Soft Materials Friction at Smaller Length Scales

尽管宏观摩擦的研究已经有几个世纪了，但在橡胶和凝胶等软表面上滑动的机制仍然难以捉摸。此外，当软表面之间接触的尺寸尺度变小时，会出现类似液体的特性，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。该奖项支持克服当前在微尺度测量和描述软材料摩擦方面的挑战的基础研究。除了促进科学进步之外，这些新知识对于实现国家健康和繁荣的广泛应用也很重要。例如，这项工作的见解预计将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。知识还有望提供设计低摩擦涂层的途径，以减轻制造或运输应用中摩擦阻力造成的能量损失。此外，该项目还将通过 PI 实验室的研究，为微机械、表面科学和材料科学跨学科领域的研究生和本科生提供教育和培训的机会，重点关注 STEM 领域代表性不足的学生。软材料的宏观摩擦通常被认为是由表面粗糙度产生的微观接触点介导的。因此，普遍预测摩擦可能需要了解单个微尺度连接的行为。尽管研究一致表明摩擦力与法向载荷、接触面积、停留时间和速度有关，但尚不清楚随着长度尺度的减小如何描述摩擦力。在小尺度下，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性，从而导致大的面外变形和毛细管现象。为了表征上述参数与摩擦力和表面变形的关系，PI 将采用一种实验方法，将使用 AFM 进行的侧向力测量与通过共焦显微镜对横截面进行原位成像相结合。将使用模型硅树脂，以便系统地调整模量和弹性毛细管长度。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预计将揭示何时用固体和/或类液体力学来描述摩擦。将生成静摩擦阈值和动摩擦机制（例如平滑滑动或粘滑）的图，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点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.