Surface Energy Minimization Research Articles

Swelling and Collapse of Cylindrical Polyelectrolyte Microgels.

In this study, we propose computer simulations of charged cylindrical microgels. The effects of cross-linking density, aspect ratio, and fraction of charged groups on the microgel swelling and collapse with a variation in the solvent quality were studied. The results were compared with those obtained for equivalent neutral cylindrical microgels. The study demonstrated that microgels' degree of swelling strongly depends on the fraction of charged groups. Polyelectrolyte microgels under adequate solvent conditions are characterized by a larger length and thickness than their neutral analogues: the higher the fraction of charged groups, the longer their length and greater their thickness. Microgels' collapse upon solvent quality decline is characterized by a decrease in length and non-monotonous behavior of its thickness. First, the thickness decreases due to the attraction of monomer units (beads) upon collapse. The further thickness increase is related to the surface tension, which tends to reduce the anisotropy of collapsed objects (the minimum surface energy is known to be achieved for the spherical objects). This reduction is opposed by the network elasticity. The microgels with a low cross-linking density and/or a low enough aspect ratio reveal a cylinder-to-sphere collapse. Otherwise, the cylindrical shape is preserved in the course of the collapse. Aspect ratio as a function of the solvent quality (interaction parameter) demonstrates the maximum, which is solely due to the electrostatics. Finally, we plotted radial concentration profiles for network segments, their charged groups, and counterions.

Experimental investigation on microstructure, mechanical and machining properties of Al‐4032/granite marble powder (GMP) composite produced through stir casting

AbstractThe aluminium‐based composites (AMCs) are quite popular in several areas such as building, aerospace, automotive, marine, and aeronautical applications. Production industries deeply emphasize improvement in productivity and saving in cost. The composites fulfil these goals because of reinforcement materials. However, to bring in the composite to usage, it generally demands certain machining operation. Al‐4032 based 6 % (by weight) of granite marble powder (GMP) composite has been developed using stir casting set‐up. The fabricated composite has been examined for its machinability in milling operation. Cutting speed, feed rate, and depth of cut are the cutting parameters selected for the machining trials. The response surface methodology‐based desirability approach has been employed to obtain the best parameters for achieving the desired objectives. Experimental results demonstrate that the machined composite is considerably influenced by built‐up edge formation and interfacial bonding of particles. Minimum surface roughness (1.57 μm) and energy consumption (0.45 kWh) have been achieved at cutting speed 200 m/min, feed rate 197 mm/min, and depth of cut 1.5 mm.

A novel bifunctional protein PNU7 in CaCO3 polymorph formation: Vaterite stabilization and surface energy minimization

CaCO3, which occurs in three crystalline anhydrous polymorphs named calcite, aragonite, and vaterite, is always found in mineralized skeletons or growing shells of many marine organisms. However, understanding how these organisms achieve this control has been a significant challenge in biomineralization. In this work, we proposed a novel vaterite stabilizer acidic matrix protein PNU7 that existed in both prism and nacre of Pinctada fucata, and identified its functional domain DDDDDDHDDVEETED. Our experiments reveal that PNU7 triggers a stable large vaterite formation with Mg2+ deficiency even under low Ca2+. Increasing PNU7 in the calcium carbonate crystallization system with Mg2+ leads to a significant shrinking in crystal size and rising in nucleation quantity. Moreover, it converts an atomically rough dome-like shape to a smooth sphere on unsiliconized glass. These effects rescind after removing the asp-rich region at the C-terminus. We also find that decreasing pnu7 mRNA in vivo leads to nacreous inner surface growth substantially lessened. Thus, PNU7 may not only supply vaterite in shell formation but also involve the nacreous regulation via surface energy minimization.

