Showing papers by "Qunyang Li published in 2020"

Abnormal conductivity in low-angle twisted bilayer graphene

Abstract: Controlling the interlayer twist angle offers a powerful means for tuning the electronic properties of two-dimensional (2D) van der Waals materials. Typically, the electrical conductivity would increase monotonically with decreasing twist angle owing to the enhanced coupling between adjacent layers. Here, we report a nonmonotonic angle-dependent vertical conductivity across the interface of bilayer graphene with low twist angles. More specifically, the vertical conductivity enhances gradually with decreasing twist angle up to a crossover angle at θc ≈ 5°, and then it drops notably upon further decrease in the twist angle. Revealed by density functional theory calculations and scanning tunneling microscopy, the abnormal behavior is attributed to the unusual reduction in average carrier density originating from local atomic reconstruction. The impact of atomic reconstruction on vertical conductivity is unique for low-angle twisted 2D van der Waals materials and provides a strategy for designing and optimizing their electronic performance.

Droplet Precise Self‐Splitting on Patterned Adhesive Surfaces for Simultaneous Multidetection

Abstract: Precise separation and localization of microdroplets are fundamental for various fields, such as high-throughput screening, combinatorial chemistry, and the recognition of complex analytes. We have developed a droplet self-splitting strategy to divide an impacting droplet into predictable microdroplets and deposit them at preset spots for simultaneous multidetection. No matter exchange was observed between these microdroplets, so they could be manipulated independently. Droplet self-splitting was attributed to anisotropic liquid recoiling on the patterned adhesive surface, as influenced by the droplet Weber number and the width of the low-adhesive stripe. A quantitative criterion was also developed to judge the droplet self-splitting capability. The precise separation and distribution of microdroplets enabled simultaneous arrayed reactions and multiple analyte detection using one droplet of sample.

Observations of 3 nm Silk Nanofibrils Exfoliated from Natural Silkworm Silk Fibers

Abstract: Silk fibers are one of the most attractive natural materials that exhibit enchanting luster and superior mechanical properties. Exploring the unique hierarchical architecture structures of natural ...

Optocapillarity-driven assembly and reconfiguration of liquid crystal polymer actuators.

Abstract: Realizing programmable assembly and reconfiguration of small objects holds promise for technologically-significant applications in such fields as micromechanical systems, biomedical devices, and metamaterials. Although capillary forces have been successfully explored to assemble objects with specific shapes into ordered structures on the liquid surface, reconfiguring these assembled structures on demand remains a challenge. Here we report a strategy, bioinspired by Anurida maritima, to actively reconfigure assembled structures with well-defined selectivity, directionality, robustness, and restorability. This approach, taking advantage of optocapillarity induced by photodeformation of floating liquid crystal polymer actuators, not only achieves programmable and reconfigurable two-dimensional assembly, but also uniquely enables the formation of three-dimensional structures with tunable architectures and topologies across multiple fluid interfaces. This work demonstrates a versatile approach to tailor capillary interaction by optics, as well as a straightforward bottom-up fabrication platform for a wide range of applications. When floating at the air water interface, the shape of objects determines their capillary menisci and mutual interaction. Hu et al. show how dynamically changing shapes of liquid-crystal polymer actuators can be used to achieve optically controlled and reconfigurable assembly.

Decohesion of a rigid flat punch from an elastic layer of finite thickness

Abstract: Interfacial bond strength is a crucial mechanical property of solid adhesive joints that plays an important role in a wide range of applications. A common way of quantifying mechanical strength of an adhered interface is to measure the apparent adhesion strength by finding the maximum pulling force in a tensile bond test and dividing it by the area. Despite the simplicity of this method, there is growing evidence suggesting that the measured quantity is not necessarily an intrinsic attribute of the interface but depends on thickness of the adhesive film and size of the interface. In this work, the critical pull-off force of a flat rigid punch adhered to an elastic film of finite thickness is studied via asymptotic analysis as well as finite element (FE) simulations. The decohesion behavior is found to be governed by two dimensionless parameters η and φ: the former represents the deformation of the interface relative to the interaction distance of interfacial forces, while the latter denotes a correction factor due to finite thickness of the film. As η changes from 0 to infinity, the interface decohesion would take place from uniform detachment (DMT-like) to crack-like propagation (JKR-like). The transition behavior can be well described by an empirical solution, which suggests a new rational procedure to consistently characterize the intrinsic adhesion properties of an adhered interface from tensile bond tests. Finally, the general empirical solution and the adhesion characterization procedure are directly validated by a series of experiments via detaching a stainless-steel flat punch from adhesive Polydimethylsiloxane (PDMS) films. This work not only offers a clear and explicit solution to capture the transition behavior of interface decohesion but also provides a guideline for modulating adhesion performance via geometry rather than chemistry of the adhesives.

