Collaborative Research: Validated Complementarity Contact Conditions for Suction-Friction of Multiphasic Soft Materials

该项目将研究粘附力和摩擦力，并在各种被液体溶胀的软材料中开发基于物理的接触模型。软膨胀材料在工程和生物医学应用中的日益广泛的应用促进了对其力学行为建模的需求。这些材料经历大的变形，并表现出依赖于加载速率的非线性机械响应。在接触加载中，速率依赖性主要源于液体在软固体网络中的扩散、溶胀液体的粘性剪切和固体网络的粘弹性变形。在最近的研究中，在合成和生物软溶胀材料中的剥离和滑动的开始期间观察到粘附和摩擦的速率依赖性。目前的接触制剂缺乏描述这些观察结果的方法。这项研究将填补这一空白，探索机制，导致率依赖性和耦合物理的粘附和摩擦，并相应地纠正目前的接触配方。基于物理学的接触公式将能够在肌肉骨骼关节中遇到的众多接触应用中建模摩擦-粘附;软机器人与其环境相互作用;软组织和替代物中的手术切割，切割，创伤和弹道影响;柔性电子产品与人体皮肤的接触;以及其他承载显著机械负载的坚韧凝胶应用。这一新的贡献接触力学将讨论在研究生水平的课程和实践研讨会的高中学生。最后，为了确保研究成果能够广泛、快速地传播到社区，该公式将在一个开源和广泛使用的软材料有限元代码中实施。研究的具体目标是检验一个主要假设，即在软膨胀材料中，固体网络控制着粘附和摩擦的强度，因此，改进的基于有效牵引的公式将解决接触分离（剥离）过程中的速率依赖性和非局部性。PI进一步假设非局部粘附（吸力）和摩擦响应的耦合由材料的固有弛豫时间常数决定。这些假设将通过新的计算和实验努力进行测试。在计算程序中，将使用变分多尺度间断Galerkin方法推导出用于软膨胀材料界面的统一接触框架，该方法可以弱地施加沿界面沿着材料点的整个加载和卸载范围，而无需数值调节参数。实验程序将补充计算框架与验证数据，从散装机械特性，粘合和摩擦测试合成软膨胀材料界面。除了整体力-位移响应，先进的激光和粒子图像测速技术将提供界面附近的全场表征。这种时间分辨的全局和局部数据将使建模框架的可靠验证成为可能。经过验证，该框架将提供第一个基于物理的接触模型，该模型具有可测量的体积和界面属性。通过揭示局部和非局部界面机制协同作用的变革性知识，并直接类比土壤和颗粒材料中的整体响应行为（内聚力和吸力），将实现这一统一的公式化。该奖项反映了NSF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 project will study adhesion and friction, and develop a physics-based contact model in various soft materials swollen by liquids. The ever-growing use of soft swollen materials in engineering and biomedical applications catalyzes demands for modeling of their mechanical behavior. Those materials experience large deformations and exhibit nonlinear mechanical response that depends on loading rate. In contact loading, rate-dependence stems primarily from liquid diffusion within soft solid network, viscous shearing of swelling liquid and viscoelastic deformation of the solid network. In recent studies, the rate-dependence of adhesion and friction were observed during the onset of peeling and sliding in synthetic and biological soft swollen materials. Current contact formulations lack a methodology to describe those observations. The research will fill this gap by exploring the mechanisms leading to rate-dependent and coupled physics of adhesion and friction, and by correcting current contact formulations accordingly. The physics-based contact formulation will enable modeling of friction-adhesion in numerous contact applications encountered in musculoskeletal joints; soft robots interacting with their environment; surgical cuts, suturing, and traumatic and ballistic impacts in soft tissues and surrogates; contact of flexible electronics with human skin; and other tough gel applications carrying significant mechanical loads. This novel contribution to contact mechanics will be discussed in graduate level courses and hands-on seminars for high school students. Finally, to ensure broad and quick dissemination of research products to the community, the formulation will be implemented in an open-source and widely used finite element code for soft materials.The specific goal of the research is to test a major hypothesis that in soft swollen materials, solid network controls the strength in both adhesion and friction, and therefore a modified effective traction-based formulation would resolve both rate-dependence and nonlocality in contact separation (peeling) processes. The PIs further hypothesize that the coupling of nonlocal adhesion (suction) and friction response is governed by the intrinsic relaxation time constants of the materials. Those hypotheses will be tested through novel computational and experimental efforts. In the computational program, a unified contact framework will be derived for the soft swollen material interfaces using the Variational Multiscale Discontinuous Galerkin method, which can weakly impose the entire range of loading and unloading of a material point along the interface without numerical tuning parameters. The experimental program will complement the computational framework with validation data to be obtained from bulk mechanical characterization, adhesion and friction tests on synthetic soft swollen material interfaces. Besides the global force-displacement responses, advanced laser and particle image velocimetry techniques will deliver full field characterization in the vicinity of interfaces. This time-resolved global and local data will enable robust validation of the modeling framework. After validation, this framework will deliver the first physics-based contact model that is fully parametrizable with measurable bulk and interface properties. This unified formulation will be achieved by exposing the transformative knowledge that local and nonlocal interfacial mechanisms act in tandem, with direct analogy to the behavior of bulk response (cohesion and suction) in soils and granular materials.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.