Surface Energy Minimization Research Articles

Multi-artery heat-pipe spreader: monolayer-wick receding meniscus transitions and optimal performance

For high heat flux and low thermal resistance, the multiple-artery heat-pipe spreader uses distributed high-permeability arteries (posts) for liquid supply and high-capillary pressure monolayer wick for liquid spreading and evaporation. Experiments indicate the receding meniscus transitions in monolayer play a role in sudden drop in thermal resistance prior to dryout. Using monolayer SEM images and the minimum surface energy principles, the meniscus dynamics up to dryout is analyzed, and the meniscus location, capillary pressure, effective thermal conductivity, and permeability are also predicted for heterogeneous, periodic sintered copper-particle (including bimodal particle size) unit cells. The liquid thickness is nonuniform within the heterogeneous unit cell, and with increase in the wick superheat local dryout occurs (meniscus snaps) in the loose-packed region influencing the wet-wick properties and the occurrence of the minimum thermal resistance. The monolayer wick continues to function under local dryout (away from post) until a receding dry front is formed followed by complete dryout. These predictions are in good agreement with experiments. The optimal wick thermal-hydraulic performance, i.e., dimensionless ratio of heat flux to thermal resistance (wick figure of merit Zm) is sought through analysis. The uniform, sintered, close-packed 30–50μm particles give the highest Zm over a range of superheat in the wet regime, and 30μm particles give a record low resistance near 2.5μK/(W/m2)

Automatic 3D garment positioning based on surface metric

AbstractPositioning virtual garments onto a 3D avatar is known to be a time‐consuming process. This task is usually done manually if the proportion of the garment needs to be kept during the process for a fitting purpose. The positioning of the garment is currently the main limitation to propose a fully automatic virtual try‐on application. This paper presents an automatic method based on surface energy minimisation that allows fully automatically positioning of a 3D virtual cloth without deformation of the patterns. The minimum of surface energy is reached by animating the humanoid avatar. Humanoid joints are manipulated to place a 3D avatar into the virtual garment. The proposed method, as well as its implementation, is described in this paper. image

Study on the dimensional, configurational and optical evolution of palladium nanostructures on c-plane sapphire by the control of annealing temperature and duration

Metallic nanostructures can find various applications such as in optoelectronic devices, nanostructure synthesis and catalytic applications and their applicability vary depending on their size, density and configuration dependent properties. In this paper, the dimensional and configurational evolution of self-assembled palladium (Pd) nanostructures is systematically studied on c-plane sapphire with the control of annealing temperature (AT) and annealing duration with the initial Pd layers of various thicknesses. Depending on the AT, two distinct growth regimes are observed based on the concurrent effect of surface diffusion, surface energy minimization and Pd sublimation: i.e. (i) agglomeration of Pd nano-clusters from voids (500<AT≤650°C) and (ii) round nanoparticle (NP) fabrication (650°C<AT≤900°C). At 950°C, due to the sublimation of Pd atoms, substantial decrease in the NP dimension is witnessed and results in the ring patterns around the NPs. Additional investigation is performed at 950°C to reveal the annealing duration effect on the NP evolution. Due to the dual effect of the Oswald ripening and atom sublimation, initially the dimension of NPs is grown and then gradually decays along with the duration, resulting in an inverted ‘V’ pattern evolution in diameter and height. Moreover, the evolution of optical properties such as absorption band and average reflectance are studied with the corresponding reflectance spectra as a function of wavelength over UV, visible and NIR region. The Raman spectra analysis depicts the variation of lattice vibration peak intensity and position based on the surface morphology of the Pd nanostructures.

Stretchable and Soft Electronics using Liquid Metals.

The use of liquid metals based on gallium for soft and stretchable electronics is discussed. This emerging class of electronics is motivated, in part, by the new opportunities that arise from devices that have mechanical properties similar to those encountered in the human experience, such as skin, tissue, textiles, and clothing. These types of electronics (e.g., wearable or implantable electronics, sensors for soft robotics, e-skin) must operate during deformation. Liquid metals are compelling materials for these applications because, in principle, they are infinitely deformable while retaining metallic conductivity. Liquid metals have been used for stretchable wires and interconnects, reconfigurable antennas, soft sensors, self-healing circuits, and conformal electrodes. In contrast to Hg, liquid metals based on gallium have low toxicity and essentially no vapor pressure and are therefore considered safe to handle. Whereas most liquids bead up to minimize surface energy, the presence of a surface oxide on these metals makes it possible to pattern them into useful shapes using a variety of techniques, including fluidic injection and 3D printing. In addition to forming excellent conductors, these metals can be used actively to form memory devices, sensors, and diodes that are completely built from soft materials. The properties of these materials, their applications within soft and stretchable electronics, and future opportunities and challenges are considered.

