53 research outputs found
Electrohydrodynamic deformation and rotation of a particle-coated drop
A dielectric drop suspended in conducting liquid and subjected to an uniform
electric field deforms into an ellipsoid whose major axis is either
perpendicular or tilted (due to Quincke rotation effect) relative to the
applied field. We experimentally study the effect of surface-adsorbed colloidal
particles on these classic electrohydrodynamic phenomena. We observe that at
high surface coverage (>90%), the electrohydrodynamic flow is suppressed,
oblate drop deformation is enhanced, and the threshold for tilt is decreased
compared to the particle-free drop. The deformation data are well explained by
a capsule model, which assumes that the particle monolayer acts as an elastic
interface. The reduction of the threshold field for rotation is likely related
to drop asphericity
Drop Behavior in Uniform DC Electric Field
Drop deformation in uniform electric fields is a classic problem. The
pioneering work of G.I.Taylor demonstrated that for weakly conducting media,
the drop fluid undergoes a toroidal flow and the drop adopts a prolate or
oblate spheroidal shape, the flow and shape being axisymmetrically aligned with
the applied field. However, recent studies have revealed a nonaxisymmetric
rotational mode for drops of lower conductivity than the surrounding medium,
similar to the rotation of solid dielectric particles observed by Quincke in
the 19th century. This fluid dynamics video demonstrates three behavioral
modes. I) toroidal recirculation inside the drop in weak fields II)
nonaxisymmetric fluid rotation in strong fields and III) drop breakup in strong
fields.Comment: APS DFD Gallery of Fluid Motion 200
Dynamics of a viscous vesicle in linear flows
An analytical theory is developed to describe the dynamics of a closed lipid
bilayer membrane (vesicle) freely suspended in a general linear flow.
Considering a nearly spherical shape, the solution to the creeping-flow
equations is obtained as a regular perturbation expansion in the excess area.
The analysis takes into account the membrane fluidity, incompressibility and
resistance to bending. The constraint for a fixed total area leads to a
non-linear shape evolution equation at leading order. As a result two regimes
of vesicle behavior, tank-treading and tumbling, are predicted depending on the
viscosity contrast between interior and exterior fluid. Below a critical
viscosity contrast, which depends on the excess area, the vesicle deforms into
a tank--treading ellipsoid, whose orientation angle with respect to the flow
direction is independent of the membrane bending rigidity. In the tumbling
regime, the vesicle exhibits periodic shape deformations with a frequency that
increases with the viscosity contrast. Non-Newtonian rheology such as normal
stresses is predicted for a dilute suspension of vesicles. The theory is in
good agreement with published experimental data for vesicle behavior in simple
shear flow
Vesicles in a Poiseuille flow
Vesicle dynamics in unbounded Poiseuille flow is analyzed using a
small-deformation theory. Our analytical results quantitatively describe
vesicle migration and provide new physical insights. At low ratio between the
inner and outer viscosity (i.e. in the tank-treading regime), the
vesicle always migrates towards the flow centerline, unlike other soft
particles such as drops. Above a critical , vesicle tumbles and
cross-stream migration vanishes. A novel feature is predicted, namely the
coexistence of two types of nonequilibrium configurations at the centreline, a
bullet-like and a parachute-like shapes.Comment: 4 pages and 5 figure
Lipid membrane instability and poration driven by capacitive charging
A new model for the interaction of an electric pulse with a lipid membrane is
proposed. Using this model we show that when a DC electric pulse is applied to
an insulating lipid membrane separating fluids with different conductivities,
the capacitive charging current through the membrane drives electrohydrodynamic
flow that destabilizes the membrane. The instability is transient and decays as
the membrane charges. The bulk conductivity mismatch plays an essential role in
this instability because it results in a different rate of charge accumulation
on the membrane's physical surfaces. Shearing stresses created by the electric
field acting on its own induced free charge are non-zero as long as the charge
imbalance exists. Accordingly, the most unstable mode is related to the ratio
of membrane charging time and the electrohydrodynamic time.Comment: 4 pages, 4 figure
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