Effect of surface stress and surface mass on elastic vibrations of nanoparticles

The elastic vibrations of nanoparticles, in the present work, have been studied theoretically by taking into account the size-dependent elasticity due to the surface effect. Surface elasticity has been resorted to where, in addition to the effect of surface stress, the effect of surface mass has also been taken into account in the model. The torsional and spherical modes for the free vibration have been obtained. Numerical calculations have been conducted, and the results have shown that both the surface stress and the surface mass affect significantly the eigenfrequencies of nanospheres. With those surface effects, the eigenfrequencies are no longer proportional to the radius inverse. Under appropriate circumstances, the surface effects may render the lowest eigenfrequency amount to the second lowest by classical elasticity.     

On the surface roughness and hydrophobicity of dual-size double-layer silica nanoparticles

An array of double-layer silica nanoparticles with two different length scales was introduced onto the glass substrates for investigating the effect of surface morphology on wettability. Silica nanoparticles of 7, 12, 20, and 40 nm in diameters were individually functionalized using 3-aminopropyltriethoxysilane or 3-glycidoxypropyltrimethoxysilane. Silica nanoparticles functionalized with the complementary amine and epoxy groups were deposited alternately on glass surfaces to obtain durable and covalently bound dual-size double-layer silica nanoparticle coatings. Order of the deposition of nanoparticles for the fabrication of dual-size double-layer coatings was found to be the most determinant factor on surface roughness and hence the wettability. Deposition of the bigger nanoparticles on top of smaller ones resulted in rougher surfaces and consequently higher hydrophobicity. Based upon these findings, it is now possible to fine-tune surface roughness and subsequent wettability by controlling the size ratio of dual-size double-layer silica nanoparticles.     

On the role of stress fluctuations in brittle fracture

Cracks in random stress fields are assumed to be originated in regions with high local tension. As a legacy of this special location, additional local tractions opening the crack in its centre are developed even in self-equilibrating stress fields. As the crack becomes a mesocrack it will deviate its path to meet the regions with higher possible local tension. The necessary statistical properties of the microcrack-generated random stress field can be calculated using the dipole asymptotics to approximate the stresses generated by each microcrack. The microcracks are assumed to be noninteracting and surrounded by nonintersecting excluded volumes. For the case of spherical excluded volumes the correlation radius is found to be less than the microcrack radius, which suggests that the stresses acting on each microcrack can be assumed to be statistically independent. In brittle fracture under uniaxial tension the effect of the stress fluctuations is shown to be able to significantly reduce the macroscopic strength. In fracture of brittle materials under uniaxial compression wing cracks are developed which, in real 3-D situations, cannot grow extensively and therefore cannot themselves cause failure. Instead, they induce stress fluctuations which generate mesocracks growing towards compression in such a way as to avoid the wing cracks. Hence, only stresses outside excluded volumes around the wing cracks will affect the mesocrack growth. These stresses have positive mean even if the full stress field is self-equilibrating. This results in a background tension acting perpendicular to the compression axis, amplifying the mesocrack growth and eventually causing failure. The growth and opening of mesocracks results in a specific dependence between dilatancy, i.e. inelastic increase of the sample volume, and the applied compressive stress. This dependence has a universal nature independent of the particular model of wing cracks. It corresponds well to the data of uniaxial compressive tests on 4 samples of Oshima granite (Sano et al. 1981) despite markedly different loading rates and resulted strengths. This revised version was published online in July 2006 with corrections to the Cover Date.     

An energy conservation approach to adsorbate-induced surface stress and the extraction of binding energy using nanomechanics

Surface stress induced by molecular adsorption in three different binding processes has been studied experimentally using a microcantilever sensor. A comprehensive free-energy analysis based on an energy conservation approach is proposed to explain the experimental observations. We show that when guest molecules bind to atoms/molecules on a microcantilever surface, the released binding energy is retained in the host surface, leading to a metastable state where the excess energy on the surface is manifested as an increase in surface stress leading to the bending of the microcantilever. The released binding energy appears to be almost exclusively channeled to the surface energy, and energy distribution to other channels, including heat, appears to be inactive for this micromechanical system. When this excess surface energy is released, the microcantilever relaxes back to the original state, and the relaxation time depends on the particular binding process involved. Such vapor phase experiments were conducted for three binding processes: physisorption, hydrogen bonding, and chemisorption. Binding energies for these three processes were also estimated.     

