在先前的實驗中，我們發現PDMS(聚二甲基矽氧烷)的顆粒流會因為剪切速度變化而呈現三種不同狀態[Phys. Rev. Lett. 126,128001 (2021)]。我們認為這個與速度相關的相變化，其原因可能來自於PDMS表面摩擦力的速度相關性。為了驗證這個解釋，我們設計了兩種實驗去量測PDMS表面滑動時的應力。第一類實驗為控制兩PDMS顆粒滑動軌跡與速度，使其接觸並測量其中一個顆粒的正向與切向應力。第二類實驗使用兩PDMS長棒狀表面，垂直交叉擺放並相互摩擦，測量其中一個表面的正向與切向應力。此兩種實驗皆在特定濃度的甘油水溶液中進行。我們改變滑動速度進行多次測量，以探討PDMS滑動時應力與速度的關係。我們發現，在這兩種實驗中，PDMS表面的切向應力都與滑動速度相關，呈現Stribeck曲線關係。在其中一段速度區間中，平均的切向應力隨著滑動速度增快而變小，且會伴隨者Stick-slip(滯著滑動)現象發生。另外我們觀察到，對同一個樣本而言，隨著累積摩擦距離的增加,發生Stick-slip現象的最低速度，有不斷變小的趨勢。這暗示著PDMS表面性質會隨著實驗發生變化，換言之，Stick-slip現象有可能成為表面些微變化的指標。我們希望這篇論文能夠對於軟表面的摩擦學，提供新的線索。;We perform experiments that simultaneously resolve the tangential and normal components of the force between fluid-immersed PDMS (polydimethylsiloxane) surfaces, as they compress and slide against each other at controlled speeds and relative positions. Two combinations of surface geometries are utilized for these experiments: the passing of two spheres (SP), and the steady sliding between two elongated samples that are arranged orthogonally with a fixed depth of overlap (FD). We use glycerol-water mixture at different concentrations as the interstitial fluid, with the sliding speeds varied over four decades. In both SP and FD experiments, the time-averaged values of tangential force share similar speed dependence that is consistent to the well-known Stribeck curve in tribology. However, the normal force reflects a Hertzian scaling and appears insensitive to the sliding speed. In addition, our experiments time-resolve the fluctuations of tangential force which reveal stick-slip patterns, in order to probe the possible mechanisms behind the reduction of "friction" from its plateau value as the system enters the mixed lubrication regime in past literatures. We also find that the onset of stick-slip patterns can serve as a sensitive indicator for a small change on the contacting surfaces. These experiments reveal clues for understanding the fluid-structure interaction that leads to the velocity weakening of friction between fluid-immersed elastic surfaces, and complete our model for interpreting the solid-fluid transition in a granular shear flow [Phys. Rev. Lett. 126,128001 (2021)] that shows stick-slip behaviors.
