Evaluation of the Influences of Surface Tension on the Bubble Measurement Using a Single-Tip Optical Fiber Probe

The optical probe method has been repeatedly improved in order to measure bubbles and droplets efficiently and reliably in gas-liquid two-phase flows, since the application of an optical fiber to measure them was proposed early eighties. However, simultaneous measurement of their 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 relation between the reflected-light intensity at the wedge-shaped probe tip and the tip-surface area covered with a phase is cleverly used to realize the simultaneous measurement. The surface tension and probe-surface wettability intensively influence the S-TOP signals. The main aim of the present study is to strictly evaluate the influences of surface tension and wettability on the bubble measurement in order to develop precise and reliable S-TOP method. In the present study, we specify 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 bubbles and droplets via S-TOP, this relation is effectively utilized. The influences of surface tension and probe-surface wettability on this relation are quantitatively discussed. At surface tension higher than about 50mN/m, the surface tension is dominant. On the other hand, at lower than this value, the wettability is dominant. On the basis of improvement in the consideration of the above results, the authors demonstrate the simultaneous measurement of diameters and velocities of small bubbles with about 200 mm in diameter.