On the determination of surface energy from cleavage experiments

An analysis is made of the strain energy distribution in a double-cantilever cleavage specimen of a type which has been used extensively in surface energy determinations. It is shown that the contribution from the uncleaved part of the specimen cannot be neglected as done hitherto by various authors. As a consequence, values for the surface energy of several materials so far determined in cleavage experiments must be revised upward to 50%. The predictions of the theoretical analysis are shown to be confirmed by the results of compliance measurements.

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Zusammenfassung Es wird der Spannungszustand in solchen Probekörpern berechnet, wie sie allgemein für die Bestimmung der Grenzflächenenergie nach der Spaltmethode benutzt werden. Dabei zeigt sich, daß der Beitrag des ungespaltenen Probeteils zur insgesamt gespeicherten elastischen Energie keineswegs vernachlässigt werden darf, wie bisher von zahlreichen Autoren angenommen wurde. Dies hat zur Folge, daß alle bisher nach der Spaltmethode bestimmten Werte der spezifischen Grenzflächenenergie um his zu 50% nach oben korrigiert werden müssen. Die theoretische Aussage konnte durch sogenannte Compliance-Messungen bestätigt werden.

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Résumé On a calculé l'état de contrainte d'une éprouvette du type de celles utilisées pour les déterminations d'énergies de surface par la méthode de la fente. Il apparait dans ces conditions que la contribution de la partie non fondue de l'éprou-vette dans l'estimation de l'energie élastique emmagasinée totale n'est en aucun cas négligeable, comme de nombreux auteurs font jusqu'à présent admis. Par suite, toutes les valeurs trouvées jusqu'à present pour les energies de surface sont trop faibles et doivent être augmentées dans une proportion atteignant 50%. La prevision théorique put être vérifiée par Compliance measurements.

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List of Symbols A cross-sectional area - A _i free constants, see (15) - B _i free constants, see (15) - C compliance - E modulus of elasticity - F external force - G shear modulus - J moment of inertia of the cross section - L crack length - L ₁ length of the uncleaved part of the specimen - m Poisson's ratio - N ₁, N ₂ shortened terms, see (16) - r, polar coordinates - r _a external radius of the semiannular disc - r _i internal radius - S surface energy - t cross-sectional height - U stored elastic energy - w cross-sectional width - W externally supplied energy - x correction factor - y shortened term, see (25) - y specific surface energy - ₀ displacement at the point of application of the force - shortened term, see (20) - _r, _t normal stresses - _r shear stresses - stress function     

On the determination of the surface free energy of quartz

Studies of water vapor adsorption on quartz by the method analogous to the dynamic gas chromatography step profile method are described. The adsorption was determined by changes in the capacitance of the capacitor (detector) between the coverings of which the quartz powder was placed. Prom the adsorption isotherm the film pressure π of the water film on quartz were determined, obtaining $\text{π}{}_{\max }\text{= 380}\text{erg}\text{cm}{}^2$.An interpretation of the π changes in relation to the film thickness and the kind of wetting process has been proposed. It is concluded that the characteristic film pressure values result from the work of spreading, immersional and adhesional wetting and correspond to thicknesses of about 2, 3 and 4 statistical water monolayers, respectively. The maximum π value, however, probably corresponds to the work of quartz- water adhesion + water cohesion work. On the basis of the thus determined values of π_s, π₁, and π_max, the value of the polar component γ_q^p of the quartz surface free energy was determined, using the value $\text{γ}{}_{\mathrm{q}}{}^{\mathrm{p}}\text{= 76}\text{erg}\text{cm}{}^2$. The calculated average of the γ_q^p value equals 115 $\text{erg}\text{cm}{}^2\text{2}$.     

