6 research outputs found
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><sub>⊥</sub><sup><i>P</i></sup>, relaxation time λ<sub>⊥</sub>, and viscosity
η<sub>⊥</sub> and an increase in the logarithmic relaxation
rate <i>r</i><sub>⊥</sub> 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