Membrane tensiometer for heavy giant vesicles

One key parameter of giant-vesicles adhesion is their membrane tension, sigma. A theoretically simple but delicate way to impose (and measure) it is to use micropipette manipulation techniques. But usually, the vesicles are free and their tension is unknown, until an adhesion patch grows. sigma can be deduced from the detailed profile of the membrane close to the substrate, but this method is limited to very low tensions. We present here a rather simple way to estimate the membrane tension of heavy vesicles, which sediment close to a surface, by observing by RIM the size of the flat region of the vesicle. As an application, we follow the slow flattening of vesicles, when the surrounding sugar solution is evaporating, and their light-induced tensioning.