Hamaker Constant Research Articles

Surface thermodynamics and extended DLVO theory of Leptospirillum ferrooxidans cells’ adhesion on sulfide minerals

The adhesion of Leptospirillum ferrooxidans bacterial cells onto the sulfide minerals pyrite and chalcopyrite was evaluated using two different physical-chemical approaches; thermodynamic and extended DLVO theory. For the parameters incorporated into calculations, the zeta potentials and contact angles of powdered solids and bacterial cells were acquired experimentally. The Hamaker constants essential for Lifshitz-van der Waals interaction calculations were calculated following two different methods: macroscopic and microscopic. Adsorption tests were carried out at physiologic conditions to estimate the amount of cells adsorbed onto a mineral surface and the extent of alteration of that mineral surface in biobeneficiation. The free energy of adhesion was found to be negative for both minerals, indicating that the adhesion is energetically favored and preferred. The interaction energy diagrams of the total interacting force was also negative for the cases where the particles were charged oppositely; in the remaining cases, the total force was attractive after overcoming an energetic barrier caused by the repulsive electrostatic forces. Under the conditions of the adsorption test, the experimental results are in agreement with the theoretical; this suggests that the physical-chemical forces are crucial for bacterial adhesion.

Settling of particles in liquids: Effects of material properties

AbstractThis article presents a numerical study on the settling of uniform spheres in liquids by means of the discrete element method. The effects of particle and liquid properties, such as particle size, Hamaker constant, liquid density, and viscosity, on the formation of packed beds or cakes were studied in terms of packing fraction, radial distribution function (RDF), and coordination number (CN). The results showed that the packing fraction of a cake increases with increasing particle size but decreases with increasing the Hamaker constant, liquid density, and viscosity. RDF and CN also change correspondingly: packings with lower packing fraction generally have RDFs with fewer peaks and smaller mean CNs. A good correlation between packing fraction and other structural properties was identified. The analysis of the particle‐particle and liquid‐particle interactions showed that the packing properties are mainly affected by the ratio of the interparticle cohesion to the effective gravity of particles. The previously proposed equation linking packing fraction with the interparticle forces has been extended to incorporate the impact‐induced pressure force in a settling process. Based on the modified equation, the effects of key variables on the relationship between packing fraction and particle size were re‐examined for general application. © 2011 American Institute of Chemical Engineers AIChE J, 2012.

Hamaker Constants of Iron Oxide Nanoparticles

The Hamaker constants for iron oxide nanoparticles in various media have been calculated using Lifshitz theory. Expressions for the dielectric responses of three iron oxide phases (magnetite, maghemite, and hematite) were derived from recently published optical data. The nonretarded Hamaker constants for the iron oxide nanoparticles interacting across water, A(1w1) = 33 - 39 zJ, correlate relatively well with previous reports, whereas the calculated values in nonpolar solvents (hexane and toluene), A(131) = 9 - 29 zJ, are much lower than the previous estimates, particularly for magnetite. The magnitude of van der Waals interactions varies significantly between the studied phases (magnetite < maghemite < hematite), which highlights the importance of a thorough characterization of the particles. The contribution of magnetic dispersion interactions for particle sizes in the superparamagnetic regime was found to be negligible. Previous conjectures related to colloidal stability and self-assembly have been revisited on the basis of the new Lifshitz values of the Hamaker constants.

Interfacial thin films rupture and self-similarity

Two superposed thin layers of fluids are prone to interfacial instabilities due to London-van der Waals forces. Evolution equations for the film thicknesses are derived using lubrication theory. Using the intrinsic scales, for a single layer, results in a system with parametric dependence of four ratios of the two layers: surface tension, Hamaker constant, viscosity, and film thickness. In contrast to the single layer case, the bilayer system has two unstable eigenmodes: squeezing and bending. For some particular parameter regimes, the system exhibits the avoided crossing behavior, where the two eigenmodes are interchanged. Based on numerical analysis, the system evolves into four different rupture states: basal layer rupture, upper layer rupture, double layer rupture, and mixed layer rupture. The ratio of Hamaker constants and the relative film thickness of the two layers control the system dynamics. Remarkably, the line of avoided crossing demarks the transition region of mode mixing and energy transfer, affecting the scaling of the dynamical regime map consequentially. Asymptotic and numerical analyses are used to examine the self-similar ruptures and to extract the power law scalings for both the basal layer rupture and the upper layer rupture. The scaling laws for the basal layer rupture are the same as those of the single layer on top of a substrate. The scaling laws for the upper layer rupture are different: the lateral length scale decreases according to (tr-t)1/3 and the film thickness decreases according to (tr-t)1/6.

