Special Seminar: “Poking and Bulging of 2D Crystals”

Abstract:

In recent years, nano-tents and nano-bubbles formed by two-dimensional (2D) materials have seen a surge of interest because they are able to induce in-plane strain and strain gradient via easy-to-apply out-of-plane deformations. We adopted the membrane limit of the Föppl–von Kármán (FvK) theory and atomic force microscopy (AFM) to unveil what sets the in-plane strains in terms of the shape characteristics and boundary conditions of nano-tents and nano-bubbles. Moreover, out-of-plane poking or bulging, also known as indentation or blister tests, are popular methods to characterize the in-plane elasticity of thin sheets. However, we find that recent results of 2D materials do not follow the well-known load-cubic-deflection relation of linear elastic sheets, which can be attributed to the slippage of atomically smooth 2D materials against their supporting substrates. Besides, the interfacial slippage could cause instabilities such as radial wrinkles in the suspended region, with finite and predictable lengths. We identify a single dimensionless parameter governing these mechanical responses—the sliding number—defined by comparing the sheet tension (that drives the slippage) with the interfacial traction (that resists the slippage). We discuss several useful asymptotic behaviors emerging at small and large sliding numbers. These understandings are helpful for determining when the effect of the interfacial slippage (as well as other substrate-associated subtleties) can be neglected in these tests. Through a parent-satellite bubble system, we have also determined the transition from membrane to plate theory and from Griffith-type interface to vdW-type interface.

Bio:

Dr. Nanshu Lu is currently a Full Professor at the University of Texas at Austin. She received her B.Eng. with honors from Tsinghua University, Beijing, Ph.D. from Harvard University, and then Beckman Postdoctoral Fellowship at UIUC. Her research concerns the mechanics, materials, manufacture, and human / robot integration of soft electronics. She is currently Associate Editor of Nano Letters and Journal of Applied Mechanics. She has been named 35 innovators under 35 by MIT Technology Review (TR 35) and iCANX/ACS Nano Inaugural Rising Star. She has received US NSF CAREER Award, US ONR and AFOSR Young Investigator Awards, 3M non-tenured faculty award, and the 2022 Thomas J.R. Hughes Young Investigator Award from the ASME Applied Mechanics Division. She has been invited to speak at multiple National Academy of Engineering Frontiers of Engineering Symposia and the National Academy of Medicine emerging leaders forum. She is the Snow Lecturer of the 2022 Churchill conferences on the deformation, yield and fracture of polymers (DYFP). She was selected as one of the five great innovators on campus and five world-changing women at the University of Texas at Austin. She is a highly cited researcher identified by Web of Science. For more information, please visit Dr. Lu’s research group webpage at https://sites.utexas.edu/nanshulu/ and follow her on Twitter or WeChat: @nanshulu.