Steric Repulsive Forces Research Articles

Stabilization activity of a new protein-carbohydrate complex in sesame paste: rheology, microstructure, and particle size analysis.

Sesame protein concentrate is a by-product of sesame processing with great nutritional and functional value. In the present study, sesame protein concentrate and a conjugated form of the protein with maltodextrin were applied to improve stability and texture of sesame paste. The rheological properties of samples were studied using dynamic measurement, controlled rate and controlled stress rheological analysis, and creep analysis to investigate the sesame paste structure and to illustrate the mechanism of their stability against oil separation. Samples containing the conjugate showed good stability, low average droplet size and uniform microstructure. Creep analyses showed as the conjugate was added to sesame paste, μ0 and μ1 values (corresponding dashpot viscosities) were higher than that of G0 and G1 (shear moduli of Maxwell and Kelvin-Voigt springs). Dynamic shear test exhibited increased storage modulus (G') and loss modulus (G″) by the increase in frequency and elastic structure of sesame paste converted to viscose behavior with the contribution of conjugate. Particle interaction in sesame protein concentrate (SPC)-maltodextrin conjugate (SMC), may be overcome by steric repulsion force. Furthermore, impression of a weak network structure can be observed. These phenomenon could resolve the phase separation problem of traditional sesame paste. © 2022 Society of Chemical Industry.

Phosphate and humic acid inhibit corrosion of green-synthesized nano-iron particles to remove Cr(VI) and facilitate their cotransport

Green tea synthesized nano-zero-valent iron (GT-nZVI), composed of a Fe0 core and tea polyphenols (TPs), has been attracting attention for effective removal of hexavalent chromium (Cr(VI)) from water and soils. However, the relevant environmental factors which may affect GT-nZVI removal of Cr(VI) are still unclear. Thus, it is necessary to investigate the interplay of naturally occurring phosphate and HA on the reactions and stability of GT-nZVI with Cr(VI) in water-saturated porous media to mimic soils and groundwater. The inherent mechanisms were revealed using multiple technologies, including TEM, SEM-EDS, FTIR, XPS, and a transport model. We found that the Cr(OH)3 and Cr2O3 were formed on GT-nZVI upon reacting with Cr(VI). When phosphate and HA were present, their adsorption inhibited the Fe0 core corrosion and the release of TPs from GT-nZVI at pH ≤ 6.0. Thus, the generation of soluble Cr(III) was reduced by 39.9% (6.7 mg∙g−1) and 70.6% (11.9 mg∙g−1) at pH 3.0, respectively. In particular, HA favors the formation of Cr2O3, and then facilitates the cotransport with GT-nZVI, forming Fe(III)–HA–Cr2O3 complex due to the steric repulsive forces of HA. In contrast, phosphate ion dereases Cr(VI) adsorption and subsequent generation of Cr2O3 on GT-nZVI and accordingly increased the Cr(VI) mobility through water-saturated porous media. To our knowledge, this is the first study unraveling the mechanisms regarding the impact of phosphate and HA on the change of physicochemical properties of GT-nZVI coupling with Cr(VI)/Cr(III) transformation. Our discovery informs a better strategy for in situ remediation of Cr(VI) by GT-nZVI where phosphate and HA commonly cooccur.

Mechanisms for efficient clay dispersing effect with tannins and sodium hydroxide

Substituting the industrial additives in building materials with natural ones can reduce the embodied carbon in the construction sector. In this paper, the dispersing effect of oak tannin as natural dispersant on clay materials is studied in combination with sodium hydroxide. Through rheological measurements, adsorption tests, and zeta potential measurements, the deflocculation mechanism of tannin under varied NaOH environments has been highlighted. The results show that the dispersing ability of tannin can be increased greatly with the incorporation of NaOH. By adding NaOH in the system, the kaolinite deflocculation mechanism of tannin is changed from steric hindrance to combined steric hindrance and electrostatic repulsion forces. This shift stems from two aspects: the transformation of the tannin chemical structure and the modification of kaolinite by NaOH. The results indicate that tannin coupled with NaOH can be a promising superplasticizer for earthen materials, with the potential of producing poured earth based on this combination. • Oak tannin is an efficient and natural dispersant for clay materials. • In combination with sodium hydroxide, the deflocculation action of tannin is amplified. • Oak tannin and NaOH disperse clay particles via combined steric repulsion forces and electrostatic repulsive forces. • Oak tannin coupled with NaOH is a promising superplasticizer for earthen materials and an environmentally friendly alternative to industrial product.

