Finite-size effects in biomimetic smectic films

Thin stacks of lipid multibilayers supported on rigid silicon and mica substrates are found to exhibit finite-size effects. Using neutron diffraction we find that the repeat spacing (d) of stacks containing up to a few tens of bilayers depends on their thickness (D), with d increasing with decreasing D. Differences in d are larger in the low-temperature Lbeta' phase consisting of rigid bilayers than in the high-temperature Lalpha phase where the bilayers are more flexible. Various scenarios that may be responsible for this counterintuitive observation are discussed.