61 research outputs found
Ultrasound pulse echo analysis of blood aggregation in microfluidics
International audienceBlood aggregation occurs when erythrocytes, red blood cells (RBC), agglutinate together into compact stacks of cells called rouleaux. Blood aggregation can then be compared to a liquid-gel transition that appears at low flow regime, and is responsible of the high value of the blood viscosity at low shear rate. This paper will present a physical device that can detect the appearing of this liquid-gel transition using the transmission and reflection properties of ultrasonic waves. In order to detect the liquid-gel transition in resting blood we use the property of an ultrasonic shear wave (5 MHz) to be transmitted or reflected at a glass-liquid interface that is populated with RBC. The setup, as sketched in Figure1, is made of a piezoelectric transducer, a PZT stack that is bonded on the bottom face of a glass cylinder that acts as an acoustic delay line [1]. A burst of signal (2µs) is sent by the PZT toward the glass coverslip, travels through the delay line, bounces at the interfaces and returns with a delay to the PZT where it is detected. Amplitudes of the echo is then compared to input signal in order to extract a reflexion coefficient (r). Experiments are performed both on droplets of healthy blood deposited on glass coverslips and in PDMS microfluidic circuits at low flow strength and at rest. In particular, a spiral shaped microchip, as can be seen in Figure 2, is used both to mimic a vascular network (160x150µm section channels) and to expose a large surface (about 25 mm2) of the sample to the ultrasonic signal [2]. The detection of r over time is coupled with observations of RBCs behaviour during the sample desiccation by optical microscopy to elucidate the dynamics of the drying process of blood. All measurements are carried out at controlled temperature (25 °C) and relative humidity (30-60%). Note that the acoustic power (<1pJ) sent to the blood is small and does not interact with cells through acoustophoresis mechanisms. (a) (b) Figure 1: (a) Schematic (not to scale) of the ultrasound pulse echo setup for the analysis of liquid gel transition in steady blood flow due to red blood cells aggregation. (b) Photograph of the spiral shaped microfluidic circuit bonded on a microscope glass blade a coupled to an ultrasonic pulse echo system. The spiral channel is 160μm wide, 150μm for a total diameter of diameter of 6mm
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.Comment: 14 pages, 6 figure
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
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.Comment: 4 pages 5 figures submitted to Physical Review Letter
Curling dynamics of naturally curved ribbons: from high to low Reynolds numbers
Curling deformation of thin elastic sheets appears in numerous structures in nature, such as membranes of red blood cells, epithelial tissues or green algae colonies to cite just a few examples. However, despite its ubiquity, the dynamics of curling propagation in a naturally curved material remains still poorly investigated. Here, we present a coupled experimental and theoretical study of the dynamical curling deformation of naturally curved ribbons. Using thermoplastic and metallic ribbons molded on cylinders of different radii, we tune separately the natural curvature and the geometry to study curling dynamics in air, water and in viscous oils, thus spanning a wide range of Reynolds numbers. Our theoretical and experimental approaches separate the role of elasticity, gravity and hydrodynamic dissipation from inertia and emphasize the fundamental differences between the curling of a naturally curved ribbon and a rod described by the classical Elastica. Ribbons are indeed an intermediate class of objects between rods, which can be totally described by one-dimensional deformations, and sheets. Since Lord Rayleigh, it is known that a thin sheet can easily be bent but not stretched. As a result, large deformations in thin sheets usually lead to the localization of deformations into small peaks and ridges as observed by crumpling a simple piece of paper. These elastic defects induce critical buckling situations studied in detail statically in the literature, while experimental and theoretical studies on their dynamics are scarce. Our work shows evidence for the propagation of such a single instability front, selected by a local buckling mechanism. Finally, we show that depending on gravity, and both the Reynolds and the Cauchy numbers, the curling speed and shape are modified by the large scale drag and the local lubrication forces, shedding a new light on microscopic experiences where curling is observed
In Vitro Red Blood Cell Segregation in Sickle Cell Anemia
Red blood cells in sickle cell anemia (sRBC) are more heterogeneous in their physical properties than healthy red blood cells, spanning adhesiveness, rigidity, density, size, and shape. sRBC with increased adhesiveness to the vascular wall would trigger vaso-occlusive like complications, a hallmark of sickle cell anemia. We investigated whether segregation occurs among sRBC flowing in micron-sized channels and tested the impact of aggregation on segregation. Two populations of sRBC of different densities were separated, labeled, and mixed again. The mixed suspension was flowed within glass capillary tubes at different pressure-drops, hematocrit, and suspending media that promoted or not cell aggregation. Observations were made at a fixed channel position. The mean flow velocity was obtained by using the cells as tracking particles, and the cell depleted layer (CDL) by measuring the distance from the cell core border to the channel wall. The labeled sRBC were identified by stopping the flow and scanning the cells within the channel section. The tube hematocrit was estimated from the number of fluorescence cells identified in the field of view. In non-aggregating media, our results showed a heterogeneous distribution of sRBC according to their density: low-density sRBC population remained closer to the center of the channel, while the densest cells segregated towards the walls. There was no impact of the mean flow velocity and little impact of hematocrit. This segregation heterogeneity could influence the ability of sRBC to adhere to the vascular wall and slow down blood flow. However, promoting aggregation inhibited segregation while CDL thickness was enhanced by aggregation, highlighting a potential protective role against vaso-occlusion in patients with sickle cell anemia
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 ( pN/nm) which
can be 2-3 orders of magnitude lower than that of standard cantilevers
( 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 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
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