Oscillatory Force Profiles Research Articles

Structural and Double Layer Forces between Silica Surfaces in Suspensions of Negatively Charged Nanoparticles.

Direct force measurements between negatively charged silica microparticles are carried out in suspensions of like-charged nanoparticles with atomic force microscopy (AFM). In agreement with previous studies, oscillatory force profiles are observed at larger separation distances. At smaller distances, however, soft and strongly repulsive forces are present. These forces are caused by double layer repulsion between the like-charged surfaces and can be quantitatively interpreted with the Poisson-Boltzmann (PB) model. However, the PB model must be adapted to a strongly asymmetric electrolyte to capture the nonexponential nature of these forces. Thereby, the nanoparticles are modeled as highly charged co-ions, while the counter ions are monovalent. This model permits extraction of the effective charge of the nanoparticles, which is well comparable to the one obtained from electrophoresis. The PB model also explains the presence of a particle-free layer close to the interface.

A Nudge or a Shove: Altering Actomyosin Pulse Profiles InVivo.

Pulsed actomyosin contractions drive morphogenetic processes, but how cyclic frequencies and amplitudes of contractions are tuned to achieve processive shrinking of cell surfaces remains unclear. In this issue of Developmental Cell, Cavanaugh etal. (2020) use optogenetics and biophysical modeling to demonstrate how cells respond to different oscillatory force profiles.

Impact of Hydrophilic/Hydrophobic Surface Chemistry on Hydration Forces in the Absence of Confinement

The oscillatory force profile, observed in liquids due to molecular ordering at interfaces, has been extensively investigated by means of atomic force microscopy, but it remains unclear whether molecular ordering is present at the tip apex. Using a displacement-sensitive, low-noise atomic force microscope (AFM) operated in dynamic mode, with a tip of radius < 1 nm, we have investigated the force profile between two approaching surfaces of the same or different hydrophilic and hydrophobic character. By directly comparing different surface chemistry interactions, we have been able to elucidate whether an oscillatory force profile is due to structured water layers adjacent to the surface, the tip, or a combination of the two. We have found that an oscillatory force profile is observed when the surface is hydrophilic in nature, irrespective of whether the tip is hydrophilic or hydrophobic. When the surface is hydrophobic, an oscillatory force profile is not measured, but rather a monotonic repulsive or a short-range attractive force is observed for interactions with a hydrophilic or a hydrophobic tip, respectively. Thus, we attribute the measurement of an oscillatory force profile, in the absence of lateral confinement effects, solely to water layers adjacent to a hydrophilic surface rather than the structuring of water at the tip apex. This is the first direct evidence that solvation forces occur solely as a result of water layers adjacent to the substrate.

Direct Imaging of Individual Intrinsic Hydration Layers on Lipid Bilayers at Ångstrom Resolution

The interactions between water and biological molecules have the potential to influence the structure, dynamics, and function of biological systems, hence the importance of revealing the nature of these interactions in relation to the local biochemical environment. We have investigated the structuring of water at the interface of supported dipalmitoylphosphatidylcholine bilayers in the gel phase in phosphate buffer solution using frequency modulation atomic force microscopy (FM-AFM). We present experimental results supporting the existence of intrinsic (i.e., surface-induced) hydration layers adjacent to the bilayer. The force versus distance curves measured between the bilayer and the AFM tip show oscillatory force profiles with a peak spacing of 0.28 nm, indicative of the existence of up to two hydration layers next to the membrane surface. These oscillatory force profiles reveal the molecular-scale origin of the hydration force that has been observed between two apposing lipid bilayers. Furthermore, FM-AFM imaging at the water/lipid interface visualizes individual hydration layers in three dimensions, with molecular-scale corrugations corresponding to the lipid headgroups. The results demonstrate that the intrinsic hydration layers are stable enough to present multiple energy barriers to approaching nanoscale objects, such as proteins and solvated ions, and are expected to affect membrane permeability and transport.

Long-Range Interactions between Soft Colloidal Particles in Slit−Pore Geometries

Combining theoretical and experimental techniques, we investigate the structure formation of charged colloidal suspensions of silica particles in bulk and in spatial confinement (slit-pore geometry). Our focus is to identify characteristic length scales determining typical quantities, such as the position of the main peak of the bulk structure factor and the period of the oscillatory force profile in the slitpore. We obtain these quantities from integral equations/SANS experiments (bulk) and Monte Carlo simulations/colloidal probe-AFM measurements (confinement), in which the theoretical calculations are based on the Derjaguin-Landau-Verwey-Overbeck (DLVO) potential. Both in bulk and in the slitpore, we find excellent qualitative and quantitative agreement between theory and experiment as long as the ionic strength chosen in the DLVO potential is sufficiently low (implying a relatively long-ranged interaction). In particular, the bulk properties of these systems obey the widely accepted density scaling of xi proportional to phi(-1/3). On the other hand, systems with larger ionic strengths and, consequently, more short-ranged interactions do not obey such power law behavior and rather resemble an uncharged hard-sphere fluid, in which the relevant length scale is the particle diameter.

