Oscillatory Structural Forces Research Articles

Interactions between Solid Surfaces Mediated by Polyethylene Oxide Polymers: Effect of Polymer Concentration

Using total internal reflection microscopy (TIRM), we have systematically measured the interactions between a microsphere and a flat hydrophilic surface in the presence of polyethylene oxide (PEO) polymer solution. Our results reveal that PEO significantly mediates the interaction forces between the two surfaces. At low polymer concentration, the interactions between two surfaces in the presence of PEO are mainly dominated by repulsive forces, originating from diffuse layer overlap. At intermediate polymer concentration, a long-range and weak attraction sets in. This force is likely attributed to the depletion attraction due to the presence of free PEO chains in bulk solution; however, a simple hard-sphere AO model fails to precisely describe the attraction. At high polymer concentration where PEO chains overlap, the attraction disappears, and levitation of the microsphere probe is detected. We argue that at this overlapping region, the correlation length of PEO chains is much smaller than the size of single PEO molecule, leading to weakening and disappearing of the depletion attraction. Finally, at very high concentration, oscillatory structural force is obviously found, indicating the significant structural ordering of the PEO chains under confinement.

Influences of oscillatory structural forces on dewetting of nanoparticle-laden ultra-thin films

To understand the influences of nanoparticles on dewetting in ultra-thin films, both linear stability theory and numerical simulations are performed in the present study, with the consideration of oscillatory structural (OS) forces. Long scale approximation is utilized to simplify the hydrodynamic and diffusion equations to a nonlinear system for film thickness and nanoparticle concentration. Results show that the presence of nanoparticles generally suppresses the dewetting process. Two physical mechanisms responsible for this phenomenon are addressed in the present study. When the oscillatory structural forces are relatively smaller, the essential feature of film evolution is similar to the case of particle-free flow. The reduction of the linear growth rate and the postponement of film rupturing can be attributed to the increment of the viscosity due to the presence of nanoparticles. On the other hand, when the intensity of the OS forces becomes stronger, the stepwise thinning of film can be observed which prevents the film from rupture. Numerical simulations indicate that this phenomenon is caused by the existence of a stable zone due to the oscillatory nature of the structural forces. Another interesting finding is that the nonuniformity of the distribution of nanoparticle concentration might destabilize a spinodally stable film, and trigger the occurrence of film dewetting.

Structural Forces in Soft Matter Systems

Oscillating structural forces arise when nanoscale colloids are confined at high concentration between two approaching surfaces. As layers of colloid are squeezed out, changes in osmotic pressure cause alternating regions of repulsion and attraction. Here, we provide direct measurements of such oscillatory structural forces between the soft interfaces of two emulsion droplets. Quantitative comparison indicates that the deformable nature of droplets allows them to act as far more sensitive probes than solid spheres. In addition, the responsive nature of soft surfaces can give rise to unexpected behaviors not encountered in rigid systems including reversible aggregation/flocculation for emulsion droplets and, potentially, spatial ordering within concentrated emulsion phases.

Structural forces in soft matter systems: unique flocculation pathways between deformable droplets

Oscillatory structural forces caused by colloidal additives including micelles, microemulsion droplets and particles were explored between rigid and deformable interfaces using direct force measurements with the atomic force microscope. The observed oscillations from rigid surfaces become distorted when confinement occurs between deformable interfaces, giving rise to a force hysteresis between approaching and separating interfaces. Small-angle neutron scattering was used to determine the bulk structure of the colloidal additives, as a basis for comparison with their behaviour when confined in thin films. It is seen that confinement itself does not appear to significantly alter the structure of the colloidal additive when compared to the bulk; however, at small separations, interactions with the confining interfaces may become important. The combined approach uncovered an unique flocculation pathway that is available to deformable emulsion droplets, and that the strength of this flocculation can be tuned by changes in the size and concentration of the structuring colloid, the emulsion droplet size, and the ionic strength of the solution.

