  We study diffusion coefficients of liquid domains by explicitly taking into
account the two-layered structure called leaflets of the bilayer membrane. In
general, the velocity fields associated with each leaflet are different and the
layers sliding past each other cause frictional coupling. We obtain analytical
results of diffusion coefficients for a circular liquid domain in a leaflet,
and quantitatively study their dependence on the inter-leaflet friction. We
also show that the diffusion coefficients diverge in the absence of coupling
between the bilayer and solvents, even when the inter-leaflet friction is taken
into account. In order to corroborate our theory, the effect of the
inter-leaflet friction on the correlated diffusion is examined.