Aggregation Kinetics of Citrate and Polyvinylpyrrolidone Coated Silver Nanoparticles in Monovalent and Divalent Electrolyte Solutions

The aggregation kinetics of silver nanoparticles (AgNPs) that were coated with two commonly used capping agents-citrate and polyvinylpyrrolidone (PVP)--were investigated. Time-resolved dynamic light scattering (DLS) was employed to measure the aggregation kinetics of the AgNPs over a range of monovalent and divalent electrolyte concentrations. The aggregation behavior of citrate-coated AgNPs in NaCl was in excellent agreement with the predictions based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, and the Hamaker constant of citrate-coated AgNPs in aqueous solutions was derived to be 3.7 × 10(-20) J. Divalent electrolytes were more efficient in destabilizing the citrate-coated AgNPs, as indicated by the considerably lower critical coagulation concentrations (2.1 mM CaCl(2) and 2.7 mM MgCl(2) vs 47.6 mM NaCl). The PVP-coated AgNPs were significantly more stable than citrate-coated AgNPs in both NaCl and CaCl(2), which is likely due to steric repulsion imparted by the large, noncharged polymers. The addition of humic acid resulted in the adsorption of the macromolecules on both citrate- and PVP-coated AgNPs. The adsorption of humic acid induced additional electrosteric repulsion that elevated the stability of both nanoparticles in suspensions containing NaCl or low concentrations of CaCl(2). Conversely, enhanced aggregation occurred for both nanoparticles at high CaCl(2) concentrations due to interparticle bridging by humic acid aggregates.

Lubricant distribution and its effect on slider air bearing performance over bit patterned media disk of disk drives

The distribution dynamics of a thin lubricant film on a bit-patterned media disk and its effect on the performance of the ultralow flying air bearing slider of disk drives are studied by direct numerical simulations. Our analysis shows that the physics governing lubricant distribution dynamics changes when deep enough sub-100-nm nanostructures are patterned on the disk surface. Air shearing under the slider that dominates lubricant flow on a flat disk may become negligible on a bit-patterned media disk. Surface tension and disjoining pressure become dominant factors instead. Our results show that disks with nanoscale patterns/roughness may no longer be treated as flat, and the air bearing load may strongly depend not only on the geometric detail of disk patterns but also on how lubricants are distributed on the patterns when slider-disk clearance is reduced to sub-10-nm. Air bearing load and consequently the slider’s flying attitude are affected by disk pattern geometry, average lubricant thickness, and material properties of lubricant such as the surface tension coefficient and Hamaker constant. The significantly expanded parameter space, upon which ultralow flying slider’s dynamics depends, has to be seriously considered in evaluating the head/disk interface tribology performance of next generation patterned media magnetic recording systems.

Preparation and stability behavior of the colloidal epoxy-1,1-iminodi-2-propanol adducts

Epoxy-amine adduct was prepared by reaction between DGEBA and 1,1-iminodi-2-propanol. The kinetic of the reaction was investigated by differential scanning calorimetry (DSC) and a model-free approach. The epoxy-amine adducts were prepared with various molar ratios of amine functionalities. Waterborne dispersions of these resins have been prepared by neutralization of amine functionalities in the epoxy-amine adducts. Fourier Transform Infrared Spectroscopy (FTIR) and DSC were also used to characterize the prepared epoxy-amine adducts. The stability behavior of the neutralized epoxy-amine adducts has been studied at 25 °C in aqueous solution of acetic acid. In each case, the experimental stability ratios ( W) versus electrolyte concentration plots were fitted using the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory and modified Fuch's model. The resulted values of diffuse potentials and Hamaker constants were obtained for the aqueous dispersions of the epoxy-amine adducts. According to the resulted Hamaker values, the applicability of the DLVO theory to the colloidal particles of epoxy amine adducts was investigated. The cathodic electrodeposition behavior of the prepared dispersions were also investigated. It was found that the samples with lower degree of neutralization result in a more efficient film deposition followed by higher dry film thickness although they show lower stability in the electrodeposition bath.