Explicit construction of C2 surfaces for meshes of arbitrary topology

ABSTRACTPresented in this paper is an approach to construct a C2-continuous surface for a mesh of arbitrary topology. The construction process is subdivision surface based, with modification performed on extra-ordinary patches to ensure C2-continuity of the resulting surface. Implementation is easy because modification is patch-based. The resulting surface has an explicit expression of the form WMG for each extra-ordinary patch where W is a parameter vector, M is a constant matrix and G is the patch-wise control point vector. Therefore, computing derivatives, normals and curvatures for points in the domain of the given mesh is very easy and, consequently, the resulting surface is suitable for operations such as shape analysis, shape optimization, surface energy minimization etc. The construction process includes constraints so that the shape of the resulting C2 surface is very similar to the surface generated by subdivision. More importantly, the resulting C2 surface satisfies the convex hull property.

Orientations of dendritic growth during solidification

Dendrites are crystalline forms which grow far from the limit of stability of the plane front and adopt an orientation which is as close as possible to the heat flux direction. Dendritic growth orientations for cubic metals, bct Sn, and hcp Zn, can be controlled by thermal conductivity, Young’s modulus, and surface energy. The control factors have been elaborated. Since the dendrite is a single crystal, its properties such as thermal conductivity that influences the heat flux direction, the minimum Young’s modulus direction that influences the strain energy minimization, and the minimum surface energy plane that influences the crystal/liquid interface energy minimization have been proved to control the dendritic growth direction. The dendritic growth directions of cubic metals are determined by the minimum Young’s modulus direction and/or axis direction of symmetry of the minimum crystal surface energy plane. The dendritic growth direction of bct Sn is determined by its maximum thermal conductivity direction and the minimum surface energy plane normal direction. The primary dendritic growth direction of hcp Zn is determined by its maximum thermal conductivity direction and the minimum surface energy plane normal direction and the secondary dendrite arm direction of hcp Zn is normal to the primary dendritic growth direction.

Scaling Relations of Lognormal Type Growth Process with an Extremal Principle of Entropy

The scale, inflexion point and maximum point are important scaling parameters for studying growth phenomena with a size following the lognormal function. The width of the size function and its entropy depend on the scale parameter (or the standard deviation) and measure the relative importance of production and dissipation involved in the growth process. The Shannon entropy increases monotonically with the scale parameter, but the slope has a minimum at p 6/6. This value has been used previously to study spreading of spray and epidemical cases. In this paper, this approach of minimizing this entropy slope is discussed in a broader sense and applied to obtain the relationship between the inflexion point and maximum point. It is shown that this relationship is determined by the base of natural logarithm e ' 2.718 and exhibits some geometrical similarity to the minimal surface energy principle. The known data from a number of problems, including the swirling rate of the bathtub vortex, more data of droplet splashing, population growth, distribution of strokes in Chinese language characters and velocity profile of a turbulent jet, are used to assess to what extent the approach of minimizing the entropy slope can be regarded as useful.

Role of Carbon Monoxide in Catalyst Reconstruction for CO Hydrogenation: First-Principles Study of the Composition, Structure, and Stability of Cu/Co(101̅2) as a Function of CO Pressure

CoCu-based catalysts are promising candidates for the large-scale application of CO hydrogenation to higher alcohols with varying hydrocarbon chain length. To mimic the Co@Cu core–shell structure of nanosized CoCu particles, we choose a Cu/Co(1012)-oriented slab and find, in agreement with chemical imaging results, that the slab surface is always Cu-terminated, with Co underneath. Using DFT calculations, we observe major surface atom exchange in the presence of adsorbed CO, with up to 50% of the Cu atoms being replaced by Co in the straight-chain steps of the slab surface. Co atom exchange beyond 50% is not observed. More specifically, this work is accomplished by scanning the configurational space of adsorbed CO, surface Co, and surface Cu and then identifying minimum-energy surface configurations. Phase diagrams are also constructed to determine the thermodynamic driving force imposed in the presence of adsorbed CO. The inclusion of surface phonon modes is shown to ensure the correctness of the calcula...

Tunable superapolar Lotus-to-Rose hierarchical nanosurfaces via vertical carbon nanotubes driven electrohydrodynamic lithography.