Rapid Geometric Screening of Low-Energy Surfaces in Crystals

A high-throughput approach to determine the equilibrium shape of a crystal by brute force is impractical due to the vast number of density functional theory (DFT) calculations required along just a single crystallographic direction. We propose a screening method that allows the bypass of performing DFT calculations for each candidate surface. Using a series of physically-motivated simplifications, we are able to consider the relative surface energy of each of the large number of candidate surfaces required to solve the surface energy minimization problem in 3 dimensions. Application of this technique to calculate the surface energy landscapes of a set of well-known crystal structures demonstrates high accuracy in the prediction of stable planes and validates its potential as a valuable tool in ab initio determination of equilibrium crystal shapes.

Tailoring intrinsic hydrophobicity and surface energy on rough surface via low‐T Cassie–Wenzel wetting transition method

AbstractWettability is an important parameter of micro/nanostructured composites. The measurement of apparent contact angle is strongly affected by surface roughness, which induces some challenges to study the intrinsic hydrophobicity correlating to the nature of chemistry. Carbon‐Nafion composites exhibited about 30° decrease in apparent contact angle from 30 to 10°C due to the condensation of water vapor into cavities, suggesting a significant Cassie–Wenzel wetting transition phenomenon. The focus of this work has been on the first‐time use of a low‐T Cassie–Wenzel wetting transition method to evaluate Young's (ideal) contact angle and surface free energy. A maximum Young's contact angle (113°) and minimum total surface energy (12 mJ/m2) were determined at Nafion content of 70 wt%, indicating the orientation effect that sulfonate groups in Nafion preferentially pointed toward polar carbon. This approach provided the reasonable prediction of intrinsic hydrophobicity, especially when a rough solid surface is not easily wetted by liquids.

Yield stress of foam flow in porous media: The effect of bubble trapping

Foam behaves as a yield-stress fluid as it flows in a porous medium. Quasi-static analysis suggests that the yield stress arises from the non-smooth motion of foam films, denoted as lamellae, in pores. In order to study the effect of trapped lamellae on the motion of a moving lamella and consequently on the yield stress of foam, we conduct numerical simulations in the quasi-static limit. We propose a new method utilizing the surface energy minimization algorithm, which explicitly considers the connectivity of pores in a porous medium. We consider two different shapes of pore and vary the number of nearby trapped lamellae to investigate the effects of bubble trapping on the non-smooth and the smooth motion of a single lamella passing through a pore, respectively. We find that the trapped lamellae lead to the increased volume-averaged pressure drop and thus the increased yield stress. Notably, the motion of a lamella through a pore with rounded corners in the pore body becomes non-smooth, due to the presence of trapped lamellae. The results contribute to a better understanding of the yield stress of foam in porous media.

Confinement-Induced Fabrication of Liquid Crystalline Polymeric Fibers.

In aqueous media, liquid crystalline droplets typically form spherical shapes in order to minimize surface energy. Recently, non-spherical geometry has been reported using molecular self-assembly of surfactant-stabilized liquid crystalline oligomers, resulting in branched and randomly oriented filamentous networks. In this study, we report a polymerization of liquid crystalline polymeric fibers within a micro-mold. When liquid crystal oligomers are polymerized in freely suspended aqueous media, curvilinear and randomly networked filaments are obtained. When reactive liquid crystalline monomers are oligomerized in a micro-channel, however, highly aligned linear fibers are polymerized. Within a top-down microfabricated mold, a bottom-up molecular assembly was successfully achieved in a controlled manner by micro-confinement, suggesting a unique opportunity for the programming architecture of materials via a hybrid approach.

WC-VC/Cr3C2 composite powders prepared by a carbothermic reduction-carburization process

Ultrafine WC-VC/Cr3C2 composite powders with uniform distribution of V/Cr were synthetized by a novel method composed of carbothermic reduction of the mixture of WO3 and V2O5/Cr2O3 at 1050 °C and the following carburization process at 1300 °C under hydrogen atmosphere. The effects of addition amount of VC or Cr3C2 on the phase constitution, particle size and micromorphology of WC-VC/Cr3C2 composite powders were investigated. The results revealed that W, C, V/Cr elements were evenly distributed in the WC-VC/Cr3C2 composite powders. Moreover, the particle size of WC-0.5wt.%VC composite powder was obviously smaller than that of pure WC powder, but decreased slightly with the further increase of VC content. Furthermore, it was found that the inhibition result of Cr3C2 on WC grain growth was significantly worse than that of VC. In terms of micromorphology, pure WC and WC-VC powder had the round shaped WC particles originated from the isotropic growth, while there were triangular prism shaped WC particles because of the faceting of WC particles caused by the minimization of surface energy in WC-Cr3C2 composite powders. Moreover, there were also a few WC particles with the layer-by-layer microstructure owing to the anisotropic growth under the action of Cr3C2.

