Diffusion of liquid domains in lipid bilayer membranes
Pietro Cicuta, Sarah L. Keller, and Sarah L. Veatch

We report diffusion coeficients of micron-scale liquid domains in giant unilamellar vesicles of
phospholipids and cholesterol. The trajectory of each domain is tracked, and the mean square
displacement grows linearly in time as expected for Brownian motion. We study domain diffusion
as a function of composition and temperature, and measure how diffusion depends on domain size.
We ¯nd mechanisms of domain diffusion which are consistent with membrane-dominated drag in
viscous Lo phases [P.G. Saffman and M. DelbrÄuck, PNAS 72, 3111 (1975)], and bulk-dominated drag
for less viscous Ld phases [B.D.Hughes et al., J. Fluid Mech. 110, 349 (1981)]. Where applicable,
we obtain the membrane viscosity and report activation energies of diffusion.