Thermodynamic Surface Energy Research Articles

Confined wetting of water on CNT web patterned surfaces

We report the formation of a thin liquid film of pre-determined shape that is achieved through wetting of water on a silicon surface patterned with aligned carbon nanotube arrays or CNT webs. We measured the profiles of liquid films on two types of silicon substrate surfaces (namely, with and without a patterned CNT web) using monochrome interferometry. We found that the CNT web patterned surface produces a much thinner liquid film with a well-controlled shape due to the roughness-induced wetting enhancement on the CNT web and the anchoring effect of contact lines at morphological edges. We further used a thermodynamic surface energy based model to interpret our experimental observations and to elucidate the underlying mechanism of wetting enhancement induced by the CNT web. Our study provides a promising approach for forming thin liquid films of controllable pre-determined shape that has numerous potential applications.

Ligand-mediated control of dislocation dynamics and resulting particle morphology of GdOCl nanocrystals.

While much progress has been achieved in the shape-controlled synthesis of nanocrystals, chemical strategies to define morphology remain primarily empirical. Here, a mechanistic study of the influence of different coordinating ligands on the kinetics and thermodynamics of crystal growth during the preparation of GdOCl by the non-hydrolytic condensation of GdCl3 and Gd(O (i) Pr)3 is reported. Growth using oleylamine, octadecylamine, trioctylamine, and didodecylamine yields 2D nanosheets with approximately square cross sections, whereas growth in trioctylphosphine oxide yields larger and thicker platelets. The nanostructures are characterized by the presence of spiral growth patterns and dislocations. Apart from preferential binding to specific crystallographic facets, the coordinating ligands are suggested to control the extent of supersaturation, thereby facilitating and tuning dislocation-mediated growth. Upon depletion of monomers, thermodynamic surface energy considerations become of paramount importance and the nanocrystals are reshaped via mass transport from edges to sides yielding their eventual equilibrium shapes. The mechanisms developed here are thought to be broadly generalizable to the ligand-directed growth of nanomaterials.

Generation of strong electric fields in an ice film capacitor

We present a capacitor-type device that can generate strong electrostatic field in condensed phase. The device comprises an ice film grown on a cold metal substrate in vacuum, and the film is charged by trapping Cs(+) ions on the ice surface with thermodynamic surface energy. Electric field within the charged film was monitored through measuring the film voltage using a Kelvin work function probe and the vibrational Stark effect of acetonitrile using IR spectroscopy. These measurements show that the electric field can be increased to ∼4 × 10(8) V m(-1), higher than that achievable by conventional metal plate capacitors. In addition, the present device may provide several advantages in studying the effects of electric field on molecules in condensed phase, such as the ability to control the sample composition and structure at molecular scale and the spectroscopic monitoring of the sample under electric field.

Solventless synthesis of monodisperse Cu2S nanorods, nanodisks, and nanoplatelets.

Cu(2)S nanocrystals with disklike morphologies were synthesized by the solventless thermolysis of a copper alkylthiolate molecular precursor. The nanodisks ranged from circular to hexagonal prisms from 3 to 150 nm in diameter and 3 to 12 nm in thickness depending on the growth conditions. High resolution transmission electron microscopy (HRTEM) revealed the high chalcocite (hexagonal) crystal structure oriented with the c-axis ([001] direction) orthogonal to the favored growth direction. This disk morphology is thermodynamically favored as it allows the extension of the higher energy [100] and [110] surfaces with respect to the [001] planes. The hexagonal prism morphology also appears to relate to increased C-S bond cleavage of adsorbed dodecanethiol along the more energetic [100] facets relative to [001] facets. Monodisperse Cu(2)S nanodisks self-assemble into ribbons of stacked platelets. This solventless approach provides a new technique to synthesize anisotropic metal chalcogenide nanostructures with shapes that depend on both the face-sensitive thermodynamic surface energy and the surface reactivity.

Theory of Tackiness

We show that the interplay between surface roughness on micron scale and air suction can yield tack energies much higher than thermodynamic surface energies. Our model provides a quantitative interpretation of highly nonlinear force-versus-separation curves which are usually observed when adhesion forces and energies are high.

Effect of prior deformation on the cleavage of zinc single crystals

Abstract The critical stress intensity factor. K ic, for basal cleavage at 77 K has been measured on a number of zinc single crystals, either as-grown or pre-deformed by tension or by torsional fatigue. The dislocation distribution and the extent of the plastic zone associated with cleavage were studied by chemical etching and by X-ray topography. K ic is found to decrease with prior deformation, and with increasing density of non-basal 'forest' dislocations. This result is consistent with the forest dislocations limiting the extent of plastic deformation which accompanies cleavage. even at 77 K. This is confirmed by the X-ray topography studies of plastic zone size. However, effective fracture surface energies are found to be considerably less than the thermodynamic surface energy for the basal plane of zinc.

Effect of prior deformation on the cleavage of zinc single crystals

Abstract The critical stress intensity factor. K ic, for basal cleavage at 77 K has been measured on a number of zinc single crystals, either as-grown or pre-deformed by tension or by torsional fatigue. The dislocation distribution and the extent of the plastic zone associated with cleavage were studied by chemical etching and by X-ray topography. K ic is found to decrease with prior deformation, and with increasing density of non-basal 'forest' dislocations. This result is consistent with the forest dislocations limiting the extent of plastic deformation which accompanies cleavage. even at 77 K. This is confirmed by the X-ray topography studies of plastic zone size. However, effective fracture surface energies are found to be considerably less than the thermodynamic surface energy for the basal plane of zinc.

