Effect of Surface Stiffness on the Friction of Confined Microgel Liquids

这项资助将通过研究被紧密限制在“软”壁之间的液体的特性来影响科学的进步，并促进国家的健康和繁荣，这些液体通常存在于生物系统和纳米结构中，受到纳米技术的影响。一个液体被紧紧地束缚在两个固体表面之间，其间隙与它的分子尺寸相当，它的行为与它的体积状态非常不同。在密闭液体中通常观察到摩擦力的增加，这通常是由于液体在密闭时凝固所致。先前对承压液体的研究主要集中在坚硬且结构良好的承压壁上。然而，许多生物和工程过程涉及高度柔软和可变形的表面，例如滑膜和软骨关节之间的润滑，毛细血管中红细胞的运动，甚至橡胶轮胎的打滑。表面刚度对流体润滑的影响仍然知之甚少。该基金支持基础研究，以了解表面刚度对强受限液体结构和摩擦的影响，这些液体的围壁刚度可以改变，与许多现实世界的系统相关。从这项研究中获得的知识可以通过优化表面机械弹性和设计低摩擦界面来控制流体输送，从而在各种工业应用中实现节能。该项目的更广泛影响还将包括招募和培训代表性不足的学生，包括女学生，作为下一代美国工程师。该项目解决了长期争论的表面诱导玻璃化转变和空间受限液体摩擦增加的问题。表面刚度对可变形表面之间受限液体动力学的影响的定量理解将通过单颗粒水平的原位微观表征来发展。通过使用不同交联度的微凝胶颗粒作为表面涂层和受限液体，可以在单个微凝胶系统中调节表面对液体的刚度比。集成了共聚焦激光扫描显微镜的微米间隙摩擦计能够测量两个可变形表面之间的非均质动力学和受限微凝胶液体在不同间隙和剪切条件下的摩擦。表面刚度、约束长度尺度、动态非均质性和受约束液体的剪切诱导流动性之间的关系将被量化。从这项研究中获得的结果将为涉及高度可变形表面的禁锢诱导的玻璃动力学和生物润滑提供分子洞察力。该项目的成功可以通过明智地设计和控制表面和涂层的机械顺应性来改变摩擦，促进界面质量传输，以及操纵表面传感和驱动，从而开辟一条新的途径。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。

This grant will impact progress of science and advance the national health and prosperity by researching the properties of liquids tightly confined between 'soft' walls often found in biological systems and in nanostructures impacted nanotechnology. A liquid tightly confined between two solid surfaces at a gap spacing comparable to its molecular dimension can behave very different from its bulk state. Increased friction is commonly observed with confined liquids, often due to the solidification of the liquid when confined. Prior research of confined liquids has mostly focused on hard and well-structured confining walls. Yet many biological and engineering processes involve highly soft and deformable surfaces, such as lubrication between synovial and cartilaginous joints, motions of red blood cells in capillaries, and even the aquaplaning of rubbery tires. The impact of surface stiffness on fluid lubrication remains poorly understood. This grant supports fundamental research to understand the effect of surface stiffness on the structure and friction of strongly confined liquids, whose confining wall stiffness can be varied to be relevant of many real world systems. The knowledge gained from this investigation can be transformed to control fluid transport by optimizing surface mechanical elasticity and design low-friction interfaces for energy saving in various industrial applications. Broader impacts of this project will also include the recruitment and training of underrepresented students, including women students, as the next generation of American engineers. This project addresses the long-debated questions of surface-induced glass transition and friction augment of spatially confined liquids. A quantitative understanding of the effect of surface stiffness on the dynamics of confined liquids between deformable surfaces will be developed by in-situ microscopic characterization at a single-particle level. The surface-to-liquid stiffness ratios will be tuned in a single microgel system by using microgel particles of varied crosslinking degrees as both surface coatings and confined liquids. A micron-gap tribometer integrated with confocal laser scanning microscopy enables the measurement of the heterogeneous dynamics and friction of confined microgel liquids between two deformable surfaces against varied gap spacings and shear conditions. The relationship among surface stiffness, confinement length scales, dynamic heterogeneity, and shear-induced fluidity of confined liquids will be quantified. The results obtained from this research will give molecular insight to confinement-induced glassy dynamics and biolubrication involving highly deformable surfaces. The success of this project can lead to a new avenue by judicious design and control of the mechanical compliance of surfaces and coatings to modify friction, facilitate interfacial mass transport, and manipulate surface sensing and actuation.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.