Pull-off force measurements between rough surfaces by atomic force microscopy

Direct measurements of the pull-off (adhesion) forces between pharmaceutical particles (beclomethasone dipropionate, a peptide-type material, and lactose) with irregular geometry and rough polymeric surfaces (series of polypropylene coatings, polycarbonate, and acrylonitrile-butadiene-styrene) were carried out using the atomic force microscope. These measurements showed that roughness of the interacting surfaces is the significant factor affecting experimentally measured pull-off forces. A broad distribution of pull-off force values was noted in the measurements, caused by a varying adhesive contact area for a particle located on rough substrate. The possibility of multiple points of contact between irregularly shaped pharmaceutical particles and substrate surfaces is demonstrated with nanoindentations of the particle in a fluoro-polymer film. Force-distance curves showing the "sawtooth" pattern are additional evidence that particles make contact with substrates at more than one point. Reduced adhesion of 10- to 14-microm-diameter lactose and peptide material particles to the polypropylene coatings with a roughness of 194 nm was found in this study. Similar pull-off force versus roughness relationships are also reported for the model spherical particles, silanized glass particle with a size of 10 microm and polystyrene particle with a diameter of 9 microm, in contact with polypropylene coatings of varying roughness characteristics. It was found that the model recently proposed by Rabinovich et al. (J. Colloid Interface Sci. 232, 1-16 (2000)) closely predicts the pull-off forces for glass and lactose particles. On the other hand, the adhesion of the peptide material and polystyrene particle to polypropylene is underestimated by about an order of magnitude with the theoretical model, in which the interacting substrates are treated as rigid materials. The underestimate is attributed to the deformation of the peptide material and polystyrene particles.     

Measuring the size dependence of Young's modulus using force modulation atomic force microscopy

The dependence of the local Young's modulus of organic thin films on the size of the domains at the nanometer scale is systematically investigated. Using atomic force microscopy (AFM) based imaging and lithography, nanostructures with designed size, shape, and functionality are preengineered, e.g., nanostructures of octadecanethiols inlaid in decanethiol self-assembled monolayers (SAMs). These nanostructures are characterized using AFM, followed by force modulation spectroscopy and microscopy measurements. Young's modulus is then extracted from these measurements using a continuum mechanics model. The apparent Young's modulus is found to decrease nonlinearly with the decreasing size of these nanostructures. This systematic study presents conclusive evidence of the size dependence of elasticity in the nanoregime. The approach utilized may be applied to study the size-dependent behavior of various materials and other mechanical properties.     

Isoelectric point of fluorite by direct force measurements using atomic force microscopy

Interaction forces between a fluorite (CaF2) surface and colloidal silica were measured by atomic force microscopy (AFM) in 1 x 10(-3) M NaNO3 at different pH values. Forces between the silica colloid and fluorite flat were measured at a range of pH values above the isoelectric point (IEP) of silica so that the forces were mainly controlled by the fluorite surface charge. In this way, the IEP of the fluorite surface was deduced from AFM force curves at pH approximately 9.2. Experimental force versus separation distance curves were in good agreement with theoretical predictions based on long-range electrostatic interactions, allowing the potential of the fluorite surface to be estimated from the experimental force curves. AFM-deduced surface potentials were generally lower than the published zeta potentials obtained from electrokinetic methods for powdered samples. Differences in methodology, orientation of the fluorite, surface carbonation, and equilibration time all could have contributed to this difference.     

Reliable measurements of interfacial slip by colloid probe atomic force microscopy. II. Hydrodynamic force measurements

Here we report a new study on the boundary conditions for the flow of a simple liquid in a confined geometry obtained by measuring hydrodynamic drainage forces with colloid probe atomic force microscopy (AFM). In this work, we provide experimental data obtained using a best practice experimental protocol and fitted with a new theoretical calculation (Zhu, L.; Attard, P.; Neto, C. Langmuir 2010, submitted for publication, preceding paper). We investigated the hydrodynamic forces acting on a silica colloid probe approaching a hydrophobized silicon surface in a single-component viscous Newtonian liquid (di-n-octylphthalate), a partially wetting system. The measured average slip lengths were in the range of 24-31 nm at approach velocities of between 10 and 80 μm/s. Using our experimental approach, the presence of nanoparticle contaminants in the system can be indentified, which is important because it has been shown that nanoparticles lead to a large apparent slip length. Under our stringent control of experimental conditions, the measurement of the slip length is reproducible and independent of the spring constant of the cantilever.     

