Title: Non-monotonic frictional behavior in the lubricated sliding of soft patterned surfaces Show Chinese title

Title: 润滑滑动中软图案表面的非单调摩擦行为

Abstract: We study the lubricated contact of sliding soft surfaces that are locally patterned but globally cylindrical, held together under an external normal force. The local patterns represent either naturally occurring surface roughness or engineered surface textures. Three dimensionless parameters govern the system: a speed, and the amplitude and wavelength of the pattern. Using numerical solutions of the Reynolds lubrication equation, we investigate the effects of these dimensionless parameters on key variables such as contact pressure and the coefficient of friction of the lubricated system. For small roughness amplitudes, the coefficient of friction increases with roughness. However, our findings reveal that increasing surface roughness beyond a critical value can decrease the friction coefficient, a result that contradicts conventional intuition and classical studies on the lubrication of rigid surfaces. For very large roughness amplitudes, we show that the coefficient of friction drops even below the corresponding smooth case. We support these observations with a combination of perturbation theory and physical arguments, identifying scaling laws for large and small speeds, and for large and small roughness amplitudes. This study provides a quantitative understanding of friction in the contact of soft, wet objects and lays theoretical foundations for incorporating the friction coefficient into haptic feedback systems in soft robotics and haptic engineering. Show Chinese abstract

Abstract: 我们研究在外部法向力作用下，滑动的软表面局部图案化但整体为圆柱形的润滑接触。局部图案可以代表自然出现的表面粗糙度或工程表面纹理。三个无量纲参数控制该系统：速度以及图案的振幅和波长。通过求解雷诺润滑方程的数值解，我们研究了这些无量纲参数对接触压力和润滑系统的摩擦系数等关键变量的影响。对于小的粗糙度振幅，摩擦系数随着粗糙度的增加而增加。然而，我们的研究结果表明，超过临界值后增加表面粗糙度会降低摩擦系数，这一结果与传统直觉和刚性表面润滑的经典研究相矛盾。对于非常大的粗糙度振幅，我们证明摩擦系数甚至会低于相应的光滑情况。我们通过摄动理论和物理论证相结合的方法支持这些观察结果，确定了高速和低速、大振幅和小振幅粗糙度的标度定律。本研究提供了对软湿物体接触中摩擦的定量理解，并为将摩擦系数纳入软体机器人和触觉工程中的触觉反馈系统奠定了理论基础。