Manouk Abkarian

Swinging of red blood cells under shear flow

We reveal that under moderate shear stress (etagamma[over ] approximately 0.1 Pa) red blood cells present an oscillation of their inclination (swinging) superimposed to the long-observed steady tank treading (TT) motion. A model based on a fluid ellipsoid surrounded by a viscoelastic 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 etagamma[over ], a etagamma[over ]-triggered transition toward a narrow etagamma[over ] range intermittent regime of successive swinging and tumbling, and a pure tumbling at low etagamma[over ] values.

Controlled assembly of jammed colloidal shells on fluid droplets

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.

Colloid science: Non-spherical bubbles

Surface tension gives gas bubbles their perfect spherical shape by minimizing the surface area for a given volume. Here we show that gas bubbles and liquid drops can exist in stable, non-spherical shapes if the surface is covered, or ‘armoured’, with a close-packed monolayer of particles. When two spherical armoured bubbles are fused, jamming of the particles on the interface supports the unequal stresses that are necessary to stabilize a non-spherical shape.

Hierarchical folding of elastic membranes under biaxial compressive stress

Mechanical instabilities that cause periodic wrinkling during compression of layered materials find applications in stretchable electronics and microfabrication, but can also limit an applications performance owing to delamination or cracking under loading and surface inhomogeneities during swelling. In particular, because of curvature localization, finite deformations can cause wrinkles to evolve into folds. The wrinkle-to-fold transition has been documented in several systems, mostly under uniaxial stress. However, the nucleation, the spatial structure and the dynamics of the invasion of folds in two-dimensional stress configurations remain elusive. Here, using a two-layer polymeric system under biaxial compressive stress, we show that a repetitive wrinkle-to-fold transition generates a hierarchical network of folds during reorganization of the stress field. The folds delineate individual domains, and each domain subdivides into smaller ones over multiple generations. By modifying the boundary conditions and geometry, we demonstrate control over the final network morphology. The ideas introduced here should find application in the many situations where stress impacts two-dimensional pattern formation.

Multiscale approach to link red blood cell dynamics, shear viscosity, and ATP release

RBCs are known to release ATP, which acts as a signaling molecule to cause dilation of blood vessels. A reduction in the release of ATP from RBCs has been linked to diseases such as type II diabetes and cystic fibrosis. Furthermore, reduced deformation of RBCs has been correlated with myocardial infarction and coronary heart disease. Because ATP release has been linked to cell deformation, we undertook a multiscale approach to understand the links between single RBC dynamics, ATP release, and macroscopic viscosity all at physiological shear rates. Our experimental approach included microfluidics, ATP measurements using a bioluminescent reaction, and rheology. Using microfluidics technology with high-speed imaging, we visualize the deformation and dynamics of single cells, which are known to undergo motions such as tumbling, swinging, tanktreading, and deformation. We report that shear thinning is not due to cellular deformation as previously believed, but rather it is due to the tumbling-to-tanktreading transition. In addition, our results indicate that ATP release is constant at shear stresses below a threshold (3 Pa), whereas above the threshold ATP release is increased and accompanied by large cellular deformations. Finally, performing experiments with well-known inhibitors, we show that the Pannexin 1 hemichannel is the main avenue for ATP release both above and below the threshold, whereas, the cystic fibrosis transmembrane conductance regulator only contributes to deformation-dependent ATP release above the stress threshold.

Dissolution Arrest and Stability of Particle-Covered Bubbles

Experiments show that bubbles covered with monodisperse polystyrene particles, with particle to bubble radius ratios of about 0.1, evolve to form faceted polyhedral shapes that are stable to dissolution in air-saturated water. We perform Surface Evolver simulations and find that the faceted particle-covered bubble represents a local minimum of energy. At the faceted state, the Laplace overpressure vanishes, which together with the positive slope of the bubble pressure-volume curve, ensures phase stability. The repulsive interactions between the particles cause a reduction of the curvature of the gas-liquid interface, which is the mechanism that arrests dissolution and stabilizes the bubbles.

A novel mechanism for egress of malarial parasites from red blood cells.

The culminating step of the intraerythrocytic development of Plasmodium falciparum, the causative agent of malaria, is the spectacular release of multiple invasive merozoites on rupture of the infected erythrocyte membrane. This work reports for the first time that the whole process, taking place in time scales as short as 400 milliseconds, is the result of an elastic instability of the infected erythrocyte membrane. Using high-speed differential interference contrast (DIC) video microscopy and epifluorescence, we demonstrate that the release occurs in 3 main steps after osmotic swelling of the infected erythrocyte: a pore opens in ~ 100 milliseconds, ejecting 1-2 merozoites, an outward curling of the erythrocyte membrane is then observed, ending with a fast eversion of the infected erythrocyte membrane, pushing the parasites forward. It is noteworthy that this last step shows slight differences when infected erythrocytes are adhering. We rationalize our observations by considering that during the parasite development, the infected erythrocyte membrane acquires a spontaneous curvature and we present a subsequent model describing the dynamics of the curling rim. Our results show that sequential erythrocyte membrane curling and eversion is necessary for the parasite efficient angular dispersion and might be biologically essential for fast and numerous invasions of new erythrocytes.

Vesicles and red blood cells in shear flow

We describe the similarities and the specificities of the behaviour of individual soft particles, namely, drops, lipid vesicles and red blood cells subjected to a shear flow. We highlight that their motion depends in a non-trivial way on the particle mechanical properties. We detail the effect of the presence of a wall with or without wall-particle attractive interaction from a biological perspective.

Continuous droplet interface crossing encapsulation (cDICE) for high throughput monodisperse vesicle design

We report on a continuous droplet interface crossing encapsulation (cDICE) technique for the high-yield production of microscopic vesicles tunable in size and content. Overcoming some of the current technique limitations, such a simple setup has a great potential in various fields from encapsulation to the design of biomimetic cells and artificial tissues.

Giant lipid vesicles filled with a gel: Shape instability induced by osmotic shrinkage

We report the properties of giant lipid vesicles enclosing an agarose gel. In this system, the lipid bilayer retains some basic properties of biological membranes and the internal fluid exhibits viscoelastic properties, thus permitting us to address the question of the deformation of a cell membrane in relation to the mechanical properties of its cytoskeleton. The agarose gel (concentration c0gel = 0.07%, 0.18%, 0.36%, and 1% w/w), likely not anchored to the membrane, confers to the internal volume elastic moduli in the range of 10-10(4) Pa. Shapes and kinetics of de-swelling of gel-filled and aqueous solution-filled vesicles are compared upon either a progressive or a fast osmotic shrinkage. Both systems exhibit similar kinetics. Shapes of solution-filled vesicles are well described using the area difference elasticity model, whereas gel-filled vesicles present original patterns: facets, bumps, spikes (c0gel < 0.36%), or wrinkles (c0gel > or = 0.36%). These shapes partially vanish upon re-swelling, and some of them are reminiscent of echinocytic shapes of erythrocytes. Their characteristic size (microns) decreases upon increasing c0gel. A possible origin of these patterns, relying on the formation of a dense impermeable gel layer at the vesicle surface and associated with a transition toward a collapsed gel phase, is advanced.