We study the motion of a small solid particle (a few micrometers in size) attached
to the membrane of a spherical giant lipid vesicle. By means of optical manipulation, the particle
is first brought near the top of the vesicle, and released. We determine the friction experienced
by the particle moving along the vesicle surface under the action of gravity. From experiments
with latex and glass beads, we check that SOPC membranes are fluid at room temperature
(static shear modulus u = 0) and estimate the shear viscosity of SOPC bilayers: nm = 3 * 10^(-6)
surface poise