Dimitrios V. Papavassiliou

Liquid water can slip on a hydrophilic surface

Understanding and predicting the behavior of water, especially in contact with various surfaces, is a scientific challenge. Molecular-level understanding of hydrophobic effects and their macroscopic consequences, in particular, is critical to many applications. Macroscopically, a surface is classified as hydrophilic or hydrophobic depending on the contact angle formed by a water droplet. Because hydrophobic surfaces tend to cause water slip whereas hydrophilic ones do not, the former surfaces can yield self-cleaning garments and ice-repellent materials whereas the latter cannot. The results presented herein suggest that this dichotomy might be purely coincidental. Our simulation results demonstrate that hydrophilic surfaces can show features typically associated with hydrophobicity, namely liquid water slip. Further analysis provides details on the molecular mechanism responsible for this surprising result.

Boundary slip and wetting properties of interfaces: Correlation of the contact angle with the slip length

Correlations between contact angle, a measure of the wetting of surfaces, and slip length are developed using nonequilibrium molecular dynamics for a Lennard-Jones fluid in Couette flow between graphitelike hexagonal-lattice walls. The fluid-wall interaction is varied by modulating the interfacial energy parameter epsilonr=epsilonsfepsilonff and the size parameter sigmar=sigmasfsigmaff, (s=solid, f=fluid) to achieve hydrophobicity (solvophobicity) or hydrophilicity (solvophilicity). The effects of surface chemistry, as well as the effects of temperature and shear rate on the slip length are determined. The contact angle increases from 25 degrees to 147 degrees on highly hydrophobic surfaces (as epsilonr decreases from 0.5 to 0.1), as expected. The slip length is functionally dependent on the affinity strength parameters epsilonr and sigmar: increasing logarithmically with decreasing surface energy epsilonr (i.e., more hydrophobic), while decreasing with power law with decreasing size sigmar. The mechanism for the latter is different from the energetic case. While weak wall forces (small epsilonr) produce hydrophobicity, larger sigmar smoothes out the surface roughness. Both tend to increase the slip. The slip length grows rapidly with a high shear rate, as wall velocity increases three decades from 100 to 10(5) ms. We demonstrate that fluid-solid interfaces with low epsilonr and high sigmar should be chosen to increase slip and are prime candidates for drag reduction.

Computational modeling of flow-induced shear stresses within 3D salt-leached porous scaffolds imaged via micro-CT

Flow-induced shear stresses have been found to be a stimulatory factor in pre-osteoblastic cells seeded in 3D porous scaffolds and cultured under continuous flow perfusion. However, due to the complex internal structure of porous scaffolds, analytical estimation of the local shear forces is impractical. The primary goal of this work is to investigate the shear stress distributions within Poly(l-lactic acid) scaffolds via computation. Scaffolds used in this study are prepared via salt leeching with various geometric characteristics (80-95% porosity and 215-402.5microm average pore size). High resolution micro-computed tomography is used to obtain their 3D structure. Flow of osteogenic media through the scaffolds is modeled via lattice Boltzmann method. It is found that the surface stress distributions within the scaffolds are characterized by long tails to the right (a positive skewness). Their shape is not strongly dependent on the scaffold manufacturing parameters, but the magnitudes of the stresses are. Correlations are prepared for the estimation of the average surface shear stress experienced by the cells within the scaffolds and of the probability density function of the surface stresses. Though the manufacturing technique does not appear to affect the shape of the shear stress distributions, presence of manufacturing defects is found to be significant: defects create areas of high flow and high stress along their periphery. The results of this study are applicable to other polymer systems provided that they are manufactured by a similar salt leeching technique, while the imaging/modeling approach is applicable to all scaffolds relevant to tissue engineering.

Computational modeling of the thermal conductivity of single-walled carbon nanotube–polymer composites

A computational model was developed to study the thermal conductivity of single-walled carbon nanotube (SWNT)-polymer composites. A random walk simulation was used to model the effect of interfacial resistance on the heat flow in different orientations of SWNTs dispersed in the polymers. The simulation is a modification of a previous model taking into account the numerically determined thermal equilibrium factor between the SWNTs and the composite matrix material. The simulation results agreed well with reported experimental data for epoxy and polymethyl methacrylate (PMMA) composites. The effects of the SWNT orientation, weight fraction and thermal boundary resistance on the effective conductivity of composites were quantified. The present model is a useful tool for the prediction of the thermal conductivity within a wide range of volume fractions of the SWNTs, so long as the SWNTs are not in contact with each other. The developed model can be applied to other polymers and solid materials, possibly even metals.

Interfacial water on crystalline silica: a comparative molecular dynamics simulation study

Understanding the properties of interfacial water at solid–liquid interfaces is important in a wide range of applications. Molecular dynamics is becoming a widespread tool for this purpose. Unfortunately, however, the results of such studies are known to strongly depend on the selection of force fields. It is, therefore, of interest to assess the extent by which the implemented force fields can affect the predicted properties of interfacial water. Two silica surfaces, with low and high surface hydroxyl density, respectively, were simulated implementing four force fields. These force fields yield different orientation and flexibility of surface hydrogen atoms, and also different interaction potentials with water molecules. The properties for interfacial water were quantified by calculating contact angles, atomic density profiles, surface density distributions, hydrogen bond density profiles and residence times for water near the solid substrates. We found that at low surface density of hydroxyl groups, the force field strongly affects the predicted contact angle, while at high density of hydroxyl groups, water wets all surfaces considered. From a molecular-level point of view, our results show that the position and intensity of peaks observed from oxygen and hydrogen atomic density profiles are quite different when different force fields are implemented, even when the simulated contact angles are similar. Particularly, the surfaces simulated by the CLAYFF force field appear to attract water more strongly than those simulated by the Bródka and Zerda force field. It was found that the surface density distributions for water strongly depend on the orientation of surface hydrogen atoms. In all cases, we found an elevated number of hydrogen bonds formed between interfacial water molecules. The hydrogen bond density profile does not depend strongly on the force field implemented to simulate the substrate, suggesting that interfacial water assumes the necessary orientation to maximise the number of water–water hydrogen bonds irrespectively of surface properties. Conversely, the residence time for water molecules near the interface strongly depends on the force field and on the flexibility of surface hydroxyl groups. Specifically, water molecules reside for longer times at contact with rigid substrates with high density of hydroxyl groups. These results should be considered when comparisons between simulated and experimental data are attempted.

