Pai-Yi Hsiao

Roles of nanolayer and particle size on thermophysical characteristics of ethylene glycol-based copper nanofluids

Calculation of thermal conductivity and the characterization of the molecular-level mechanisms of ethylene-glycol-based copper nanofluid are conducted using the molecular dynamics (MD) Simulation when the nanoparticle size ranges from 6 to 14 A. Layer–Maxwell model is developed for the calculation of effective thermal conductivity of the nanofluid with nanoparticle size up to 2000 A by the application of distinct thermal conductivity in the nanolayers around nanoparticle obtained from MD simulations. The comparison between computational and experimental results reveals the roles of interfacial layer and nanoparticle size in the thermal conductivity enhancement.

Enhanced thermal conductivity of nanofluids diagnosis by molecular dynamics simulations.

Boehmite nanoparticles covered with a polymer shell enhancing the organophilicity of the surface were prepared by physical adsorption of a polyelectrolyte atom transfer radical polymerization (ATRP) macroinitiator followed by graft-polymerization of methyl methacrylate or 2-hydroxyethyl methacrylate. The presence of polymer chains adsorbed/grafted on the Boehmite was confirmed by attenuated total reflection infrared (ATR-IR) spectroscopy and by thermo-gravimetric analysis (TGA), which showed a significant amount of polymer covering the particles. The methodology of polymerization and the kinetics suggested the possibility to modulate the amount, type and thickness of grafted polymer shell. These organic-inorganic hybrid materials were melt compounded in a Brabender mixer with isotactic polypropylene in the presence of functionalized polypropylene. The dispersion degree of Boehmite nanoparticles in the polypropylene matrix as well as their reinforcing effect were studied by morphology characterization [scanning electron microscopy (SEM) and X-ray diffraction (XRD)], whereas thermal and thermo-mechanical properties were assessed by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA).

Constructing a force interaction model for thermal conductivity computation using molecular dynamics simulation: Ethylene glycol as an example

This study aims to construct a force interaction model for thermal conductivity computation and to analyze the liquid properties in atomic level for liquid ethylene glycol (EG) using molecular dynamic simulation. The microscopic details of the molecular system and the macroscopic properties of experimental interest are connected by Green-Kubo relations. In addition, the major contributions of heat transfer modes for thermal conductivity due to convection, interaction, and torque are obtained quantitatively. This study reveals that the intramolecular interaction force fields result in different conformations of the EG in the liquid and thus the molecular shapes. The trans∕gauche ratio for EGs O-Me-Me-O torsional angle and the number of intermolecular∕intramolecular H-bonds are found to be important parameters affecting the thermal conductivity.

Thermodynamic investigations using molecular dynamics simulations with potential of mean force calculations for cardiotoxin protein adsorption on mixed self-assembled monolayers.

Understanding protein adsorption onto solid surfaces is of critical importance in the field of bioengineering, especially for applications such as medical implants, diagnostic biosensors, drug delivery systems, and tissue engineering. This study proposed the use of molecular dynamics simulations with potential of mean force (PMF) calculations to identify and characterize the mechanisms of adsorption of a protein molecule on a designed surface. A set of model systems consisting of a cardiotoxin (CTX) protein and mixed self-assembled monolayer (SAM) surfaces were used as examples. The set of mixed SAM surfaces with varying topographies were created by mixing alkanethiol chains of different lengths. The results revealed that CTX proteins underwent similar conformal changes upon adsorption onto the various mixed SAMs but showed distinctive characteristics in free energy profiles. Enhancement of the adsorption affinity, i.e., the change in free energy of adsorption, for mixed SAMs was demonstrated by using atomic force microscopic measurements. A component analysis conducted to quantify the physical mechanisms that promoted CTX adsorption revealed contributions from both SAMs and the solvent. Further component analyses of thermodynamic properties, such as the free energy, enthalpy, and entropy, indicated that the contribution from SAMs was driven by enthalpy, and the contribution from the solvent was driven by entropy. The results indicated that CTX adsorption was an entropy-driven process, and the entropic component from the solvent, i.e., the hydrophobic interaction, was the major driving force for CTX adsorption onto SAMs. The study also concluded that the surfaces composed of mixtures of SAMs with different chain lengths promoted the adsorption of CTX protein.

Dynamic information for cardiotoxin protein desorption from a methyl-terminated self-assembled monolayer using steered molecular dynamics simulation.

