Excess Surface Energy Research Articles

Efficient Self-Propelling of Small-Scale Condensed Microdrops by Closely Packed ZnO Nanoneedles.

Realizing the efficient self-propelling of small-scale condensed microdrops is very challenging but extremely important to design and develop advanced condensation heat transfer nanomaterials and devices, for example, for power generation and thermal management. Here, we present the efficient self-propelling of small-scale condensed microdrops on the surface of closely packed ZnO nanoneedles, as-synthesized by facile, rapid, and inexpensive wet chemical crystal growth followed by hydrophobic modification. Compared with flat surfaces, the nanostructured surfaces with the same low-surface-energy chemistry possess far higher time-averaged density of condensed droplets at the microscale, among which those with diameters below 10 μm occupy more than 80% of the total drop number of residual condensates. Theoretical analyses clearly reveal that this remarkable property should be ascribed to the extremely low solid-liquid adhesion of the surface nanostructure, where excess surface energy released from the coalescence of smaller condensed microdrops can be sufficient to ensure the self-propelled jumping of merged microdrops.

Unsaturated coordination and surface stresses in metal nanoparticles

This work focuses on the relationship between structure and thermodynamic properties in metal nanoparticles. It is shown that the many-body character of interactions results in a dependence of interatomic distances on coordination numbers. At the particle surface, interatomic distances contract due to the unsaturation of atomic coordination shells, giving rise to relatively intense surface stresses. These, and the excess surface energy, induce a depression of the vacancy concentration respect to the bulk phase, and the emergence of pre-melting phenomena at the particle surface.

Surface free energy of the crystal-liquid interface on the metastable extension of the melting curve

The surface free energy γ, excess surface energy e, and excess surface stress τ at the crystal-liquid interface of a Lennard-Jones system have been determined by molecular dynamics simulation. The calculations have been performed for temperatures both above and below the triple-point temperature for the region where each of the coexisting phases is metastable and is at a negative pressure. The asymptotic behavior of γ, e, and τ has been analyzed near the endpoint of the melting curve, which is a point of the contact of the metastable extension of the melting curve and the spinodal of the stretched liquid [V.G. Baidakov and S.P. Protsenko, Phys. Rev. Lett. 95, 015701 (2005)]. It has been found that γ, e, and τ at this point are finite and the excess surface entropy is zero.

Electric-Field-Enhanced Condensation on Superhydrophobic Nanostructured Surfaces

When condensed droplets coalesce on a superhydrophobic nanostructured surface, the resulting droplet can jump due to the conversion of excess surface energy into kinetic energy. This phenomenon has been shown to enhance condensation heat transfer by up to 30% compared to state-of-the-art dropwise condensing surfaces. However, after the droplets jump away from the surface, the existence of the vapor flow toward the condensing surface increases the drag on the jumping droplets, which can lead to complete droplet reversal and return to the surface. This effect limits the possible heat transfer enhancement because larger droplets form upon droplet return to the surface, which impedes heat transfer until they can be either removed by jumping again or finally shedding via gravity. By characterizing individual droplet trajectories during condensation on superhydrophobic nanostructured copper oxide (CuO) surfaces, we show that this vapor flow entrainment dominates droplet motion for droplets smaller than R ≈ 30 μm at moderate heat fluxes (q″ > 2 W/cm(2)). Subsequently, we demonstrate electric-field-enhanced condensation, whereby an externally applied electric field prevents jumping droplet return. This concept leverages our recent insight that these droplets gain a net positive charge due to charge separation of the electric double layer at the hydrophobic coating. As a result, with scalable superhydrophobic CuO surfaces, we experimentally demonstrated a 50% higher overall condensation heat transfer coefficient compared to that on a jumping-droplet surface with no applied field for low supersaturations (<1.12). This work not only shows significant condensation heat transfer enhancement but also offers avenues for improving the performance of self-cleaning and anti-icing surfaces as well as thermal diodes.

