Time-Resolved Measurements of Interfacial Tension and Flow Speed of the Inclined Water Surface around a Self-propelled Camphor Boat by the Quasi-elastic Laser Scattering Method.

An inclined liquid surface, such as a meniscus, plays an important role in advection and transport phenomena at a liquid's surface. However, there is no time-resolved measurement method for the interfacial tension of an inclined liquid-air interface. Here, a noninvasive method for simultaneous measurements of the interfacial tension and surface flow speed for an inclined water surface is described. This is an upgrade of the quasi-elastic laser scattering method with a closed-loop control system that introduces the dynamically tracked scattered and referential light into the detector. For the evaluation of the tilt compensation by dynamic tracking, the relationship between the apparent interfacial tension and surface inclination was examined for a water meniscus at 0-5° inclinations. It was also demonstrated that simultaneous measurements of the interfacial tension and surface flow speed around a self-propelled camphor boat on a pure water surface inclined by >3° at the back end of the boat are difficult to conduct accurately without dynamic tracking. Both the interfacial tension difference and the backward flow speed increased as the boat speed increased to 0.1 m/s; that had not been evaluated to date because of the high velocity of the boat and the surface inclination of the water around it. The direct experimental evaluation of the interfacial tension and the flow speed supported the model that the driving force of the camphor boat is the interfacial tension difference and the resistance force proportional to the boat velocity reduces its acceleration.