Function Of Surface Separation Research Articles

Geocolloidal interactions and relaxation dynamics under nanoconfinement: Effects of salinity and particle concentration

HypothesisEnergy-related contaminants are frequently associated with geocolloids that translocate in underground fissures with dimensions comparable with geocolloids. To assess the transport and impact of energy-related contaminants in geological systems, fundamental understandings of interfacial behaviors of nanoparticles under confinement is imperative. We hypothesize that the dynamic properties of geocolloids, as well as their dependence on aqueous medium conditions would deviate from bulk behaviors under nanoconfinement. ExperimentsForce profiles and rheological properties of 50 nm silica nanoparticles in aqueous media confined between mica surfaces as a function of surface separation, particle concentrations, and salinity were measured utilizing the surface forces apparatus. FindingsForce profiles revealed the critical surface separation for nonlinear rheological behaviors coincides with the onset of exponential repulsion between mica surfaces. When salts were absent, the normal forces and viscosity values of colloidal suspensions resembled pure water. In contrast, with salts, the force profiles and corresponding critical length scales were found to be highly sensitive to the particle concentration and the degree of confinement. A Newtonian to shear-thinning transition was captured with increasing degrees of confinement. Our results show that the interplay among confinement, particle, and ionic concentrations can alter the interparticle forces and rheological responses of true nanosized-colloidal suspensions and thus their transport behaviors under nanoconfinement for the first time.

Influence of surface commensurability on the structure and relaxation dynamics of a confined monatomic fluid.

Molecular dynamics simulations are carried out for a single component, monatomic Lennard-Jones fluid confined between two mica surfaces to investigate the structure and relaxation dynamics of the confined fluid as a function of surface separation. Due to the underlying symmetry of the potassium ions on the mica surface, the contact layers prefer to adopt an incommensurate square or rhombic symmetry. The inner layers adopt a symmetry varying between rhombic, triangular, and square, depending on the density and surface separation. When the surface separation is an integral multiple of the particle diameter, distinct layering is observed, whereas jammed layers are formed at intermediate surface separations. This leads to the formation of both commensurate and incommensurate layering with varying intralayer symmetry. The self-intermediate scattering function exhibits a gamut of rich dynamics ranging from a distinct two-step relaxation indicative of glassy dynamics to slow relaxation processes where the correlations do not relax to zero over a microsecond for specific surface separations. An extended β relaxation is observed for both commensurate and incommensurate layering. Stretched exponential fits are used to obtain the relaxation times for the late α-relaxation regime of the self-intermediate scattering function. In some cases, we also observed dynamic and structural heterogeneities within individual layers. Although a single-component Lennard-Jones fluid does not exhibit a glass transition in the bulk, this study reveals that such a fluid can display, without supercooling, complex relaxation dynamics with signatures of a fluid approaching a glass transition upon confinement at constant temperature.

AFM measurements of forces between silica surfaces

Interaction forces and adhesion between a silica sphere and a flat silica surface in aqueous electrolyte solutions were investigated by atomic force microscopy. The forces were measured as a function of surface separation, pH and NaCl concentration as the surfaces were approaching each other. The adhesion force was determined upon retraction with respect to pH, NaCl concentration and contact time. The magnitude of the long range repulsive force was decreasing with decreasing pH. A short range repulsive force was observed at pH = 2, but no long range repulsive forces were observed at this pH. Force measurements showed that adhesion of silica surfaces in water was obstructed by short and long range repulsive forces. Adhesion was enhanced when both the long and the short range repulsive force was mitigated. A maximum adhesion force of 7.8 mN/m was measured at pH = 12.5 when the short range force vanished and the long range repulsive force was reduced by increasing the NaCl concentration. At pH = 12.5, the work of adhesion was calculated to be 1.2 mJ/m2 according to the Derjaguin–Muller–Toporov (DMT) model. Adhesion energy was much less at pH = 2 (0.3 mJ/m2) due to persistive short range repulsion.

Dynamics of confined water reconstructed from inelastic x-ray scattering measurements of bulk response functions

Nanoconfined water and surface-structured water impacts a broad range of fields. For water confined between hydrophilic surfaces, measurements and simulations have shown conflicting results ranging from "liquidlike" to "solidlike" behavior, from bulklike water viscosity to viscosity orders of magnitude higher. Here, we investigate how a homogeneous fluid behaves under nanoconfinement using its bulk response function: The Green's function of water extracted from a library of S(q,ω) inelastic x-ray scattering data is used to make femtosecond movies of nanoconfined water. Between two confining surfaces, the structure undergoes drastic changes as a function of surface separation. For surface separations of ≈9 Å, although the surface-associated hydration layers are highly deformed, they are separated by a layer of bulklike water. For separations of ≈6 Å, the two surface-associated hydration layers are forced to reconstruct into a single layer that modulates between localized "frozen' and delocalized "melted" structures due to interference of density fields. These results potentially reconcile recent conflicting experiments. Importantly, we find a different delocalized wetting regime for nanoconfined water between surfaces with high spatial frequency charge densities, where water is organized into delocalized hydration layers instead of localized hydration shells, and are strongly resistant to `freezing' down to molecular distances (<6 Å).

