Soft Materials Friction at Smaller Length Scales

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Although macroscale friction has been studied for centuries, the mechanism for sliding on soft surfaces, such as rubbers and gels, remains elusive. Moreover, when the size scale of contact between soft surfaces becomes small, liquid-like characteristics arise and it is unclear if solid or liquid descriptions are appropriate, limiting the design of soft surfaces. This award supports fundamental research that overcomes current challenges in measuring and describing friction of soft materials at the microscale. In addition to promoting scientific advancement, this new knowledge is important for a broad range of applications towards national health and prosperity. For example, insights from this work are expected to afford design guidelines for soft biomaterials, resulting in advanced biotechnologies and healthcare. Knowledge is also expected to offer routes to designing low-friction coatings to mitigating energy losses due to frictional drag for manufacturing or transportation applications. Additionally, the project will provide opportunities to educate and train graduate and undergraduate students in the cross-disciplinary areas of micromechanics, surface science, and materials science, through research in the PI's lab, with a focus on underrepresented students in STEM.Macroscale friction of soft materials is often assumed to be mediated by microscale contact points, arising from surface asperities. Therefore, universally predicting friction arguably requires understanding the behavior of single microscale junctions. Although research has consistently shown that friction is related to normal loading, contact area, dwell time, and velocity, it is unclear how it is described as the length scale is decreased. At small scales, near the so-called elastocapillary length, surface tension becomes critical and liquid-like characteristics are expected, leading to large out-of-plane deformations and capillarity. To characterize how the above parameters relate to friction force and surface deformation, the PI will utilize an experimental method that combines lateral force measurements using an AFM with in-situ imaging of cross-sections via confocal microscopy. A model silicone will be used, allowing for systematic tuning of the modulus and elastocapillary length. Experiments that control the normal loading, velocity, dwell time, and interfacial tension, while measuring the contact area, out-of-plane deformations, and friction force, are anticipated to reveal when friction is described by solid and/or liquid-like mechanics. A map of static friction thresholds and kinetic friction mechanisms, such as smooth sliding or stick slip, will be generated and new knowledge will be applied to more comprehensively predict micro and macroscale friction.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.