and volume fraction. Our results contradict the current paradigm stating that RBCs should align and elongate in the flow direction thanks to their membrane circulation around their center of mass, reducing flow-lines disturbances. On the contrary, we observe both experimentally and with simulations, rich morphological transitions that relate to global blood rheology. For increasing shear stresses, RBCs successively tumble, roll, deform into rolling stomatocytes and finally adopt highly deformed and polylobed shapes even for semi-dilute volume fractions analogous to microcirculatory values. Our study suggests that any pathological change in plasma composition, RBCs cytosol viscosity or membrane mechanical properties will impact the onset of shape transitions and should play a central role in pathological blood rheology and flow behavior.

physics.bio-ph↗

arXiv2007

Dissolution Arrest and Stability of Armored Bubbles

Dissolving armored bubbles stabilize with nonspherical shapes by jamming the initially Brownian particles adsorbed on their interfaces. In a gas-saturated solution, these shapes are characterized by planar facets or folds for decreasing ratios of the particle to bubble radii. We perform numerical simulations that mimic dissolution, and show that the faceted shape represents a local minimum of energy during volume reduction. This minimum is marked by the vanishing of the Laplace overpressure $ΔP$, which together with the existence of a $V$-interval where $dΔP/dV>0$ guarantees stability against dissolution. The reduction of $ΔP$ is due to the saddle-shape deformation of most of the interface which accompanies the reduction in the mean curvature of the interface.

cond-mat.soft↗

arXiv2007

Swinging of red blood cells under shear flow

We reveal that under moderate shear stress (of the order of 0.1 Pa) red blood cells present an oscillation of their inclination (swinging) superimposed to the long-observed steady tanktreading (TT) motion. A model based on a fluid ellipsoid surrounded by a visco-elastic membrane initially unstrained (shape memory) predicts all observed features of the motion: an increase of both swinging amplitude and period (1/2 the TT period) upon decreasing the shear stress, a shear stress-triggered transition towards a narrow shear stress-range intermittent regime of successive swinging and tumbling, and a pure tumbling motion at lower shear stress-values.

physics.bio-ph↗

arXiv2006

Mechanics of Interfacial Composite Materials

Recent experiments and simulations have demonstrated that particle-covered interfaces can exist in stable non-spherical shapes as a result of the steric jamming of the interfacially trapped particles, which confers the interface with solid-like properties. We provide an experimental and theoretical characterization of the mechanical properties of these armored objects, with attention given to the two-dimensional granular state of the interface. Small inhomogeneous stresses produce a plastic response while homogeneous stresses produce a weak elastic response. Shear-driven particle-scale rearrangements explain the basic threshold needed to obtain the near-perfect plastic deformation that is observed. Furthermore, the inhomogeneous stress state of the interface is exhibited experimentally by using surfactants to destabilize the particles on the surface. Since the interfacially trapped particles retain their individual characteristics, armored interfaces can be recognized as a kind of composite material with distinct chemical, structural and mechanical properties.

cond-mat.soft↗

arXiv2006

Microstructure, morphology and lifetime of armored bubbles exposed to surfactants

We report the behavior of particle-stabilized bubbles (armored bubbles) when exposed to various classes and concentrations of surfactants. The bubbles are non-spherical due to the jamming of the particles on the interface and are stable to dissolution prior to the addition of surfactant. We find that the dissolving bubbles exhibit distinct morphological, microstructural, and lifetime changes, which correlate with the concentration of surfactant employed. For low concentrations of surfactant an armored bubble remains non-spherical while dissolving, while for concentrations close to and above the surfactant CMC a bubble reverts to a spherical shape before dissolving. We propose a microstructural interpretation, supported by our experimental observations of particle dynamics on the bubble interface, that recognizes the role of interfacial jamming and stresses in particle stabilization and surfactant-mediated destabilization of armored bubbles.

cond-mat.soft↗

arXiv2006

Controlled interfacial assembly of 2D curved colloidal crystals and jammed shells

Assembly of colloidal particles on fluid interfaces is a promising technique for synthesizing two-dimensional micro-crystalline materials useful in fields as diverse as biomedicine1, materials science2, mineral flotation3 and food processing4. Current approaches rely on bulk emulsification methods, require further chemical and thermal treatments, and are restrictive with respect to the materials employed5-9. The development of methods that exploit the great potential of interfacial assembly for producing tailored materials have been hampered by the lack of understanding of the assembly process. Here we report a microfluidic method that allows direct visualization and understanding of the dynamics of colloidal crystal growth on curved interfaces. The crystals are periodically ejected to form stable jammed shells, which we refer to as colloidal armour. We propose that the energetic barriers to interfacial crystal growth and organization can be overcome by targeted delivery of colloidal particles through hydrodynamic flows. Our method allows an unprecedented degree of control over armour composition, size and stability.

cond-mat.soft↗