Role of Structure and Dynamics of Molecules at the Interface in Controlling Friction, Adhesion and Adhesion Hysteresis

我们建议将联合收割机表面敏感的红外-可见和频发生光谱（SFG）与原位摩擦和粘附测量相结合。Chaudhury和Brown使用聚二甲基硅氧烷（PDMS）透镜和Israelachvili使用表面力装置的实验结果激发了实验的动机;他们发现了摩擦力、粘附滞后和界面处分子流动性之间的关系，但是没有关于聚合物/固体或隐藏界面处与本体中相比松弛过程有何不同的信息。最近的聚合物薄膜的玻璃化转变温度的实验表明，在空气界面处的弛豫可以比在本体中快几个数量级。高分子链的弛豫时间在与不同表面能的表面接触时会发生变化。此外，在运动或滑动界面上，没有分子结构变化的直接信息。理解这些性质对于理解摩擦力是至关重要的。最近，我们证明了SFG可以用于研究弹性PDMS透镜与聚合物或固体基底之间的接触界面。在这里，我们利用这种设计来同时测量两个滑动表面之间的结构变化，摩擦和粘附。此外，我们建议使用SFG测量界面处的弛豫动力学，以了解迁移率与摩擦系数之间的关系。作为第五区的主任，我在过去的五年里一直积极参与为K5-K12的学生组织第五区科学日。除了这项推广活动之外，这项提案还为当地一所高中的两名高中生提供了研究机会，让他们在研究生的监督下在我的实验室里工作。这些高中项目的研究可以作为他们的科学项目的一部分提交给英特尔科学与工程博览会。这项工作将与阿克伦大学现有的NSF-REU本科课程相结合。 更广泛的影响：两个表面之间的摩擦在各种技术和生物应用中至关重要。对这些现象的认识和控制将对节约和利用宝贵的能源资源产生巨大的影响。由于对界面处分子的结构和性质缺乏了解，限制了这一领域的研究进展。这一建议为探索分子在界面上的运动或变形及其与摩擦力和粘附力的关系提供了一种新的实验方法。这将广泛影响表面、润滑剂和微纳米致动器的设计。教育和外展活动针对第5区的当地学校，通过鼓励学生承担科学项目并在地区，州和国际层面上竞争，促进对科学的兴趣。

We propose to combine surface sensitive infrared-visible sum frequency generationspectroscopy (SFG) with insitu friction and adhesion measurements. The experimentsare motivated by results of Chaudhury and Brown using poly (dimethylsiloxane) (PDMS)lenses and Israelachvili using surface force apparatus; where they find a relationshipbetween friction, adhesion hysteresis and mobility of the molecules at the interface.However, there is no information on how different relaxation processes are atpolymer/solid or hidden interfaces in comparison to that in bulk. Recent experiments ofglass transition temperature of polymer thin films indicate that the relaxation at the airinterface can be several orders of magnitude faster than in the bulk. The relaxationtimes of polymer chains are expected to be altered in contact with surfaces of differentsurface energies. Furthermore, there is no direct information of the change in thestructure of molecules at moving or sliding interfaces. Understanding these properties iscrucial to understand friction. Recently, we demonstrated that SFG can be applied tostudy the contact interface between an elastomeric PDMS lens and a polymer or solidsubstrate. Here, we take advantage of this design to measure simultaneously thechanges in structure between two sliding surfaces, friction and adhesion. In addition, wepropose to measure the relaxation dynamics at interfaces using SFG to understand therelationship between mobility and friction coefficient. As a Director of District 5, I havebeen actively involved in organizing District 5 Science Day for students from K5-K12 forthe past five years. In addition to this outreach activity, this proposal offers researchopportunities to two high school students from a local high school to work duringsummer in my laboratory under the supervision of graduate students. The research ofthese high school projects can be submitted as a part of their science project forcompetition in the Intel Science and Engineering Fair. This effort will be integrated withthe NSF-REU undergraduate program already existing at The University of Akron. Broader Impact: Friction between two surfaces is of utmost importance in a variety oftechnological and biological applications. The understanding and control of thesephenomena will have a tremendous impact in conserving and using precious energyresources. The advances in this field are limited by the lack of understanding of thestructure and properties of molecules at the interface. This proposal provides a newexperimental method to probe molecular motion or deformation taking place at theinterface and studies its relationship to friction and adhesion. This will broadly impactour design of surfaces, lubricants and micro or nano-actuators. The education andoutreach activities target local schools in District 5 to promote an interest in science byencouraging students to undertake science projects and compete at district, state andinternational level.