Interparticle Forces of a Native and Encapsulated Metal-Organic Framework and Their Effects on Colloidal Dispersion.

The colloidal properties of suspended metal-organic frameworks (MOFs) are critical for device fabrication and application. Herein, van der Waals attractive, electric double layer repulsive, and steric repulsive forces of a native and encapsulated MOF are quantified for the first time. The van der Waals attractive forces were investigated by conducting environmental ellipsometric porosimetry (EEP) and spectroscopic ellipsometry (SE) on submicron, optical-quality nanoparticle films. The repulsive forces were determined from colloid and material characterization measurements. These data were used to predict suspension properties via extended Derjaguin, Landau, Verwey, and Overbeek theory. The state of dispersion was quantified for comparison with theoretical predictions for nine solvents. The MOF encapsulated with a surface-selective modification showed superior suspension in hydrophobic solvents. These findings should expedite the formulation of MOF colloidal suspensions for future works.

Atomic Force Microscopy Measurement in the Lignosulfonate/Inorganic Silica System: From Dispersion Mechanism Study to Product Design

Designing and preparing high-performance lignin-based dispersants are crucial steps in realizing the value-added utilization of lignin on an industrial scale. Such process depends heavily on an understanding of the dispersion mechanism of lignin-based dispersants. Here, atomic force microscopy (AFM) is employed to quantitatively investigate the dispersion mechanism of a lignosulfonate/silica (LS/SiO2) system under different pH conditions. The results show that the repulsive force between SiO2 particles in LS solution is stronger than it is in water, resulting in better dispersion stability. The Derjaguin–Landau–Verwey–Overbeek (DLVO) formula as well as the DLVO formula combined with steric repulsion is utilized for the fitting of the AFM force/distance (F/D) curves between the SiO2 probe and substrate in water and in LS solution. Based on these fitting results, electrostatic and steric repulsive forces are respectively calculated, yielding further evidence that LS provides strong steric repulsion between SiO2 particles. Further studies indicate that the adsorbance of LS on SiO2 (Q), the normalized interaction constant (A), and the characteristic length (L) are the three critical factors affecting steric repulsion in the LS/SiO2 system. Based on the above conclusions, a novel quaternized grafted-sulfonation lignin (QAGSL) dispersant is designed and prepared. The QAGSL dispersant exhibits good dispersing performance for SiO2 and real cement particles. This work provides a fundamental and quantitative understanding of the dispersion mechanism in the LS/inorganic particle system and provides important guidance for the development of high-performance lignin-based dispersants.

Reconfigurable Chirality of DNA-Bridged Nanorod Dimers.

Gold nanorods assembled in a side-by-side chiral configuration have potential applications in sensing due to their strong chiroptical surface plasmon resonances. Recent experiments have shown that dimers of gold nanorods bridged by double-stranded DNA exhibit variable chiral configurations depending on the chemical and ionic properties of the solvent medium. Here, we uncover the underlying physics governing this intriguing chiral behavior of such DNA-bridged nanorods by theoretically evaluating their configurational free energy landscape. Our results reveal how chiral configurations emerge from an interplay between the twist-stretch coupling of the intervening DNA and the intermolecular interactions between the nanorods, with dimers exhibiting left-handed chirality when the interparticle interactions are dominated by attractive depletion or van der Waals forces and right-handed chirality when dominated by repulsive electrostatic or steric forces. We demonstrate how changes in the depletant or ion concentration of the solvent medium lead to different classes of configurational responses by the dimers, including chirality-switching behavior, in good agreement with experimental observations. Based on extensive analyses of how material properties like nanorod aspect ratio, DNA length, and graft height modulate the free energy landscape, we propose strategies for tuning the environmentally responsive reconfigurability of the nanorod dimers. Overall, this work should help control the chirality and related optical activity of nanoparticle dimers and higher-order assemblies for various applications.