Strong Metal–Support Interaction in Heterogeneous Catalysts

AbstractStrong metal–support interaction (SMSI) in supported metal catalysts, typically accompanied by the formation of encapsulation layers over metal nanoparticles, has drawn intense research attention owing to a variety of intriguing behaviors. In particular, recent years have witnessed enormous progress in constructing SMSI between novel components as well as in understanding the nature of SMSI. Notably, SMSI also provides substantial possibilities for designing and modulating advanced heterogeneous catalysts toward a wide range of applications. Therefore, a systematic and critical overview is highly desirable to highlight the recent advances in SMSI and to discuss its applications in heterogeneous catalysts. In this review, the formation process of SMSI is described based on the typical material systems, and then the surface energy minimization mechanism is discussed by fully taking account of new material systems as well as novel construction strategies. Moreover, the principles of using SMSI to control the activity, selectivity, and stability of supported metal catalysts are demonstrated with an emphasis on thermocatalysis and electrocatalysis. To conclude, personal perspectives on the opportunities and challenges for SMSI are provided.

High-resolution CH stretch spectroscopy of jet-cooled cyclopentyl radical: First insights into equilibrium structure, out-of-plane puckering, and IVR dynamics.

First, high-resolution sub-Doppler infrared spectroscopic results for cyclopentyl radical (C5H9) are reported on the α-CH stretch fundamental with suppression of spectral congestion achieved by adiabatic cooling to Trot ≈ 19(4) K in a slit jet expansion. Surprisingly, cyclopentyl radical exhibits a rotationally assignable infrared spectrum, despite 3N - 6 = 36 vibrational modes and an upper vibrational state density (ρ ≈ 40-90 #/cm-1) in the critical regime (ρ ≈ 100 #/cm-1) necessary for onset of intramolecular vibrational relaxation (IVR) dynamics. Such high-resolution data for cyclopentyl radical permit detailed fits to a rigid-rotor asymmetric top Hamiltonian, initial structural information for ground and vibrationally excited states, and opportunities for detailed comparison with theoretical predictions. Specifically, high level ab initio calculations at the coupled-cluster singles, doubles, and perturbative triples (CCSD(T))/ANO0, 1 level are used to calculate an out-of-plane bending potential, which reveals a C2 symmetry double minimum 1D energy surface over a C2v transition state. The inversion barrier [Vbarrier ≈ 3.7(1) kcal/mol] is much larger than the effective moment of inertia for out-of-plane bending, resulting in localization of the cyclopentyl wavefunction near its C2 symmetry equilibrium geometry and tunneling splittings for the ground state too small (<1MHz) to be resolved under sub-Doppler slit jet conditions. The persistence of fully resolved high-resolution infrared spectroscopy for such large cyclic polyatomic radicals at high vibrational state densities suggests a "deceleration" of IVR for a cycloalkane ring topology, much as low frequency torsion/methyl rotation degrees of freedom have demonstrated a corresponding "acceleration" of IVR processes in linear hydrocarbons.