Raman spectra of Sr3(PO4)2 and Ba3(PO4)2 orthophosphates at various temperatures

The Raman spectra for Sr3(PO4)2 and Ba3(PO4)2 were investigated in the temperature range from 80 to 1623K at atmospheric pressure. An unexpected melting of each sample was observed around 1573–1583K in this study. In the temperature range from 80 to 1323K, the Raman wavenumbers of all observed bands for Sr3(PO4)2 and Ba3(PO4)2 continuously decrease with increasing temperature. A quantitative analysis on the wavenumbers of Raman bands for both samples reveals that the ν3 antisymmetric stretching vibrations show the strongest temperature dependence and the ν2 symmetric bending vibration displays the weakest temperature dependence. The effects of cations on Raman bands are discussed. The reason for the unexpected melting of both samples is mainly attributed to the significant contribution from excess surface energy and the grain-boundary energy that has apparently lowered the melting points of the small samples, i.e., Gibbs–Thomson effect.

Electrostatic charging of jumping droplets

With the broad interest in and development of superhydrophobic surfaces for self-cleaning, condensation heat transfer enhancement and anti-icing applications, more detailed insights on droplet interactions on these surfaces have emerged. Specifically, when two droplets coalesce, they can spontaneously jump away from a superhydrophobic surface due to the release of excess surface energy. Here we show that jumping droplets gain a net positive charge that causes them to repel each other mid-flight. We used electric fields to quantify the charge on the droplets and identified the mechanism for the charge accumulation, which is associated with the formation of the electric double layer at the droplet-surface interface. The observation of droplet charge accumulation provides insight into jumping droplet physics as well as processes involving charged liquid droplets. Furthermore, this work is a starting point for more advanced approaches for enhancing jumping droplet surface performance by using external electric fields to control droplet jumping.

Understanding Copper Activation and Xanthate Adsorption on Sphalerite by Time-of-Flight Secondary Ion Mass Spectrometry, X-ray Photoelectron Spectroscopy, and in Situ Scanning Electrochemical Microscopy

In situ scanning electrochemical microscopy (SECM) was applied for the first time to study the copper activation and subsequent xanthate adsorption on sphalerite. The corresponding surface compositions were analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). The probe approach curve (PAC) using SECM shows that unactivated and activated sphalerite surfaces have negative current feedback and partially positive current feedback, respectively, suggesting that CuxS is formed on the sphalerite after copper activation. The copper activation of sphalerite strongly depends on the surface heterogeneity (e.g., presence of polishing defects, chemical composition), impacting the subsequent xanthate adsorption process. The SECM, ToF-SIMS, and XPS analyses show that during the copper activation the polishing defects, which have high excess surface energy, tend to consume more copper ions, resulting in Cu-rich regions by forming CuS-like species, while Fe oxid...

Studies on properties of ionic liquids 1-alkyl-3-methylimidazolium lactate at temperatures from (288.15 to 333.15) K

Two chiral ionic liquids based on lactic acid (LAILs), 1-ethyl-3-methylimidazolium lactate ([C2mim][Lact]) and 1-pentyl-3-methylimidazolium lactate ([C5mim][Lact]), have been prepared by the neutralization method and characterized by 1H NMR and DSC. Their density, surface tension and refractive index were measured at T {(288.15 to 333.15)±0.05}K. Since the LAILs can form strong hydrogen bonds with water, small amounts of water are difficult to remove by conventional methods. In order to eliminate the effect of the impurity as water, the standard addition method (SAM) was applied to these measurements. The molar volume of [Cnmim][Lact] (n=2,5), the thermal expansion coefficient, α, the surface excess entropy, Sa, the surface excess energy Ea, parachor, P, and molar refraction, Rm, were obtained. According to the methods of both Kabo and Rebelo, the molar enthalpy of evapouration of the LAILs was estimated using experimental values for the surface tension and molar volume. At the same time, the surface tension of the LAILs may be estimated using the data of the parachor and refractive index. The estimated values of the surface tension correlate quite well with the corresponding experimental values.

Effect of temperature and composition on the density, viscosity surface tension and excess quantities of binary mixtures of 1-ethyl-3-methylimidazolium tricyanomethanide with thiophene

