Passages and droplets in lamellar fluid membrane phases

We consider thermally excited passages and droplets formed between fluid membranes in a lamellar phase. We show that they induce entropic attractive interactions which compete with Helfrich’s entropic intermembrane repulsion. This competition determines the intermembrane distance ${\mathit{l}}_{\max }$ lamellar phases reach at transitions to isotropic fluid membrane phases. In general, ${\mathit{l}}_{\max }$ is much smaller than the membrane persistence length and crucially depends on membrane saddle-splay (‘‘Gaussian’’) rigidity.