Mechanistic underpinning of cell aspect ratio-dependent emergent collective motions in swarming bacteria.

Self-propelled bacteria can exhibit a large variety of non-equilibrium self-organized phenomena. Swarming is one such fascinating dynamical scenario where a number of motile individuals group into dynamical clusters and move in synchronized flows and vortices. While precedent investigations into rod-like particles confirm that an increased aspect-ratio promotes alignment and order, recent experimental studies in bacteria Bacillus subtilis show a non-monotonic dependence of the cell-aspect ratio on their swarming motion. Here, by computer simulations of an agent-based model of self-propelled, mechanically interacting, rod-shaped bacteria under overdamped conditions, we explore the collective dynamics of a bacterial swarm subjected to a variety of cell-aspect ratios. When modeled with an identical self-propulsion speed across a diverse range of cell aspect ratios, simulations demonstrate that both shorter and longer bacteria exhibit slow dynamics whereas the fastest speed is obtained at an intermediate aspect ratio. Our investigation highlights that the origin of this observed non-monotonic trend of bacterial speed and vorticity with the cell-aspect ratio is rooted in the cell-size dependence of motility force. The swarming features remain robust for a wide range of surface density of the cells, whereas asymmetry in friction attributes a distinct effect. Our analysis identifies that at an intermediate aspect ratio, an optimum cell size and motility force promote alignment, which reinforces the mechanical interactions among neighboring cells leading to the overall fastest motion. Mechanistic underpinning of the collective motions reveals that it is a joint venture of the short-range repulsive and the size-dependent motility forces, which determines the characteristics of swarming.

Inter-particle interactions of alumina powders in UV-curable suspensions for DLP stereolithography and its effect on rheology, solid loading, and self-leveling behavior

For the UV-curable alumina suspensions used in digital light processing (DLP) stereolithography, optimizing the dispersant type is important for achieving low viscosity, high solid loading, and remarkable self-leveling behavior. However, the inter-particle interactions in UV-curable alumina suspensions dispersed using different dispersants are overlooked. Herein, the effect of inter-particle interactions on rheology, solid loading, and self-leveling behavior of UV-curable alumina suspensions was systematically investigated. Three different commercial dispersants were used: oleic acid (OA), alkane-acrylic phosphate ester (PM1590), and copolymer dispersant (BYK111). After dispersing, BYK111 endowed alumina powders with thicker adsorption polymer layer to provide stronger steric repulsion force and facilitated better wetting of alumina powers in the photocurable resin, resulting in a reduced network structure degree, which decreased the viscosity (1.04 Pa s at 30 s−1); homogeneous packing of alumina powders, which enhanced the maximum solid loading (55 vol%); and inhibition of particle flocculation, which facilitated the spontaneous spreading of suspension.

The Size-Accelerated Kinetic Resolution of Secondary Alcohols.

The factors responsible for the kinetic resolution of alcohols by chiral pyridine derivatives have been elucidated by measurements of relative rates for a set of substrates with systematically growing aromatic side chains using accurate competitive linear regression analysis. Increasing the side chain size from phenyl to pyrenyl results in a rate acceleration of more than 40 for the major enantiomer. Based on this observation a new catalyst with increased steric bulk has been designed that gives enantioselectivity values of up to s=250. Extensive conformational analysis of the relevant transition states indicates that alcohol attack to the more crowded side of the acyl‐catalyst intermediate is favoured due to stabilizing CH‐π‐stacking interactions. Experimental and theoretical results imply that enantioselectivity enhancements result from accelerating the transformation of the major enantiomer through attractive non‐covalent interactions (NCIs) rather than retarding the transformation of the minor isomer through repulsive steric forces.