Molecular modeling of proteinlike inclusions in lipid bilayers: Lipid-mediated interactions

We investigated the insertion of transmembrane structures in a lipid bilayer and their interactions using self-consistent field theory. The lipids are coarse-grained on a united-atom level and consist of a phosphatidylcholinelike headgroup and two hydrophobic tails. The inclusions, acting as simple models for proteins that span biological membranes, are rigid rods (radius R ) with a hydrophobic surface and hydrophilic end caps. The insertion free energy Omega of an individual rod is strongly regulated by the affinity between its hydrophobic surface and the lipid tails. This affinity also controls the best match of the hydrophobic length of the rod with that of the bilayer. The line tension tau(=Omega/2piR) is practically independent of R . The perturbations in the bilayer as a function of distance from the inclusion, have the shape of a damped oscillation. The wavelength and decay length are related to the elastic properties of the bilayer and do not depend on R . These results are used to analyze how the lipid matrix affects the interaction between transmembrane objects, for computational reasons considering the limit of R-->infinity . Contributions on different length scales can be distinguished: (i) a long-range elastic interaction, which is an exponentially decaying oscillation; (ii) an exponentially decaying repulsion on an intermediate length scale, resulting from the loss of conformational entropy of the lipid tails; and (iii) a short-range interaction due to the finite compressibility of the lipid tails, which manifests either as a depletion attraction if there is no affinity between the tails and the inclusions' surface or, otherwise, as an oscillatory structural force.

Layering and shear properties of an ionic liquid, 1-ethyl-3-methylimidazolium ethylsulfate, confined to nano-films between mica surfaces

We report high-resolution measurements of the forces between two atomically smooth solid surfaces across a film of 1-ethyl-3-methylimidazolium ethylsulfate ionic liquid, for film thickness down to a single ion diameter. For films thinner than approximately 2 nm oscillatory structural forces are observed as the surface separation decreases and pairs of ion layers are squeezed out of the film. Strikingly, measurements of the shear stress of the ionic liquid film reveal low friction coefficients which are 1-2 orders of magnitude smaller than for analogous films of non-polar molecular liquids, including standard hydrocarbon lubricants, up to ca. 1 MPa pressure. We attribute this to the geometric and charge characteristics of the ionic liquid: the irregular shapes of the ions lead to a low shear stress, while the strong coulombic interactions between the ions and the charged confining surfaces lead to a robust film which is maintained between the shearing surfaces when pressure is applied across the film.

The Importance of Oscillatory Structural Forces in the Sedimentation of a Binary Hard-Sphere Colloidal Suspension

Concentrated colloidal dispersions are complex systems comprised of colloidal particles of various sizes. This paper summarizes recent experimental and theoretical findings on understanding the interactions between microspheres in the presence of submicrometer particles; the submicrometer particles can be solvent molecules, surfactant micelles, or other nanometer-sized particles. In a hard-sphere suspension containing microspheres in the presence of submicrometer particles, oscillatory structural (entropic) forces between large particles, caused by small particles, arise as a result of the collective particle−particle interactions. The magnitude of the oscillatory structural forces is theoretically estimated based on the statistical mechanics approach, Monte Carlo simulations, and stochastic particle dynamics methods. Also, sedimentation experiments using spherical low-charged, binary hard-sphere particle suspensions with a large size ratio are presented to elucidate the importance of the oscillatory stru...

Generalization of Kelvin's equation for compressible liquids in nanoconfinement

The Kelvin equation for a compressible liquid in nanoconfinement is written in a form that takes into account not only Laplace's pressure, but also the oscillatory compression pressure. This leads to a simple analytical equation for pressure in nanocapillaries. The corrected equation is used to analyze properties of aqueous systems, including the oscillatory structural forces between attractive surfaces and inert surfaces, repulsive “hydration” forces between hydrophilic surfaces, and attractive “hydrophobic” forces between hydrophobic surfaces. Relative vapor pressure in a nanocapillary also is discussed.

Direct Comparison of Atomic Force Microscopic and Total Internal Reflection Microscopic Measurements in the Presence of Nonadsorbing Polyelectrolytes

We have investigated the structural and depletion forces between silica glass surfaces in aqueous, salt-free solutions of sodium poly(styrene sulfonate). The interaction forces were investigated by two techniques: total internal reflectance microscopy (TIRM) and colloid probe atomic force microscopy (AFM). The TIRM technique measures the potential energy of interaction directly, while the AFM is a force balance. Comparison between the data sets was used to independently calibrate the AFM data since the separation distances cannot be unequivocally determined by this technique. Oscillatory structural forces are excellent for this work since they give multiple reference points against which to analyze. Comparison of the data from the two techniques highlighted significant uncertainties in the AFM data. At low polymer concentrations, a significant uncertainty in the apparent zero separation distance was seen as a result of the AFM cantilever reaching an apparent constant compliance region prior to any real contact between the surfaces. Further complications arising from the number and position of the measured minima were also seen in the dilute polymer concentration regime as a result of hydrodynamic drainage between the approaching surfaces in the AFM perturbing the delicate structural components in the fluid.

