Unexpected Dynamics in Shape Fluctuations of Bilayer Vesicles

Fluid lipid bilayers are composed of two mono-molecular sheets held together by weak van der Waals forces. Except for viscous resistance, the monolayers are free to slide relative to one another — giving rise to a “hidden” degree of freedom within the composite structure. In this paper, important effects of interlayer coupling are demonstrated through their influence on the static and dynamic features of undulating bilayer vesicles. In particular, it is shown that the non-local bending elasticity can modulate mean square undulatory amplitudes with a long wavelength suppression similar to that of membrane tension; the two effects are indistinguishable on the basis of spectral analysis. With regard to conformational dynamics, we show that the two dissipative mechanisms — namely interlayer drag and conventional hydrodynamics — are important on different length scales. With the crossover length being a fraction of a μm, it is concluded that viscous resistance to conformational changes is dominated by interlayer dissipation in the mesoscopic regime.