Interactions between Adsorbed Hydrogenated Soy Phosphatidylcholine (HSPC) Vesicles at Physiologically High Pressures and Salt Concentrations

Using a surface force balance, we measured normal and shear interactions as a function of surface separation between layers of hydrogenated soy phosphatidylcholine (HSPC) small unilamellar vesicles (SUVs) adsorbed from dispersion at physiologically high salt concentrations (0.15 M NaNO3). Cryo-scanning electron microscopy shows that each surface is coated by a close-packed HSPC-SUV layer with an overlayer of liposomes on top. A clear attractive interaction between the liposome layers is seen upon approach and separation, followed by a steric repulsion upon further compression. The shear forces reveal low friction coefficients (μ = 0.008–0.0006) up to contact pressures of at least 6 MPa, comparable to those observed in the major joints. The spread in μ-values may be qualitatively accounted for by different local liposome structure at different contact points, suggesting that the intrinsic friction of the HSPC-SUV layers at this salt concentration is closer to the lower limit (μ = ∼0.0006). This low friction is attributed to the hydration lubrication mechanism arising from rubbing of the hydrated phosphocholine-headgroup layers exposed at the outer surface of each liposome, and provides support for the conjecture that phospholipids may play a significant role in biological lubrication.

Mesoscale Molecular Dynamics Simulations of the Force between Surfaces with Grafted Poly(ethylene oxide) Chains Derived from Atomistic Simulations

The force between parallel surfaces with grafted poly(ethylene oxide) chains was calculated using molecular dynamics simulations. First, a single chain surrounded by water molecules was simulated with full atomistic detail. On the basis of that, a coarse-grained model with implicit solvent was developed. The force between surfaces with grafted coarse-grained chains was calculated as a function of surface separation (3−18 nm), chain length (12−20 repeat units), grafting density (Γ = 0.01−2.04 chains/nm2), and interactions between surface and polymer. Results showed that the force was mainly repulsive and increased monotonically as distance decreased. Chain length had a stronger influence on the force than Γ. For purely repulsive interactions between polymer and surfaces, chains had extended conformations, leading to higher repulsive forces between surfaces. For surfaces with higher affinity for polymer, adsorption took place. The force between surfaces was significant only for high Γ because chains then ac...

A molecular simulation study of the structure and tribology of polymer brushes: Comparison of behavior in theta and good solvents

Implicit solvent Brownian dynamics simulations of the structure and tribology of opposing polymer-brush covered surfaces have been carried out as a function of surface separation and solvent quality. Consistent with experiment, shear forces were found to be greater under theta solvent conditions than in a good solvent at equal relative separations (normalized by the respective height of the brushes in theta and good solvents). Much higher relative compression is required before the onset of significant shear forces in good solvent compared to theta solvent. The dependence of shear force for a given relative separation on solvent quality can be accounted for by differences in interpenetration of the brushes. When compared as a function of absolute surface separation, greater interpenetration and greater shear force are observed at large separations for the brushes in good solvent than in theta solvent, consistent with the greater brush height in good solvent. At shorter separations, corresponding to moderate to high compression, brush–brush interactions result in significant deformation of the brushes. In this regime, greater interpenetration and greater shear forces are observed in theta solvent at a given separation, in qualitative agreement with experiment.

