107 research outputs found
Extended surfaces modulate and can catalyze hydrophobic effects
Interfaces are a most common motif in complex systems. To understand how the
presence of interfaces affect hydrophobic phenomena, we use molecular
simulations and theory to study hydration of solutes at interfaces. The solutes
range in size from sub-nanometer to a few nanometers. The interfaces are
self-assembled monolayers with a range of chemistries, from hydrophilic to
hydrophobic. We show that the driving force for assembly in the vicinity of a
hydrophobic surface is weaker than that in bulk water, and decreases with
increasing temperature, in contrast to that in the bulk. We explain these
distinct features in terms of an interplay between interfacial fluctuations and
excluded volume effects---the physics encoded in Lum-Chandler-Weeks theory [J.
Phys. Chem. B 103, 4570--4577 (1999)]. Our results suggest a catalytic role for
hydrophobic interfaces in the unfolding of proteins, for example, in the
interior of chaperonins and in amyloid formation.Comment: 22 pages, 5 figure
Heat transfer from nanoparticles: a corresponding state analysis
In this contribution, we study situations in which nanoparticles in a fluid
are strongly heated, generating high heat fluxes. This situation is relevant to
experiments in which a fluid is locally heated using selective absorption of
radiation by solid particles. We first study this situation for different types
of molecular interactions, using models for gold particles suspended in octane
and in water. As already reported in experiments, very high heat fluxes and
temperature elevations (leading eventually to particle destruction) can be
observed in such situations. We show that a very simple modeling based on
Lennard-Jones interactions captures the essential features of such experiments,
and that the results for various liquids can be mapped onto the Lennard-Jones
case, provided a physically justified (corresponding state) choice of
parameters is made. Physically, the possibility of sustaining very high heat
fluxes is related to the strong curvature of the interface that inhibits the
formation of an insulating vapor film
Water Dynamics at Protein Interfaces: Ultrafast Optical Kerr Effect Study
The behavior of water molecules surrounding a protein can have an important bearing on its structure and function. Consequently, a great deal of attention has been focused on changes in the relaxation dynamics of water when it is located at the protein surface. Here we use the ultrafast optical Kerr effect to study the H-bond structure and dynamics of aqueous solutions of proteins. Measurements are made for three proteins as a function of concentration. We find that the water dynamics in the first solvation layer of the proteins are slowed by up to a factor of 8 in comparison to those in bulk water. The most marked slowdown was observed for the most hydrophilic protein studied, bovine serum albumin, whereas the most hydrophobic protein, trypsin, had a slightly smaller effect. The terahertz Raman spectra of these protein solutions resemble those of pure water up to 5 wt % of protein, above which a new feature appears at 80 cm–1, which is assigned to a bending of the protein amide chain
Thermal Conductivity of Carbon Nanotubes and their Polymer Nanocomposites: A Review
Thermally conductive polymer composites offer new possibilities for replacing metal parts in several applications, including power electronics, electric motors and generators, heat exchangers, etc., thanks to the polymer advantages such as light weight, corrosion resistance and ease of processing. Current interest to improve the thermal conductivity of polymers is focused on the selective addition of nanofillers with high thermal conductivity. Unusually high thermal conductivity makes carbon nanotube (CNT) the best promising candidate material for thermally conductive composites. However, the thermal conductivities of polymer/CNT nanocomposites are relatively low compared with expectations from the intrinsic thermal conductivity of CNTs. The challenge primarily comes from the large interfacial thermal resistance between the CNT and the surrounding polymer matrix, which hinders the transfer of phonon dominating heat conduction in polymer and CNT. This article reviews the status of worldwide research in the thermal conductivity of CNTs and their polymer nanocomposites. The dependence of thermal conductivity of nanotubes on the atomic structure, the tube size, the morphology, the defect and the purification is reviewed. The roles of particle/polymer and particle/particle interfaces on the thermal conductivity of polymer/CNT nanocomposites are discussed in detail, as well as the relationship between the thermal conductivity and the micro- and nano-structure of the composite
Improving Thermal Conduction Across Cathode/Electrolyte Interfaces in Solid-State Lithium-Ion Batteries by Hierarchical Hydrogen-Bond Network