Texture and stability of emulsions and suspensions: role of oscillatory structural forces

Texture and stability of emulsions and suspensions: role of oscillatory structural forces

Texture and stability of emulsions and suspensions: role of oscillatory structural forces

The stability of macro-dispersions, such as emulsions and particle suspensions, is characterized in different ways—creaming or sedimentation, flocculation of drops/particles, coalescence between drops or phase separation. Several novel experimental techniques have been developed in our laboratory to examine both the texture and stability of emulsions and suspensions. These methods include direct image analysis to extract the emulsion radial distribution function and to determine the effective inter-droplet interaction, and the Kossel diffraction technique, which is used to obtain the structural factors. The film thinning interferometric technique employing our capillary force balance is used to study the role of the surfactant micelles/colloidal particle-layering phenomenon and the in-layer structure formation. Monte Carlo simulations and a theoretical model based on the Ornstein–Zernike equation of statistical mechanics are used to discern the effects of the micelle/particle structuring and layering phenomenon in confined films between two droplets/particles. These experiments and theoretical calculations are used to gain a fundamental understanding of the role of long-range oscillatory (repulsion/attraction) structural interactions on the stability of both mono- and polydispersed systems. During the past 10 years, our research group has worked on several problems of interest to industry in which structural forces in emulsions and suspensions appear to play an important role. This paper is an overview of some of these relevant examples.

Texture and Stability of Emulsions: Role of Oscillatory Structural Forces

ABSTRACT A digitized optical imaging technique was used to obtain the droplet size distribution, texture, and the radial distribution function which determines the inter-droplet interactions in emulsion systems. The effects of sucrose ester and polyglycerin stearic acid ester as emulsifiers on the stability (i.e., creaming) of oil-in-water food emulsions were investigated, ft was observed that as the concentration of the emulsifier was increased, the droplet size decreased, and the emulsion became more monodispersed and the stability increased. This was confirmed by the experimentally determined radial distributions and the structure factors. It was found that the emulsion made with the fatty acid ester was more stable than that with sucrose ester, and was less polydisperse with better texture. A statistical thermodynamic model was applied which accounts for the droplet-droplet interaction forces, i.e., oscillatory structural forces, and the polydispersity effect to predict the creaming velocity of an oil-in-water emulsion. Good agreement was found between the experimentally determined creaming velocity and the model predictions.

Atomic Force Microscopy at Solid−Liquid Interfaces

An atomic force microscope (AFM) operating in force modulation mode is used to study solvation forces at the interface between a graphite (HOPG) surface and the liquids octamethylcyclotetrasiloxane (OMCTS) and n-dodecanol. Simple analytical models are used that adequately describe the response of the cantilever as the modulation frequency and tip−sample interaction change. The analysis of AFM force curves yields the tip−sample interaction stiffness and damping. Hydrodynamic damping is significant for all the levers used and at present this limits the sensitivity of detecting weak tip−surface damping effects. The main results are: (i) Confinement of liquid between two surfaces can lead to oscillatory structural forces even when one of the surfaces has very high curvature. This could influence topographic images at the atomic level in liquids. In these experiments the typical radius for a sharp AFM tip is measured as Rtip ≈ 14 nm. (ii) The effective viscosity increases by ∼4 orders of magnitude for a sharp tip interacting with the OMCTS solvation layers nearest the surface. (iii) The AFM data are compared to published results obtained using the surface force apparatus (SFA). The AFM data for the interaction stiffness and damping are qualitatively similar to the SFA results, but the magnitude of the effects is smaller. This most likely arises from the limited interaction area over which the confined molecules can exhibit cooperative behavior (∼100 nm2). (iv) Oscillatory solvation forces can also be observed with very blunt tips (Rtip ≈ 350 nm), and the data suggest that in this case tip microasperities dominate the tip−sample interaction.

Role of Oscillatory Structural Forces for Interactions in Thin Emulsion Films Containing Micelles

We study thin aqueous films sandwiched between two oil phases (styrene). The system is stabilized by an anionic surfactant. These films exhibit stepwise thinning (stratification), due to the presence of micellar aggregates. By means of dynamic and static light scattering, we determine the hydrodynamic diameter and the aggregation number of the micelles. Using effective micellar volume fraction, we calculate the contribution of the oscillatory structural forces to the energy of interaction between the two film surfaces. Adding also the van der Waals and the electrostatic interactions, we are able to predict the contact angles of films which contain one layer of micelles. These angles are measured by interferometry, and the obtained experimental values agree very well with the theoretical estimates. It is proved that the oscillatory structural energy dominates in the total energy of interaction. The addition of electrolyte leads to smaller contact angles. This is unexpected in view of the conventional DLVO ...