Thermal properties of the ionic liquid (IL) 1-ethyl-3-methylimidazolium tricyanomethanide, ([EMIM][TCM]), measured by the Differential scanning calorimetry (DSC) as glass transition temperature (Tg,1), heat capacity change at glass transition temperature (ΔCp(g),1), melting temperature (Tfus), enthalpy of melting (ΔHfus), phase transition temperature (Ttr), enthalpy of phase transition (ΔtrH) and decomposition temperature (Td) were presented. The density and viscosity for the binary mixtures containing [EMIM][TCM] and thiophene at six temperatures (298.15, 308.15, 318.15, 328.15, 338.15 and 348.15K) and ambient pressure were reported. The density and viscosity as a function of temperature were tested by an empirical second-order polynomial and by the Vogel–Fucher–Tammann equation. The density and viscosity variations with compositions were described by polynomials. Excess molar volumes were described by the Redlich–Kister polynomial expansion. Viscosity deviations were calculated and correlated by the Redlich–Kister polynomial expansions. The surface tensions of pure ionic liquid and binary mixtures of [EMIM][TCM] and thiophene were measured at atmospheric pressure at five temperatures (298.15, 308.15, 318.15, 328.15, and 338.15K). The surface tension deviations were calculated and correlated by the Redlich–Kister polynomial expansion. The temperature dependence of interfacial tension was used to evaluate the excess surface energy, the surface enthalpy, the critical temperature, parachor and speed of sound for pure ionic liquid. These measurements have been provided to complete information of the influence of temperature on physico-chemical properties for the selected IL, which was chosen as a possible new entrainer in the separation of sulfur compounds from fuels. A qualitative analysis on these quantities in terms of molecular interactions is reported. The obtained results indicate that ionic liquid interactions with alcohols are strong dependent on the special trend of packing effects and hydrogen bonding of this ionic liquid with polar solvent.

Size dependence of radiation-induced amorphization and recrystallization of synthetic nanostructured CePO4 monazite

Monazite, CePO4, is considered an important phosphate-type structure for the incorporation and disposal of actinides. Nanocrystalline monazite with particle sizes ranging from 20nm to greater than 100nm was synthesized. The displacive and ionizing effects of radiation were investigated, separately and simultaneously, for 1MeV Kr2+ and 200keV electron irradiations. In situ transmission electron microscopy observations indicated that CePO4 nanoparticles can be readily amorphized by 1MeV Kr2+ irradiation. Nanostructured monazite displays greater critical amorphization doses and lower critical temperatures than that of bulk natural monazite, suggesting enhanced amorphization tolerance. A strong size dependence on radiation-induced amorphization was observed in which the smaller-sized particles (20nm) are less resistant to amorphization compared with larger-sized particles (40nm). The excess surface energy of nanostructured materials, as suggested by the larger surface area upon the reduction of particle size, may alter the energy difference between amorphous and crystalline phases, thus affecting the radiation stability. With 200keV electron-beam irradiation, CePO4 previously amorphized by 1MeV Kr2+ experienced an ionizing-radiation-induced recrystallization. A greater recrystallization rate was observed for the smaller-sized particles. Under simultaneous electron and displacive ion irradiations, CePO4 displayed greater tolerance against amorphization, probably as a result of radiation-induced recovery of displacive damage by the ionizing radiation. The strong size dependence of displacive radiation-induced amorphization and ionizing-radiation-enhanced recrystallization processes for nanostructured CePO4 implies that an optimized size regime may exist in which nanostructured materials are more tolerant of radiation-induced amorphization.

Variability of the surface radiation budget over the United States from 1996 through 2011 from high‐quality measurements

Sixteen years of high‐quality surface radiation budget (SRB) measurements over seven U.S. stations are summarized. The network average total surface net radiation increases by +8.2 Wm−2 per decade from 1996 to 2011. A significant upward trend in downwelling shortwave (SW‐down) of +6.6 Wm−2 per decade dominates the total surface net radiation signal. This SW brightening is attributed to a decrease in cloud coverage, and aerosols have only a minor effect. Increasing downwelling longwave (LW‐down) of +1.5 Wm−2 per decade and decreasing upwelling LW (LW‐up) of −0.9 Wm−2 per decade produce a +2.3 Wm−2 per decade increase in surface net‐LW, which dwarfs the expected contribution to LW‐down from the 30 ppm increase of CO2 during the analysis period. The dramatic surface net radiation excess should have stimulated surface energy fluxes, but, oddly, the temperature trend is flat, and specific humidity decreases. The enigmatic nature of LW‐down, temperature, and moisture may be a chaotic result of their large interannual variations. Interannual variation of the El Niño/Southern Oscillation (ENSO) ONI index is shown to be moderately correlated with temperature, moisture, and LW‐down. Thus, circulations associated with ENSO events may be responsible for manipulating (e.g., by advection or convection) the excess surface energy available from the SRB increase. It is clear that continued monitoring is necessary to separate the SRB's response to long‐term climate processes from natural variability and that collocated surface energy flux measurements at the SRB stations would be beneficial.