Characterization of distance-dependent damping in tapping-mode atomic force microscopy force measurements in liquid

We have used a spectral analysis method to characterize changes in the local damping coefficient for an acoustically driven cantilever as it approaches a hard surface in liquid. We show a significant distance dependence of the damping coefficient (and associated quality factor) that must be accounted for to achieve successful theoretical reproduction of experimental tapping-mode force curves. We model the cantilever dynamics using a forced damped harmonic oscillator model and solve the equation of motion using the method of finite differences. Experiments in solutions of differing viscosities show that bulk viscous damping is not the source of the system dissipation, while simulations of the cantilever dynamics including adhesion hysteresis also eliminate this as the origin of the dissipation. We conclude that frictional dissipation that occurs with the intermittent contact is the likely source of dissipation in the system. Our results identify a semiquantitative means of interpreting tapping-mode force curves on nondeformable surfaces in liquid.     

Molecular views and measurements of hemostatic processes using atomic force microscopy

Hemostasis and thrombosis are highly complex and coordinated interfacial responses to vascular injury. In recent years, atomic force microscopy (AFM) has proven to be a very useful approach for studying hemostatic processes under near physiologic conditions. In this report, we review recent progress in the use of AFM for studying hemostatic processes, including molecular level visualization of plasma proteins, protein aggregation and multimer assembly, and structural and morphological details of vascular cells under aqueous conditions. AFM offers opportunities for visualizing surface-dependent molecular and cellular interactions in three dimensions on a nanoscale and for sensitive, picoNewton level, measurements of intermolecular forces. AFM has been used to obtain molecular and sub-molecular, resolution of many biological molecules and assemblies, including coagulation proteins and cell surfaces. Surface-dependent molecular processes including protein adsorption, conformational changes, and subsequent interactions with cellular components have been described. This review outlines the basic principles and utility of AFM for imaging and force measurements, and offers objective perspectives on both the advantages and disadvantages. We focus primarily on molecular level events related to hemostasis and thrombosis, particularly coagulation proteins, and blood platelets, but also explore the use of AFM in force measurements and surface property mapping.     

Flat hydrogel substrate for atomic force microscopy to observe liposomes and lipid membranes

In order to avoid denaturation of biomolecules due to strong adsorption on solid surfaces, a soft substrate has to be used for atomic force microscopy (AFM) observation. We propose a hydrophilic agarose gel surface as a soft substrate for AFM to observe liposomes and lipid membranes. Although our simple method does not require any delicate control at the molecular level, an agarose gel surface can be simply flattened to 0.3 nm in roughness using an atomically flat solid surface during gelation. The AFM images revealed that liposomes were unruptured on the gel surface at low liposome density, whereas an unruptured state was difficult to obtain on a solid surface like mica. This indicates that the weak interaction between the liposome and the soft surface inhibits the liposome from rupturing, and also that the surface rougher than the solid surface prevents lateral diffusion of the liposomes along the surface to be fused. Increasing the liposome density resulted in a lipid membrane at various thicknesses forming on the hydrogel surface by the fusion and rupture of liposomes. Using the soft substrate, it can be expected to promote investigations of structures and functions of biomolecules at the nanometer scale under physiological conditions with AFM.     

Capillary force in atomic force microscopy

Under ambient conditions, a water meniscus generally forms between a nanoscale atomic force microscope tip and a hydrophilic surface. Using a lattice gas model for water and thermodynamic integration methods, we calculate the capillary force due to the water meniscus for both hydrophobic and hydrophilic tips at various humidities. As humidity rises, the pull-off force rapidly reaches a plateau value for a hydrophobic tip but monotonically increases for a weakly hydrophilic tip. For a strongly hydrophilic tip, the force increases at low humidities (<30%) and then decreases. We show that mean-field density functional theory reproduces the simulated pull-off force very well.     

