Techniques for measuring surface forces

The forces acting between colloidal particles and between surfaces are of utmost importance for determining the behaviour of dispersed systems and adhesion phenomena. Several techniques are now available for direct measurement of these surface forces. In this review we focus on some of these methods. Two techniques for measuring forces between macroscopic solid surfaces; the interferometric surface force apparatus, known as the SFA, and a novel instrument which is based on a bimorph force sensor, the so-called MASIF, are described in some detail. Forces between a macroscopic surface and a particle can be measured with the atomic force microscope (AFM) using a colloidal probe, or by employing total internal reflection microscipy (TIRM) to monitor the position of a colloidal particle trapped by a laser beam. We also describe two different techniques that can be used for measuring forces between “soft” interfaces, the thin film balance (TFB) for single foam, emulsion and solid/fluid/fluid films, and osmotic stress methods, commonly used for studying interactions in liquid crystalline surfactant phases or in concentrated dispersions. The advantages and limitations of each of these techniques are discussed and typical results are presented.     

Direct measurement of depletion and hydrodynamic forces in solutions of a reversible supramolecular polymer

In this paper, the investigation of surface forces in semidilute solutions of a nonadsorbing hydrogen-bonded reversible supramolecular polymer is described. Colloidal probe atomic force microscopy was used for direct measurement of depletion forces. Hydrodynamic drag on the AFM cantilever with the colloidal probe was measured both far away from and close to the planar substrate surface. The results indicate that the presence of the depletion layer causes slip at the surfaces with a large apparent slip length. Our analysis explains how the presence of slip enables the measurement of (relatively weak) depletion forces in solutions with a high viscosity by significantly reducing the hydrodynamic forces. The range and magnitude of the measured depletion forces are qualitatively in agreement with previous experiments and theoretical predictions. Due to the relatively large experimental error, no quantitative conclusions can be drawn. Depletion-induced phase separation of suspensions of stearylated silica particles was also observed. Phase separation becomes more pronounced with increasing polymer concentration.     

Forces between a rigid probe particle and a liquid interface. II. The general case

The semianalytic theory developed previously (Chan, D. Y. C., Dagastine, R. R., and White, L. R., J. Colloid Interface Sci. 236, 141 (2001)) to predict the force curve of an AFM measurement at a liquid interface using a colloidal probe has been expanded to incorporate a general force law with both attractive and repulsive forces. Expressions for the gradient of the force curve are developed to calculate the point at which the probe particle on the cantilever will spontaneously jump in toward the liquid interface. The calculation of the jump instability is reduced to a straightforward embroidery of the simple algorithms presented in Chan et al. In a variety of sample calculations using force laws including van der Waals, electrostatic, and hydrophobic forces for both oil/water and bubble/water interfaces, we have duplicated the general behaviors observed in several AFM investigations at liquid interfaces. The behavior of the drop as a Hookean spring and the numerical difficulties of a full numerical calculation of F(deltaX) are also discussed.     

In situ hydrodynamic lateral force calibration of AFM colloidal probes

Lateral force microscopy (LFM) is an application of atomic force microscopy (AFM) to sense lateral forces applied to the AFM probe tip. Recent advances in tissue engineering and functional biomaterials have shown a need for the surface characterization of their material and biochemical properties under the application of lateral forces. LFM equipped with colloidal probes of well-defined tip geometries has been a natural fit to address these needs but has remained limited to provide primarily qualitative results. For quantitative measurements, LFM requires the successful determination of the lateral force or torque conversion factor of the probe. Usually, force calibration results obtained in air are used for force measurements in liquids, but refractive index differences between air and liquids induce changes in the conversion factor. Furthermore, in the case of biochemically functionalized tips, damage can occur during calibration because tip-surface contact is inevitable in most calibration methods. Therefore, a nondestructive in situ lateral force calibration is desirable for LFM applications in liquids. Here we present an in situ hydrodynamic lateral force calibration method for AFM colloidal probes. In this method, the laterally scanned substrate surface generated a creeping Couette flow, which deformed the probe under torsion. The spherical geometry of the tip enabled the calculation of tip drag forces, and the lateral torque conversion factor was calibrated from the lateral voltage change and estimated torque. Comparisons with lateral force calibrations performed in air show that the hydrodynamic lateral force calibration method enables quantitative lateral force measurements in liquid using colloidal probes.     