DISPERSION PROCESS: ROLE IN THE FORMULATION OF PARTICULATE DISPERSE SYSTEM OF POORLY SOLUBLE DRUGS

As significant number of products manufactured for personal care/health care, either in the final stage or at some stage of their production, dispersion of particulate materials dispersed into liquid or solid vehicles, often at high volume fraction. The vast array of cosmetics/personal care and pharmaceutical products reveal the importance of adequate dispersion. With active pharmaceutical ingredients possessing poor aqueous solubility, optimal dispersion is necessary for maximizing the bioavailability and uniformity of dose. Fine particulate dry powders always contain agglomerates that require de-agglomeration and stabilization to obtain optimal dispersion. All particulate dispersion systems are inherently thermodynamically unstable. Unless properly stabilized, through the random motion of the particles over time, they will get aggregate due to the natural and dominant tendency to decrease the large specific surface area and excess surface energy. Adequate concentrations of wetting agents, de-agglomerating agents and stabilizers are required to produce stable particulate dispersions. The dispersion process is accomplished via three distinct steps: wetting the particulate material, de-agglomeration of the particles and stabilization of the same, and these steps should be performed in the correct order to get a stable product. Key words: de-agglomeration, stabilization, Ostwald ripening, particulate dispersion, aggregates.

Jumping-Droplet-Enhanced Condensation on Scalable Superhydrophobic Nanostructured Surfaces

When droplets coalesce on a superhydrophobic nanostructured surface, the resulting droplet can jump from the surface due to the release of excess surface energy. If designed properly, these superhydrophobic nanostructured surfaces can not only allow for easy droplet removal at micrometric length scales during condensation but also promise to enhance heat transfer performance. However, the rationale for the design of an ideal nanostructured surface as well as heat transfer experiments demonstrating the advantage of this jumping behavior are lacking. Here, we show that silanized copper oxide surfaces created via a simple fabrication method can achieve highly efficient jumping-droplet condensation heat transfer. We experimentally demonstrated a 25% higher overall heat flux and 30% higher condensation heat transfer coefficient compared to state-of-the-art hydrophobic condensing surfaces at low supersaturations (<1.12). This work not only shows significant condensation heat transfer enhancement but also promises a low cost and scalable approach to increase efficiency for applications such as atmospheric water harvesting and dehumidification. Furthermore, the results offer insights and an avenue to achieve high flux superhydrophobic condensation.

Controlling the wetting properties of the Asakura–Oosawa model and applications to spherical confinement

We demonstrate for the Asakura–Oosawa model and an extension of this model that uses continuous rather than hard potentials, how wetting properties at walls can be easily controlled. By increasing the interaction range of the repulsive wall potential acting on the colloids (while keeping the polymer–wall interactions constant) polymers begin to substitute colloids at walls and the system can be driven from complete wetting of colloids via partial wetting to complete wetting of polymers. As an application, we discuss the morphology and wetting behavior of colloid–polymer mixtures in spherical confinement. We apply the recently developed ‘ensemble switch method’ where the Hamiltonian is extended to a combination of a system with walls and of a system without walls to calculate the surface excess free energies of colloid-rich and polymer-rich phases. The contact angle then is inferred from Young’s equation.

The surface tension, density and refractive index of amino acid ionic liquids: [C3mim][Gly] and [C4mim][Gly

The amino acid ionic liquids (AAILs) [C3mim][Gly] (1-propyl-3-methylimidazolium glycine) and [C4mim][Gly] (1-butyl-3-methylimidazolium glycine) have been prepared by the neutralization method and characterized by 1H NMR spectroscopy and differential scanning calorimetry (DSC). The values of their density, surface tension and refractive index were measured at different temperatures. Since the AAILs can form strong hydrogen bonds with water, small amounts of water are difficult to remove from the AAILs by common methods. In order to eliminate the effect of the impurity as water, the standard addition method (SAM) was applied to these measurements. The values of thermal expansion coefficients, α, surface excess energy, Ea, surface excess entropy, Sa, and molar refraction, Rm, for the AAILs were determined.

Thermodynamic properties of silver nanoclusters

Icosahedral models of magic number silver clusters have been constructed using molecular dynamics simulations with the Doyama-Kogure potential. We have calculated the energy, heat capacity, entropy, Gibbs energy change, and excess surface energy as functions of cluster size and temperature. The vibrational contributions to the heat capacity and entropy are weak functions of cluster size (N), and the statistical degeneracy is lifted starting at 250–300 K. The surface energy density of the silver clusters is size-independent down to N = 13. The saturated vapor pressure over the clusters has been estimated. The calculated thermodynamic properties of fcc silver agree well with standard thermodynamic data.