Single-photon atomic force microscopy

In the last few years, an array of novel technologies, especially the big family of scanning probe microscopy, now often integrated with other powerful imaging tools such as laser confocal microscopy and total internal reflection fluorescence microscopy, have been widely applied in the investigation of biomolecular interactions and dynamics. But it is still a great challenge to directly monitor the dynamics of biomolecular interactions with high spatial and temporal resolution in living cells. An innovative method termed “single-photon atomic force microscopy” (SP-AFM), superior to existing techniques in tracing biomolecular interactions and dynamics in vivo, was proposed on the basis of the combination of atomic force microscopy with the technologies of carbon nanotubes and single-photon detection. As a unique tool, SP-AFM, capable of simultaneous topography imaging and molecular identification at the subnanometer level by synchronous acquisitions and analyses of the surface topography and fluorescent optical signals while scanning the sample, could play a very important role in exploring biomolecular interactions and dynamics in living cells or in a complicated biomolecular background.     

Single molecule charging by atomic force microscopy

AFM/KPM charging and charge mapping of polyamine charge carriers in a PMMA matrix is reported. Selective charging of the designed charge carrier is demonstrated at concentrations down to a single molecule. This works constitutes electrochemical charging and detection of single redox-active organic molecules in low dielectric matrices by probe microscopy.     

Measuring molecular weight by atomic force microscopy

Absolute-molecular-weight distribution of cylindrical brush molecules were determined using a combination of the Langmuir Blodget (LB) technique and Atomic Force Microscopy (AFM). The LB technique gives mass density of a monolayer, i.e., mass per unit area, whereas visualization of individual molecules by AFM enables accurate measurements of the molecular density, i.e., number of molecules per unit area. From the ratio of the mass density to the molecular density, one can determine the absolute value for the number average molecular weight. Assuming that the structure of brush molecules is uniform along the backbone, the length distribution should be virtually identical to the molecular weight distribution. Although we used only brush molecules for demonstration purpose, this approach can be applied for a large variety of molecular and colloidal species that can be visualized by a microscopic technique.     

Halo-like structures studied by atomic force microscopy

Nanometer-sized clusters of copper have been produced in a hollow cathode sputtering source and deposited on SiO_x. Halo-like structures consisting of micrometer sized protrusions in the silicon oxide surface surrounded by thin rings of smaller particles are observed. The area in between seems to be depleted of particles. We propose that the halo-like structures are a result of electrostatic forces acting between the incoming charged clusters and charged regions on the surface. A simple computer simulation supports this suggestion.     

Characterization of elastomeric blends by atomic force microscopy

The bulk mechanical properties of a blend of elastomers are found to depend on the micro and nano scale morphology of the phases of the materials in the blend. In this study, we examine the phase morphology of blends of incompatible elastomers using Atomic Force Microscopy (AFM). Specifically, nanoindentation and Tapping Mode AFM (TMAFM) imaging techniques are used as experimental tools for mapping the composition of unfilled elastomeric blends. Depending on the composition of the blend, either co‐continuous or discontinuous domain/matrix morphology is observed. To identify the different components in bromobutyl (BIIR)/natural rubber (NR) blends, nanoscale indentation measurements were made on the observed phase‐separated regions. Results from force mode AFM and mechanical measurements of bulk NR and BIIR are used to assist in the interpretation of the TMAFM results for the BIIR/NR blends. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 492–503, 2006     

Investigation of aerosol particles by atomic force microscopy

AFM has been applied for studying morphology and size distribution of nanometer-sized particles adsorbed on flat surfaces. In order to optimize imaging of these ultrafine particles different substrates were evaluated with respect to their roughness and stability under the influence of the sensing tip. Moreover, a method for calculating particle volumes from the three-dimensional AFM data is described. This greatly enhances the information content of AFM images, because a large number of particles in the raw data can be evaluated automatically in order to derive information on size distribution or surface coverage. This evaluation method has also been applied successfully to quantitatively describe changes on particles induced by different humidity of the surrounding atmosphere. Received: 15 July 1996 / Revised: 18 December 1996 / Accepted: 3 January 1997     

Thin liquid films studied by atomic force microscopy

Since its invention twenty years ago the atomic force microscope (AFM) has become one of the most important instruments in colloid and interface science. The ability of tracing force profiles between single particles or particles and flats in liquid environment makes it a tool-of-choice for investigating thin liquid films. In this paper we review experimental work on confined Newtonian and non-Newtonian liquids using the AFM.     