The development of a robust, cost-effective, scalable and simple technique that enables the design and construction of well-controlled large area superhydrophobic surface structures which can be easily tuned from lotus-leaf to rose-petal state is essential to enable progress in realising the full applied potential of such surfaces. In this study, we introduce the tuneable carbon nanotubes-based electrohydrodynamic lithography (CNT-EHL) to fabricate unique multiscale structured cones and nanohair-like architectures with various periodicities and dimensions, successfully enabling surface energy minimization. The possibility of contact-less lithography via the CNT-EHL morphology replication combined with the electric field coupling to smaller self-assembled patterns within the film, provides a way for hierarchical structure control spanning many length scales along with tuneable wetting capabilities. By controlling the hierarchy of micro- to nano cones and spikes, these morphologies provide a range of architectures with sufficient roughness for very low wettability, with the highest contact angle achieved of 173° and their properties can be easily switched between lotus-leaf to rose-petal behaviour.

Fabrication of Ag nanostructures by the systematic control of annealing temperature and duration on GaN (0001) via the solid state dewetting

The gradual evolution of Ag nanostructures in terms of the configuration, size, and density of via the solid state dewetting is demonstrated on GaN (0001) by the systematic control of annealing temperature and duration with various Ag film thickness. Upon the annealing between 150 and 700 °C with the 20 nm Ag film, the hillocks, voids, irregular, and round dome shaped Ag nanoparticles (NPs) are fabricated based on various growth mechanisms: i.e., thermal diffusion and sublimation as well as the surface energy minimization. On the other hand, along with the systematic control of annealing duration between 0 and 3600 s with 10 nm Ag film 700 °C, the Ag NPs show gradually decreased size and density based on the concurrent influence of the Ostwald's ripening and sublimation of Ag atoms. The Raman, photoluminescence, and reflectance spectroscopy of the resulting Ag NPs are characterized in accordance with the various morphology evolution of Ag nanostructures.

Ag Nanostructures on GaN (0001): Morphology Evolution Controlled by the Solid State Dewetting of Thin Films and Corresponding Optical Properties

Silver (Ag) nanostructures have demonstrated the feasibility of being utilized in various optoelectronic, catalytic, biomedical, and sensor devices due to their excellent surface plasmon resonance characteristics. The geometrical structure, spacing, and spatial arrangement of nanostructures are crucial for controlling the properties and device performance. Herein, we demonstrate the fabrication of various configurations of self-assembled Ag nanostructures on GaN (0001) by the systematic control of deposition thickness and annealing duration. The surface morphology evolution is thoroughly investigated, and the corresponding influence on optical properties is probed. The evolution of Ag nanostructures in response to thermal annealing is described based on the dewetting of thin films, Volmer–Weber growth model, coalescence growth, and surface energy minimization mechanism. For the deposition amount variation between 1 and 100 nm, the Ag nanostructures show gradual morphological transitions such as small nano...

Atomic-Scale Mechanism on Nucleation and Growth of Mo2C Nanoparticles Revealed by in Situ Transmission Electron Microscopy.

With a similar electronic structure as that of platinum, molybdenum carbide (Mo2C) holds significant potential as a high performance catalyst across many chemical reactions. Empirically, the precise control of particle size, shape, and surface nature during synthesis largely determines the catalytic performance of nanoparticles, giving rise to the need of clarifying the underlying growth characteristics in the nucleation and growth of Mo2C. However, the high-temperature annealing involved during the growth of carbides makes it difficult to directly observe and understand the nucleation and growth processes. Here, we report on the use of advanced in situ transmission electron microscopy with atomic resolution to reveal a three-stage mechanism during the growth of Mo2C nanoparticles over a wide temperature range: initial nucleation via a mechanism consistent with spinodal decomposition, subsequent particle coalescence and monomer attachment, and final surface faceting to well-defined particles with minimum surface energy. These microscopic observations made under a heating atmosphere offer new perspectives toward the design of carbide-based catalysts, as well as the tuning of their catalytic performances.