Universal Statistical Laws for the Velocities of Collective Migrating Cells.

Abstract: Migratory dynamics of collective cells is central to the morphogenesis of biological tissues. The statistical distribution of cell velocities in 2D confluent monolayers is measured through large-scale and long-term experiments of various cell types lying on different substrates. A linear relation is discovered between the variability and the mean of cell speeds during the jamming process of confluent cell monolayers, suggesting time-invariant distribution profile of cell velocities. It is further found that the probability density function of cell velocities obeys the non-canonical q-Gaussian statistics, regardless of cell types and substrate stiffness. It is the Tsallis entropy, instead of the classical Boltzmann-Gibbs entropy, that dictates the universal statistical laws of collective cell migration. The universal statistical law stems from cell-cell interactions, as demonstrated by the wound healing experiments. This previously unappreciated finding provides a linkage between cell-level heterogeneity and tissue-level ensembles in embryonic development and tumor growth.

Metal Nanoparticle Harvesting by Continuous Rotating Electrodeposition and Separation

Abstract: Summary Current synthetic approaches to metal nanoparticles are mostly batch processes that use a large quantity of reagents and surfactants, producing enormous amount of solid and liquid waste. Here, we developed a rotating electrodeposition and separation (REDS) technique, which entails electrochemically depositing nanoparticles onto a continuously rotating metal foil and subsequently harvesting them through mechanical delamination. A wide array of elemental nanoparticles (e.g., Ag, Au, Ni, Cu), alloys nanoparticles (e.g., FeCoNi and FeCoNiW), and metal oxide nanomaterials (e.g., Co3O4) were synthesized by REDS. We further controlled the growth direction of metals during the electrodeposition to fabricate more complex structures such as polyhedrons, nanoplates, dendrites, and flower nanostructures. Based on the metallic nanoparticles, we obtained conducting inks and fabricated near-field communication tags and touch screen panels. Our technique provides a novel approach for rapid, scalable, and green preparation of low-cost and high-quality nanoparticles in which electrodeposition chemistry can be controlled with a roll-to-roll system.

Effect of airborne contaminants on the macroscopic anti-wear performance of chemical vapor deposition graphene

Abstract: With ultra-high mechanical strength and inherent lubrication property, graphene exhibits great potential as an atomically thin solid lubricant to address wear problems that affect mechanical function and longevity. Despite the excellent wear resistance of graphene at the nanoscale, wear of graphene is difficult to avoid at micro- to macroscale friction tests in ambient conditions and sensitive to the environmental atmosphere and humidity. As an atomically thin material, contaminants from the environment have a non-ignorable effect on graphene surface properties. To study the influence of airborne contaminants on graphene macroscale wear behavior, we performed sliding tests on new (as-prepared) and aged monolayer chemical vapor deposition grown graphene using a 5 mm-radius lens under load down to 0.1mN. The atomic force microscope characterization demonstrated the existence of airborne contaminants on aged graphene. The tribological experiments showed the aged graphene exhibited better wear resistance than the new one due to that the airborne contaminants separated the counterpart from the graphene wrinkles, from where graphene wear was initiated.

Length Scale Effect in Frictional Aging of Silica Contacts.

Abstract: Friction between two solid surfaces often exhibits strong rate and slip-history dependence, which critically determines the dynamic stability of frictional sliding. Empirically, such an evolutional effect has been captured by the rate-and-state friction (RSF) law based on laboratory-scale experiments; but its applicability for generic sliding interfaces under different length scales remains unclear. In this Letter, frictional aging, the key manifestation of the evolutional behavior, of silica-silica contacts is studied via slide-hold-slide tests with apparent contact size spanning across 3 orders of magnitude. The experimental results demonstrate a clear and strong length scale dependency in frictional aging characteristics. Assisted by a multiasperity RSF model, we attribute the length scale effect to roughness-dependent true contact area evolution as well as scale-dependent friction stress due to nonconcurrent slip.