By exploiting the nanometer sensitivity of the confocal response to the position of an in-focus reflecting surface, we measured the bending rigidity of lipid-bilayer vesicles with a noninvasive all-optical method. The vesicles were weakly deformed with femtonewton optical force, and the bending rigidity was measured continuously from the $L_{\alpha }$ through the $P_{{\beta }^{\prime }}$ to the $L_{{\beta }^{\prime }}$ phases on the same specimen for the first time. The bending modulus is found to increase by an order of magnitude from the $L_{\alpha }$ phase to the $L_{{\beta }^{\prime }}$ phase, as a result of the increasing area-compressibility modulus and bilayer thickness. The dips of bending modulus give precisely the main-transition and pretransition temperatures, which supports the recently proposed chain-melting model of pretransition.

• Received 19 January 2001

DOI:https://doi.org/10.1103/PhysRevE.64.020901