On the role of low-energy electrons in the radiosensitization of DNA by gold nanoparticles

Four different gold nanoparticle (GNP) preparations, including naked GNPs and GNPs coated either with thiolated undecane (S-C(11)H(23)), or with dithiolated diethylenetriaminepentaacetic (DTDTPA) or gadolinium (Gd) DTDTPA chelating agents, were synthesized. The average diameters, for each type of nanoparticle, are 5 nm, 10 and 13 nm, respectively. Dry films of plasmid DNA pGEM-3Zf(-), DNA with bound GNPs and DNA with coated GNPs were bombarded with 60 keV electrons. The yields of single and double strand breaks were measured as a function of exposure by electrophoresis. The binding of just one GNP without coating to DNA containing 3197 base pairs increases single and double strand breaks by a factor of 2.3 while for GNPs coated with S-C(11)H(23) this factor is reduced to 1.6. The GNPs coated with DTDTPA and DTDTPA:Gd in the same ratio with the DNA, produce essentially no increment in damage. These results could be explained by the attenuation by the coatings of the intensity of the low-energy photoelectrons emitted from the GNPs. Thus, coatings of GNPs may considerably attenuate the short-range low-energy electrons emitted from gold, leading to a considerable decrease of radiosensitization. According to our results, the highest radiosensitization should be obtained with GNPs having the shortest possible ligand, directed to the DNA of cancer cells.     

The role of surface charge on the uptake and biocompatibility of hydroxyapatite nanoparticles with osteoblast cells

The objective of this study is to evaluate the effect of hydroxyapatite (HAP) nanoparticles with different surface charges on the cellular uptake behavior and in vitro cell viability and proliferation of MC3T3-E1 cell lines (osteoblast). The nanoparticles' surface charge was varied by surface modification with two carboxylic acids: 12-aminododecanoic acid (positive) and dodecanedioic acid (negative). The untreated HAP nanoparticles and dodecanoic acid modified HAP nanoparticles (neutral) were used as the control. X-ray diffraction (XRD) revealed that surface modifications by the three carboxylic acids did not change the crystal structure of HAP nanoparticles; Fourier transform infrared spectroscopy (FT-IR) confirmed the adsorption and binding of the carboxylic acids on the HAP nanoparticles' surfaces; and zeta potential measurement confirmed that the chemicals successfully modified the surface charge of HAP nanoparticles in water based solution. Transmission electron microscopy (TEM) images showed that positively charged, negatively charged and untreated HAP nanoparticles, with similar size and shape, all penetrated into the cells and cells had more uptake of HAP nanoparticles with positive charge compared to those with negative charge, which might be attributed to the attractive or repulsive interaction between the negatively charged cell membrane and positively/negatively charged HAP nanoparticles. The neutral HAP nanoparticles could not penetrate the cell membrane due to their larger size. MTT assay and LDH assay results indicated that as compared with the polystyrene control, greater cell viability and cell proliferation were measured on MC3T3-E1 cells treated with the three kinds of HAP nanoparticles (neutral, positive, and untreated), among which positively charged HAP nanoparticles showed the strongest improvement for cell viability and cell proliferation. In summary, the surface charge of HAP nanoparticles can be modified to influence the cellular uptake of HAP nanoparticles and the different uptake also influences the behavior of cells. These in vitro results may also provide useful information for investigations of HAP nanoparticle applications in gene delivery and intracellular drug delivery.     

The equivalency of surface tension,surface energy and surface free energy

In much of the available literature, there is confusion regarding the correct use of the terms surface tension, surface energy and surface free energy. As a result, these three terms have been used interchangeably to describe the same quantity. This problem is particularly serious in the area of solid surface science. Linford has examined and discussed such inconsistencies but failed to differentiate the three quantities clearly. In the present paper, the definitions and the relationships between surface tension, surface energy and surface free energy are examined and their proper usage clarified.     