Role of Surface Forces in the Stability of Evaporating Thin Liquid Films That Contain Surfactant Micelles

We consider an evaporating liquid film which lies on a planar heated solid substrate. The film contains a dissolved surfactant at a high concentration, so that micellar aggregates exist in the bulk. Linear stability analysis of this system is performed by investigating the time evolution of the amplitude of fluctuation waves. The liquid-vapor interface is regarded as a two-dimensional continuum characterized by intrinsic viscosity, specific adsorption, and surface tension. The latter quantities depend on the instantaneous subsurface concentration of surfactant monomers (subject to fluctuation) and upon the temperature. At small Reynolds and large Peclet numbers, and for thin films, the lubrication approximation model can be applied to solve the hydrodynamic problem. In the balance of normal stress at the fluid interface, we account for the contribution of intermolecular forces. There are van der Waals, electrostatic, steric, and oscillatory structural interactions, which are described in terms of separate components of disjoining pressure. The oscillatory structural forces are due to the presence of surfactant micelles or other colloidal particles in the film. These forces turn out to have the highest magnitude, and are of great importance for the stability. We solve numerically the evolution equation for the fluctuation, thus finding the critical thickness of film rupture and the critical lateral wave number. The influence of the surfactant type and concentration and the relative significance of the particular interactions under different conditions are discussed in detail.

Stability of emulsions under equilibrium and dynamic conditions

In the present article we analyse the influence of various factors, both thermodynamic and hydrodynamic, on the stability of emulsion systems. The effect of the droplet size on the droplet lifetime in an emulsion cream is quantified. The comparative importance of kinetic factors such as surface and bulk diffusion fluxes, or viscous and elastic surface stresses, is investigated. The fact that the emulsion films drain much more slowly when the surfactant is dissolved in the continuous phase (rather than in the disperse phase) provides a new understanding of the Bancroft rule and the process of chemical demulsification. New thermodynamic aspects of emulsion stability are also discussed. One of them is the relatively high surface electric potential of pure oil-water interfaces and adsorption monolayers of non-ionic surfactants. Another aspect is the role of non-DLVO surface forces, such as the hydration repulsion, oscillatory structural and depletion forces due to the presence of surfactant micelles. A criterion of emulsion stability is formulated synthesizing the effects of the major factors. Finally we consider the importance of the kinetic phenomena in emulsions formed from non-pre-equilibrated phases.

Analytical expression for the oscillatory structural surface force

A semiempirical expression is proposed to describe the dependence of the oscillatory surface force on both film thickness and particle volume fraction for a hard sphere fluid between hard walls. The expression is tested successfully against the predictions of integral equations, computer simulations and experimental data for liquid films containing ionic and nonionic surfactant micelles. The proposed expression can be used to interpret experimental data for stability of liquid films and colloidal dispersions, or can be incorporated in complex theoretical models of dynamics of liquid film drainage, or kinetics of flocculation in emulsions and suspensions.

Adhesion and short-range forces between surfaces. Part II: Effects of surface lattice mismatch

The adhesion forces and interaction potentials between two mica surfaces as a function of the orientation (twist angle) of their surface lattices are reported. The forces were measured in air, in water, and in an aqueous KCl solution where oscillatory structural forces are present. In air, the adhesion is relatively independent of the twist angle θ in the range −10° < θ < +10° due to a 0.4 nm thick amorphous layer at the interface. In water, apart from a relatively angle-independent baseline adhesion, a sharp adhesion peak (energy minimum) occurs at θ = 0°, corresponding to maximum alignment of the surface lattices. As little as ±1° away from this peak the energy decreases by 50%. In aqueous KCl solution, due to potassium ion adsorption the water between the surfaces becomes ordered, resulting in an oscillatory structural force where the adhesive minima occur at discrete separations corresponding to an integral number of water layers. The adhesion energies corresponding to the first three potential minima were angle dependent near θ = 0° (again decreasing by 50% at ±1° away from θ = 0°). The repulsive maxima were also affected near θ = 0°. The results show that the whole interaction potential between two surfaces in liquids depends on the orientation of the surface lattices, and that these effects can extend at least four molecular layers. We discuss the consequences of these findings for material properties such as grain boundary energies, cracks, and friction.