Dynamic information, such as force, structural change, interaction energy, and potential of mean force (PMF), about the desorption of a single cardiotoxin (CTX) protein from a methyl-terminated self-assembled monolayer (SAM) surface was investigated by means of steered molecular dynamics (SMD) simulations. The simulation results indicated that Loop I is the first loop to depart from the SAM surface, which is in good agreement with the results of the nuclear magnetic resonance spectroscopy experiment. The free energy landscape and the thermodynamic force of the CTX desorption process was represented by the PMF and by the derivative of PMF with respect to distance, respectively. By applying Jarzynskis equality, the PMF can be reconstructed from the SMD simulation. The PMFs, calculated by different estimators based upon Jarzynskis equality, were compared with the conventional umbrella sampling method. The best estimation was obtained by using the fluctuation-dissipation estimator with a pulling velocity of v = 0.25 nm/ns for the present study.

Mixed-SAM Surfaces Monitoring CTX-Protein, Part II: Analysis Using Molecular Dynamics Simulations

Molecular dynamics simulations are performed to study the physical mechanism of cobra cardiotoxin (CTX) proteins adsorption on alkanethiol self-assembled monolayers (SAMs) composed of S( CH₂)₅ CH₃ and S( CH₂)₉ CH₃. The binding energy of the CTX protein to the SAM surface of different mixing ratios of alkanethiol chains is calculated. The results show that the affinity of CTX to SAM reaches a maximum value when the ratio S(CH₂)₅ CH₃: S(CH₂)₉ CH₃ is 1:1, which agrees with the measurements of atomic force microscope obtained in Part I of our dual paper. Moreover, the binding energy is found to be linearly proportional to the CTX-SAM contact area. The hydrophobicity on CTX residues, the flexibility of SAMs and the behavior of water molecules near the SAM surface are examined to understand how these parameters affect the adsorption of a CTX protein on SAM surfaces. In addition, the importance of modeling water molecules explicitly in the study of protein adsorption is demonstrated by applying different solvent models.

Comment on “Critical and slow dynamics in a bulk metallic glass exhibiting strong random magnetic anisotropy” [Appl. Phys. Lett. 92, 011923 (2008)]

In this comment, by using Monte Carlo simulation, we show that the perpendicular shift of hysteresis loops reported in the commented work is nothing special but simply due to the fact that the range of field does not surpass the reversible field beyond which the two branches of the loop merge. If the reversible field is exceeded, the shift is no longer observed. Moreover, we point out that even using a small range of field, the shift will not be observed if the observation time is long enough for the reversible field to drop within the range.

An ac field probe for the magnetic ordering of magnets with random anisotropy

A Monte Carlo simulation is carried out to investigate the magnetic ordering in magnets with random anisotropy (RA). Our results show peculiar similarities to recent experiments that the real part of ac susceptibility presents two peaks for weak RA and only one for strong RA regardless of glassy critical dynamics manifested for them. We demonstrate that the thermodynamic nature of the low-temperature peak is a ferromagneticlike dynamic phase transition to quasilong range order (QLRO) for the former. Our simulation, therefore, is able to be incorporated with the experiments to help clarify the existence of the QLRO theoretically predicted so far.

Studies of Cardio Toxin Protein Adsorption on Mixed Self-Assembled Monolayers Using Molecular Dynamics Simulations

To understand protein adsorption on a surface is very important in bio-related domains of technology and application such as biomaterials, implant biocompatibility, and biosensor technology. (Gray 2004) Self-assembled monolayers (SAMs) are excellent model surfaces for biology and biochemistry because they are stable, highly ordered, easy to prepare, and provide a wide range of organic functionality. (Love et al. 2005) Previous experimental studies (Ostuni et al. 2003; Prime & G.M. Whitesides 1991) have indicated that hydrophobic interactions are the major mechanism for protein adsorption on surfaces, and the dehydration of both the protein and hydrophobic SAMs provides an entropic driving force for protein adsorption. (Ostuni et al. 2003) Nonetheless, there are still many questions needed to be clarified and difficult to be investigated in experiments. Molecular dynamics (MD) simulation is a powerful tool that is able to investigate the atomic details of a molecular system. This paper summarizes the investigations of proteins adsorption on alkanethiol SAMs by means of MD simulations in order to expand the limited information that the experiments can provide. (Hung et al.2006; Hung et al.2010; Hung et al. 2011)