The surface energy of zinc

The influence of temperature and associated dislocation microstructure on the energetics of basal plane cleavage in zinc crystals has been investigated using the method of Hull, Beardmore, and Valentine (HBV). A marked temperature dependence was observed in the zinc surface energy, over the range 77–298 °K, contrary to previous expectations. Plastic relaxation was associated with crack initiation at 77 °K, but not propagation; while at room temperature a plastic zone of 1200–1500 μm in depth was produced by crack extension. The surface energy could be estimated, independent of the usual Griffith analysis, by measuring the energy dissipation in a fully relaxed deformed zone associated with an explosively formed precursor crack. This method yielded surface energies of 0.066 to 0.079 J m−2 which was in good agreement with previous work. It is demonstrated that the cleavage surface energy of zinc is well below the thermodynamic surface energy and that this discrepancy is not related to plastic deformation.

The deformation and fracture of β-HMX

A study has been made of the mechanical deformation properties of β-HMX, an important secondary explosive. It is shown that under compressive loading twinning takes place on the (101)-plane. At low loads, this twinning is elastic and usually precedes fracture. Cleavage in β-HMX takes place on the {011}-planes. The fracture surface energy of 0.06 J m -2 has been determined by a micro-indentation technique. This compares with a value of 0.045 J m -2 obtained for the thermodynamic surface energy from contact-angle measurements. The values suggest that there is relatively little energy loss by plastic deformation associated with crack propagation in HMX compared with, for example, the secondary explosives PETN and RDX. Despite this brittleness the twin deformation allows β-HMX to undergo large changes of shape: the significance of this in plastic-bonded explosives is commented on.

The debonding of two linear chains of atoms connected by alternate bonds having different force laws

In this paper a model representing the debonding of two linear chains of atoms connected by alternate bonds having different force laws is analysed. Such a model may be used to discuss the effect of alloy of impurity atoms on brittle crack propagation, e.g. when brittle cracks propagate along grain boundaries or precipitate-matrix interfaces. In general, little effect on the effective fracture energy is expected unless there is extensive alloy or impurity atom segregation; the effect of this prediction on interface fracture is discussed. It is also demonstrated, for the force laws considered, that the macroscopic thermodynamic surface energy lies between the critical strain energy release rate levels corresponding to rapid crack extension and contraction.

Crack tip lattice trapping and the relation between the macroscopic and microscopic thermodynamic surface energies

Analysis of a simple one-dimensional crack tip simulation model shows that there is always some degree of crack tip lattice trapping, irrespective both of the force law between the atomic planes being fractured and of the elastic characteristics of the material. Furthermore the macroscopic and microscopic thermodynamic surface energies are shown to be identical, which implies that the macroscopic thermodynamic surface energy always lies between the critical strain energy release rate levels corresponding to rapid growth and rapid healing. Results from the simple model analyses support views recently expressed by Rice, Thomson and Fuller.

Fracture surface energy of olivine

A relatively simple indentation technique for the rapid measurement of fracture surface energy, Γ, of small samples is described. The reliability of this technique is assessed by testing soda-lime glass for which there are good independent fracture mechanics determinations of fracture surface energy. The indentation technique gives a value for Γ of 4.33 J m−2 which compares favourably with the accepted value of 3.8 J m−2. Fracture surface energies of the {010} and {001} cleavage planes of single crystal olivine (modal composition Fo88Fa12) are then determined and compared with theoretical estimates of the thermodynamic surface energy, γ, calculated from atomistic parameters (γ is equal to Γ in the absence of dissipative processes during crack extension). The experimental values for Γ{010} and Γ{001} are respectively 0.98 J m−2 and 1.26 J m−2. The calculated values of γ{010} and γ{001} are respectively in the range from 0.37 J m−2 to 8.63 J m−2 and 12.06 J m−2. The particular advantages of the indentation technique for the study of the fracture surface energies of geological materials are outlined.

Fracture Surface Energies and Dislocation Processes during Dynamical Cleavage of LiF. II. Experiments

The dynamics of cleavage of long slender single crystals of LiF has been observed with high-speed ciné-photography in order to measure the surface energy and the dislocation processes in a propagating semiplastic crack. The fracture surface energy deduced from these observations could be separated into two parts, a reversible thermodynamic surface energy and a contribution due to dissipative plastic deformation, using the theoretical analysis given in Part I of this series. A specific reversible surface energy of 480±50 erg/cm2 was obtained and the plastic contribution was found to persist to fracture velocities > 104 cm/sec. Dislocation mechanisms of dissipation persisting to very high crack velocities and to temperatures as low as 90°K were deduced from studies of etch-pit patterns and atomic step structures on the cleaved surfaces. The atomic step structures were observed by imaging in the electron microscope replicas of eptiaxial metal deposits grown from the vapor at the step sites. The plastic flow processes were found to be consistent with the theory of Part I and the observed magnitude of the plastic contribution to the surface energy.