Energy minimizing twinning with variable volume fraction, for two nonlinear elastic phases with a single rank-one connection

In materials that undergo martensitic phase transformation, distinct elastic phases often form layered microstructures — a phenomenon known as twinning. In some settings the volume fractions of the phases vary macroscopically; this has been seen, in particular, in experiments involving the bending of a bar. We study a two-dimensional (2D) model problem of this type, involving two geometrically nonlinear phases with a single rank-one connection. We adopt a variational perspective, focusing on the minimization of elastic plus surface energy. To get started, we show that twinning with variable volume fraction must occur when bending is imposed by a Dirichlet-type boundary condition. We then turn to paper’s main goal, which is to determine how the minimum energy scales with respect to the surface energy density and the transformation strain. Our analysis combines ansatz-based upper bounds with ansatz-free lower bounds. For the upper bounds we consider two very different candidates for the microstructure: one that involves self-similar refinement of its length scale near the boundary, and another based on piecewise-linear approximation with a single length scale. Our lower bounds adapt methods previously introduced by Chan and Conti to address a problem involving twinning with constant volume fraction. The energy minimization problem considered in this paper is not intended to model twinning with variable volume fraction involving two martensite variants; rather, it provides a convenient starting point for the development of a mathematical toolkit for the study of twinning with variable volume fraction.

Optimization of Surface Quality and Power Consumption in Machining Hardened AISI 4340 Steel

Hard turning has become an attractive method of machining for most manufacturers in the last few years due to its low cost and superior surface quality compared to grinding. In this experimental study, the machinability of hardened steel under dry machining on a CNC lathe is undertaken to optimize the cutting parameters for minimum surface roughness and energy consumption with the cutting speed (320, 450, and 575), tool type (coated and uncoated), and feed rate (0.1, 0.18, and 0.26) as the input parameters. The Taguchi method, based on the L18 orthogonal array, the variance analysis, the signal-to-noise ratios, and the response surface methodology have been used to optimize surface roughness (Ra) and cutting power (Cp). Optimum cutting parameters and levels were determined, and the relationship between cutting parameters and output variables was analyzed with the aid of two-dimensional and three-dimensional graphics. The results show that the most effective parameter on the surface roughness was the tool type (78%), while the most effective parameter on energy consumption was the cutting speed (90%). The combination of low feed rate and high cutting speed is necessary for minimizing the surface roughness. Besides, the impact of two-factor interactions of the feed rate-cutting speed and depth of cut-cutting speed appears to be substantial. The linear regression models were validated using confirmation tests. Finally, regression coefficients were determined as a mathematical model, and it was observed that this estimated model yielded results that were very similar to those achieved via real experiment (correlation values: 97.64% for surface roughness and 98.72% for energy consumption).

Significant slowdown of plasma-optimized surface energy deactivation by vacuum sealing for efficient adhesive bonding

This work proposes an approach to minimize surface energy deactivation of plasma-treated metal and carbon fiber-reinforced polymer (CFRP) surfaces by vacuum sealing. Plasma treatments enhance adhesive wettability on post-treated surfaces for adhesive joints, but the treated surfaces deactivate quickly in air. The surface energy of aluminum alloy AA6061 and carbon fiber-reinforced polymer-polyamide (CFRP-PA66) optimally treated by a blown air plasma instrument returns to the original surface energy within 1 h. Vacuum sealing of AA6061 and CFRP-PA66 reduced the surface energy deactivation over 7 days by at least 230 times and 970 times compared to in air. Double Cantilever Beam (DCB) tests performed on adhesively-bonded AA6061/CFRP-PA66 joints showed that the total energy release and energy dissipation before failure of plasma-treated and vacuum-sealed materials was up to 60% more than plasma-treated materials without vacuum sealing and up to 125% more than non-plasma-treated materials.

Microscopic modelling of gas diffusivity in unsaturated cementitious materials considering multiple diffusion regimes

Common degradations of concrete including reinforcement corrosion and carbonation are highly associated with the transport of aggressive gases. However, it is still challenging to accurately understand the gas diffusion behaviour and determine the gas diffusivity in unsaturated cementitious materials. This study proposes a pore-scale model for simulating gas diffusivity in unsaturated cement paste covering multiple gas diffusion mechanisms. The representative volume elements of colloidal calcium silicate hydrate and heterogeneous cement paste with various water saturation degrees are established, capturing the structural attributes of hierarchical cement paste and water-gas distribution with minimum surface energy. A finite difference model for diffusion considering Knudsen effect and molecular diffusion is developed to simulate the diffusion process of various gases through partially saturated cement pastes with different pore structures. Results indicate that gas diffusivity decreases with the increasing water saturation level, the evolution of which can be divided into stable, decreasing and non-diffusive stages. The gas diffusion in partially saturated cement paste is highly dependent on the pore structure and diffusive gas type. The results obtained from this study are expected to provide a deeper insight into the solution of diffusion-related durability problems and the prediction of service life of concrete.