Silica anchored colloidal suspension of magnetite nanorods

This study focuses on the interaction of colloidal silica nanoparticles with the magnetite (Fe3O4) magnetic fluids (MF), which eventually forms nanorod like structure. The aqueous magnetic fluid consists of magnetite nanoparticles having double layers of lauric acid surfactant. This surfactant provides stability towards short-range van der Waals attractive and steric repulsive forces, as well as long-range dipole-dipole interactive force. Whereas, in the colloidal silica, the sodium ions provide stability to the silica nanoparticles. The colloidal silica and magnetic fluid both were mixed in different proportions to understand the interaction between the silica and magnetite nanoparticles. Thus, the interaction present in the system is studied using FTIR, TGA, and magnetic field induced microscopy. The FTIR and TGA data reveal that silica interacts with the outer layer of lauric acid through the Si–O bond and eventually provides stability to the system. The length of the lauric acid sheath varies with the concentration of silica nanoparticles. The SEM images indicate nanorod formation, and its structure dimensions vary with the silica concentrations, which is also reflected in the magnetic field-induced structure formations. The structure observed using microscopy correlated with the interaction derived from FTIR and TGA data analysis. XRD data of iron oxide (magnetite) nanoparticles are present here again for completion.

Influence of the degree of esterification of soluble soybean polysaccharide on the stability of acidified milk drinks

Soluble soybean polysaccharide (SSPS), an anionic polysaccharide extracted from the by-product of soybean protein, has been widely used as a stabilizer for the stabilization of casein dispersions. The methyl esterified carboxylic group of galacturonic acid (GalA) in SSPS has a vital influence on its physicochemical and functional properties. In particular, the degree of esterification (DE) of SSPS plays an important role in the stability of acidified milk drinks (AMDs). Thus, in this work, the DE of SSPS was determined by 1H NMR spectroscopy and titration method. The influence of DE of SSPS on the stability of AMDs was comprehensively investigated and evaluated by using LUMiSizer. It was found that the SSPS with a low DE showed better stabilization in AMDs compared to SSPS with a high DE. Zeta potential measurements can aid in elucidating the mechanisms responsible for the improved stability of AMDs, which showed that the decrease in DE resulted in the more negative zeta potential of SPSS coated casein particles. These results demonstrated that SSPS with a low DE has a high ability to stabilize AMDs, mostly due to the large electrostatic repulsive force of SSPS and the steric repulsive force of adsorption layer that is formed on the surface of casein particles.

Magnetite Hydrosols with Positive and Negative Surface Charge of Nanoparticles: Stability and Effect on the Lifespan of Drosophila melanogaster.

This paper presents sols of uncoated and citric acid-coated Fe3O4 nanoparticles obtained by a combination of coprecipitation and sonochemistry methods. A stable concentrated CA-Fe3O4 sol synthesized by a combination of coprecipitation with an inconvenient Fe2+/Fe3+ ratio, modification with citric acid and US treatment was obtained for the first time. A comparative analysis of the composition and morphology of nanoparticles was performed. The sols are oppositely charged and behave as a typical ferrofluid. The citric acid-modified sol is aggregatively stable over wider ranges of pH and electrolyte concentration, but it becomes less stable with the temperature increase. DLVO calculations showed that steric repulsion forces are a vital factor contributing to increased aggregative stability in a modified Fe3O4 sol. The experiments have revealed the magneto-optical effect in a modified Fe3O4 sol with an electrolyte concentration of 0.025-0.075 M caused by a high potential barrier and a deep secondary minimum in pairwise interaction curves. The "pK spectroscopy" mathematical model to describe the potentiometric curves of synthesized magnetite sols was used for the first time. According to potentiometric titration, the ions of the electrolyte practically do not contribute to formation of a surface charge in modified Fe3O4 with a change in pH due to blocking the magnetite surface by citric acid molecules. Drosophila melanogaster was used as a model to show that Fe3O4 in chronic exposure has a low toxic effect.

Interpreting the role of NO3-, SO42-, and extracellular polymeric substances on aggregation kinetics of CeO2 nanoparticles: Measurement and modeling.

