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

Electric-field-driven deformation, poration, and phase separation in biomimetic membranes

When exposed to a DC electric pulse, giant vesicles made of bilayer membranes adopt peculiar drum-like shapes and may collapse. We present experimental results and theoretical model suggesting that the edge separates porated (conducting) and intact (insulating) regions of the membrane. The time dependence of the edge location can serve as a quick method to estimate the critical voltage for membrane poration. Electrohydrodynamic analysis of the deformation and collapse dynamics provides a novel means to measure the membrane viscosity. In the case of multicomponent membranes, the miscibility temperature (at which domains form in an initially homogeneous membrane) decreases with applied electric field strength

Living on the edge: voltage-driven extreme deformation of bilayer membranes

Living cells maintain electric potential difference across their plasma membrane. Changes in the transmembrane electric field are biologically important signals in physiological processes such as electric-field-directed cell migration in development and regeneration. Pulsed electric fields are used to open pores in the membrane, which enable the delivery of material inside the cell, e.g. for gene transfection. I will present our theoretical and experimental work on the deformation and stability of biomimetic membranes (bilayers assembled from lipids or polymers) in electric fields. Gant vesicles exposed to DC pulses adopt peculiar drum-like shapes and may collapse. Our experiments suggest that the edge separates porated (conducting) and intact (insulating) regions of the membrane. The time dependence of the edge location can serve as a quick method to estimate the critical voltage for membrane poration. Electrohydrodynamic analysis of the collapse dynamics shows that membrane conductance and asymmetry in the embedding electrolyte solutions destabilize the interface; membrane viscosity slows down the growth of the instability. In the case of multicomponent membranes, electric fields induce reversible phase separation

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 $\lambda$ (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 $\lambda$, 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