Evolution of self-assembled Ag nanostructures on c-plane sapphire by the systematic control of annealing temperature

The morphological evolution of various Ag nanostructures on c-plane sapphire is systematically investigated through the solid-state-dewetting by the control of annealing temperature between 200 and 950 °C with the Ag films of 60 and 10 nm. Various configurations of Ag nanostructures including Ag voids, wiggly and round nanoparticles are fabricated with the 60 nm due to the annealing temperature dependent surface diffusion, surface energy minimization and Ag sublimation. As a sharp distinction, highly dense semi-spherical nanoparticles are fabricated with the 10 nm set throughout the temperature range due to a much higher dewetting degree induced by the enhanced surface diffusion with a much thinner film. The reflectance spectra shows a spectral shift along with the size variation, i.e. blue shift with the decreased size and vice versa. The reflectance and Raman spectra reveal a sharp distinction between the two sets along with the strong correlation to the nanostructure morphology.

Modeling morphology evolution and densification during solid-state sintering via kinetic Monte Carlo simulation

Microstructure control is an important subject in solid-state sintering and plays a crucial role in determining post-sintering material properties, such as strength, toughness and density, to name but a few. The preponderance of existing numerical sintering simulations model the morphology evolution and densification process driven by surface energy minimization by either dilating the particles to be sintered or using the vacancy annihilation model. Here, we develop a novel kinetic Monte Carlo model to model morphology evolution and densification during free sintering. Specifically, we derive analytically a heterogeneous densification rate of the sintering system by considering sintering stress induced mass transport. The densification of the system is achieved by modeling the sintering stress induced mass transfer via applying effective particle displacement and grain boundary migration with an efficient two-step iterative interfacial energy minimization procedure. Coarsening is also considered in the later stages of the simulations. We show that our model can accurately capture the diffusion-induced evolution of particle morphology, including neck formation and growth, as well as realistically reproduce the overall densification of the sintered material. The computationally obtained dynamic density evolution curves for both two-particle sintering and many-particle material sintering are found to be in excellent agreement with the corresponding experimental master sintering curves. Our model can be utilized to control a variety of structural and physical properties of the sintered materials, such as the pore size and final material density.

Dynamics of H2 dissociation on the close-packed (111) surface of the noblest metal: H2 + Au(111).

We have performed calculations on the dissociative chemisorption of H2 on un-reconstructed and reconstructed Au(111) with density functional theory, and dynamics calculations on this process on un-reconstructed Au(111). Due to a very late barrier for dissociation, H2 + Au(111) is a candidate H2-metal system for which the dissociative chemisorption could be considerably affected by the energy transfer to electron-hole pairs. Minimum barrier geometries and potential energy surfaces were computed for six density functionals. The functionals tested yield minimum barrier heights in the range of 1.15-1.6 eV, and barriers that are even later than found for the similar H2 + Cu(111) system. The potential energy surfaces have been used in quasi-classical trajectory calculations of the initial (v,J) state resolved reaction probability for several vibrational states v and rotational states J of H2 and D2. Our calculations may serve as predictions for state-resolved associative desorption experiments, from which initial state-resolved dissociative chemisorption probabilities can be extracted by invoking detailed balance. The vibrational efficacy ηv=0→1 reported for D2 dissociating on un-reconstructed Au(111) (about 0.9) is similar to that found in earlier quantum dynamics calculations on H2 + Ag(111), but larger than found for D2 + Cu(111). With the two functionals tested most extensively, the reactivity of H2 and D2 exhibits an almost monotonic increase with increasing rotational quantum number J. Test calculations suggest that, for chemical accuracy (1 kcal/mol), the herringbone reconstruction of Au(111) should be modeled.

Modeling adhesive forces caused by nanobubble capillary bridging

Adhesive forces between particles immersed in a poorly wetting liquid can be much higher than the predicted forces by the DLVO-theory. This could be caused by so called nanobubbles at the solid surfaces. If these bubbles are between two approaching solid surfaces the formation of gaseous bridges between them becomes possible. These capillary bridges lead to capillary forces as known from liquid bridges. In contrast to the calculation of adhesive forces cause by liquid bridges the compressibility of the gaseous bridge as a function of the pressure difference must be taken into account. The proposed model uses the minimization of the free surface energies to calculate the adhesive forces cause by gaseous bridges. By adding the ideal gas law to the set of equations the compressibility of the bridge is respected. Beside adhesive forces also the pressure inside the bridges as well as contact angles and dipping angles are calculated. The results show a direct correlation between the particles radius, the surface energy of the liquid gas interface as well as the macroscopic contact angle and the adhesive force. However the amount of molecules has no influence on the adhesive forces. Furthermore the well-known stepping behavior of the measured force distance curves shown in literature can be explained by the model.

Effect of Low Pressure on Surface Roughness and Morphological Defects of 4H-SiC Epitaxial Layers.

