Yusuke Ozawa

Measurement of tiny droplets using a newly developed optical fibre probe micro-fabricated by a femtosecond pulse laser

Optical fibre probing is widely applied to measurements in gas–liquid two-phase flows. We have already developed and reported a four-tip optical fibre probe (F-TOP) for millimetre-size bubbles/droplets, and a single-tip optical fibre probe (S-TOP equipped with a wedge-shaped tip) for sub-millimetre-size bubbles/droplets. However, it is difficult to measure micrometre-size bubbles/droplets by S-TOP. The main purpose of the present study was to develop a new type of optical fibre probe micro-fabricated by a femtosecond pulse laser (Fs-TOP). First, we confirmed the performance of the new probe by examining millimetre- and sub-millimetre-size droplets; the results from the Fs-TOP were compared with those obtained from the visualization of the droplets by high-speed video camera, and showed satisfactory agreement. In addition, we demonstrated the measurement of velocities and chord lengths of micrometre-size droplets (about 50 µm in diameter) using the Fs-TOP and compared them with those from the S-TOP to clarify the limits and strengths of each type of probe.

Bubbles and Droplets Measurement via Optical Fiber Probe Processed by Femtosecond Pulse Laser

An optical fiber has interesting and useful characteristics, which are able to be applied to scientific measurement. Its phase detection characteristic based on refraction difference between two phases is applied to bubbles/droplets measurement. Recently, demands for measurement of tiny bubbles/droplets increase in research fields of steam injectors, sprays, automotive engines, fine chemical reactors, and so on. Meanwhile, laser science and engineering, particularly femtosecond pulse laser, has made remarkable advances lately. Their unique properties of the interaction between the femtosecond pulses and materials can be utilized for microfabrication. The optical fiber probe methods have been repeatedly improved in order to measure bubbles/droplets efficiently and reliably in gas-liquid two-phase flows. However, one has been taking it for granted that simultaneous measurement of their diameters and velocities needs at least two optical fiber probes. To break through this situation, we newly developed a Single-Tip Optical Fiber Probe (F-STOP) microfabricated by femtosecond pulse laser (fs-pulses) which realizes simultaneous measurement of diameters and velocities of tiny bubbles/droplets. In the F-STOP measurement, the following properties are used to realize the simultaneous measurement of diameters and velocities of bubbles/droplets: the relation between the reflected-light intensity at the wedge-shaped probe tip and the tip-surface area covered with a phase; reflected-light intensity at the groove microfabricated by fs-pulses. The first aim of the present study is to provide data for evaluation of the influences of surface tension and wettability on the bubble measurement in order to develop precise and reliable F-STOP method. The second aim is to describe the process to make F-STOP via fs-pulses. The third aim is demonstration of the newly developed probe in real measurement of bubbles/droplets. On the basis of these, the performance of the new probe is discussed.Copyright

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.Copyright