Reflow of supported sub-100 nm polymer films as a characterization process for NanoImprint lithography

In this paper, we address the reflow behavior of polymer thin films, focusing our effort on the accuracy of surface shape recognition. Although much work was already performed to control resist reflow during lens manufacturing for instance, our approach is significantly different since no contact line (substrate/polymer/atmosphere) needs to be considered. Here, a linear stability approach is successfully developed to describe the thin film evolution which is also compared with experiments. Polystyrene films, with thickness ranging from few tens of nanometers up to several hundred of nanometers were patterned with NanoImprint lithography technique. Atomic force microscopy measurements were used to characterize smooth or steep shapes, respectively. Mechanical measurements of earlier stages of pattern reflow were directly accessible without any assumption, contrary to the diffraction method usually employed. We show that by controlling the reflow process of any complex surface shape during the course of time, measurements of material parameters such as thin film viscosity, surface tension, or even Hamaker constant can be made possible.

Reversible uptake of water on NaCl nanoparticles at relative humidity below deliquescence point observed by noncontact environmental atomic force microscopy

The behavior of NaCl nanoparticles as a function of relative humidity (RH) has been characterized using non-contact environmental atomic force microscopy (e-AFM) to measure the heights of particles deposited on a prepared hydrophobic surface. Cubic NaCl nanoparticles with sides of 35 and 80 nm were found to take up water reversibly with increasing RH well below the bulk deliquescence relative humidity (DRH) of 75% at 23(∘)C, and to form a liquid-like surface layer of thickness 2 to 5 nm, with measurable uptake (>2 nm increase in particle height) beginning at 70% RH. The maximum thickness of the layer increased with increasing RH and increasing particle size over the range studied. The liquid-like behavior of the layer was indicated by a reversible rounding at the upper surface of the particles, fit to a parabolic cross-section, where the ratio of particle height to maximum radius of curvature increases from zero (flat top) at 68% RH to 0.7 ± 0.3 at 74% RH. These observations, which are consistent with a reorganization of mass on the solid NaCl nanocrystal at RH below the DRH, suggest that the deliquescence of NaCl nanoparticles is more complex than an abrupt first-order phase transition. The height measurements are consistent with a phenomenological model that assumes favorable contributions to the free energy of formation of a liquid layer on solid NaCl due both to van der Waals interactions, which depend partly upon the Hamaker constant, A(film), of the interaction between the thin liquid film and the solid NaCl, and to a longer-range electrostatic interaction over a characteristic length of persistence, ξ; the best fit to the data corresponded to A(film)= 1 kT and ξ = 2.33 nm.

Computational and Experimental Study of Colloidal Stability of CuPc Aqueous Dispersions

Dispersions of copper phthalocyanine (CuPc) pigments are commonly used in digital printing. Their colloidal stability is essential for fulfilling desirable shelf life and achieving good printing performance. In order to understand the role of electrostatic and van der Waals forces in their stability, a combined computational and experimental study has been performed to investigate the stability of the CuPc aqueous dispersions.The Hamaker constant was calculated with a novel combination of an ab initio and an empirical methodology (J. Chem. Theory Comput., 2010, 6 (2), pp 491&#x2013;498) to address the van der Waals attractions between the pigment particles. The calculated Hamaker constant was then employed in the DLVO (Derjaguin-Landau-Verwey-Overbeek) equation for dispersion stability simulation. The predictions allow comparisons with experimental colloidal stability study of CuPc aqueous dispersions (Langmuir, 2010, 26 (10), pp 6995&#x2013;7006)). The effect of a nonionic surfactant on the dispersion stability of CuPc particles was taken into consideration. The experimental dispersion stability ratio (W) was determined from dynamic light scattering (DLS) data and based on the Rayleigh-Debye-Gans (RDG) scattering theory. The adsorption isotherms show that the adsorption density increases with increasing equilibrium concentration of surfactant up to the cmc, and then reaches a plateau. Desorption tests were conducted to determine the reversibility or irreversibility of adsorption. Preliminary results suggest that a portion of the surfactant adsorbs irreversibly. The comparisons reveal that electrostatic forces play a significant role in the dispersion stabilization.

