CAREER: Mechanics and Physics at the Boundary Between Solid and Fluid: Probing the Thermodynamic and Kinetic Properties of Gels

这个学院早期职业发展（Career）项目将开发一种新的实验技术来表征凝胶的非线性热力学和动力学性质。与固体和流体的经典定义不同，凝胶既像固体又像流体。它们是自然生物中普遍存在的成分，也是重要的工程材料。尽管这些材料得到了广泛的应用，但由于缺乏对凝胶复杂的热力学和动力学行为的基本理解，这些材料的设计在现阶段仍然主要是试错。这项工作的成功将导致一种强大的、高通量的技术，能够在广泛的条件下测量软凝胶的热力学和动力学特性，并为工程师提供一个标准工具箱，以实现基于这些材料的定量设计。PI还将扩展一个软湿实验室，将视觉、触觉和动手模块结合起来，将人类对宏观特性的感知与K-12学生适当水平的软材料的潜在微观结构联系起来。凝胶由交联聚合物网络和溶剂分子组成。交联可防止长聚合物在溶剂中溶解；相反，凝胶会随着小分子的进出而膨胀和收缩。溶剂的吸收是双向的：当溶剂扩散到网络中时，网络发生变形，导致尺寸和形状发生变化，同时网络的变形也影响溶剂扩散进入或流出网络的速度和数量。凝胶的概念和行为都非常复杂，因此有足够的空间进行额外的工作，将力学、热力学和动力学原理与实验联系起来。提出的研究将发展一种基于压痕法的技术来表征凝胶的非线性热力学和动力学性质。这项新技术将允许在不同环境条件下系统地表征各种类型的刺激反应凝胶。基于系统测量的一套完整的数据，可以深入了解凝胶的结构-性质关系。因此，将建立一个基于物理的本构模型。

This Faculty Early Career Development (CAREER) project will develop a new experimental technique to characterize the nonlinear thermodynamic and kinetic properties of gels. Defying the classical definitions of solid and fluid, gels are both solid-like and fluid-like. They are both ubiquitous components of natural organisms and important engineering materials. Despite their wide applications, the design of these materials at this stage remains mostly trial-and-error due to a lack of fundamental understanding of the complex thermodynamic and kinetic behaviors of gels. The success of this work will lead to a robust and high throughput technique capable of measuring the thermodynamic and kinetic properties of soft gels under wide range of conditions and provide a standard toolbox for engineers to realize quantitative designs based on these materials. The PI will also expand a Soft Squishy Lab to combine visual, tactile and hands-on modules to connect human perception of macroscopic properties to the underlying microstructures of soft materials at appropriate levels for K-12 students.Gels are composed of crosslinked polymer network and solvent molecules. The crosslinks prevent the long polymers from dissolving in the solvent; rather the gel swells and shrinks as the small molecules migrate in and out. The solvent uptake is a two-way street: as the solvent diffuses into the network, the network deforms, leading to size and shape changes, while the deformation of the network also affects the rate and amount of solvent diffusing into or out of the network. Both the concepts and the behaviors of gels are sufficiently complex such that ample room exists for additional work to connect principles of mechanics, thermodynamics, and kinetics to experiments. The proposed study will develop a technique based on an indentation method for characterizing the nonlinear thermodynamic and kinetic properties of gels. The new technique will allow for systematic characterization of various types of stimuli-responsive gels under different environmental conditions. Based on a complete set of data from the systematic measurements, an in-depth understanding of the structure-property relations of gels can be achieved. Consequently, a physics-based constitutive model will be built.