Numerical computations of the dynamics of fluidic membranes and vesicles

TL;DR

This paper introduces a parametric finite element method to numerically simulate the complex dynamics of fluidic membranes and vesicles, capturing various shape transformations and motion modes in three dimensions.

Contribution

It presents the first 3D numerical computations of vesicle dynamics based on the full Navier-Stokes system, including effects of membrane viscosity, spontaneous curvature, and area difference elasticity.

Findings

Membrane viscosity influences the transition from tank treading to tumbling.

A new transition mode between classical motions is numerically identified.

Shape features like budding and starfish forms are studied under shear flow.

Abstract

Vesicles and many biological membranes are made of two monolayers of lipid molecules and form closed lipid bilayers. The dynamical behaviour of vesicles is very complex and a variety of forms and shapes appear. Lipid bilayers can be considered as a surface fluid and hence the governing equations for the evolution include the surface (Navier--)Stokes equations, which in particular take the membrane viscosity into account. The evolution is driven by forces stemming from the curvature elasticity of the membrane. In addition, the surface fluid equations are coupled to bulk (Navier--)Stokes equations. We introduce a parametric finite element method to solve this complex free boundary problem, and present the first three dimensional numerical computations based on the full (Navier--)Stokes system for several different scenarios. For example, the effects of the membrane viscosity, spontaneous curvature and area difference elasticity (ADE) are studied. In particular, it turns out, that even in the case of no viscosity contrast between the bulk fluids, the tank treading to tumbling transition can be obtained by increasing the membrane viscosity. Besides the classical tank treading and tumbling motions, another mode (called the transition mode in this paper, but originally called the vacillating-breathing mode and subsequently also called trembling, transition and swinging mode) separating these classical modes appears and is studied by us numerically. We also study how features of equilibrium shapes in the ADE and spontaneous curvature models, like budding behaviour or starfish forms, behave in a shear flow.