Using a novel fluorescence microscope configuration on a lipid monolayer, we demonstrate for the first time that $\rho \mathrm{H}$-dependent liquid-to-solid phase transitions can occur locally on the meniscus in a typical Langmuir-Blodgett transfer, at surface pressure substantially below the phase transition pressure on the bulk. A $\rho \mathrm{H}$ gradient is proposed, originating from acidification induced by absorption of ambient ${\mathrm{CO}}_2$ and the restricted hydroxide-ion exchange between the bulk and meniscus subphases.

• Received 27 June 1988

DOI:https://doi.org/10.1103/PhysRevLett.61.2492