Synthesis and characterization of Sn-rich Ni–Sb–Sn nanosolders

Nano-crystalline samples of pure Sn and of Sn-rich ternary Ni–Sb–Sn alloys, with compositions ranging from 80 to 97.5 at% Sn and a Ni to Sb molar ratio of 1:1, were synthesized by reduction of stoichiometric metal chloride solutions with NaBH 4 at 0 °C in alkaline medium. The particle sizes of the obtained alloys, measured by TEM/SEM, were found to be in the range of 40–350 nm. A relative decrease in melting temperature of up to 15 °C was observed for these alloys compared to a bulk sample. A sample with 95 at% Sn was subjected to heat treatment at 180 °C for various time periods up to 5.0 h to study particles with well defined average sizes between 50 and 135 nm. The melting temperatures of the annealed samples were found to increase progressively with increasing particle size. At the same time, the excess surface energy of these heat treated samples was measured as a function of particle size using a Calvet-type calorimeter. From the differences of the ( H 573– H 299) values between nano and bulk samples, the excess enthalpies for nano-sized samples were derived to be between 18.8 ± 1.9 and 0.8 ± 1.4 kJ g −1 for particle sizes between 50 and 135 nm.

Treatment of Waste Cutting Oil Emulsions by Leaching Solutions of White Mud

A leaching solution of white mud was prepared from this waste material of soda production, and used for treatment of waste cutting oil emulsion. In alkaline conditions, the leaching solution of white mud generates hydroxide precipitates, which have relatively high specific surface area and excess surface energy, and readily adsorb the pollutants in waste cutting oil emulsion. The chemical composition of the white mud was determined and the hydroxide precipitates were characterized. The maximum removal efficiency (of turbidity, oil contents and TOC) was obtained at an adsorbent dosage of 4.0 g/L, pH 12.0 and 25°C. Leaching solutions of white mud could be used as an effective and low-cost material for treatment of waste cutting oil emulsion by the precipitation method.

Removal of anionic dyes from aqueous solution by leaching solutions of white mud

A leaching solution of white mud was prepared from this waste material of soda production, and used in the removal of the model anionic dyes reactive bright red K-2BP, reactive light yellow K-6G, acid orange II and direct yellow R. In alkaline conditions, the leaching solution of white mud generates hydroxide precipitates, which have relatively high specific surface area and excess surface energy, and readily adsorb the dyestuffs. In this study, the chemical composition of the white mud was determined by X-ray fluorescence and the hydroxide precipitates were characterized by dynamic light scattering, transmission electron microscopy and Fourier-transform infrared spectroscopy. The maximum adsorption capacity was observed at an adsorbent dosage of 2.0 g/L, pH 12.0 and 293 K, which for reactive bright red K-2BP, reactive light yellow K-6G, acid orange II and direct yellow R were 416.7, 454.5, 769.2 and 2186.4 mg/g, respectively. The adsorption was consistent with the Langmuir and Freundlich isotherms and was exothermic. Leaching data showed that the treated water was nontoxic. It is especially suitable for the treatment of alkaline dye effluents. The leaching solution also displayed significant removal of color and COD for the textile effluent from a printing and dyeing factory.

Tensile Stress Evolution During the Early-Stage Constrained Sintering of Gadolinium-Doped Ceria Films

In situ measurements during the constrained sintering of Gd-doped ceria reveal tensile stresses up to ∼250 MPa. These large tensile stresses are likely to contribute to the reduced densification (compared with freely sintered material) typically observed during constrained sintering. While existing models postulate that the tensile stress in a densifying constrained film cannot exceed the “sintering stress,”σS, the observed tensile stresses are significantly larger than the estimated σS for these materials. To explain this observation, we propose that the formation and extension of interparticle grain boundaries induce substantial tensile stresses in constrained films. A model of this phenomenon shows that converting excess surface energy to elastic strain energy can produce stresses that are comparable to the measured values. Further, if these “cohesive” stresses exceed σS, grain-boundary diffusion should initially move material from the neck regions into the grain boundaries, not out of the grain boundaries as described by traditional sintering models.