Normal and Frictional Forces between Surfaces Bearing Polyelectrolyte Brushes

Normal and shear forces were measured as a function of surface separation, D, between hydrophobized mica surfaces bearing layers of a hydrophobic-polyelectrolytic diblock copolymer, poly(methyl methacrylate)- block-poly(sodium sulfonated glycidyl methacrylate) copolymer (PMMA- b-PSGMA). The copolymers were attached to each hydrophobized surface by their hydrophobic PMMA moieties with the nonadsorbing polyelectrolytic PSGMA tails extending into the aqueous medium to form a polyelectrolyte brush. Following overnight incubation in 10 (-4) w/v aqueous solution of the copolymer, the strong hydrophobic attraction between the hydrophobized mica surfaces across water was replaced by strongly repulsive normal forces between them. These were attributed to the osmotic repulsion arising from the confined counterions at long-range, together with steric repulsion between the compressed brush layers at shorter range. The corresponding shear forces on sliding the surfaces were extremely low and below our detection limit (+/-20-30 nN), even when compressed down to a volume fraction close to unity. On further compression, very weak shear forces (130 +/- 30 nN) were measured due to the increase in the effective viscous drag experienced by the compressed, sliding layers. At separations corresponding to pressures of a few atmospheres, the shearing motion led to abrupt removal of most of the chains out of the gap, and the surfaces jumped into adhesive contact. The extremely low frictional forces between the charged brushes (prior to their removal) is attributed to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments.

A Langevin dynamics simulation study of the tribology of polymer loop brushes

The tribology of surfaces modified with doubly bound polymer chains (loops) has been investigated in good solvent conditions using Langevin dynamics simulations. The density profiles, brush interpenetration, chain inclination, normal forces, and shear forces for two flat substrates modified by doubly bound bead-necklace polymers and equivalent singly bound polymers (twice as many polymer chains of 12 the molecular weight of the loop chains) were determined and compared as a function of surface separation, grafting density, and shear velocity. The doubly bound polymer layers showed less interpenetration with decreasing separation than the equivalent singly bound layers. Surprisingly, this difference in interpenetration between doubly bound polymer and singly bound polymer did not result in decreased friction at high shear velocity possibly due to the decreased ability of the doubly bound chains to deform in response to the applied shear. However, at lower shear velocity, where deformation of the chains in the flow direction is less pronounced and the difference in interpenetration is greater between the doubly bound and singly bound chains, some reduction in friction was observed.

Adsorbed layer structure of a weak polyelectrolyte studied by colloidal probe microscopy and QCM-D as a function of pH and ionic strength

The adsorbed layer structure of a weak polyelectrolyte poly(2-vinylpyridine), on silica and mica, has been studied using colloidal probe microscopy (CPM) and the quartz crystal microbalance (QCM-D). The adsorbed layer structure was found to be highly dependent on both the solution conditions from which it was adsorbed and the molecular weight. Conditions were altered to favour both flat adsorbed layers and the development of significant steric layers. The structural reconformation of the large steric layer was furthermore studied as a function of pH. The equilibrium interaction force, measured using CPM as a function of surface separation, was used to determine the relative extension of the polyelectrolyte away from the interface. At low pH (3.2) and high ionic strength, the adsorbed polyelectrolyte molecules adopt a highly extended conformation; as the pH is increased (to 4.6 then to 5.5) the polymer chains collapse progressively towards the interface. Reversal of this pH cycle causes a re-protonation of the adsorbed polymers and highlights the finite timescale for this process to occur. The kinetics of this process was measured from the amount of trapped solvent within the polyelectrolyte layer as determined from the QCM-D. Upon decrease in pH, the adsorbed chains swell before partially collapsing. We propose that this is due to cross-linking of the polymer in the protonated state. Furthermore, the combination of techniques used in this study, namely optical reflectometry, CPM and QCM-D, allows the determination of the solvent content of the adsorbed film of polyelectrolyte as well as the extension of the polymer from the interface as a function of pH.

Depletion and Structural Forces in Confined Polyelectrolyte Solutions

Monte Carlo simulations and density functional calculations have been performed for charged macromolecules confined to planar slits. The force between the confining walls has been evaluated as a function of separation, while keeping the chemical potential of the macromolecules constant. Highly charged spherical particles and flexible polyelectrolyte chains in confinement give rise to depletion and structural oscillatory forces as a function of surface separation. The sign and magnitude of the surface charge of the confining walls have no dramatic effect on the qualitative behavior of the confined liquid. With neutral or oppositely charged surfaces, an accumulation of charged macroions is seen in the slit driven by the repulsive interaction between the macroions, while equally charged surfaces give rise to a pure depletion. The net charge, the range of interaction, and the particle density affect the details of the force curve. For spherical macroions, the period of the oscillations scales approximately as the bulk aggregate concentration, cbulk-1/3. Confined polyelectrolyte chains share some of these properties, but they partly display a different behavior. One clear difference is that the polyelectrolyte net charge, that is, the degree of polymerization, has no effect on the osmotic pressure. This is an indication that polyelectrolyte chains pack not as spheres but rather as cylindrical objects. Another difference is that the effective repulsive interaction between polyelectrolyte chains can be more long ranged and oscillatory forces can appear more readily than for a corresponding solution of equally charged spherical macroions.

