A cornerstone of the DYFCOM team activity is a multiscale approach of the flow dynamics of complex fluids, from an investigation of the microscopic dynamics involved in suspensions to rheology at the macroscale and suspension dynamics in situations involving hydrodynamic interactions and coupling with external fields.
A prominent topic is the dynamics of suspensions of soft particles in flow, governed by their deformability and mediated by complex hydrodynamics or contacts. Beyond general and challenging questions of fundamental interest, suspensions of living cells provide a source of inspiration from model systems to applications.
Vesicle and capsules as model of red blood cells
In its bottom-up approach of blood flow (i.e. taking explicitly the dynamics of blood elements into account), the team has consolidated its expertise in a relevant modelling of Red Blood Cells (RBCs), the major component of blood, either as vesicles (closed fluid membranes) or inextensible capsules (endowed with membrane shear elasticity mimicking the RBCs cytoskeleton). The large deformations undergone by RBCs in flow represent a theoretical and numerical challenge which has been tackled thanks to the development of complementary methods.
Hydrodynamic interactions and shear induced diffusion
In semi-dilute suspensions of soft particles such as vesicles and RBCs, hydrodynamic interactions are an essential ingredient of the structure and dynamics of the suspension under flow. A detailed experimental and numerical study of the pairwise interaction of vesicles in shear flow has been performed10, a mechanism that leads to hydrodynamic repulsion between soft objects and to shear-induced diffusion at the scale of the suspension, a key ingredient in the structuration of blood flow. In blood cell suspensions, an experimental investigation11 has revealed that this phenomenon is sub-diffusive and that pairwise interactions dominate even in the semi-dilute regime, with sharp concentration profiles and anisotropic diffusivity
Dynamics of active suspensions
Active suspensions are ubiquitous in nature (sperm, bacteria, plankton, etc.) as well as in industry where suspensions with controllable viscosity are of considerable interest because of their occurrence in many kinds of devices such as active dampers, clutches or brakes. They also represent a fascinating non equilibrium system exhibiting rich behaviors in particular in terms of hydrodynamics and statistical mechanics. We work on fundamental properties of active and passive suspensions, combining experimental investigations, modeling and numerical simulations. We study microscopic properties like the way of swimming at low Reynolds number of motile microalgae (Chlamydomonas Rheinhardtii) We provided the first direct experimental evidence of the effect of motility on an increase of the effective viscosity of a puller-type microswimmer suspension. Recently we showed how the coupling between phototaxis and a Poiseuille flow leads to a spontaneous migration of microswimmers at the center of the flow channel.
Shapes of vesicles and RBCs in confined flow : A significant theoretical and experimental effort has been put on the understanding of the shapes of individual cells in microchannel flow, an essential ingredient of the rheology of confined blood and of the structuring of cell files, a route towards clustering. We have shown that surprising transitions from parachute to bullet shapes exist for vesicles, with counter-intuitive effects of an asymmetry of the channel’s cross section due to the coupling with surface flow patterns24. On the other hand, even in a symmetric flow, very deflated objects like RBCs adopt an asymmetric slipper shape, a previously unexplained phenomenon reported by Skalak (Science 1969). We have shown that the slipper shape is a robust feature resulting from the instability of the symmetric (parachute) shape25, highliting the fact that neither three dimensionality, nor confinement or cytoskeleton (present within RBCs) are necessary ingredients. Two outcomes of this study are : (i) the slipper moves faster than the parachute and (ii) the slipper is associated with membrane tanktreading (TT), which mixes the internal RBC solution thereby enhancing oxygen and ATP exchange.
Clustering of red blood cells in capillary flow : The formation of clusters of red blood cells in flow, besides its influence on blood rheology through a modification of local viscous dissipation, and hematocrit homogeneity, may have an impact on pathological events such as thrombus formation, which can lead to occlusion of blood vessels, embolism and ischemia.
Acoustic manipulation of objects in microflows
Suspended particles, cells, droplets or bubbles usually follow the flow in microfluidic conditions. Here we show how acoustic waves may create forces than can compete to drag so as to manipulate them individually. Acoustic waves do not only propagate through object bodies : they can also exert a radiation force on them, when the amplitude is high enough. We took advantage of this non- linear property to manipulate objects in microchannels. Any objects are potentially submitted to these acoustic forces. We could achieve the demonstration of fast "acoustic tweezers", by shaping the ultrasonic field and shifting its position in the xy plane. Red or white blood cells, droplets can be handled and moved around within micro-channels at frequencies larger than 30 Hz30. Bubbles can be pushed easily31 because of their resonance at a specific frequency.
At resonance, bubbles also emit a wave that exerts a (secondary) acoustic radiation force to other neighbour bubbles. We discovered that when the bubbles are confined in a microchannel, the waves propagate at the surface of channel walls, and this leads to a very strong interaction. Bubbles then self-organize in at a specific distance given by the wavelength. We observe patterns that we called "acoustic crystals", because these regular arrangements are solely mediated by an interaction force, of acoustic nature32. Note that the resonance of bubbles is at the origin of a large pulsation of their volume, helpful to design contrast agents that efficiently scatter back the ultrasonic emissions in ultrasound echography.