AFM colloidal forces measured between microscopic probes and flat substrates in nanoparticle suspensions

Colloidal forces between atomic force microscopy probes of 0.12 and 0.58 N/m spring constant and flat substrates in nanoparticle suspensions were measured. Silicon nitride tips and glass spheres with a diameter of 5 and 15 μm were used as the probes whereas mica and silicon wafer were used as substrates. Aqueous suspensions were made of 5–80 nm alumina and 10 nm silica particles. Oscillatory force profiles were obtained using atomic force microscope. This finding indicates that the nanoparticles remain to be stratified in the intervening liquid films between the probe and substrate during the force measurements. Such structural effects were manifested for systems featuring attractive and weak repulsive interactions of nanoparticles with the probe and substrate. Oscillation of the structural forces shows a periodicity close to the size of nanoparticles in the suspension. When the nanoparticles are oppositely charged to the probes, they tend to coat the probes and hinder probe–substrate contact.

Structuring of Macroions Confined between Like-Charged Surfaces

We investigate the structuring of charged spherical nanoparticles and micelles (i.e., "macroions") between two surfaces as a function of bulk macroion concentration. Structuring is deduced from measured force profiles between a silica particle and a silica plate in the presence of an aqueous macroion (Ludox silica nanoparticle or sodium dodecyl sulfate micelle) solution, obtained with an atomic force microscope. We observe oscillatory force profiles that decay with separation. We find that the wavelength of the force profiles scales with the bulk number density as rho(-)(1/3), rather than with the effective macroion size. Only at very high silica nanoparticle concentration (above 10 vol %) in a low ionic strength solution does the wavelength become smaller than that predicted by the simple rho(-)(1/3) scaling; however, the original scaling is recovered upon the addition of a small amount of electrolyte. A comparison between the measured wavelength and the predicted spacing between the macroions in the bulk shows that the two variables differ in both magnitude and bulk density scaling. This finding suggests that confined macroions are more ordered than those in the bulk and the nature of this ordering is maintained over a relatively wide range of bulk concentration.

Solvation forces in branched molecular liquids.

The solvation force of squalane confined between a silicon tip and a graphite surface has been measured by atomic force microscopy. This highly branched molecule shows oscillatory force profiles similar to those of spherical and linear chain molecules. Squalane molecules closest to the substrate are tightly bound and finer details imply that interdigitation occurs. This agrees with computer simulations for branched molecules but differs qualitatively from force balance experiments. These differences arise from the smaller confinement area and the different chemical nature of the surfaces.

Forces between aqueous nonuniformly charged colloids from molecular simulation

NVT Monte Carlo simulation results are presented for the forces between charged colloids within the primitive model for electrolytes. The calculations show that when charged colloids have a net dipole moment, a strong attraction can arise at short separations. The attractive force is not purely electrostatic; significant contributions follow from hard-sphere collisions between the electrolyte ions and the colloidal particles. In divalent electrolyte solutions, nonuniformly charged colloids show an oscillatory force profile as a function of separation, due to layering of electrolyte ions around the interacting colloids. Simulation results are compared to two analytical models derived from classical Debye–Hückel screened potentials. In the first model, contributions from charge–charge, dipole–dipole, and charge–dipole interactions are independently angle-averaged and then added to obtain the colloid–colloid potential. In the second model, the pair potential is obtained by simultaneously angle-averaging all interactions. Our results show that simultaneous angle-averaging of anisotropic interactions provides significant improvement over the commonly used additivity approximation.

