Optical energy propagates along metal surfaces as collective oscillations of free electrons when those surfaces are irradiated with optical waves in accordance with the resonant condition. The oscillations with electrical fields are called surface plasmon polaritons (SPPs) and are used in medium sensors. Here, using SPPs, the flow of a liquid–liquid two-phase fluid is visualized, and the flow-rate distribution is derived. A channel on a silver-film surface deposited on a glass slide is filled with an ethanol aqueous solution. SPPs are excited on the silver surface by a helium–neon laser. Then, water is injected into the channel in a laminar flow. As the water approaches the silver surface, the SPP excitation is disturbed. This disturbance is observed as decreasing reflectance, from which we can estimate the distance between the water layer and the silver surface. The method does not require any tracer particles or coloring even though the sample fluids are clear and colorless.

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