The early stage of aggregation of cerium oxide nanoparticles (CeO2 NPs) in anion solutions was inspected in the absence and presence of extracellular polymeric substance (EPS) with a help of time-resolved dynamic light scattering (DLS). The aggregation kinetics and attachment efficiencies were calculated according to measured hydrodynamic diameter across a range of 1-500mM NaNO3 and 0.01-100. mM Na2SO4. The aggregation of CeO2 NPs in both NaNO3 and Na2SO4 solution conformed with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. In NaNO3 solution, the critical coagulation concentrations (CCC) of CeO2 NPs was calculated to be about 47mM; in Na2SO4 solution, CeO2 NPs showed a re-stabilization process and thus there was no CCC value. SO42- had intenser effects on CeO2 NPs aggregation than NO3- might because of the distinction between their polarization, consisting in Hofmeister series. The presence of bound EPS (B-EPS), tightly bound EPS (TB-EPS) and loosely bound EPS (LB-EPS) in NaNO3 solutions all lead to significant decrease in CeO2 NPs aggregation. Steric repulsive force produced by absorbed EPS on CeO2 NPs might take main responsibility in stabilizing CeO2 NPs. Besides, Extended Derjaguin-Landau-Verwey-Overbeek (EDLVO) model successfully predicted the energy barrier between CeO2 NPs with B-EPS, TB-EPS and LB-EPS as a function of NaNO3 concentration. Furthermore, the difference in impeding the CeO2 NPs aggregation with B-EPS, TB-EPS and LB-EPS may be caused by the divergence in molecular weight and component mass fraction especially protein content. These results might subserve the assessment on the fate and transport behaviors of CeO2 NPs released in wastewater treatment plants.

A computational model for molecular interactions between curved slender fibers undergoing large 3D deformations with a focus on electrostatic, van der Waals, and repulsive steric forces

SummaryThis contribution proposes the first three‐dimensional (3D) beam‐beam interaction model for molecular interactions between curved slender fibers undergoing large deformations. While the general model is not restricted to a specific beam formulation, in the present work, it is combined with the geometrically exact beam theory and discretized via the finite element method. A direct evaluation of the total interaction potential for general 3D bodies requires the integration of contributions from molecule or charge distributions over the volumes of the interaction partners, leading to a six‐dimensional integral (two nested 3D integrals) that has to be solved numerically. Here, we propose a novel strategy to formulate reduced section‐section interaction laws for the resultant interaction potential between a pair of cross‐sections of two slender fibers such that only two one‐dimensional integrals along the fibers' length directions have to be solved numerically. This section‐section interaction potential (SSIP) approach yields a significant gain in efficiency, which is essential to enable the simulation of relevant time and length scales for many practical applications. In a first step, the generic structure of SSIP laws, which is suitable for the most general interaction scenario (eg, fibers with arbitrary cross‐section shape and inhomogeneous atomic/charge density within the cross‐section) is presented. Assuming circular, homogeneous cross‐sections, in a next step, specific analytical expressions for SSIP laws describing short‐range volume interactions (eg, van der Waals (vdW) or steric interactions) and long‐range surface interactions (eg, Coulomb interactions) are proposed. Besides ready‐to‐use expressions for the total interaction potential, also the resulting virtual work contributions, its finite element discretizations, as well as a suitable numerical regularization for the limit of zero separation are derived. The validity of the SSIP laws, as well as the accuracy and robustness of the general SSIP approach to beam‐beam interactions, is thoroughly verified by means of a set of numerical examples considering steric repulsion, electrostatic, or vdW adhesion.

