Slow sedimentation and deformability of charged lipid vesicles

TL;DR

This paper combines computer simulations and experiments to analyze how charged lipid vesicles sediment and deform near surfaces, revealing the importance of electrostatic interactions and membrane charge on vesicle behavior.

Contribution

It introduces a boundary-integral simulation method incorporating electrostatics and membrane mechanics to study charged vesicle sedimentation, validated by experimental data.

Findings

Electrostatic interactions are crucial for accurate sedimentation predictions.

Membrane charge increases vesicle rigidity.

Simulations match experimental sedimentation rates and equilibrium gaps.

Abstract

The study of vesicles in suspension is important to understand the complicated dynamics exhibited by cells in vivo and in vitro. We developed a computer simulation based on the boundary-integral method to model the three dimensional gravity-driven sedimentation of charged vesicles towards a flat surface. The membrane mechanical behavior was modeled using the Helfrich Hamiltonian and near incompressibility of the membrane was enforced via a model which accounts for the thermal fluctuations of the membrane. The simulations were verified and compared to experimental data obtained using suspended vesicles labelled with a fluorescent probe, which allows visualization using fluorescence microscopy and confers the membrane with a negative surface charge. The electrostatic interaction between the vesicle and the surface was modeled using the linear Derjaguin approximation for a low ionic concentration solution. The sedimentation rate as a function of the distance of the vesicle to the surface was determined both experimentally and from the computer simulations. The gap between the vesicle and the surface, as well as the shape of the vesicle at equilibrium were also studied. It was determined that inclusion of the electrostatic interaction is fundamental to accurately predict the sedimentation rate as the vesicle approaches the surface and the size of the gap at equilibrium, we also observed that the presence of charge in the membrane increases its rigidity.