VESICULAR INSTABILITIES - THE PROLATE-TO-OBLATE TRANSITION AND OTHER SHAPE INSTABILITIES OF FLUID BILAYER-MEMBRANES

The equilibrium shapes of fluid-phase phospholipid vesicles in an aque ous solution are controlled by bending elasticity. The regime of nonve siculated shapes at reduced volume v greater than or equal to 1/root 2 involves the interplay of five branches of distinct stationary shapes : pears, prolates, oblates, stomatocytes, plus a branch of nonaxisymme tric shapes with the symmetry D-2h. We exploit a method for calculatin g explicitly the stability of arbitrary axisymmetric shapes to map out in a numerically exact way both the stable phases and the metastabili ty of the low-lying shape branches. To obtain additional required info rmation about nonaxisymmetric shapes, we calculate these by numerical minimization of the curvature energy on a triangulated surface. Combin ing these two methods allows us to construct the full (shape) phase di agram and the full stability diagram in this region. We provide explic it results for values of the bending constants appropriate to stearoyl -oleoyl-phosphatidylcholine; generalization to other values is straigh tforward.