Nanoparticle Adhesion Models: Applications in Particulate Contaminant Removal from Extreme Ultraviolet Lithography Photomasks

Photomask contamination is a critical issue in the microelectronics manufacturing industry, and this problem has grown more challenging with the advent of extreme ultraviolet lithography (EUVL). Experimental studies of nanoscale contaminant particle adhesion and removal have limitations associated with metrology to accurately detect and quantify the nanoscale contaminants themselves. In this work, we have utilized our experimentally-validated simulation scheme as a tool to estimate the adhesion forces between hypothetical nanosized silicon nitride particles and EUVL mask materials. Specifically, this study focuses on the importance of different physical parameters such as the shape, size and surface roughness of the interacting bodies on the force of adhesion between them, with particular attention to particle size as the size is varied from the micro- to the nanoscale. The effective Hamaker constant (a force constant for the van der Waals force) for nanoscale particle–substrate systems was derived from the force measurements between a nanoscale silicon nitride AFM probe and the substrates of interest. By using hypothetical surface descriptions (geometry and roughness) of the nanoparticles, it was determined that surface roughness generally reduces adhesion of both micro- and nanoscale particulates. However, the effect of roughness on the adhesion force is greatly reduced for particles with diameters below 50 nm. The particle geometry can directly influence the adhesion force by altering the area of interaction between the particle and substrate. As a result, it remains important for systems of all particle sizes. Finally, an adhesion force map (a 'force-band diagram') was developed to bound the adhesion forces for any given particle–substrate system irrespective of the particle geometry and surface morphology.

Tip-sample interactions on graphite studied using the wavelet transform

Wavelet transform analysis is applied to a thermally excited cantilever to get insights into fundamental thermodynamical properties of its motion. The shortcomings of the widely used Fourier analysis are briefly discussed to put into perspective the wavelet transform analysis, used to describe the temporal evolution of the spectral content of the thermal oscillations of a cantilever with an interacting tip. This analysis allows to retrieve the force gradients, the forces and the Hamaker constant in a measurement time of less than 40 ms.

Thin-Film Properties of Some Functionalized Perfluoropolyether Boundary Lubricants Based on the n-Perfluoropropylene Oxide Monomer Unit

The thin-film properties of some perfluoropolyether boundary lubricants based on hydroxyl-termination of the n-perfluoropropylene oxide monomer units are investigated as a function of molecular polarity. The n-perfluoropropylene oxide monomer units provide a stiffer main chain than perfluoropolyethers comprised of a copolymer of perfluoro-methylene oxide and -ethylene oxide monomer units. Terraced flow and bonding kinetics show that the stiffer main chain reduces lubricant mobility on the disk surface but provides a lower profile lubricant film. The lack of mobility attributed to the main chain can be compensated for by decreasing the number of OH end groups. The Hamaker constants, derived from surface energy vs. thickness measurements are larger than the corresponding flexible main chains, suggesting an increased adhesion to the underlying carbon surface.