Conformation of poly(styrene sulfonate) layers physisorbed from high salt solution studied by force measurements on two different length scales

The conformation of poly(styrene sulfonate) (PSS) layers physisorbed from 1 M NaCl is determined by force measurements and imaging on two length scales. With colloidal probe technique steric forces as predicted for neutral grafted brushes are observed. On decrease and increase of the NaCl concentration, the grafting density remains constant, yet the brush thickness swells and shrinks reversibly with the salt concentration with an exponent of -0.3. At low salt conditions, the brush length amounts to 30% of the contour length, a behavior known for polyelectrolyte brushes and attributed to the entropy of the counterions trapped in the brush. Between a PSS layer and a pure colloidal silica sphere, the same steric forces are observed, and additionally at large separations (beyond the range of the steric repulsion) an electrostatic force is found. A negatively charged AFM tip penetrates the brush--a repulsive electrostatic force between the tip and surface is found, and single chains can be imaged. Thus, with the nanometer-sized AFM tip, the flatly adsorbed fraction of the PSS chains is seen, whereas the micrometer-sized colloidal probe interacts with the fraction of the chains penetrating into solution.     

Bubble colloidal AFM probes formed from ultrasonically generated bubbles

Here we introduce a simple and effective experimental approach to measuring the interaction forces between two small bubbles (approximately 80-140 microm) in aqueous solution during controlled collisions on the scale of micrometers to nanometers. The colloidal probe technique using atomic force microscopy (AFM) was extended to measure interaction forces between a cantilever-attached bubble and surface-attached bubbles of various sizes. By using an ultrasonic source, we generated numerous small bubbles on a mildly hydrophobic surface of a glass slide. A single bubble picked up with a strongly hydrophobized V-shaped cantilever was used as the colloidal probe. Sample force measurements were used to evaluate the pure water bubble cleanliness and the general consistency of the measurements.     

Characterization of the physical properties of model biomembranes at the nanometer scale with the atomic force microscope

Interaction forces and topography of mixed phospholipid-glycolipid bilayers were investigated by atomic force microscopy (AFM) in aqueous conditions with probes functionalized with self-assembled monolayers terminating in hydroxy groups. Short-range repulsive forces were measured between the hydroxy-terminated probe and the surface of the two-dimensional (2-D) solid-like domains of distearoyl-phosphatidylethanolamine (DSPE) and digalactosyldiglyceride (DGDG). The form and range of the short-range repulsive force indicated that repulsive hydration/steric forces dominate the interaction at separation distances of 0.3-1.0 nm after which the probe makes mechanical contact with the bilayers. At loads < 5 nN the bilayer was elastically deformed by the probe, while at higher loads plastic deformation of the bilayer was observed. Surprisingly, a short-range repulsive force was not observed at the surface of the 2-D liquid-like dioleoylphosphatidylethanolamine (DOPE) film, despite the identical head groups of DOPE and DSPE. This provides direct evidence for the influence of the structure and mechanical properties of lipid bilayers on their interaction forces, an effect which may be a major importance in the control of biological processes such as cell adhesion and membrane fusion. The step height measured between lipid domains in the AFM topographic images was larger than could be accounted for by the thickness and mechanical properties of the molecules. A direct correlation was observed between the repulsive force range over the lipid domains and the topographic contrast, which provides direct insight into the fundamental mechanisms of AFM imaging in aqueous solutions. This study demonstrates that chemically modified AFM probes can be used in combination with patterned lipid bilayers as a novel and powerful approach to characterize the nanometer scale chemical and physical properties of heterogeneous biosurfaces such as cell membranes.     

