arXiv subjects

Thomas Podgorski

Publications and source records attributed to Thomas Podgorski.

22 records · Page 2Linked to original sources

arXiv2008

Hydrodynamic lift of vesicles under shear flow in microgravity

The dynamics of a vesicle suspension in a shear flow between parallel plates has been investigated under microgravity conditions, where vesicles are only submitted to hydrodynamic effects such as lift forces due to the presence of walls and drag forces. The temporal evolution of the spatial distribution of the vesicles has been recorded thanks to digital holographic microscopy, during parabolic flights and under normal gravity conditions. The collected data demonstrates that vesicles are pushed away from the walls with a lift velocity proportional to $\dotγ R^3/z^2$ where $\dotγ$ is the shear rate, $R$ the vesicle radius and $z$ its distance from the wall. This scaling as well as the dependence of the lift velocity upon vesicle aspect ratio are consistent with theoretical predictions by Olla [J. Phys. II France {\bf 7}, 1533--1540 (1997)].

cond-mat.soft↗

arXiv2007

Rheology of dilute suspensions of vesicles and red blood cells

We present rheology experiments on dilute solutions of vesicles and red blood cells (RBC). Varying the viscosity ratio $λ$ between internal and external fluids, the microscopic dynamics of suspended objects can be qualitatively changed from tank-treading ($tt$) to tumbling ($tb$). We find that in the tt regime the viscosity $η$, decreases when $λ$ increases, in contrast with droplet emulsions and elastic capsule theories which are sometimes invoked to model RBC dynamics. At a critical $λ$ (close to the tt-tb transition) $η$ exhibits a minimum before it increases in the tb regime. This is consistent with a recent theory for vesicles. This points to the nontrivial fact that the cytoskeleton in RBC does not alter the qualitative evolution of $η$ and that, as far as rheology is concerned, vesicle models might be a better description.

cond-mat.soft↗

arXiv2007

Dynamics and rheology of a dilute suspension of vesicles: higher order theory

Vesicles under shear flow exhibit various dynamics: tank-treading ($tt$), tumbling ($tb$) and vacillating-breathing ($vb$). A consistent higher order theory reveals a direct bifurcation from $tt$ to $tb$ if $C_a\equiv τ\dotγ$ is small enough ($τ$= vesicle relaxation time towards equilibrium shape, $\dotγ$=shear rate). At larger $C_a$ the $tb$ is preceded by the $vb$ mode. For $C_a\gg 1$ we recover the leading order original calculation, where the $vb$ mode coexists with $tb$. The consistent calculation reveals several quantitative discrepancies with recent works, and points to new features. We analyse rheology and find that the effective viscosity exhibits a minimum at $tt-tb$ and $tt-vb$ bifurcation points.

cond-mat.soft↗

arXiv2007

Fingering instabilities of a reactive micellar interface

We present an experimental study of the fingering patterns in a Hele-Shaw cell, occurring when a gel-like material forms at the interface between aqueous solutions of a cationic surfactant cetyltrimethylammonium bromide) and an organic salt (salicylic acid), two solutions known to form a highly elastic wormlike micellar fluid when mixed homogeneously. A variety of fingering instabilities are observed, depending on the velocity of the front (the injection rate), and on which fluid is injected into which. We have found a regime of non-confined stationary or wavy fingers for which width selection seems to occur without the presence of bounding walls, unlike the Saffman-Taylor experiment. Qualitatively, some of our observations share common mechanisms with instabilities of cooling lava flows or growing biofilms.

nlin.PS↗