Stresses in the crystalline and amorphous components as well as the interface segments as a function of their relative velocity.

<p>(A) The crystalline cube was pulled horizontally along the amorphous rectangular plate of 0.5 nm thickness. (B) The crystalline cube was pulled with a 10 degree angle with respect to horizontal plane along the amorphous component of 2.85 nm thickness.</p

Viscous Friction between Crystalline and Amorphous Phase of Dragline Silk

<div><p>The hierarchical structure of spider dragline silk is composed of two major constituents, the amorphous phase and crystalline units, and its mechanical response has been attributed to these prime constituents. Silk mechanics, however, might also be influenced by the resistance against sliding of these two phases relative to each other under load. We here used atomistic molecular dynamics (MD) simulations to obtain friction forces for the relative sliding of the amorphous phase and crystalline units of <i>Araneus diadematus</i> spider silk. We computed the coefficient of viscosity of this interface to be in the order of 10² Ns/m² by extrapolating our simulation data to the viscous limit. Interestingly, this value is two orders of magnitude smaller than the coefficient of viscosity within the amorphous phase. This suggests that sliding along a planar and homogeneous surface of straight polyalanine chains is much less hindered than within entangled disordered chains. Finally, in a simple finite element model, which is based on parameters determined from MD simulations including the newly deduced coefficient of viscosity, we assessed the frictional behavior between these two components for the experimental range of relative pulling velocities. We found that a perfectly relative horizontal motion has no significant resistance against sliding, however, slightly inclined loading causes measurable resistance. Our analysis paves the way towards a finite element model of silk fibers in which crystalline units can slide, move and rearrange themselves in the fiber during loading.</p></div

Setup of an FPMD simulation for assessing molecular friction between seven bundles of the amorphous phase and two crystalline units.

<p>(A) Schematic representation of the model before equilibration (left). The two crystalline units (red) are 3 nm apart, and seven bundles of the amorphous phase (blue) are placed around it. The loading and boundary conditions of the model are indicated (right). A harmonic spring that moves with constant velocity <i>V</i> was connected to the termini of the seven bundles. The crystalline units were position-restrained in pulling and in one lateral direction. (B) The MD simulation system (left) with a front and top view of mid-sections (middle), and an enlarged view of interactions between the crystalline and amorphous component (right).</p

Simulated coefficient of viscosity per residue as a function of shear stress .

<p>Red and black lines present fits of the stochastic model to the simulation data with varying <i>ma</i> and , respectively. The solid red line shows the best fit to the data.</p

Friction force per residue () as a function of pulling velocity () when pulling the amorphous phase along the crystalline units.

<p>Both amorphous-crystalline and amorphous-water friction contribute to the total friction.</p

Enhancing the Mechanical Durability of Icephobic Surfaces by Introducing Autonomous Self-Healing Function

Ice accretion presents a severe risk for human safety. Although great efforts have been made for developing icephobic surfaces (the surface with an ice adhesion strength below 100 kPa), expanding the lifetime of state-of-the-art icephobic surfaces still remains a critical unsolved issue. Herein, a novel icephobic material is designed by integrating an interpenetrating polymer network (IPN) into an autonomous self-healing elastomer, which is applied in anti-icing for enhancing the mechanical durability. The molecular structure, surface morphology, mechanical properties, and durable icephobicity of the material were studied. The creep behaviors of the new icephobic material, which were absent in most relevant studies on self-healing materials, were also investigated in this work. Significantly, the material showed great potentials for anti-icing applications with an ultralow ice adhesion strength of 6.0 ± 0.9 kPa, outperforming many other icephobic surfaces. The material also exhibited an extraordinary durability, showing a very low long-term ice adhesion strength of ∼12.2 kPa after 50 icing/deicing cycles. Most importantly, the material was able to exhibit a self-healing property from mechanical damages in a sufficiently short time, which shed light on the longevity of icephobic surfaces in practical applications

Displacement Mechanism of Oil in Shale Inorganic Nanopores by Supercritical Carbon Dioxide from Molecular Dynamics Simulations

Supercritical CO₂ (scCO₂), as an effective displacing agent and clean fracturing fluid, exhibits a great potential in enhanced oil recovery (EOR) from unconventional reservoirs. However, the microscopic translocation behavior of oil in shale inorganic nanopores has not been well understood yet in the scCO₂ displacement process. Herein, nonequilibrium molecular dynamics (NEMD) simulations were performed to study adsorption and translocation of scCO₂/dodecane in shale inorganic nanopores at different scCO₂ injection rates. The injected scCO₂ preferentially adsorbs in proximity of the surface and form layering structures due to hydrogen bonds interactions between CO₂ and −OH groups. A part of scCO₂ molecules in the adsorption layer retain the mobility, due to the cooperation of slippage, Knudsen diffusion, and imbibition of scCO₂. The adsorbed dodecane are separated partly from the surface by scCO₂, as a result the competitive adsorption between scCO₂ and dodecane, and thus enhancing the mobility of oil and improving oil production. In the scCO₂ displacement front, interfacial tension (IFT) reduction and dodecane swelling enhance the mobilization of dodecane molecules, which plays the crucial role in the CO₂ EOR process. The downstream dodecane, adjacent to the displacement front, is found to aggregate and pack tightly. The analysis of contact angle, meniscus, and interfacial width shows that the small scCO₂ injection rate with a large injection volume is favorable for CO₂ EOR. The morphology of meniscus changes in the order convex–concave–CO₂ entrainment with the increase of the injection rate. The microscopic insight provided in this study is helpful to understand and effectively design CO₂ exploitation of shale resources