Casimir Force between Nanoparticles Immersed in a Crosslinked Polymer Blend

We consider a crosslinked polymer blend made of two polymers of different chemical nature. We suppose that such a system incorporates small colloidal particles, which prefer to be attracted by one polymer, close to the spinodal temperature. This is the so-called critical adsorption. As assumption, the particle diameter, d0, is considered to be small enough in comparison with the size of microdomains (mesh size) ξ∗ ∼ an, with a — the monomer size and n — the number of monomers between consecutive crosslinks. The critical fluctuations of the crosslinked polymer mixture induce a pair-potential between particles located in the non-preferred phase. The purpose is the determination of the Casimir pair-potential, U2(r), as a function of the interparticle distance r. To achieve calculations, use is made of an extended de Gennes field theory that takes into account the colloid-polymer interactions. Within the framework of this theory, we first show that the pair-particle is attractive. Second, we find for this potential the exact form: U2(r)/kBT = −AH(d0/r) exp(−r/ξ∗) − BH(d0/r) exp(−2r/ξ∗), with the known universal amplitudes AH > 0 and BH > 0 (the Hamaker constants). This expression clearly shows that the pair-potential differs from its homologue with no crosslinks only by the two exponential factors exp(−r/ξ∗) and exp(−2r/ξ∗). The main conclusion is that the presence of reticulations reduces substantially the Casimir effect in crosslinked polymer blends.

Energetic and Entropic Contributions to Self-Assembly of Binary Nanocrystal Superlattices: Temperature as the Structure-Directing Factor

We studied the effect of temperature on self-assembly of monodisperse colloidal nanocrystals into single-component and binary superlattices. Temperature, which serves as a weighting factor for the internal energy (U) and entropy (S) contributions to the Helmholtz free energy F = U - TS, allows tailoring relative weights of the interparticle interactions and free-volume entropy during the formation of nanocrystal superlattices. Temperature also provides a convenient tool for directing self-assembly of nanocrystals toward desired superlattice structures. We found that temperature strongly affects the structures of binary superlattices self-assembled from the mixtures of CdSe + PbS nanocrystals and PbSe + Pd nanocrystals. In the former case, small Hamaker constants for CdSe and PbS nanocrystals led to a relatively simple phase diagram, including only high-density NaZn(13)-, AlB(2)-, and NaCl-type binary superlattices. In contrast, binary superlattices self-assembled at different temperatures from PbSe and Pd nanocrystals showed a number of low-density complex phases stabilized by strong local van der Waals interactions between Pd nanocrystals. The structural diversity of nanoparticle superlattices is shown to be a result of the cooperative effect of the entropy-driven crystallization and the interparticle interactions. Both DeltaU and TDeltaS terms associated with the superlattice formation should be of the same order of magnitude, with |DeltaU| < |TDeltaS| for the assembly of CdSe and PbS nanocrystals and |DeltaU| > |TDeltaS| for the PbSe and Pd nanocrystals.

Aggregation and Deposition of Engineered Nanomaterials in Aquatic Environments: Role of Physicochemical Interactions

The ever-increasing use of engineered nanomaterials will lead to heightened levels of these materials in the environment. The present review aims to provide a comprehensive overview of current knowledge regarding nanoparticle transport and aggregation in aquatic environments. Nanoparticle aggregation and deposition behavior will dictate particle transport potential and thus the environmental fate and potential ecotoxicological impacts of these materials. In this review, colloidal forces governing nanoparticle deposition and aggregation are outlined. Essential equations used to assess particle-particle and particle-surface interactions, along with Hamaker constants for specific nanoparticles and the attributes exclusive to nanoscale particle interactions, are described. Theoretical and experimental approaches for evaluating nanoparticle aggregation and deposition are presented, and the major findings of laboratory studies examining these processes are also summarized. Finally, we describe some of the challenges encountered when attempting to quantify the transport of nanoparticles in aquatic environments.

Heat Transfer in the Microlayer Under a Bubble During Nucleate Boiling

Many studies have shown that a very thin liquid microlayer forms under vapor bubbles during nucleate boiling. The heat transfer from the surface to the bubble is then significantly affected by this microlayer and the curved region leading into the microlayer. Various models have been developed to predict the microlayer shape and the heat transfer along the curved interfacial region, but they tend to have inconsistent boundary conditions or unrealistic results. This paper presents a theoretical model to predict the microlayer thickness and the heat transfer rates for a variety of conditions. The results show how the wall superheat, the Hamaker constant, the bubble radius, and the accommodation coefficient at the interface affect the evaporation heat transfer rates and the microlayer shape for a large range of conditions for water and FC 72. The microlayer results are then shown to compare well with predictions made by solving the Navier-Stokes equations in the microlayer.

