Soft Materials Friction at Smaller Length Scales

尽管宏观尺度的摩擦已经研究了几个世纪，但在柔软表面（如橡胶和凝胶）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尺度变小时，会产生类似液体的特性，不清楚是固体还是液体的描述合适，这限制了软表面的设计。该奖项支持基础研究，以克服当前在测量和描述微尺度软材料摩擦方面的挑战。除了促进科学进步外，这种新知识对于促进国家健康和繁荣的广泛应用也很重要。例如，这项工作的见解有望为软生物材料的设计提供指导，从而产生先进的生物技术和医疗保健。这些知识也有望为设计低摩擦涂层提供途径，以减轻制造或运输应用中由于摩擦阻力造成的能量损失。此外，该项目将提供机会，通过在PI实验室的研究，在微观力学、表面科学和材料科学等跨学科领域教育和培训研究生和本科生，重点关注STEM领域代表性不足的学生。软质材料的宏观摩擦通常被认为是由表面凹凸不平引起的微尺度接触点介导的。因此，普遍预测摩擦可能需要了解单个微尺度结的行为。尽管研究一致表明，摩擦与正常载荷、接触面积、停留时间和速度有关，但尚不清楚它如何被描述为长度尺度的减小。在小尺度上，接近所谓的弹性毛细管长度，表面张力变得至关重要，预计会出现类似液体的特征，导致大的面外变形和毛细作用。为了描述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进行横向力测量与通过共聚焦显微镜进行截面的原位成像相结合。模型硅胶将被使用，允许系统调整模量和弹性毛细管长度。控制正常载荷、速度、停留时间和界面张力的实验，同时测量接触面积、面外变形和摩擦力，预计将揭示固体和/或类液体力学描述摩擦的情况。将生成静摩擦阈值和动态摩擦机制（如光滑滑动或粘滑）的地图，并将新知识应用于更全面地预测微观和宏观摩擦。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.