Development of rapid ripplon spectroscopy and its application to liquid surface physics

本研究的目的是开发一种时间分辨率为1ms的新型光散射系统来检测波纹子的传播。Ripplon是一种热激发的表面张力波，它的传播提供了液体表面分子动力学的信息，如表面活性剂的吸附过程和Langmuir膜的二维相变。研究结果表明，通过引入高相干激光器和改进的光学外差干涉仪，可以显著提高探测灵敏度。在纯水上波纹板的测量频率上限提高到50 MHz，这是光散射的几何极限。将该系统应用于ripple plon的快速测量，利用长记忆数字示波器检测并记录由ripple plon引起的光跳动信号，计算随机表面运动的相关函数。在1 ms的数据采集时间内，测量精度优于纹波频率的1%和阻尼的5%。将该系统应用于肉豆酱酸钠等典型表面活性剂的动态吸附过程的测量，成功地观察到清洗表面后表面张力的时间变化。用分子扩散模型考察了表面恢复的行为，发现表面能势对表面形成的时间常数也起着重要的作用。我们还观察到表面电位的振荡现象。

The purpose of this study is to develop a new light scattering system to detect the ripplon propagation with the time resolution of 1 ms. Ripplon is a thermally excited surface tension wave, whose propagation gives information on the molecular dynamics on the liquid surface, such as the adsorption process of surfactants and the two-dimensional phase transition of Langmuir films.As the result of the study, we could remarkably improve the sensitivity of the detection by introducing the highly coherent laser as well as the refined optical heterodyne interferometer. The higher frequency limit of the measurement was increased up to 50 MHz for the ripplon on pure water, which is the geometric limit of the light scattering. The system was then applied for the rapid measurement of the ripplon, in which the optical beating signal due to the ripplon is detected and recorded by the digital oscilloscope with long memory and the correlation function of the random surface motion is calculated. The accuracy of the measurement during the data acquisition time of 1 ms is better than 1 % for ripplon frequency and 5 % for the damping.The system is then applied for the measurement of the dynamic adsorption process of the typical surface-active agents of sodium myristic acid and we could successfully observed the temporal change of the surface tension after the cleaning the surface. The behavior of the surface recovery is examined with the model of the molecular diffusion and we found that the surface energy potential also plays an important role to determine the time constant of the surface formation. We also observed the oscillatory phenomenon of surface potential.