Person: Bai, Ruobing
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Bai
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Ruobing
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Bai, Ruobing
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Publication Optomechanics of Soft Materials(ASME International, 2015) Bai, Ruobing; Suo, ZhigangSome molecules change shape upon receiving photons of certain frequencies, but here we study light-induced deformation in ordinary dielectrics with no special optical effects. All dielectrics deform in response to light of all frequencies. We derive a dimensionless number to estimate when light can induce large deformation. For a structure made of soft dielectrics, with feature size comparable to the wavelength of light, the structure shapes the light, and the light deforms the structure. We study this two-way interaction between light and structure by combining the electrodynamics of light and the nonlinear mechanics of elasticity. We show that optical forces vary nonlinearly with deformation and readily cause optomechanical snap-through instability. These theoretical ideas may help to create optomechanical devices of soft materials, complex shapes, and small features.Publication Fatigue Fracture of Self-Recovery Hydrogels(American Chemical Society (ACS), 2018-02-16) Bai, Ruobing; Yang, Jiawei; Morelle, Xavier; Yang, Canhui; Suo, ZhigangHydrogels of superior mechanical behavior are under intense development for many applications. Some of these hydrogels can recover their stress-stretch curves after many loading cycles. These hydrogels are called self-recovery hydrogels, or even fatigue-free hydrogels. Such a hydrogel typically contains a covalent polymer network, together with some non-covalent, reversible interactions. Here we show that self-recovery hydrogels are still susceptible to fatigue fracture. We study a hydrogel containing both covalently crosslinked polyacrylamide and uncrosslinked polyvinyl alcohol. For a sample without pre-cut crack, the stress-stretch curve recovers after thousands of loading cycles. For a sample with a pre-cut crack, however, the crack extends cycle by cycle. The threshold for fatigue fracture depends on the covalent network, but negligibly on non-covalent interactions. Above the threshold, the non-covalent interactions slow down the extension of the crack under cyclic loads.Publication Fatigue fracture of tough hydrogels(Elsevier BV, 2017) Bai, Ruobing; Yang, Quansan; Tang, Jingda; Morelle, Xavier P.; Vlassak, Joost; Suo, ZhigangTough hydrogels of many chemical compositions have been developed in recent years, but their fatigue fracture has not been studied. The lack of study hinders further development of hydrogels for applications that require long lifetimes under cyclic loads. Examples include tissue engineering, soft robots, and stretchable electronics. Here we study the fatigue fracture of a polyacrylamide-alginate tough hydrogel. We find that the stress-stretch curve changes cycle by cycle, and reaches a steady state after thousands of cycles. The threshold for fatigue fracture is about 53 J/m2, much below the fracture energy (~10,000 J/m2) measured under monotonic load. Nonetheless, the extension of crack per cycle in the polyacrylamide-alginate tough hydrogel is much smaller than that in a single-network polyacrylamide hydrogel.Publication Localized Deformation in Plastic Liquids on Elastomers(ASME International, 2017) Morelle, Xavier; Bai, Ruobing; Suo, ZhigangA plastic liquid such as toothpaste and butter deforms like an elastic solid under a small stress and like a plastic solid under a large stress. Recently, plastic liquids have been used as compliant electrodes for elastomeric transducers. Here, we study the deformation of a plastic liquid adherent on an elastomer when the elastomer is stretched monotonically. We observe that deformation in the plastic liquid localized into shear bands and necks. We further observe that the plastic liquid slips near the interface between the plastic liquid and the elastomer. Each pulling edge of the plastic liquid develops a shear tail, a thin layer of the plastic liquid adherent to the elastomer. As the elastomer is stretched, the tail conforms to the deformation of the elastomer, and the plastic liquid above the tail slips. Finite element simulations confirm that localization occurs even for a relatively simple elastic–plastic model, but require a boundary condition that allows the near-interface slip.