Simulation insights into thermally conductive graphene-based nanocomposites

Dispersing nanoparticles in a polymer can enhance both mechanical and transport properties. Nanocomposites with high thermal conductivity could be obtained by using thermally conductive nanoparticles. Carbon-based nanoparticles are extremely promising, although high resistances to heat transfer from the nanoparticles to the polymer matrix could cause significant limitations. This work focuses on graphene sheets (GS) dispersed within n-octane. Although pristine GS agglomerate, equilibrium molecular dynamic simulations suggest that when the GS are functionalized with short branched hydrocarbons along the GS edges, they remain well dispersed. Results are reported from equilibrium and non-equilibrium molecular dynamics simulations to assess the effective interactions between dispersed GS, the self-assembly of GS, and the heat transfer through the GS–octane nanocomposite. Tools are designed to understand the effect of GS size, solvent molecular weight and molecular architecture on GS dispersability and GS–octane thermal conductivity. Evidence is provided for the formation of nematic phases when the GS volume fraction increases within octane. The atomic-level results are input for a coarse-grained Monte Carlo simulation that predicts anisotropic thermal conductivity for GS-based composites when the GS show nematic phases.

Random walks in nanotube composites: Improved algorithms and the role of thermal boundary resistance

Random walk simulations of thermal walkers are used to study the effect of interfacial resistance on heat flow in randomly dispersed carbon nanotube composites. The adopted algorithm effectively makes the thermal conductivity of the nanotubes themselves infinite. The probability that a walker colliding with a matrix-nanotube interface reflects back into the matrix phase or crosses into the carbon nanotube phase is determined by the thermal boundary (Kapitza) resistance. The use of “cold” and “hot” walkers produces a steady state temperature profile that allows accurate determination of the thermal conductivity. The effects of the carbon nanotube orientation, aspect ratio, volume fraction, and Kapitza resistance on the composite effective conductivity are quantified.

Inter-carbon nanotube contact in thermal transport of controlled-morphology polymer nanocomposites

Directional thermal conductivities of aligned carbon nanotube (CNT) polymer nanocomposites were calculated using a random walk simulation with and without inter-CNT contact effects. The CNT contact effect has not been explored for its role in thermal transport, and it is shown here to significantly affect the effective transport properties including anisotropy ratios. The primary focus of the paper is on the non-isotropic heat conduction in aligned-CNT polymeric composites, because this geometry is an ideal thermal layer as well as constituting a representative volume element of CNT-reinforced polymer matrices in hybrid advanced composites under development. The effects of CNT orientation, type (single-versus multi-wall), inter-CNT contact, volume fraction and thermal boundary resistance on the effective conductivities of CNT composites are quantified. It is found that when the CNT-CNT thermal contact is taken into account, the maximum effective thermal conductivity of the nanocomposites having their CNTs parallel to the heat flux decreases by approximately 4 times and approximately 2 times for the single-walled and the multi-walled CNTs, respectively, at 20% CNT volume fraction.

Turbulent transport from continuous sources at the wall of a channel

Abstract Dispersion from a continuous line source located at the wall of a turbulent channel and transport over a step change in wall heat flux are studied for fluids with Prandtl numbers between 0.1 and 2400. Direct Numerical Simulation is used to develop the velocity flow field, which is then coupled with a particle tracking algorithm to describe the behavior of heat or mass markers released from instantaneous sources on the wall. The positions in time and space of these markers, which have been available as a database created by Papavassiliou and Hanratty [Int. J. Heat Mass Transfer 40 (6) (1997) 1303], are used as the building block for the study of passive scalar transport from the wall of the channel. Qualitative and quantitative results are obtained with particular emphasis on transport parameters from the wall.

Scalar dispersion from an instantaneous line source at the wall of a turbulent channel for medium and high Prandtl number fluids

Lagrangian methods have been used recently to reconstruct temperature profiles for relatively high Prandtl number, Pr, fluids (up to Pr ¼ 2400) in direct numerical simulations (DNS) of turbulent channel flow. The basic concept is that a heated surface is formed by an infinite number of continuous sources of heat. For example, the behavior of a heated plane can be synthesized by the behavior of an infinite number of continuous sources of heat that cover the plane. The building block for such a reconstruction is the behavior of a single instantaneous heat source located at the wall. The present work studies the behavior of such sources in turbulent channel flow. The trajectories of heat markers are monitored in space and time as they move in a hydrodynamic field created by a DNS. The fluids span several orders of magnitude of Pr (or Sc), Pr ¼ 0:1, 1, 10, 100, 200, 500, 2400, 7000, 15 000, 50 000, (liquid metals, gases, liquids, lubricants and electrochemical fluids). The effects of Pr in the evolution of the marker cloud are examined and quantified. The marker cloud is found to evolve in three stages, two of which are Pr dependent. 2002 Elsevier Science Inc. All rights reserved.