Thermal Marangoni flows and macroscopic domain movement in monolayer surface potential experiments

Thermal Marangoni flows directed outward from the zone under the potential probe are shown to affect the surface potential of Langmuir monolayers in the gas-condensed phase coexistence region, by altering the movement and distribution of macroscopic domains. These flows originate from a gradient in surface tension, owing to a surface thermal gradient caused by suppression of water evaporation underneath the probe. The repelling effect on the domains is visualised using fluorescence emission over the entire Langmuir monolayer. Since domain movement is highly dependent on domain size and rheological properties, the degree of reproducibility in surface potential isotherms is bound to vary with the type of monolayer, which is observed experimentally for monolayers with distinct properties. The model proposed also explains the observation of area-independent surface potentials in the gas-condensed phase coexistence region and why tiny amounts of impurity in the aqueous subphase cause the surface potential to be irreproducible in such a region.