Fullerene C60: Surface Energy and Interfacial Interactions in Aqueous Systems

The underlying mechanisms of fullerene-fullerene, fullerene-water, and fullerene-soil surface interactions in aqueous systems are not well understood. To advance our understanding of these interfacial interactions, the surface properties of Buckminsterfullerene (C60) and quartz surfaces were investigated. From application of the van Oss-Chaudhury-Good model and the Young-Dupre equation, the Lifshitz-van der Waals, acid-base, and the total surface energies of C60 powder and quartz surfaces were calculated from contact angle measurements using the sessile drop technique. C60 powder measurements indicate low to medium energy surfaces of 41.7 mJ/m2 with a dominant Lifshitz-van der Waals component. In aqueous systems, hydrophobic attraction due to the high cohesion of water is the driving force for C60 aggregation. The high free energy of hydration (DeltaG(pw)(total) = -90.5 mJ/m2) indicates the high affinity of C60 particles for water. Hamaker constants of 4.02 x 10(-21) J (A(pwp)) and 2.59 x 10(-21) J (A(pws)) were derived for C60-C60 and C60-quartz interactions in aqueous systems. The results of this study indicate that surface energy is an important physical parameter that should be considered as a basic characterization property of fullerene nanomaterials.

Adhesion of Ferroplasma acidiphilum onto pyrite calculated from the extended DLVO theory using the van Oss–Good–Chaudhury approach

The adhesion behavior of Ferroplasma acidiphilum archaeon to pyrite mineral was investigated experimentally and theoretically . F. acidiphilum showed high affinity to adhere to pyrite surface at acidic regions, however low affinity was observed at neutral and alkaline regions. The microbe–mineral adhesion was assessed by the extended DLVO theory. Hamaker constants, electron donors, electron acceptors and surface charges for the microbe and the mineral were experimentally determined. The extended DLVO theory was used to explain the adhesion results. Significant changes to the pyrite surface properties after being treated with the microbial cells were observed. Pyrite lost its hydrophobic nature and became hydrophilic, the contact angle of untreated pyrite was 61° and this decreased to 36° after the treatment. As a consequence, the flotation experiment results showed that F. acidiphilum strain could act as a good depressant for pyrite in xanthat flotation; where in absence of F. acidiphilum cells, over 95% of pyrite can be recovered as a float. However, when the mineral was pretreated with F. acidiphilum cells, less than 20% can be recovered as a float.

Aggregation kinetics of cerium oxide nanoparticles in monovalent and divalent electrolytes

As a result of the commercial availability and use of nanoparticulate cerium oxide, CeO 2, it is extremely likely that this material will be introduced into the environment requiring knowledge of its fate, transport and bioavailability in natural aquatic systems. To this end, this work probes the physicochemical interactions that govern the aggregation kinetics of cerium oxide nanoparticles using time-resolved dynamic light scattering (TR-DLS) over a range of monovalent, Na +, and divalent, Ca 2+, electrolyte concentrations. Sets of nanoparticles were synthesized by precipitation in aqueous solutions containing varying concentrations of methanol. The point of zero charge (pzc) of these nanoparticles changes as a result of synthesis method. Those produced in the absence of methanol had pzc = 6.5. As predicted by the theories of Derjaguin–Landau–Verwey–Overbeek (DLVO), both reaction-limited and diffusion-limited aggregation were observed in each solution type. The experimental critical coagulation concentrations (CCC) at pH 11.0 was ca. 80 mM and ca. 16 mM for the monovalent (NaCl) and divalent (CaCl 2) salts, respectively. DLVO theory proved to be an adequate predictor for the interactions between cerium oxide nanoparticles albeit the derived Hamaker constant of 1.0 × 10 −20 J was somewhat smaller than experimental Hamaker constants determined for other metal oxide nanoparticles. Deviations between experimental data and DLVO theory are discussed.