Experimental and theoretical investigation of the lateral Casimir force between corrugated surfaces

The lateral Casimir force acting between a sinusoidally corrugated gold plate and sphere was calculated and measured. The experimental setup was based on the atomic force microscope specially adapted for the measurement of the lateral Casimir force. The measured force oscillates sinusoidally as a function of the phase difference between the two corrugations. Both systematic and random errors are analyzed and a lateral force amplitude of $3.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\mathrm{N}$ was measured at a separation distance of 221 nm with a resulting relative error 24% at a 95% confidence probability. The dependence of the measured lateral force amplitude on separation was investigated and shown to be consistent with the inverse fourth power distance dependence. The complete theory of the lateral Casimir force is presented including finite conductivity and roughness corrections. The obtained theoretical dependence was analyzed as a function of surface separation, corrugation amplitudes, phase difference, and plasma wavelength of a metal. The theory was compared with the experimental data and shown to be in good agreement. The constraints on hypothetical Yukawa-type interactions following from the measurements of the lateral Casimir force are calculated. The possible applications of the lateral vacuum forces to nanotechnology are discussed.

Debye Length and Double-Layer Forces in Polyelectrolyte Solutions

We report experimental results and a theoretical analysis of the Debye length in aqueous solutions of nonadsorbing polyelectrolytes. The measurements were done using a surface forces apparatus, in which the normal forces between smooth mica surfaces in aqueous hyaluronic acid (HA) solutions were measured as a function of surface separation (to ±1 Å). HA is negatively charged and does not adsorb to the negatively charged surface of mica, as was established by optical and viscosity measurements and in agreement with the measured force−distance curves. From these measurements it appears that the multivalent polyelectrolyte is “depleted” from the gap between the surfaces. We use the mean-field Poisson−Boltzmann theory to theoretically predict the effective Debye length and double-layer force under such conditions and compare the predictions with the experimental results. The comparison gives excellent agreement and shows that the effective Debye length is determined solely by the monovalent ions in the solution. Specifically, the effective Debye length κeff-1 for the double-layer interaction is determined by an effective ionic concentration given by , where ns and nc are the bulk negative and positive monovalent ion concentrations, respectively.

Molecular dynamics studies of the effects of chain branching on the properties of confined alkanes

Using a recently developed approach, we perform molecular-dynamics simulations to probe the influence of chain branching on the properties of fluids confined between two solid surfaces. Two decane isomers are investigated: n-decane and 2,2-dimethyloctane. Under confinement, n-decane forms a layered structure consistent with structures observed for n-alkanes in previous simulation studies. In contrast, 2,2-dimethyloctane forms a “pillared-layered” structure consisting of a mix of molecules oriented parallel and perpendicular to the confining surfaces. Although both molecules exhibit solvation forces that oscillate between attractive and repulsive as a function of surface separation, pronounced differences are observed between the two isomers. The number of confined n-decane molecules changes in a stepwise manner as the surface separation is varied, while the number of 2,2-dimethyloctane molecules varies in a smooth fashion due to perpendicular “pillar” molecules gradually switching between parallel and perpendicular orientations. In addition, chain branching reduces the densities and structural changes in the adjacent layers, causing solvation forces and force oscillations to be less pronounced than those of linear chains. It also weakens the influence of pressure on the structure and properties. At separations corresponding to well-ordered films, the translational diffusivity of n-decane is the lowest, while at separations corresponding to disordered films, we recover the bulk diffusivity for n-decane. The diffusivity of 2,2-dimethyloctane is lower and its variation with surface separation is less than that seen for n-decane.

Shear and Frictional Interactions between Adsorbed Polymer Layers in a Good Solvent

The forces between layers of poly(ethylene oxide) (PEO), of molecular weights M = 37 × 103 (PEO37) and M = 112 × 103 (PEO112) adsorbed onto smooth, curved solid (mica) surfaces across the good solvent toluene have been determined using a surface force balance (SFB). The SFB used is capable of measuring both normal interactions Fn(D) as a function of surface separation D and, with extreme sensitivity, shear or frictional forces Fs(D,vs) between them as they slide past each other at velocity vs. The Fn(D) profiles are closely similar to those measured in earlier studies between adsorbed PEO layers. The shear or frictional forces between the sliding PEO-bearing surfaces are very low up to moderate compressions of the adsorbed layers (local pressures up to ca. 105 N m-2), corresponding to effective friction coefficients μeff = (Fs/Fn) of order 0.003 or less. This is attributed to the fluid interfacial layer between the adsorbed layers resulting from their weak mutual interpenetration. At higher loads Fs incre...

