A coarse grained transport model for nanofluidic systems

Molecular Dynamics (MD) is an important tool to simulate flows at the nanoscale. The limitation of MD in simulating important biological and chemical systems having a large length and time scale, increased the interest in efficient coarse-grained (CG) models. Although many existing CG models for various fluids are able to capture structure and dynamics of the bulk fluid accurately, these models are not suited to describe transport phenomena involving explicit walls in nano-channels. Previous coarse-grained models for confined fluids are only optimized to match the structure of the confined fluid. Here we introduce a complete CG transport model for a single component fluid in nano-channels having explicit walls. The model, which was applied to the water-graphene system, was able to demonstrate a very good match, with the structure (error< 7%) and dynamical (error<1%) equilibrium properties of MD simulations. Moreover, the CG model was able to reproduce the MD results for water transport in a Poiseuille flow configuration with an error < 5%. The accuracy of the model was transferable through different configurations and forcing conditions up to a critical force, where the MD slip velocity starts to deviate from the equilibrium prediction. Finally, the CG model was able to achieve ≈ 20x speedup compared to MD simulations, making it more suitable for flows close to experimental conditions, where MD produces a poor signal to noise ratio

Internally mixed nanoparticles from oscillatory spark ablation between electrodes of different materials

<p>The increasing need for engineered alloy nanoparticles (NPs) in diverse fields has spurred efforts to explore efficient/green synthesis methods. In this respect, spark ablation provides a scalable and viable way for producing widely different types of mixed NPs. Most importantly, implementation of the spark has the great advantage to combine a wider range of materials, thereby allowing the synthesis of mixed NPs with virtually unlimited combinations. Here we show that polarity reversal of spark discharges between two electrodes consisting of different materials enables synthesis of alloy NPs, while having a good potential to control the broadness of their composition distribution. A model developed in this work provides a tool for tuning the ablation ratio between the electrodes by adjusting the electric characteristics of the spark circuit. The ablation ratio is equal to the mean composition of the resulting NPs. The model predictions are in accordance with measurements obtained here and in earlier works. The unique way of producing alloy NPs by spark ablation shown in this work becomes especially useful when the starting electrode materials are immiscible at macroscopic scale.</p> <p>Copyright © 2018 American Association for Aerosol Research</p

Molecular design rules for imparting multiple damping modes in dynamic covalent polymer networks

Imparting multiple, distinct dynamic processes at precise timescales in polymers is a grand challenge in soft materials design with implications for applications including electrolytes, adhesives, tissue engineering, and additive manufacturing. Many competing factors including the polymer architecture, molecular weight, backbone chemistry, and presence of solvent affect the local and global dynamics, and in many cases are interrelated. One approach to imparting distinct dynamic processes is through the incorporation of dynamic bonds with widely varying kinetics of bond exchange. Here, statistically crosslinked polymer networks are synthesized with mixed fast and slow dynamic bonds with four orders of magnitude different exchange kinetics. Oscillatory shear rheology shows that the single component networks (either fast or slow) exhibit a single relaxation peak, while mixing fast and slow crosslinkers in one network produces two peaks in the relaxation spectrum. This is in stark contrast to telechelic networks with the same mixture of dynamic bonds where only one mixed mode is observed, and here we develop the molecular design rules necessary to have each dynamic bond contribute a distinct relaxation mode. By controlling the polymer architecture and difference in the number of dynamic bonds per chain, we have elucidated the role of network architecture on imparting multimodal behavior in dynamic networks. A highly tunable and recyclable material has been developed with control of rubbery plateau modulus (through crosslink density), relaxation peak locations and ratio (through crosslinker selection and molar fractions), and tan δ (through the relationships of the rubbery plateau and relaxation peak locations)

