Hydrodynamics of Capillary Imbibition under Nanoconfinement

Understanding fluid flow in nanoconfined geometries is crucial for a broad range of scientific problems relevant to the behavior of porous materials in biology, nanotechnology, and the built environment. Because of the dominant importance of surface effects at the nanoscale, long-standing assumptions that are valid for macroscopic systems must be revisited when modeling nanoconfined fluids, because boundary conditions and the confined behavior of liquids are challenging to discern from experiments. To address this issue, here we present a novel coarse-grained model that combines parameters calibrated for water with a dissipative particle dynamics thermostat for the purpose of investigating hydrodynamics under confinement at scales exceeding current capabilities with all-atomistic simulations. Conditions pertaining to slip boundary conditions and confinement emerge naturally from particle interactions, with no need for assumptions a priori. The model is used to systematically investigate the imbibition dynamics of water into cylindrical nanopores of different diameters. Interestingly, we find that the dynamic contact angle depends on the size of the nanopore in a way that cannot be explained through a relationship between contact line velocity and dynamic contact angle, suggesting nonlocal effects of the flow field may be important. Additionally, a size-dependent characteristic time scale for imbibition is found, which could be useful for the interpretation of experiments and design of novel nanofluidic devices. We present the first systematic study that explains how contact angle dynamics and imbibition dynamics vary with nanopore radius. Our modeling approach lays the foundation for broader investigations on the dynamics of fluids in nanoporous materials in conjunction with experimental efforts

Anisotropy of Shear Relaxation in Confined Thin Films of Unentangled Polymer Melts

The anisotropic shear relaxation functions of confined thin films of unentangled polymer melts are measured via nonequilibrium step–strain simulations of in-plane and out-of-plane shear using the finitely extensible, nonlinear-elastic (FENE) model. We show that the classical Rouse model unsurprisingly fails to predict the thin-film relaxation functions in response to out-of-plane shear, due in part to non-Gaussian conformation statistics in the dimension perpendicular to the sub/superstrate. Using an alternate empirical model for the out-of-plane response, we quantify decreases in the plateau modulus <i>G</i>_⊥^<i>P</i>, relaxation time λ_⊥, and viscosity η_⊥ and an increase in the logarithmic relaxation rate <i>r</i>_⊥ as functions of film thickness, and we discuss these anisotropic changes in stress-relaxation properties in terms of structural/conformation changes on the microscopic level, namely the relative contraction and non-Gaussian quality of polymer conformations in the dimension normal to the substrate and the resulting phenomenon of cooperative relaxation. We then incorporate these into a semiempirical extension to the Rouse model which closely predicts our computational results and which will be useful for further study of polymer thin films

A domain-reduction approach to bridging-scale simulation of one-dimensional nanostructures

We present a domain-reduction approach for the simulation of one-dimensional nanocrystalline structures. In this approach, the domain of interest is partitioned into coarse and fine scale regions and the coupling between the two is implemented through a bridging-scale interfacial boundary condition. The atomistic simulation is used in the fine scale region, while the discrete Fourier transform is applied to the coarse scale region to yield a compact Green‟s function formulation that represents the effects of the coarse scale domain upon the fine/coarse scale interface. This approach facilitates the simulations for the fine scale, without the requirement to simulate the entire coarse scale domain. After the illustration in a simple 1D problem and comparison with analytical solutions, the proposed method is then implemented for carbon nanotube structures. The robustness of the proposed multiscale method is demonstrated after comparison and verification of our results with benchmark results from fully atomistic simulations

Surface Ripples of Polymeric Nanofibers under Tension: The Crucial Role of Poisson’s Ratio

Molecular dynamics and finite element simulations are performed to study the phenomenon of surface rippling in polymeric nanofibers under tension. Each nanofiber is modeled as a core–shell system that resembles most relevant features extracted from detailed molecular simulations and experiments. Accordingly, our model nanofiber consists of a dense glassy core embedded in a less dense, more flexible, rubbery shell. Poisson’s ratios of the core and shell layers are assumed close to that of compressible and incompressible materials, respectively. Surface rippling of the nanofiber is found, via combined finite element analysis and continuum theory, to be governed by a “polarization” mechanism at the core–shell interphase regime that is ultimately induced by the mismatch between Poisson’s ratios while a mismatch between Young’s moduli seems to play a secondary role. Plastic deformation is a prerequisite for the formation of rippled surfaces, that evolve from initial imperfections, and grow in the presence of uniaxial tension. For this reason, both strain rate and yield stress greatly influence the onset and modes of the observed rippled surface. Our findings are consistent with experimental observations on surface ripples of electrospun nanofibers and pave the way to design polymeric nanofibers with distinct surface morphologies

Predicting the Macroscopic Fracture Energy of Epoxy Resins from Atomistic Molecular Simulations

Predicting the macroscopic fracture energy of highly cross-linked glassy polymers from atomistic simulations is challenging due to the size of the process zone being large in these systems. Here, we present a scale-bridging approach that links atomistic molecular dynamics simulations to macroscopic fracture properties on the basis of a continuum fracture mechanics model for two different epoxy materials. Our approach reveals that the fracture energy of epoxy resins strongly depends on the functionality of epoxy resin and the component ratio between the curing agent (amine) and epoxide. The most intriguing part of our study is that we demonstrate that the fracture energy exhibits a maximum value within the range of conversion degrees considered (from 65% to 95%), which can be attributed to the combined effects of structural rigidity and postyield deformability. Our study provides physical insight into the molecular mechanisms that govern the fracture characteristics of epoxy resins and demonstrates the success of utilizing atomistic molecular simulations toward predicting macroscopic material properties

Multiscale Simulation as a Framework for the Enhanced Design of Nanodiamond-Polyethylenimine-Based Gene Delivery

Nanodiamonds (NDs) are emerging carbon platforms with promise as gene/drug delivery vectors for cancer therapy. Specifically, NDs functionalized with the polymer polyethylenimine (PEI) can transfect small interfering RNAs (siRNA) in vitro with high efficiency and low cytotoxicity. Here we present a modeling framework to accurately guide the design of ND-PEI gene platforms and elucidate binding mechanisms between ND, PEI, and siRNA. This is among the first ND simulations to comprehensively account for ND size, charge distribution, surface functionalization, and graphitization. The simulation results are compared with our experimental results both for PEI loading onto NDs and for siRNA (c-Myc) loading onto ND-PEI for various mixing ratios. Remarkably, the model is able to predict loading trends and saturation limits for PEI and siRNA while confirming the essential role of ND surface functionalization in mediating ND-PEI interactions. These results demonstrate that this robust framework can be a powerful tool in ND platform development, with the capacity to realistically treat other nanoparticle systems