Effects of Micropillar Array Strain on Sliding Friction at Soft Interfaces

Soft surfaces with strain-dependent friction properties are desirable for applications involving compliant objects, such as medical devices and food handling. Here, we investigate friction properties of soft micropillars while varying equibiaxial strain, material stiffness, and lubrication status. Specifically, we present friction measurements between strained, soft micropillar arrays and soft substrates. Coefficient of friction was determined with a custom device that measures both normal and shear forces during sliding. Friction was measured at different strain levels with or without a lubricant (lubricated and unlubricated cases, respectively) for soft micropillars of varying stiffness (referred to as “softer” and “stiffer” micropillars). The measured friction coefficient is found to depend on the micropillar modulus, applied strain, and interface lubrication. A two-dimensional plane strain finite element model was developed to obtain a qualitative understanding of the mechanisms underlying the friction tunability. In these simulations, a substrate is indented into a micropillar array with a prescribed normal load, followed by a prescribed translation across the micropillars. Correlation of experimental data and modeling results suggests that the primary factor governing the friction between soft micropillars and soft substrates depends on the lubrication condition. For the lubricated case, substrate deformation, specifically how the substrate bulges around the indented micropillars, governs the friction response. For the unlubricated case, the friction response is primarily influenced by the true contact area due to the adhesion-dominated nature of the friction behavior.