The use of potato starch/xanthan gum combinations as a thickening agent in the formulation of tomato ketchup

ABSTRACT Tomato ketchup was prepared using potato starch (PS)/xanthan gum (XG) combinations as a thickening agent, and its sensory textural properties, viscosity stability, low-field nuclear magnetic resonance (LF-NMR) imaging and rheological properties were evaluated. Results showed that PS/XG as a thickener enjoyed the highest score (4.6) with good sensory evaluation. Due to the electrostatic and steric repulsive forces between PS and XG, the tomato ketchup prepared by 2.5% w/w PS/XG (PS:XG was 80:1) had better acid resistance, superior hardness, adhesion adhesiveness and cohesiveness than potato starch (PS), modified starch (MS), and no thickener (Control). Particularly, PS/XG enhanced the viscosity of ketchup for its electrostatic repulsion which delays starch retrogradation. LF-NMR imaging demonstrated that the moisture distribution in ketchup prepared by PS/XG was the most uniform. With a thixotropic behavior, PS/XG had a similar flow curve as MS. Therefore, PS/XG exhibited a compelling potential as a thickener in the formulation of ketchup.

Shear rate dependence of viscosity and normal stress differences in ferrofluids

In this work we investigate numerically the viscosity and normal stress differences of mono and polydisperse ferrofluids undergoing a simple shear flow in the presence of a transverse applied magnetic field. The simulations are carried out by means of a Brownian Dynamics integration algorithm. For all simulations we assume that the dipole moments are fixed to the magnetic particles. In the case of polydisperse suspensions the particles present a typical log-normal distribution of magnetic diameters which are covered with a layer of surfactant of constant width. The long-range dipolar interaction between the magnetic moments are computed by Ewald summation technique and the surfactant influence is modeled as a steric repulsive force between pairs of particles. The attractive van der Waals force is also incorporated in the numerical simulations. The numerical procedure is validated by comparing the simulation results of the non-dimensional relative viscosity increase as a function of the magnetic field intensity with experimental data. The present numerical simulations show that the non-dimensional relative viscosity increment as a function of the non-dimensional shear rate is strongly influenced by the particle volume fraction and the strength of the dipolar interactions parameter. By examining the suspension microstructure into the numerical box, we can see anisotropic structures like chains of particles for the low shear rate regimes, specially in the polydisperse cases. The results also indicate the presence of a suspension shear-thinning behavior with two different non-dimensional shear rate dependence. In addition, non-dimensional normal stress differences are observed even in the absence of dipolar interactions, as a consequence of stress anisotropy produced by the repulsive forces during the particle collisions induced by the imposed steady-shear flow. The dipolar interactions increase these non-dimensional normal stress differences and make their dependence on the non-dimensional shear rate non-monotonic. For low non-dimensional shear rates, the dipolar interactions dominate and the formation of large-structures like chains oriented in the direction of the magnetic fields leads to an increase in the non-dimensional first normal stress difference. For very high non-dimensional shear rates, the flow breaks the anisotropic structures and the effects of polydispersity and dipolar interactions become unimportant. The numerical simulations provide a useful tool for understanding the complex microphysical and rheology of real ferrofuids under shear flows.

Epoxy/CNT@X nanocomposite: Improved quasi-static, dynamic fracture toughness, and conductive functionalities by non-ionic surfactant treatment

The present work investigated the effects of non-ionic surfactant treatment on the dispersibility, surface chemistry and structure of carbon nanotube (CNT) particles. Subsequently, the fracture experiments of as-prepared epoxy/CNT@X nanocomposites were carried out under quasi-static and dynamic loading conditions. By simply introducing the steric repulsive force between CNT@X filler and epoxy matrix, improved mode-I critical-stress-intensity factor (KIc) and dynamic crack initiation toughness (KIid) of the epoxy/CNT@X nanocomposite were simultaneously obtained without compromising other desired physical properties, such as electrical properties and electro-thermal behavior. In the case of SHPB impact loading, high-speed imaging along with digital-image-correlation (DIC) technology was utilized to determine dynamic fracture parameters. The results showed a notable reinforcement for the epoxy/CNT@X nanocomposite category, producing maximum increase of ~79% and ~153% in KIc and KIid values relative to epoxy/CNT nanocomposite at such maximum content of 1.0 wt%, respectively. The most delayed crack initiation time (59.9–68.4 μs) and slowest crack-tip velocity (229 ± 28 m/s) were also observed in the epoxy/CNT@X_1.0 case. These results may be explained by improved dispersibility and interfacial adhesion after surfactant treatment.

