Fall 2016 - Fall 2019. This award investigates the mechanics of how synthetic surfaces with micrometer-sized pillars adhere to and slide on soft and wet substrates. Micro-pillar arrays have been introduced on the wheel treads of robotic devices to improve their mobility on soft tissues, but the underlying mechanism is yet to be understood. Most existing theoretical models on the contact mechanics of micro-structured surfaces assume stiff substrates, and thus are not directly transferable to the case of soft substrates which can deform significantly during adhesive and frictional contact. Results of this research will improve the design of in vivo robotic devices for the next generation technology of non-invasive medical diagnosis and surgery. More broadly, new knowledge in soft material contact mechanics can also enable robotic handling of food, medical transplants and implants, thus benefiting the food and healthcare industries. Education and outreach programs will be developed to engage high school though graduate school students, exposing them to the fundamental concepts and exciting forefront of mechanics. Activities include course development, undergraduate student research program, and outreach lessons.
A soft substrate can undergo large deformation upon contact with a micro-pillar array, which is three-dimensional in nature and inherently nonlinear. The large substrate deformation is expected to lead to a strong coupling between the normal and shear loadings of the micro-pillars, as well as between neighboring pillars. Understanding this coupling will facilitate the search for optimal pillar arrangement to achieve desired adhesion and friction properties. The PIs will develop a new experimental apparatus to achieve in situ mapping of the three-dimensional deformation fields in soft hydrogel substrates under contact, adhesion and friction. The soft gel substrate serves as a model material to simulate biological tissue or other soft and wet materials. The in situ deformation mapping capability will be combined with adhesion and friction tests and finite element modeling. The finite element model will connect the local micromechanics at the level of individual pillars to the global adhesion and friction, through an experimentally validated pillar-surface interface model. Results will offer new theoretical insights on the contact mechanics between micro pillar arrays and soft substrates, and enable high-fidelity simulations to drive the design of micro-pillar structures for optimized adhesion and friction on soft substrates.