Application of data dependent systems approach for evaluation of fracture modes during a single-grit scratching

Brittle fracture and the associated material removal mechanisms in model brittle materials were investigated using single-grit scratch experiments. The objective was to develop a fundamental understanding of the mechanisms of brittle material removal during machining (e.g., grinding) processes and mathematically describe the induced topological features. Initial experiments were conducted on metals and the ductile material removal response was captured in the form of smooth lenticular shaped scratch trace and a smooth semi-sinusoidal force profile. Scratch experiments on model brittle materials, namely, Homalite-100 and Pyrex glass, revealed intermittent material removal patterns and oscillatory force profiles. In Homalite, material removal occurred in periodic bursts with the size of the burst proportional to the instantaneous depth of cut, and accordingly the force profiles reflected periodic oscillations. In Pyrex glass, the material removal occurred in random bursts at irregular intervals and accordingly the force profiles reflected irregular/random force oscillations. To further characterize the nature of material removal process, the experimentally obtained force data were analyzed using data dependent systems (DDS) approach. The analysis revealed that the physical features of the induced damage in the scratch groove are effectively captured in the respective Green's functions. Based on the energy contributions arising from the characteristic roots of the DDS models, a brittleness measure, that characterizes the propensity for brittle fracture in ceramics, has been proposed. This measure can be used in more complex machining situations for on-line monitoring of the nature of material removal process.

Casimir effect in fluids above the isotropic-lamellar transition.

We study fluctuation-induced interaction in confined fluids above the isotropic-lamellar transition. At an ideally continuous transition, the disjoining pressure has the asymptotic form Pi(d-->infinity) approximately -Ck(B)Tq(2)(0)/d, where d is the interwall distance, q(0) is the wave number of the scattering peak, and C = 1/4pi in the strong anchoring limit. The long-rangedness is enhanced due to continuous distribution of soft modes in the q space. An unconventionally strong Casimir force with a range of several lamella thicknesses is realistic above the transition. We also find an oscillatory force profile near a surface-induced transition.

Oscillatory Forces from Polyelectrolyte Solutions Confined in Thin Liquid Films

Several combinations of polyelectrolyte−surfactant mixtures are studied to probe the various structure−property relationships these systems produce in thin liquid foam films. We show that either bulk solution properties or surface specific adsorption of the polymer−surfactant complexes formed can dominate the thin-film behavior. Above the polymer overlap concentration, Cp*, oscillatory force profiles are observed. In agreement with previous studies, we find that the oscillation period, Δh, scales as the negative square root of the polymer concentration, Δh ∼ Cp-1/2. In addition, by comparing directly the bulk polymer correlation length, ξ, to the oscillation period, we establish for the first time that Δh = ξ for these systems. These findings are compared to oscillatory force behavior seen with micellar solutions which display a scaling relation with micelle concentration, Cm, which follows Δh ∼ Cm-1/3. The different scaling exponents are explained by the local cylindrical and spherical symmetry of the molecular structures seen in the corresponding systems. Finally, it is also shown that the adsorption of polymer and/or complexes to the interface can govern the thin-film properties and swamp out the influence of bulk structural effects in certain systems.

Measurement of the interactions between two ordering surfaces under symmetric and asymmetric boundary conditions.

With a surface forces apparatus we have measured the interaction between two surfaces immersed in the isotropic and the nematic phases of a lyotropic solution near its lamellar phase. A smectic ordering shows up near walls, giving a specific oscillatory force profile that is shown to be the sum of two contributions. The oscillations are the elastic response of the stack mechanically constrained by the confinement. The shape of the base line supporting the oscillations derives from the distribution of the smectic ordering between the two walls. In the case of fixed symmetric boundary conditions, the background is always attractive. However, under fixed asymmetric boundary conditions, the background turns out to be repulsive at short separations and becomes even repulsive at every separation when one of the two surfaces does not induce any order. \textcopyright{} 1996 The American Physical Society.

Direct Measurement of the Interaction between Two Ordering Surfaces Confining a Presmectic Film

With a surface forces apparatus, we have investigated the isotropic and the nematic phases of a lyotropic solution near its lamellar phase. A smectic ordering shows up near walls, giving a specific oscillatory force profile which is shown to be the sum of two contributions. The symmetric distribution of the smectic order between the walls leads to an attractive background upon which oscillations arising from the elastic response of the layers are superimposed. In the isotropic phase, the oscillatory force is preceded by an attractive regime. We ascribe the attraction to a deeper penetration of the orientational ordering compared to the positional ordering range.

Direct measurement of depletion and structural forces in a micellar system.

The force as a function of separation is measured between two mica surfaces coated with adsorbed bilayers of cetyltrimethylammonium bromide (CTAB) and immersed in aqueous ionic micellar solutions of CTAB. At low separations double-layer forces are observed and arise solely from the dissociated counterions and free CTA + . At large separations, the repulsion is reduced and a secondary minimum in the interaction potential deepens when the micellar concentration is increased. Finally, at higher concentrations, structural erects superimpose an oscillatory force profile at further separations