The Tendency of Nature towards Hexagon Shape Formation due to Minimizing Surface Energy

Nature, for instance, bubble and honeycomb, tend to form a hexagon shape naturally. The array of bubbles and honeycomb is formed by merging and sharing the common wall with the adjacent unit. Even though each of the unit shapes size may vary, the noticeable elements that built up the array are hexagons. There are many regular and irregular shapes available in nature, but the shape formation still leads to hexagon at the end of the shape evolving due to surface tension. Based on the phenomenon, this study was carried out to investigate the effect of surface tension, energy, and geometry features, which affect the tendency of hexagon formation. The study was carried out by comparing hexagon with triangle, trapezium, and square. From the result, it is found that the reduction of surface energy ranged from 10-23 percent from the initial shape. As expected, the hexagon shape is packed with the lowest surface parameter and very stable in single unit or array form by showing the lowest energy reduction. The energy content is a reflection to structure equilibrium and its stability for nature tendency.

Wetting States and Departure Diameters of Bubbles on Micro-/Nanostructured Surfaces.

Wetting states for droplets have been extensively investigated in the past. As the counter phase of the droplets, bubbles' wetting states have rarely been systematically explored. The wetting state of a bubble is closely related to its departure diameter, which plays significant roles in bubble-generated processes in boiling heat transfer and gas-evolving reactions. Based on the principle of minimum surface energy, we explicitly define three equilibrium wetting states (hemi-wicking state, Wenzel state, and Cassie-Baxter state) for bubbles on micro-/nanostructured surfaces in this paper. We analyze the three-phase contact line profiles for bubbles under these wetting states and propose theoretical models for predicting departure diameters of hemi-wicking-state bubble and Wenzel-state bubble on micro-/nanostructured surfaces. We identify competing effects of bubble departure in Wenzel state: the augmentation of contact line length due to the roughness, which would delay bubble departure, and the decrease of contact line length due to the reduced apparent contact angle, which would facilitate bubble departure. We demonstrate that hemi-wicking-state bubble exhibits a much smaller departure diameter on the textured surfaces. These findings are supported by numerical simulations by the three-dimensional (3D) multiple-relaxation-time lattice Boltzmann method. It is found that the length of the outermost contact lines instead of all contact lines determines the departure diameter of hemi-wicking-state bubble based on bubble detachment processes captured by our 3D numerical simulations. This work offers an avenue for the accurate prediction and control of bubble departure behaviors from micro-/nanostructured surfaces, and therefore can guide optimal designs of micro-/nanostructured surfaces in a variety of applications in boiling, desalination, and hydrogen production by electrolysis.

Effect of Surface Nanostructures and Speciation on Undercooling for Low-Temperature Solder Alloys

Role of surface structure/composition in altering solidification behavior is demonstrated through undercooling of core–shell metal particles. Autonomous surface speciation in the passivating oxide plays a critical role in the relaxation of a molten metal due to divergence in the concentration and composition of oxidizing species across the thickness of the oxide layer. In an unreactive environment, surface speciation is dictated by flux, cohesive energy density, and surface energy minimization. Under oxidizing conditions (e.g., ambient), however, reduction potential, curvature, and surface plasticity dictate the spatial order and concentration(s) across thin passivating oxide layers. It is therefore important to redefine solubility beyond the limitations of Hume–Rothery rules by substituting electronegativity for redox potential and cohesive energy density. Increasing number of components in an alloy does not necessarily lead to increased undercooling, but a maximum is observed around two to three components. These results lead to an empirical observation that ΔGLS (Gibbs’ free energy for liquid–solid transition) around the freezing point can be understood from enthalpy and surface tension balance, with the degree of undercooling being a proportionality term for the enthalpic component. This work extends our understanding of classical nucleation theory by illustrating the importance of asymmetry in surface work in phase change, largely due to changes in structure of nanoscale passivating oxides. This phenomena is critical in development of low-temperature solders.

