Measuring Real Contact Area and Its Consequences on Adhesion and Friction in Dry and Wet Conditions

第一部分：粘合剂和摩擦在工程和生物学中发挥着重要作用，全球粘合剂市场价值数百亿美元。许多研究工作，如提高轮胎的燃油效率，需要了解摩擦和附着力，这在节能方面有重要的影响。对于生物医学应用，例如绷带、组织粘合剂和健康监测装置，还强烈需要改善在潮湿环境中的粘合性。这些应用中的主要挑战是了解粗糙度如何影响粘附力和摩擦力。该研究项目的重点是测量具有弹性的软聚合物（如橡胶）与粗糙表面之间的实际接触面积，因为该测量对于了解摩擦力和粘附力的大小至关重要。 该项目将涉及使用先进的光学仪器和进一步开发技术，以直接探测在干燥条件下和水下滑动期间弹性体与表面之间的接触面积。 这些接触面积的直接测量将有助于发展摩擦和粘附的速度依赖性的基本理解，并将有助于测试目前在这一领域的理论模型。 两名研究生将在本项目期间接受培训，研究经验将作为外联活动的一部分提供给本科生和高中生。该项目还将利用阿克伦大学的仿生中心（BRIC）来提高参与科学项目的学生的多样性，并增加市中心学校学生的参与。第二部分：技术总结首先由大卫塔博尔和同事强调，粘附和摩擦都与真实的接触面积和界面粘附强度有关。在过去的二十年中，新的分析模型已经发展到预测如何真实的接触面积的粗糙度，弹性模量，和滑动速度的影响。最近，它已被证明，这些模型是不足以预测的摩擦变化，由于粗糙度或软弹性体的滑动速度的变化。解决理论和实验之间的差异将需要直接测量真实的接触面积，特别是在滑动期间。拟议的研究将利用表面敏感和频发生光谱（SFG）的最新发展，直接测量表面-OH峰的位移，这可能与界面相互作用的强度有关。SFG将与振荡摩擦单元（速度范围为10-7至0.2 m/s）耦合，以直接测量滑动期间的接触面积。这些测量将在干燥条件下和水下进行，以了解边界润滑状态下流体动力学的影响。接触面积的直接测量将有助于发展摩擦和粘附的速度依赖性的基本理解，并将有助于测试该领域的当前理论模型。该奖项反映了NSF的法定使命，并被认为是值得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的支持。

PART 1: NON-TECHNICAL SUMMARYAdhesives and friction play important roles in engineering and biology, and the global market for adhesives is valued in the tens of billions of dollars. Many research efforts, such as improving the fuel efficiency of tires, require an understanding of friction and adhesion, which have important consequences in terms of energy savings. There is also a strong need for improving adhesion in wet environments for biomedical applications such as bandages, tissue adhesives, and health monitoring devices. The main challenge in these applications is understanding how roughness can affect adhesion and friction. This research project is focused on measuring the actual area of contact between a soft polymer that has elastic properties (such as rubber) and a rough surface, as this measure is critical for gaining insight on the magnitude of the forces of friction and adhesion. The project will involve the use of sophisticated optical instruments and the further development of techniques to directly probe the contact area between the elastomer and the surface during sliding under both dry conditions and under water. These direct measurements of the contact area will help in developing a fundamental understanding of the velocity dependence of friction and adhesion and will be useful in testing the current theoretical models in this field. Two graduate students will be trained during this project, and research experiences will be provided to undergraduate and high school students as part of the outreach activities. The project will also leverage the Biomimicry Center (BRIC) at the University of Akron to improve the diversity of students engaged in science projects and increase participation of students from inner-city schools. PART 2: TECHNICAL SUMMARYIt was first highlighted by David Tabor and colleagues that both adhesion and friction are related to the real contact area and the strength of interfacial adhesion. In the last two decades, new analytical models have been developed to predict how the real contact area is influenced by roughness, elastic modulus, and the sliding velocity. Recently, it has been shown that these models are inadequate for predicting changes in friction due to roughness or changes in the sliding velocity for soft elastomers. Resolving the discrepancy between theory and experiments will require a direct measurement of the real contact area, particularly during sliding. The proposed research will leverage recent developments in surface-sensitive sum frequency generation spectroscopy (SFG) to directly measure the shift in the surface -OH peak, which can be related to the strength of the interfacial interactions. SFG will be coupled with an oscillatory friction cell (with a velocity in the range of 10-7 to 0.2 m/s) to directly measure the contact area during sliding. These measurements will be conducted in both dry conditions and underwater to understand the influence of hydrodynamics in the boundary lubrication regime. The direct measurement of the contact area will help in developing a fundamental understanding of the velocity dependence of friction and adhesion and will be useful in testing the current theoretical models in this field..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.