Microrheological modeling of lithium ion battery anode slurry

The rheological properties of electrode slurries used in the manufacturing of lithium-ion batteries affect the manufacturing processes as well as the battery quality, such as electrochemical and durability performance. There is thus an urgent need to develop a physics-based model to describe the rheological properties of these electrode slurries to enable further optimization of battery electrode production. In this paper, we present a microrheological model for estimating the viscosity of electrode slurries that incorporates colloidal forces such as van der Waals and polymeric steric repulsion forces. The model shows good agreement with the experimentally determined viscosities of a graphite-based anode slurry and should be extendable to describe the rheological properties of cathode slurries.

Adhesion and Self-Assembly of Lubricin (PRG4) Brush Layers on Different Substrate Surfaces.

Lubricin (LUB, aka PRG4), a mucin-like glycoprotein, is best known for the significant role it plays in the boundary lubrication, wear protection, and adhesion control systems in human joints. However, LUB exhibits a number of diverse and useful properties, including a remarkable ability to self-assemble into a telechelic brush structure onto virtually any substrate. This self-assembly behavior has spawned the emergence of numerous nontraditional applications of LUB coatings in numerous areas such as microfluidics, electrochemical sensors, contact lenses, antifouling surfaces, and bionic neural interfaces. Although LUB will readily self-assemble on most substrates, it has become apparent that the substrate has a significant influence on the LUB layer's demonstrated lubrication, antiadhesion, electrokinetic, and size-selective transport properties; however, investigations into LUB-substrate interactions and how they influence the self-assembled LUB layer structure remain a neglected aspect of LUB research. This study utilizes AFM force spectroscopy to directly assess the adhesion energy of LUB molecules adsorbed to a wide variety of different substrates which include inorganic, polymeric, and metallic materials. An analysis of the steric repulsive forces measured on approach provides a qualitative assessment of the LUB layer's mechanical modulus, related to the chain packing density, across substrates. These modulus measurements, combined with characteristic features and the dwell time dependence of the LUB adhesion forces provide insight into the organization and uniformity of the LUB brush structure. The results of these measurements indicate that LUB interactions with different substrates are highly variable and substrate-specific, resulting in a surprisingly broad spectrum of adhesion energies and layer properties (i.e., chain density, uniformity, etc.) which are not, themselves, correlated or easily predicted by substrate properties. In addition, this study finds exceptionally poor LUB adhesion to both mica and poly(methyl methacrylate) surfaces that remain widely used substrates for constructing model surfaces in fundamental tribology studies which may have significant implications for the findings of a number of foundational studies into LUB tribology and molecular synergies.

Influence of macromolecules on aggregation kinetics of diesel soot nanoparticles in aquatic environments

Soot nanoparticles (SNPs) produced from incomplete combustion have strong impacts on aquatic environments as they eventually reach surface water, where their environmental fate and transport are largely controlled by aggregation. This study investigated the aggregation kinetics of SNPs in the presence of macromolecules including fulvic acid (FA), humic acid (HA), alginate polysaccharide, and bovine serum albumin (BSA, protein) under various environmentally relevant solution conditions. Our results showed that increasing salt concentrations induced SNP aggregation by suppressing electrostatic repulsion and that CaCl2 exhibited stronger effect than NaCl in charge neutralization, which is in agreement with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The aggregation rates of SNPs were variously reduced by macromolecules, and such stabilization effect was the greatest by BSA, followed by HA, alginate, and FA. Steric repulsion resulting from macromolecules adsorbed on SNP surfaces was mainly responsible for enhancing SNP stability. Such steric repulsion appeared to be affected by macromolecular structure, as BSA having a more compact globular structure on SNP surfaces imparted long-range steric repulsive forces and retarded the SNP aggregation rate by 10–100 times. In addition, alginate was shown to enhance SNP aggregation by ∼10 times at high CaCl2 concentrations due to alginate gel formation via calcium bridging. The results may bear strong significance for the fate and transport of SNPs in both natural and controlled environmental systems.