Surface charge relaxation and the pearling instability of charged surfactant tubes

TL;DR

This paper investigates how charge relaxation influences the pearling instability of charged surfactant tubes, showing it can significantly affect stability under certain screening conditions, with implications for understanding membrane deformations.

Contribution

It provides a detailed calculation of charge relaxation effects during tube deformation and quantifies its impact on the electrostatic energy and stability.

Findings

Relaxation lowers the energy cost of pearling, increasing instability.

Effect is significant when the tube radius is smaller than the screening length.

In typical experimental conditions, relaxation contributes less than 20% to electrostatic energy.

Abstract

The pearling instability of bilayer surfactant tubes was recently observed during the collapse of fluid monolayers of binary mixtures of DMPC$-$POPG and DPPC$-$POPG surfactants. We suggested it has the same physics as the well-known Raleigh instability under the action of the bilayer surface tension whose magnitude is dictated by the electrostatic interaction between charged surfactants. In this paper, we calculate the relaxation of charge molecules during the deformation of the tubes into pearling structure. We find the functional dependence of the relaxation energy on the screening length $\kappa^{-1}$ explicitly. Relaxation effect lowers the cost of bending a tube into pearls making the cylindrical tube even more unstable. It is known that for weak screening case where the tube radius is smaller than the screening length of the solution, this relaxation effect is important. However, for the case of strong screening it is negligible. For the experiments mentioned, the situation is marginal. In this case, we show this relaxation effect remains small. It gives less than 20% contribution to the total electrostatic energy.