We have investigated the migrations of phospholipid vesicles under the concentration gradients of metal ions. We microinjected metal chloride solutions, monovalent (NaCl and KCl), divalent (CaCl2 and MgCl2), and trivalent (LaCl3) salts, toward phospholipid giant vesicles (GVs) composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). For NaCl, CaCl2, and MgCl2 solutions, the GVs migrated straight toward the tip of the micropipette in response to the concentration gradients, whereas for KCl and LaCl3, GVs moved to the opposite direction. Our motion tracking of lipid domains in a vesicle membrane showed no unidirectional flow in the membrane during the vesicle migration, indicating that the Marangoni mechanism is not responsible for the observed vesicle migration. We calculated the diffusiophoretic velocities for symmetric and asymmetrical electrolytes by solving the Stokes' equation numerically. The theoretical diffusiophoretic velocities described the observed migration velocities well. Thus, we can control the migration of vesicle in response to the concentration gradient by adapting the electrolytes and the lipids.
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