Soft Materials Friction at Smaller Length Scales

尽管宏观摩擦已经被研究了几个世纪，但在橡胶和凝胶等柔软表面上滑动的机制仍然难以捉摸。此外，当软表面之间的接触尺寸变小时，会出现类似液体的特征，并且不清楚固体或液体描述是否合适，从而限制了软表面的设计。该奖项支持基础研究，这些研究克服了目前在微尺度测量和描述软材料摩擦方面的挑战。除了促进科学进步，这一新知识对于广泛应用于国家健康和繁荣也很重要。例如，这项工作的见解有望为软生物材料提供设计指南，从而产生先进的生物技术和医疗保健。知识还有望为设计低摩擦涂层提供途径，以减少制造或运输应用中因摩擦阻力造成的能量损失。此外，该项目将通过PI实验室的研究，提供教育和培训微观力学、表面科学和材料科学跨学科领域的研究生和本科生的机会，重点是STEM中代表性较低的学生。软材料的宏观摩擦通常被认为是由表面粗糙的微观接触点引起的。因此，普遍预测摩擦力可以说需要了解单个微尺度结点的行为。虽然研究一直表明摩擦力与法向载荷、接触面积、停留时间和速度有关，但如何描述它是长度比例减小的原因还不清楚。在小尺度上，在所谓的弹性毛细管长度附近，表面张力变得关键，预计会出现类似液体的特征，导致较大的面外变形和毛细现象。为了表征上述参数与摩擦力和表面变形的关系，PI将利用一种实验方法，将使用AFM测量侧向力与通过共焦显微镜对横截面进行原位成像相结合。将使用硅胶模型，允许系统地调节模数和弹性毛细血管长度。控制法向载荷、速度、停留时间和界面张力，同时测量接触面积、面外变形和摩擦力的实验，有望揭示摩擦何时由固体和/或液体力学描述。将生成静态摩擦阈值和动态摩擦机制的地图，如平滑滑动或粘滑，并将应用新知识来更全面地预测微观和宏观摩擦。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.