Measuring the wavy motion of the liquid-film interface is necessary for controlling the transportation efficiency of heat and mass transfer. Such a locally and temporally minor perturbation interacts with the flow rate, liquid-film thickness, surface wave velocity, interfacial-/wall-shear stresses, and more. To reveal those interaction, we conducted the present study to develop a new intrusive optical technique for an optical fiber probe micro-fabricated by a femtosecond pulse laser (Fs-TOP) for the direct and simultaneous measurements of the liquid-film thickness and wave velocity. The Fs-TOP is a two-in-one sensor with a single fiber. Its tapered and wedge-shaped 10-μm-dia. tip is one sensor, and another is micro-fabricated in the tip's immediate vicinity. Both sensors detect the surrounding phase, and therefore the time difference of the local phase detection with the sensors is used to accurately calculate the wave velocity with superior spatial resolution. Herein, we fixed the Fs-TOP horizontally to the channel bottom and set the sensing tip point against the gas flow. The results confirmed a basic measurement principle for the time-average thickness of the liquid film and wave velocity, respectively, in verification experiments. In the verification experiment for the thickness measurement, we vertically traversed an installed height of the Fs-TOP and calculated the liquid-film ratio from the output signal at every height. We observed that the installed height equaled the time-average thickness of 0.46 [mm] when the liquid-film ratio as 56%. An analysis using random waves indicated that the de-wetting process of the Fs-TOP caused 3% error of the average thickness. In the wave-velocity measurement experiment, we compared the results obtained with the Fs-TOP with those of a laser-induced fluorescence visualization. The results demonstrated that the Fs-TOP data were in good agreement with those of the visualization within 15% accuracy. We also evaluated the uncertainties in the wave-velocity measurement by performing a 3D ray-tracing simulation of the Fs-TOP.

测量液膜界面的波动是控制传热和传质效率的必要条件。这样的局部和时间上的微小扰动与流速，液膜厚度，表面波速度，界面/壁剪切应力等相互作用。为了揭示这些相互作用，我们进行了本研究，以开发一种新的侵入式光学技术，用于由飞秒脉冲激光（Fs-TOP）微制造的光纤探针的直接和同时测量液膜厚度和波速。Fs-TOP是具有单根光纤的二合一传感器。其锥形和楔形直径为10微米。尖端是一个传感器，另一个是在尖端附近的微型传感器。两个传感器都检测周围的相位，因此，利用传感器进行局部相位检测的时间差可以精确地计算出具有较高空间分辨率的波速。在这里，我们将Fs-TOP水平固定在通道底部，并根据气流设置传感尖端。结果证实了在验证实验中分别用于液膜的时间平均厚度和波速的基本测量原理。在厚度测量的验证实验中，我们垂直穿越了Fs-TOP的安装高度，并根据每个高度处的输出信号计算了液膜比。我们观察到，当液膜比为56％时，安装高度等于0.46 [mm]的时间平均厚度。使用随机波进行的分析表明，Fs-TOP的去湿过程导致平均厚度的误差为3％。在波速测量实验中，我们将Fs-TOP的结果与激光诱导的荧光可视化的结果进行了比较。结果表明，Fs-TOP数据与可视化数据吻合良好，准确度在15％以内。我们还通过执行Fs-TOP的3D射线追踪仿真评估了波速测量中的不确定性。