Innate Reprocessability in Engineering Thermosets

While valued for their durability and exceptional performance, crosslinked thermosets are challenging to recycle and reuse. Here, we unveil inherent reprocessability in industrially relevant polyolefin thermosets. Unlike prior methods, our approach eliminates the need to introduce exchangeable functionality to regenerate the material, relying instead on preserving the activity of the metathesis catalyst employed in the curing reaction. Frontal ring opening metathesis polymerization (FROMP) proves critical to preserving this activity. We explore conditions controlling catalytic viability to successfully reclaim performance across multiple generations of material, thus demonstrating long-term reprocessability. This straightforward and scalable remolding strategy is poised for widespread adoption. Given the anticipated growth in polyolefin thermosets, our findings represent an important conceptual advance in the pursuit of a fully circular lifecycle for thermoset polymers

Molecular Design of Multimodal Viscoelastic Spectra Using Vitrimers

Imparting multiple, distinct dynamic processes at precise time scales in polymers is a grand challenge in soft materials design with implications for applications including electrolytes, adhesives, tissue engineering, and additive manufacturing. Many competing factors, including the polymer architecture, molecular weight, backbone chemistry, and presence of a solvent, affect the local and global dynamics and in many cases are interrelated. One approach to imparting distinct dynamic processes is through the incorporation of dynamic bonds with widely varying kinetics of bond exchange. Here, statistically cross-linked polymer networks are synthesized with mixed fast and slow dynamic bonds with 3 orders of magnitude different exchange kinetics. Oscillatory shear rheology shows that the single component networks (either fast or slow) exhibit a single relaxation peak while mixing fast and slow cross-linkers in one network produces two peaks in the relaxation spectrum. This is in stark contrast to telechelic networks with the same mixture of dynamic bonds, where only one mixed mode is observed, and here we provide molecular design guidelines for having each dynamic bond contribute a distinct relaxation mode. By comparing the polymer architecture and the difference in the number of dynamic bonds per chain, we have elucidated the role of network architecture in imparting multimodal behavior in dynamic networks. A highly tunable and recyclable material has been developed with control of rubbery plateau modulus (through cross-link density), relaxation peak locations and ratio (through cross-linker selection and molar fractions), and tan δ (through the relationships of the rubbery plateau and relaxation peak locations)

Scalable and Environmentally Benign Process for Smart Textile Nanofinishing

A major challenge in nanotechnology is that of determining how to introduce green and sustainable principles when assembling individual nanoscale elements to create working devices. For instance, textile nanofinishing is restricted by the many constraints of traditional pad-dry-cure processes, such as the use of costly chemical precursors to produce nanoparticles (NPs), the high liquid and energy consumption, the production of harmful liquid wastes, and multistep batch operations. By integrating low-cost, scalable, and environmentally benign aerosol processes of the type proposed here into textile nanofinishing, these constraints can be circumvented while leading to a new class of fabrics. The proposed one-step textile nanofinishing process relies on the diffusional deposition of aerosol NPs onto textile fibers. As proof of this concept, we deposit Ag NPs onto a range of textiles and assess their antimicrobial properties for two strains of bacteria (i.e., Staphylococcus aureus and Klebsiella pneumoniae). The measurements show that the logarithmic reduction in bacterial count can get as high as ca. 5.5 (corresponding to a reduction efficiency of 99.96%) when the Ag loading is 1 order of magnitude less (10 ppm; i.e., 10 mg Ag NPs per kg of textile) than that of textiles treated by traditional wet-routes. The antimicrobial activity does not increase in proportion to the Ag content above 10 ppm as a consequence of a "saturation" effect. Such low NP loadings on antimicrobial textiles minimizes the risk to human health (during textile use) and to the ecosystem (after textile disposal), as well as it reduces potential changes in color and texture of the resulting textile products. After three washes, the release of Ag is in the order of 1 wt %, which is comparable to textiles nanofinished with wet routes using binders. Interestingly, the washed textiles exhibit almost no reduction in antimicrobial activity, much as those of as-deposited samples. Considering that a realm of functional textiles can be nanofinished by aerosol NP deposition, our results demonstrate that the proposed approach, which is universal and sustainable, can potentially lead to a wide number of applications.</p