Shape transformations of a flat fluid membrane with polymer lipids subject to lateral tension are studied as an approximation for multimembrane systems. Using Monte Carlo simulations, we find several characteristic structures of membranes, such as thermal fluctuations, large localized deformations, and undulations due to a curvature instability. The effect of thermal fluctuations on the curvature instability is investigated, and the undulations are characterized by a single-mode approximation for unstable modes. It is shown that thermal fluctuations, the curvature instability and vesiculation determine the parameter region in which an undulating membrane is observed. The phase diagram of undulation obtained through simulations closely resembles that measured experimentally for “biogels.” This suggests that the undulation corresponds to defects in multimembrane systems and that the curvature instability is the main cause of defect formation in “biogels.”

• Received 20 August 1997

DOI:https://doi.org/10.1103/PhysRevE.57.6815