The mechanism of shape instability for a vesicle in extensional flow (original) (raw)

Critical Dynamics of Vesicle Stretching Transition in Elongational Flow

Physical Review Letters, 2008

We present results on the stretching of single tubular vesicles in an elongation flow toward dumbbell shapes, and on their relaxation. A critical strain rate _ c exists; for strain rates _ < _ c , the vesicle remains tubular but fluctuates, though its steady state extension increases with the strain rate _. Above _ c , first a shape transition to dumbbell occurs, and then high order shape modes become unstable, leading to a pearling state. We have quantitatively characterized the transition and found a scaling of _ c with the system parameters. A remarkable feature of vesicle tube behavior around the critical point is a slowdown of the vesicle relaxation to the final extended state in the vesicle stretching. Such feature is similar to that found in continuous phase transitions and to the critical effects recently observed for polymer molecules near the coil-stretch transition in elongation flow.

Vesicle dynamics in elongation flow: Wrinkling instability and bud formation

Physical Review Letters, 2007

We present experimental results on the relaxation dynamics of vesicles subjected to a time-dependent elongation flow. We observed and characterized a new instability, which results in the formation of higher order modes of the vesicle shape (wrinkles), after a switch in the direction of the gradient of the velocity. This surprising generation of membrane wrinkles can be explained by the appearance of a negative surface tension during the vesicle deflation, due to compression in a sign-switching transient. Moreover, the formation of buds in the vesicle membrane has been observed in the vicinity of the dynamical transition point.

Vesicle Dynamics in Time-Dependent Elongation Flow: Wrinkling Instability

Physical Review Letters, 2007

We present experimental results on the relaxation dynamics of vesicles subjected to a timedependent elongation flow. We observed and characterized a new instability, which results in the formation of higher order modes of the vesicle shape (wrinkles), after a switch in the direction of the gradient of the velocity. This surprising generation of membrane wrinkles can be explained by the appearance of a negative surface tension during the vesicle deflation, due to compression in a sign-switching transient. Moreover, the formation of buds in the vesicle membrane has been observed in the vicinity of the dynamical transition point.

Dynamic shape transformations of fluid vesicles

Soft Matter, 2010

We incorporate a volume-control algorithm into a recently developed one-particle-thick mesoscopic fluid membrane model to study vesicle shape transformation under osmotic conditions. Each coarsegrained particle in the model represents a cluster of lipid molecules and the inter-particle interaction potential effectively captures the dual character of fluid membranes as elastic shells with out-of-plane bending rigidity and 2D viscous fluids with in-plane viscosity. The osmotic pressure across the membrane is accounted for by an external potential, where the instantaneous volume of the vesicles is calculated via a local triangulation algorithm. Through coarse-grained molecular dynamics simulations, we mapped out a phase diagram of the equilibrium vesicle shapes in the space of spontaneous curvature and reduced vesicle volume. The produced equilibrium vesicle shapes agree strikingly well with previous experimental data. We further found that the vesicle shape transformation pathways depend on the volume change rate of the vesicle, which manifests the role of dynamic relaxation of internal stresses in vesicle shape transformations. Besides providing an efficient numerical tool for the study of membrane deformations, our simulations shed light on eliciting desired cellular functions via experimental control of membrane configurations.

Wrinkling instability of vesicles in steady linear flow

EPL (Europhysics Letters), 2014

We present experimental observations and numerical simulations of a wrinkling instability that occurs at sufficiently high strain rates in the trembling regime of vesicle dynamics in steady linear flow. Spectral and statistical analysis of the data shows similarities and differences with the wrinkling instability observed earlier for vesicles in transient elongation flow. The critical relevance of thermal fluctuations for this phenomenon is revealed by a simple model using coupled Langevin equations that reproduces the experimental observations quite well.

Dynamics of Nearly Spherical Vesicles in an External Flow

Physical Review Letters, 2007

Tank-treading, tumbling, and trembling are different types of the vesicle behavior in an external flow. We derive a dynamical equation enabling us to establish a state of nearly spherical vesicles. For a 2D external flow, the character of the vesicle dynamics is determined by two dimensionless parameters, depending on the vesicle excess area, fluid viscosities, membrane viscosity and bending modulus, strength of the flow, and ratio of the elongational and rotational components of the flow. The tank-treading to tumbling transition occurs via a saddle-node bifurcation, whereas the tank-treading to trembling transition occurs via a Hopf bifurcation. A slowdown of vesicle dynamics should be observed in a vicinity of a point separating the transition lines. We show that the slowdown can be described by a power law with two different critical exponents 1=4 and 1=2 corresponding to the slowdown of tumbling and trembling cycles.

Shape instabilities in vesicles: A phase-field model

The European Physical Journal Special Topics, 2007

A phase field model for dealing with shape instabilities in fluid membrane vesicles is presented. This model takes into account the Canham-Helfrich bending energy with spontaneous curvature. A dynamic equation for the phase-field is also derived. With this model it is possible to see the vesicle shape deformation dynamically, when some external agent instabilizes the membrane, for instance, inducing an inhomogeneous spontaneous curvature. The numerical scheme used is detailed and some stationary shapes are shown together with a shape diagram for vesicles of spherical topology and no spontaneous curvature, in agreement with known results.

Dynamic model and stationary shapes of fluid vesicles

The European Physical Journal E, 2006

A phase-field model that takes into account the bending energy of fluid vesicles is presented. The Canham-Helfrich model is derived in the sharp-interface limit. A dynamic equation for the phasefield has been solved numerically to find stationary shapes of vesicles with different topologies and the dynamic evolution towards them. The results are in agreement with those found by minimization of the Canham-Helfrich free energy. This fact shows that our phase-field model could be applied to more complex problems of instabilities.

Large Deformations of Giant Floppy Vesicles in Shear Flow

Physical Review Letters, 1998

The flow deformation and rheology of vesicles of a soluble surfactant are studied. Direct observation under shear flow reveals that the vesicles become strongly elongated to form an entangled structure of connected bilayer tubes. The large deformation is due to the permeability of the membrane and the large solubility of the surfactant. The formation of the entangled structure is observed in the rheology as a strong increase of the viscosity with time. [S0031-9007(98)07534-6] PACS numbers: 87.45.Ft