Shape stabilization and laser triggered shape transformation of magnetic particle functionalized liquid metal motors

Liquid metal motors made from biologically benign gallium are promising candidates for various applications ranging from drug delivery to targeting and killing cancer cells directly. One of the main problems with this novel technology is the need to utilize a membrane, making it possible to maintain a defined shape in order to perform the required functions. For magnetic remote guidance, liquid metal motors can be doped with magnetic iron microparticles, forming a transition magnetic liquid. In an alternative approach liquid metal structures are coated with magnetite nanoparticles. We hereby present an approach to laminate biologically benign gallium-based magnetic liquid metal motors with a biodegradable and biocompatible macromolecular thin film to retain the initial shape. Thanks to the polymer lamination and by the help of magnetic fields, the presented liquid metal motors can be remotely guided. The shape retaining macromolecular thin film can be liquefied by photothermal effects such as laser irradiation in order to change the shape of the liquid metal motor into a droplet due to surface energy minimization, allowing for penetration of structures smaller than the initial motor size. This work uses a relatively large technical demonstrator to show the technical realization and properties of this novel system, which opens up new paths and potential applications.

Non-equilibrium epitaxy of metastable polymorphs of ultrawide-bandgap gallium oxide

Precision synthesis of ultrawide-bandgap semiconductors with a desired crystal phase is of broad interest for developing advanced electronic devices. However, it is highly challenging for gallium oxide (Ga2O3), which is known for versatile crystal phase transition. Here, we report a non-equilibrium epitaxy strategy to confine the crystallization pathways of Ga2O3 toward two distinct metastable polymorphs during the pulsed laser deposition (PLD) growth. This is achieved by synergic control of the substrate orientation and intentional tin (Sn) doping, which dramatically modifies the nucleation and growth kinetics of Ga2O3. Using a-plane sapphires and a medium Sn doping level, we overcome the commonly observed growth limitations of α-phase Ga2O3 (α-Ga2O3) films that are only stable for the initial few monolayers in previous PLD studies. Instead, we stabilize epitaxial α-Ga2O3 films with excellent phase uniformity and crystallinity for a thickness beyond 200 nm. This contrasts to the otherwise formed ε-phase Ga2O3 films by simply switching the sapphire substrate orientation to c-plane regardless of the Sn doping level. Density functional theory calculations reveal the critical role of the surface energy minimization for selective stabilization of metastable phases. This study provides a perspective to improve the non-equilibrium synthesis capability for exploring emerging ultrawide-bandgap semiconductors.

Phase-field modeling of the morphological evolution of ringlike structures during growth: Thermodynamics, kinetics, and template effects

Understanding and controlling the morphology of crystals growing from prepatterned shapes, as in selective-area epitaxy, is fundamental for the development of novel device architectures. Here, an in-depth analysis of the faceted growth of ring-shaped crystal structures is presented. This is a prototypical case for assessing the role of thermodynamic and kinetic driving forces, as they induce different faceting between the outer convex region and the inner concave one. The well-established concepts of equilibrium crystal shape, set by surface energy minimization, and its kinetic counterpart, determined by orientation-dependent growth rates, provide a qualitative indication of the outcome of growth experiments. However, they are insufficient to capture the whole evolution pathway. A phase-field growth model including deposition, incorporation, and surface diffusion dynamics is considered, and extensive two- and three-dimensional simulations are performed. A continuum transition in the crystal morphology is recognized by varying the magnitude of the anisotropic surface energy vs the incorporation times. The in-depth analysis of a realistic case, mimicking experiments in the literature, highlights the prominent role of kinetics and of material redistribution from the outer to the inner perimeter of the ring. Finally, the templating effect of the initial pattern is investigated by considering polygonal profiles with different orientations, showing their effect on the facet formation and evolution.