Dynamics of passive and active membrane tubes

TL;DR

This paper develops a comprehensive dynamical model for fluid membrane tubes, incorporating both passive and active forces, and predicts fluctuation behaviors that could be experimentally measured to understand active membrane properties.

Contribution

It introduces a variational framework for membrane tube dynamics including active forces and provides new predictions for higher azimuthal modes and fluctuation spectra.

Findings

Derived relaxation rates for membrane tube perturbations.

Predicted fluctuation amplitudes and effective temperature for active forces.

Suggested experimental methods to measure active membrane fluctuations.

Abstract

Utilising Onsager's variational formulation, we derive dynamical equations for the relaxation of a fluid membrane tube in the limit of small deformation, allowing for a contrast of solvent viscosity across the membrane and variations in surface tension due to membrane incompressibility. We compute the relaxation rates, recovering known results in the case of purely axis-symmetric perturbations and making new predictions for higher order (azimuthal) $m$-modes. We analyse the long and short wavelength limits of these modes by making use of various asymptotic arguments. We incorporate stochastic terms to our dynamical equations suitable to describe both passive thermal forces and non-equilibrium active forces. We derive expressions for the fluctuation amplitudes, an effective temperature associated with active fluctuations, and the power spectral density for both the thermal and active fluctuations. We discuss an experimental assay that might enable measurement of these fluctuations to infer the properties of the active noise. Finally we discuss our results in the context of active membranes more generally and give an overview of some open questions in the field.