In this work, 4H-SiC epilayers are performed on 4° off-axis substrates under low pressure condition by horizontal hot wall chemical vapor deposition (HWCVD) with a standard chemistry of silane-propane-hydrogen, which focuses on the effects of growth pressure on morphology, basal plane dislocations (BPDs) and crystalline quality. It is found that morphological defects reduce with the decreasing of growth pressure, since the surface diffusion length of absorbed adatoms increases under low growth pressure, which suppresses the nucleation of adatoms on terraces and the formation of morphological defects. However, as the surface diffusion length increases under low growth pressure, the difference of growth velocity at steps is enhanced, which leads to the extension of the steps’ width and the formation of step-bunching. Besides variation of surface diffusion length, the phenomenon described above can be correlated with different dominate modes for the minimization of surface energy at varied growth pressure. Because of the contrary influence of increased C/Si ratio and enhanced step-flow growth on the propagation of BPDs, the dislocation densities of BPDs and threading edge dislocations (TEDs) in epilayers grown at varied pressures remain basically unchanged. The crystalline quality is almost independent of growth pressure based on high resolution X-ray diffraction (HRXRD) measurements.

Tuning the configuration of Au nanostructures: from vermiform-like, rod-like, triangular, hexagonal, to polyhedral nanostructures on c-plane GaN

The systematic control over the configuration, size, and density of Au nanostructures can directly improve or optimize the physical, chemical, and optoelectronic properties and thus the functionality in the related applications. In this work, we successfully demonstrate the systematic configurational transition of self-assembled Au nanostructures on c-plane GaN via the precise control of annealing temperature, deposition amount, and annealing duration. Depending on the control of annealing temperature, self-assembled Au vermiform-like nanostructures are fabricated and evolve into the faceted Au nanorods and Au hexagons with the minimization of overall surface energy based on the Volmer–Weber growth model. With the deposition amount control, the volume-dependent transition of Au nanostructure configurations from triangles to hexagons and then to polyhedral is clearly observed and discussed based on the combinational effects of growth kinetics and surface free energy distribution. The configurational transition from irregular Au clusters to faceted nanostructures is witnessed along with the incremental variation of annealing duration based on the Ostwald ripening.

The effect of the deposition rate on the crystallographic texture, microstructure evolution and magnetic properties in sputter deposited Ni-Mn-Ga thin films

The development of crystallographic texture (preferred orientation), structure and microstructure has been investigated and correlated with the magnetic properties of sputter-deposited Ni-Mn-Ga thin films. The overall compositions of the films were strongly dependent on the sputtering power. X-ray diffraction based crystallographic texture measurement and transmission electron microscopy have been performed to examine the preferred orientation and structure of the films. All the films exhibit strong 〈400〉 and 〈220〉 double fiber orientation with no sign of in-plane preferred orientation. The strengthening of 〈400〉 and weakening of 〈220〉 preferred orientation in the films are observed with an increase in sputtering power. The development of the texture in films is mainly governed by the minimization of surface energy and strain energy. All the films exhibit a uniform microstructure with an increase in grain size as the sputtering power increases. An analysis of the magnetic hysteresis of the films revealed that all the films are ferromagnetic in nature, irrespective of the sputtering power. However, coercivity and the saturation field are found to be dependent on the sputtering power, and lead to the crystallographic texture of the films.

Effect of meniscus on the permeability of mono-layered and multi-layered packed spheres

This study aims to numerically investigate the effect of meniscus curvature on the permeability of closely-packed spheres, which have widely used for heat pipes and vapor chambers. The shape of the meniscus curvature in the hexagonally-packed spheres is estimated based on surface energy minimization algorithm. The estimated meniscus shape is then imported into a CFD study for numerically predicting the permeability of the packed spheres. Based on the numerical results, the effect of contact angle and the number of sphere layers are investigated. The results show that the permeability is nearly independent to the contact angle when the contact angle is larger than 10°. However, the permeability significantly increases with the contact angle decreases when the contact angle is lower than 10°. The permeability quasi-linearly increases as the liquid filling ratio increases when 1<l∗<2, where l∗ is liquid filling ratio. When contact angle is 5°, the permeability is shown to slightly deviate from the linear behavior with respect to the liquid filling ratio. As the number of layers increases, the permeability significantly increases and approaches that obtained from Blake-Kozeny’s equation. As the number of layers increases, the influence of meniscus curvature on the permeability decreases whereas the bottom surface plays an important role in determining the permeability. Based on extensive numerical results, correlations for predicting the packed spheres are proposed.