Shape dynamics of lipid vesicles forced by holographic optical tweezers

The shapes of unilamellar lipid vesicles are driven out of equilibrium by direct forcing with holographic optical tweezers. Vesicles have been studied extensively due to their relevance as a model for the membrane of cells as well as their potential practical uses e.g. for drug delivery or chemical confinement. We use multipoint laser tweezers formed by a spatial light modulator (holographic optical tweezers) to apply forces to such vesicles in several points simultaneously. To apply forces we utilize an index of refraction difference between the fluid inside the vesicle and the external fluid. Since this higher index of refraction material is fluid, the vesicle shape can changes in response to the optical forces. This shape change reveals the mechanical properties of vesicles subject to multiple stresses. We find that the surface forces on the membrane are localized near the points of forcing. Restoring forces from lipid tethers are used to estimate the total applied optical forces, which are below the pN level. The relaxation of deformations can be decomposed into its Fourier modes. The relaxation of all observable modes can be described well by a third order Landau equation. Ellipsoidal deformations relax more slowly than higher order deformation modes.