(535a) Dynamics of Lipid Lamellae and Lamellar Vesicles in Shear and Extensional Flows

Mechanics and dynamics of lipid bilayers and vesicles play a crucial role in cellular processes and cell functions that range from cell division to targeted drug delivery. Liposomes, spherical vesicles made up of lipids, have been widely used in drug delivery and medical diagnostics and, as a model structure for studying cell mechanics. We use coarse grained molecular dynamics (MD) simulations to study the influence of shear and extensional flow deformations on vesicle shape and dynamics as well as experimentally observed shear-induced structure transition from bilayers to multi-lamellar vesicles.

Specifically, we simulate self-assembly of dodecyldimethylammoniumbromide (DDAB) lipids in water using a coarse-grained Martini force field. Large-scale MD simulations are performed using GROMACS and LAMMPS packages on the XSEDE supercomputing platform. MD simulations have revealed various equilibrium morphologies such as micelles, uni-lamellar and multi-lamellar vesicles as well as layered structures with increasing lipid concentration. In this presentation, we will focus on the following points.

Dynamics of vesicles in shear flow: We have developed a numerical method to precisely calculate the reduced volume of a vesicle by expanding the vesicle-water interface topology obtained from MD simulations in terms of spherical harmonics. We then tracked the orientation of non-spherical (ellipsoidal) vesicles by calculating their principal axes from the inertia tensor. Our simulations suggest that the reduced volume of vesicles is a key determinant of their shear flow dynamics. A deflated vesicle shows tumbling dynamics while a nearly spherical vesicle is likely to exhibit tank-treading. We will discuss how the tumbling frequency changes with the applied shear rate.

Lamellar Structures in Shear Flow: We will discuss the mechanical response of lamellar structures in shear flow. As the deformation rate increases, we see a buckling instability leading to a morphology transition from lamellae to vesicles which is accompanied by an increase in the shear viscosity. Our results suggest that the linear dimension of the vesicles thus formed is approximately equal to the wavelength of the primary buckling mode.

Uniaxial Extension of Lamellae: We will discuss the response of lamellar structures under uniaxial extension. We have developed a method to calculate the Poisson's ratio of lamellae. We will discuss the effect of electrostatic screening induced by co-amphiphilic molecules stacked parallel to the lipids on their mechanical properties. Specifically, our simulations show that stiffness and Poisson ratio of the lamellae increases and decreases respectively with increasing concentration of co-surfactants. We will discuss the mechanisms underlying this observation.