Forces between Adsorbed Layers of β-Casein

The forces between β-casein layers adsorbed onto mica surfaces have been measured as a function of surface separation by using the interferometric surface force apparatus. Both hydrophilic pure mica and a mica surface, which has been made hydrophobic by Langmuir−Blodgett deposition of dimethyldioctadecylammonium bromide, were used. A long-range repulsive force, most probably of electrostatic origin, was observed between β-casein layers adsorbed on hydrobized mica. The results suggest that β-casein forms a monolayer on this surface, where the outer part is less densely packed and protrudes far out into the solution. This open brushlike structure can readily be compressed on which an attractive force arises. The portion of the monolayer closest to the hydrophobized surface is much more compact and has low compressibility. On the pure hydrophilic surface a bilayer structure is more likely, with the a compact inner layer and an outer layer which has a similar structure to the monolayer formed on a hydrophobic...

Solvation Force and Confinement-Induced Phase Transitions of Model Ultra Thin Films

Abstract The static (equilibrium) properties of atomically thin films confined between two surfaces are studied as a function of surface separation by Grand Canonical Monte Carlo and Molecular Dynamics simulations. A model was used, in which the fluid and wall species consist of two different Lennard-Jones rare gas atoms. This was designed to mimic the static SFA experiments in which it is known that epitaxy is not necessary for inducing an oscillatory solvation force in simple non polar liquids. We have been able to simulate, using this simple system, many aspects of the equilibrium properties observed in the experiments. The solvation force is an exponentially damped, periodic curve. All peaks of maximum amplitude in the solvation force correspond to solid-like structures. These structures melt in increasing the surface separation. A further increase in separation leads to the addition of a whole layer and the recrystallisation of the film. In addition this model displays an interesting phenomenon of co...

Confinement-Induced Phase Transitions in Simple Liquids

The liquid-to-solid transition of a simple model liquid confined between two surfaces was studied as a function of surface separation. From large surface separations (more than 1000 angstroms) down to a separation corresponding to seven molecular layers, the confined films displayed a liquid-like shear viscosity. When the surface separation was further decreased by a single molecular spacing, the films underwent an abrupt, reversible transition to a solid. At the transition, the rigidity of the confined films (quantified in terms of an "effective viscosity") increased reversibly by at least seven orders of magnitude.

Effects of solvent exclusion on the force between charged surfaces in electrolyte solution

Effects of hard-core exclusions of solvent molecules on the force between charged surfaces immersed in electrolyte solution are investigated theoretically. A three-component model of electrolyte solution is used, in which the solvent is modeled as a fluid of neutral hard spheres. A free energy density functional theory, which has been applied previously to study the interaction between primitive electrical double layers, is extended to include ion–solvent and solvent–solvent interactions. Hard-core correlations are incorporated in a nonlocal free energy density functional, while electrostatic correlations are evaluated using the mean spherical approximation. The force between charged surfaces as a function of surface separation is found highly oscillatory, with a period approximately equal to the particle diameter. Because of the dominant role of the hard-core exclusions of the solvent fluid, the force does not depend strongly on either electrolyte concentration or surface charge density.

Nanotribology

Friction, lubrication, wear, and adhesion are all phenomena familiar from everyday experience. We experience friction when we go for a walk on ordinary ground—we do not expect to fall flat on our faces, as we might when walking on ice. Wear is particularly well known because it leads to catastrophic failure and represents one of the most costly problems facing industry today. These phenomena are part of the field of tribology—the science of interacting surfaces in relative motion. A macroscopic understanding of tribology is crucial for the design and engineering of mechanical components. As dimensional tolerances of these components approach the nanometer scale and novel new materials are used in their fabrication, macroscopic understanding must give way to a microscopic atomic-scale understanding. This fundamental microscopic understanding and its practical application remain limited.With the advent of surface proximity probes, however, those limitations are rapidly disappearing. Researchers are now able to test and develop a microscopic understanding of such tribological phenomena as boundary-layer lubrication and asperity-surface interactions—ideas that underlie our current macroscopic understanding. This emerging field is known as nanotribology or molecular tribology, the focus of this issue of the MRS Bulletin. The new experimental probes include the surface force apparatus for measuring interfacial forces as a function of surface separation and, more recently, rheological properties of molecularly thin layers; a quartz-crystal microbalance for measuring the atomic-scale friction of adsorbed monolayer films; and the atomic force microscope (AFM) for measuring nanomechanical properties of surfaces, atomicscale friction of clean surfaces, and frictional properties of surfaces coated with molecular films.