运动速度和固体晶面对边界滑移影响的实验研究

Experimental Research on Effect of Velocity and Solid Crystal Plane on Boundary Slip

为了研究微纳米间隙中流体的流动特性，采用原子力显微镜（AFM）对微纳米间隙中的受限液体润滑剂与基底固体间的边界滑移进行实验研究，探索运动速度和固体晶面对边界滑移的影响．实验中对AFM的探针进行了修饰，在探针顶端粘了一个小球．实验采用的液体样品为季戊四醇油酸酯，分子式为C77H140O8，固体样品为Si（100）、Si（110）和Si（111）3种晶面的Si片．采用相对速度法对实验数据进行处理．结果表明：在19～89μm／s的相对趋近速度下，Si（100）与季戊四醇油酸酯界面均会产生边界滑移，并且随速度增大，滑移增大．通过对实验数据分段拟舍得到不同球一盘间距、速度和晶面条件下的滑移长度．在趋近速度为19μm／s的情况下，Si的3种晶面均会产生边界滑移，当球一盘间距小于100nm时，晶面对边界滑移的影响较大，滑移量按Si（100）〈Si（111）〈Si（110）依次增大．原因是边界滑移量还与晶面的表面自由能有关，Si（100）晶面表面自由能最大，对液体分子的吸引力最大，从而使得剪切力不易克服吸引力而产生的边界滑移最小．

In order to study the characteristics of fluid confined to micro-nano gap, the boundary slip between confined fluid and solid base in micro-nano gap was studied using atomic force microscope （AFM） , mainly from the perspectives of probe approaching velocity and solid crystal plane. The AFM probe was modified with a small ball stuck on the top. The liquid sample is pentaerythritol oleate, its molecular formula being C77 H14008, and the solid samples are Si （100）, Si （110） and Si （111 ）. The experi- mental data were processed using relative velocity method. Results show that boundary slip occurs on the surface of Si （100）when the relative velocity is within 19--89 μm/s ; furthermore, the degree of bounda- ry slip increases with the increase of velocity. Slip length at different ball-disk gap, velocity and crystal plane was obtained by piecewise fitting of experimental data. It is found that boundary slip occurs on the surface of Si（100）, Si（111） and Si（110） when the velocity is 19 μm/s. When the ball-disk gap is smaller than 100 nm, crystal plane has great influence on boundary slip, and the degree of boundary slip increases in the order of Si（100） 〈 Si（ 111 ） 〈 Si（110）. The reason is that Si（100） has maximum sur-face free energy, and thus maximum attraction to liquid molecules, so that it is hard for the shear force to overcome attraction to produce greater boundary slip.