Measurement technique of surface tension using a single-tip optical fiber probe

An optical fiber probe method is one of the efficient and reliable measurement methods for gas-liquid two-phase flows. Generally, simultaneous measurement of bubble diameters and velocities has been thought to need at least two optical fiber probes. To break through this situation, we newly developed a Single-Tip Optical fiber Probe (S-TOP) which realizes simultaneous measurement of diameters and velocities of minute bubbles/droplets. In the S-TOP measurement, the surface tension intensively influences the S-TOP signals. The main aim of this investigation is to strictly evaluate the influences of surface tension on the bubble measurement in order to develop the S-TOP which can take the surface tension into consideration. In the present study, we found out that the gradient of leading edge (or trailing edge) of the S-TOP signal is proportional to the gas-liquid interface velocity. In the measurement of gas-liquid interface velocity via S-TOP, this relationship is effectively utilized. The influences of surface tension and probe-surface wettability on this relationship were discussed. The gradients of the leading and trailing edges were changed with decrease in the surface tension. At surface tension higher than about 50 mN/m, the surface tension is dominant. On the other hand, at lower than this value, wettability is dominant. (author)