Mechanical surface tension governs membrane thermal fluctuations

TL;DR

This paper clarifies the different definitions of surface tension in bilayer membranes, demonstrating through analysis and simulations that membrane thermal fluctuations are governed by mechanical tension, not intrinsic tension, and critiques the quadratic approximation's limitations.

Contribution

It provides a detailed comparison of intrinsic, renormalized, and mechanical tensions, and shows that the quadratic Helfrich Hamiltonian's non-rotational invariance affects tension calculations.

Findings

Thermal fluctuations are governed by mechanical tension.

Quadratic Helfrich Hamiltonian is not rotationally invariant.

Mechanical and intrinsic tensions vanish simultaneously.

Abstract

Motivated by the still ongoing debate about the various possible meanings of the term surface tension of bilayer membranes, we present here a detailed discussion that explains the differences between the "intrinsic", "renormalized", and "mechanical" tensions. We use analytical considerations and computer simulations to show that the membrane spectrum of thermal fluctuations is governed by the mechanical and not the intrinsic tension. Our study highlights the fact that the commonly used quadratic approximation of Helfrich effective Hamiltonian is not rotationally invariant. We demonstrate that this non-physical feature leads to a calculated mechanical tension that differs dramatically from the correct mechanical tension. Specifically, our results suggest that the mechanical and intrinsic tensions vanish simultaneously, which contradicts recent theoretical predictions derived for the approximated Hamiltonian.