Effect of Surface Stiffness on the Friction of Confined Microgel Liquids

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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.