We describe a method for making and erasing barriers to the lateral diffusion of membrane components in fluid lipid bilayers supported on glass substrates. When a bilayer is mechanically partitioned by scratching the membrane-coated surface at basic pH, barriers to lateral diffusion are formed which prevent mixing between the regions separated by the scratches. Upon lowering the pH, the bilayer is observed to spread over the scratch boundary, allowing diffusive mixing between the previously separated regions. This is exploited in combination with electrophoresis within the membrane to separate fluorescently labeled charged lipid probes, partition them with a scratch, and allow remixing to occur when the scratch is healed. This method for membrane manipulation can be used to transform a homogeneous membrane into an array of corrals with different compositions while preserving the ability to allow subsequent remixing. This approach should be useful for examining the kinetics of reactions and the assembly of fluid membrane-associated components in a native setting, and for investigating the dynamics of two-dimensional fluids.