Effective thermal management is an important issue to ensure safety and performance of lithium-ion batteries. Fast heat removal is highly desired but has been obstructed by the high thermal resistance across cathode/electrolyte interface. In this study, self-assembled monolayers (SAMs) are used as the vibrational mediator to tune interfacial thermal conductance between an electrode, lithium cobalt oxide (LCO), and a solid state electrolyte, polyethylene oxide (PEO). Embedded at the LCO/PEO interface, SAMs are specially designed to form hierarchical hydrogen-bond (H-bond) network with PEO. Molecular dynamics simulations demonstrate that all SAM-decorated interfaces show enhanced thermal conductance and dominated by H-bonds types. The incorporation of poly(acrylic acid) (PAA) SAM drastically enhances interfacial thermal conductance by approximately 211.69%, largely due to the formation of a strong H-bond, -COOH···:O, between PAA and PEO. Even with weaker H-bonds such as -OH···:O, it still outperforms the pristine interface as well as interfaces decorated with non-H-bonded SAMs, e.g. PE. Such improvement is attributed to the unique hierarchical H-bond network at the interface, which removes discontinuities in temperature field, straighten SAM chains, make materials strongly adhere, and couple the vibrational modes of materials. The study is expected to guide surface engineering for more effective thermal management in lithium-ion batteries
Graphene -- Based Nanocomposites as Highly Efficient Thermal Interface Materials
We found that an optimized mixture of graphene and multilayer graphene -
produced by the high-yield inexpensive liquid-phase-exfoliation technique - can
lead to an extremely strong enhancement of the cross-plane thermal conductivity
K of the composite. The "laser flash" measurements revealed a record-high
enhancement of K by 2300 % in the graphene-based polymer at the filler loading
fraction f =10 vol. %. It was determined that a relatively high concentration
of single-layer and bilayer graphene flakes (~10-15%) present simultaneously
with thicker multilayers of large lateral size (~ 1 micrometer) were essential
for the observed unusual K enhancement. The thermal conductivity of a
commercial thermal grease was increased from an initial value of ~5.8 W/mK to
K=14 W/mK at the small loading f=2%, which preserved all mechanical properties
of the hybrid. Our modeling results suggest that graphene - multilayer graphene
nanocomposite used as the thermal interface material outperforms those with
carbon nanotubes or metal nanoparticles owing to graphene's aspect ratio and
lower Kapitza resistance at the graphene - matrix interface.Comment: 4 figure
Nanoscale heat transfer - from computation to experiment
Heat transfer can differ distinctly at the nanoscale from that at the macroscale. Recent advancement in
computational and 5 experimental techniques has enabled a large number of interesting observations and
understanding of heat transfer processes at the nanoscale. In this review, we will first discuss recent
advances in computational and experimental methods used in nanoscale thermal transport studies,
followed by reviews of novel thermal transport phenomena at the nanoscale observed in both
computational and experimental studies, and discussion on current understanding of these novel
10 phenomena. Our perspectives on challenges and opportunities on computational and experimental
methods are also presented.University of Notre Dame (Startup fund)United States. Dept. of Energy. Office of Basic Energy Sciences (Solid-State Solar-Thermal Energy Conversion Center
Molecular Modeling Approach to Prediction of Thermo-Mechanical Behavior of Thermoset Polymer Networks
Molecular dynamics and molecular mechanics simulations have been used to study thermo-mechanical response of highly cross-linked polymers composed of epoxy resin DGEBA and hardener DETDA. The effective cross-linking approach used in this work allowed construction of a set of stress-free molecular models with high conversion degree containing up to 35000 atoms. The generated structures were used to investigate the influence of model size, length of epoxy strands, and degree of cure on thermo-mechanical properties. The calculated densities, coefficients of thermal expansion, and glass transition temperatures of the systems are found to be in good agreement with experimental data. The computationally efficient static deformation approach we used to calculate elastic constants of the systems successfully compensated for the large scattering of the mechanical properties data due to nanoscopically small volume of simulation cells and allowed comparison of properties of similar polymeric networks having minor differences in structure or chemistry. However, some of the elastic constants obtained using this approach were found to be higher than in real macroscopic samples. This can be attributed to both finite-size effect and to the limitations of the static deformation approach to account for dynamic effects. The observed dependence of properties on system size, in this work, can be used to estimate the contribution of large-scale defects and relaxation events into macroscopic properties of the thermosetting materials
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