On the stress field and crack nucleation behavior of a disclinated nanowire with surface stress effects

This paper studies the stress field and crack nucleation behavior in a disclinated nanowire with a continuum model. The surface stress effects of the nanowire is accounted for with the Gurtin-Murdoch model. The Green’s functions for the stress fields of a single wedge disclination and a single edge dislocation in a cylindrical nanowire are solved respectively with the complex variable method. To make the superposition principle valid, the stress field induced by the residual surface tension is properly handled in the Green’s functions. After that, the distributed dislocation method is applied to simulate the crack nucleation behavior. The influences of the surface stress effects on the stress fields of the wedge disclination and edge dislocation as well as on the Griffith crack nucleation behavior are systematically discussed.     

On the role of stress waves in dynamic rupture of cylindrical tubes

A systematic experimental/computational study was performed to investigate the role of stress waves in ductile fracture of cylindrical tubes. The stress waves were created by high‐speed moving load, which was produced by detonation of explosive cord inside two intact and two pre‐flawed steel tubes. Several distinct phenomena like cyclic crack growths in Modes I and III, crack flap bulging and crack curving/branching were observed and simulated by finite element (FE) method. The FE models were composed of 3D brick elements equipped with interface cohesive elements. The analysis results showed that the crack growths in Modes I and III were governed by the detonation‐induced stress waves. The crack speeds were obtained based on the increments of cyclic crack growth and the time period of the stress waves. The estimated crack speed range was 63–230 m s^?1 for the axial growth, whereas the average speed for growth in Mode III was 100 m s^?1.     

Resonance energy transfer-amplifying fluorescence quenching at the surface of silica nanoparticles toward ultrasensitive detection of TNT

This paper reports a resonance energy transfer-amplifying fluorescence quenching at the surface of silica nanoparticles for the ultrasensitive detection of 2,4,6-trinitrotoluene (TNT) in solution and vapor environments. Fluorescence dye and organic amine were covalently modified onto the surface of silica nanoparticles to form a hybrid monolayer of dye fluorophores and amine ligands. The fluorescent silica particles can specifically bind TNT species by the charge-transfer complexing interaction between electron-rich amine ligands and electron-deficient aromatic rings. The resultant TNT-amine complexes bound at the silica surface can strongly suppress the fluorescence emission of the chosen dye by the fluorescence resonance energy transfer (FRET) from dye donor to the irradiative TNT-amine acceptor through intermolecular polar-polar interactions at spatial proximity. The quenching efficiency of the hybrid nanoparticles with TNT is greatly amplified by at least 10-fold that of the corresponding pure dye. The nanoparticle-assembled arrays on silicon wafer can sensitively detect down to approximately 1 nM TNT with the use of only 10 microL of solution (approximately 2 pg TNT) and several ppb of TNT vapor in air. The simple FRET-based nanoparticle sensors reported here exhibit a high and stable fluorescence brightness, strong analyte affinity, and good assembly flexibility and can thus find many applications in the detection of ultratrace analytes.     

On the dependence of crack surface morphology and energy dissipation on microstructure in ductile plate tearing

The two distinct tearing mechanisms observed in ductile metal plates are the void-by-void advance of the crack tip, and the simultaneous interaction of multiple voids on the plane ahead of the crack tip. Void-by-void crack advance, which leads to a cup-cup crack surface morphology, is the dominant mechanism if the plate contains a low number of small void nucleation sites (i.e., second phase particles). Conversely, a large number and/or size of nucleation sites trigger the simultaneous interaction of multiple voids resulting in a slanted crack. The present work aims to provide further insight into the parameters controlling the mechanisms and energy dissipation of plate tearing by focusing on the shape and, thereby, the orientation of the nucleation sites. The study uses a two-dimensional plane strain finite element domain to model the cross section of a plate, subject to mode I tearing, with discretely modeled, randomly distributed, finite-sized elliptic void nucleation sites. The developed finite element setup can capture the dependence of the crack surface morphology on the microstructure of the plate. The simulation results confirm that cup-cup crack propagation develops by intense plastic straining throughout the thinning region of the plate. Conversely, slanted and cup-cone cracks propagate in thin localized shear deformation bands. The energy dissipation is, therefore, greater for cup-cup cracks. The study demonstrates that the damage-related microstructure has a significant role in determining the overall hardening capacity of a plate, which in turn dictates the tearing mode and energy.