Confined polymer melts studied by atomic force microscopy

Using an atomic force microscope (AFM) the interaction between an AFM tip and different planar solid surfaces have been measured across a long-chain poly(dimethyl siloxane) (PDMS, M_W = 18,000 g/mol), a short-chain PDMS (M_W = 4200 g/mol), a poly(ethylmethyl siloxane) (PEMS, M_W = 16,800 g/mol), and a diblock copolymer consisting of one PDMS and one PEMS block (PDMS-b-PEMS, M_W = 15,100 g/mol). The interaction changed significantly during the first 10 h after immersing the solids in the polymer melt. This demonstrates that the time scale of structural changes at a solid surface is much slower than in the bulk. On mica and silicon oxide both polymers formed an immobilized “pinned” layer beyond which a monotonically decaying repulsive force was observed. Attractive forces were observed with short-chain PDMS on silicon oxide and PEMS on mica and silicon oxide. On the basal plane of graphite PEMS caused a stable, exponentially decaying oscillatory force.     

Structure of small actin-containing liposomes probed by atomic force microscopy: effect of actin concentration & liposome size

Actin-containing liposomes were prepared via extrusion through 400 and 600 nm pore diameter membranes at different monomeric actin concentrations in low ionic strength buffer (G-buffer). After subjecting the liposome dispersions to high ionic strength polymerization buffer (F-buffer), topological changes in liposome structure were studied using atomic force microscopy (AFM). Paired dumbbell, horseshoelike, and disklike assemblies were observed for actin-containing liposomes extruded through 400 and 600 nm pore diameter membranes. The topology of actin-containing liposomes was found to be highly dependent on both liposome size and actin concentration. At 1 mg/mL actin, the actin-containing liposomes transformed into a disklike shape, whereas, at 5 mg/mL actin, the actin-containing liposomes retained a spherical shape. On the basis of these observations, we hypothesize that actin could either polymerize on the surface of the inner leaflet of the liposome membrane or polymerize in the aqueous core of the liposome. We explain the associated shape changes induced in actin-containing liposomes on the basis of the hypothesized mechanism of actin polymerization inside the liposomes. At higher actin concentrations (5 mg/mL), we observed membrane-induced actin self-assembly in G-buffer, which implies that G-actin is able to interact directly with lipid bilayers at sufficiently high concentrations.     

Silicalite crystal growth investigated by atomic force microscopy

Atomic force microscopy has been used to image the various facets of two morphologically distinct samples of silicalite. The smaller (20 microm) sample A crystals show 1 nm high radial growth terraces. The larger (240 microm) sample B crystals show growth terraces 1 to 2 orders of magnitude higher than the terraces on sample A with growth edges parallel to the crystallographic axes. Moreover, the terraces on the (010) face are significantly higher than the terraces on the (100) face - inconsistent with the previously proposed 90 degrees intergrowth structure. Sample A highlights that under certain synthetic conditions, silicalite grows in a manner akin to zeolites Y and A, via the deposition of layers comprising, in the case of silicalite, pentasil chains. It is probable that the rate of terrace advance is identical on the (010) and (100) faces, and it is the rate of terrace nucleation that dictates the overall growth rate of each facet and hence the relative size expressed in the final crystal morphology. Analysis of the growth terraces of sample B and detailed consideration of the structures of both MFI, and a closely related material MEL, lead to the proposal of a generalized growth mechanism for silicalite including the incorporation of defects within the structure. These defects are thought to be responsible for both the relative and the absolute terrace heights observed and may also explain the hourglass phenomenon observed by optical microscopy. The implications of this growth mechanism, supported by results of infrared microscopy, generate a new dimension to the continuing debate on the existence of intergrowths within one of the most important structures relevant to zeolite catalysis.