Determination of mechanical properties of nanometer-thick liquid lubricant films for thin film lubrication technology

为了建立纳米厚液体润滑膜的力学模型，实现对薄膜剪切过程中的力学响应和接触面积的同时测量，采用了我们在前期研究中发展的高灵敏度剪切力测量方法--纤维摆动法（FWM）来测量薄膜的力学响应.在四波混频中，我们采用球头光纤作为剪切探针。为了构建力学模型，我们必须从通过FWM测量获得的力学响应确定膜的物理性质，例如粘度和弹性。这需要精确测量探针尖端和润滑剂膜之间的接触面积。为此，我们将FWM装置集成到倒置显微镜上，并从样品衬底的背面观察接触区域。我们发现，微分干涉相差显微镜是有效的，以减少背景噪声，并成功地测量接触面积在四波混频测量。从实验结果中定量地得到了薄膜的粘度。

To establish mechanical model of nanometer-thick liquid lubricant films, we aimed to achieve simultaneous measurements of mechanical responses and contact areas during the shearing of the film.Highly sensitive shear force measurement method developed in our previous study, which we called the fiber wobbling method (FWM), was used to detect the mechanical r esponses. In the FWM, we used the ball-ended optical fiber as a shearing probe. To construct the mechanical model, we have to determine the physical properties of the films, such as viscosity and elasticity, from the mechanical responses obtained by the FWM measurements. This requires precise measurements of the contact area between the probe tip and lubricant film. For this purpose, we integrated the FWM setup on to the inverted microscope and observe the contact area from the backside of the sample substrate. We found that the differential interference contrast microscopy was effective to reduce the background noise and succeeded the measurement of contact area during the FWM measurements. From the experimental results we quantitatively obtained the thin film’ s viscosity.