Evaluation of interaction forces between profilin and designed peptide probes by atomic force microscopy

We evaluated the binding affinity of peptide probes for profilin (protein) using force curve measurement techniques and atomic force microscopy (AFM). The peptide probes designed and synthesized for this investigation were H-A3GP5GP5GP5G-OH (1), H-A3GP5G-OH (2), H-A3G7-OH (3), and H-A3G-OH (4). Each peptide probe was immobilized on a cantilever tip, and the interaction force to profilin, immobilized on a mica substrate, was examined by force curve measurements. The retraction forces obtained showed a sequence-dependent affinity of the peptide probe for profilin. The retraction force for peptide probe 1 was the largest of the four probes examined, and it confirmed that peptide probe 1 has high affinity for profilin. The single molecular retraction force between peptide probe 1 and profilin was estimated to be 96 pN, as determined by Gaussian fitting to the histogram of the retraction forces.     

Combining colloidal probe atomic force and reflection interference contrast microscopy to study the compressive mechanics of hyaluronan brushes

We describe a method that combines colloidal probe atomic force microscopy (AFM) and reflection interference contrast microscopy (RICM) to characterize the mechanical properties of thin and solvated polymer films. When analyzing polymer films, a fundamental problem in colloidal probe AFM experiments is to determine the distance at closest approach between the probe and the substrate on which the film is deposited. By combining AFM and RICM in situ, forces and absolute distances can be measured simultaneously. Using the combined setup, we quantify the compressive mechanics of films of the polysaccharide hyaluronan that is end-grafted to a supported lipid bilayer. The experimental data, and comparison with polymer theory, show that hyaluronan films are well-described as elastic, very soft and highly solvated polymer brushes. The data on these well-defined films should be a useful reference for the investigation of the more complex hyaluronan-rich coats that surround many living cells.     

DLVO interactions of tungsten oxide and cobalt oxide surfaces measured with the colloidal probe technique

We have investigated the DLVO surface forces of oxidized tungsten and cobalt surfaces using the atomic force microscope (AFM) colloidal probe technique. It was shown by X-ray photoelectron spectroscopy (XPS) and electrokinetic measurements that this model system is representative of industrial tungsten carbide (WC) and cobalt powders used in the production of hard metals. We found that the attractive van der Waals forces are well described by Hamaker constants, calculated from optical data for WO(3) and CoOOH. The repulsive electrostatic double layer forces between WO(3) surfaces increase with increasing pH due to an increasingly negative surface potential. This surface potential decreases with increasing ionic strength at pH 7.5. The electrostatic interaction between WO(3) and CoOOH is attractive at pH 10, suggesting a positively charged CoOOH surface.     

Single-colloidal-particle microcontact printing

We have utilized colloidal probe atomic force microscopy (AFM) in conjunction with microinterferometry to study the forces acting in the microcontact printing process. The procedure we have developed yields well defined asymmetric imprints on colloidal probes, which could find further application in interaction force measurements between Janus-type particles and surfaces.     

Hydration forces between silica surfaces: experimental data and predictions from different theories

Silica is a very interesting system that has been thoroughly studied in the last decades. One of the most outstanding characteristics of silica suspensions is their stability in solutions at high salt concentrations. In addition to that, measurements of direct-interaction forces between silica surfaces, obtained by different authors by means of surface force apparatus or atomic force microscope (AFM), reveal the existence of a strong repulsive interaction at short distances (below 2 nm) that decays exponentially. These results cannot be explained in terms of the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, which only considers two types of forces: the electrical double-layer repulsion and the London-van der Waals attraction. Although there is a controversy about the origin of the short-range repulsive force, the existence of a structured layer of water molecules at the silica surface is the most accepted explanation for it. The overlap of structured water layers of different surfaces leads to repulsive forces, which are known as hydration forces. This assumption is based on the very hydrophilic nature of silica. Different theories have been developed in order to reproduce the exponentially decaying behavior (as a function of the separation distance) of the hydration forces. Different mechanisms for the formation of the structured water layer around the silica surfaces are considered by each theory. By the aid of an AFM and the colloid probe technique, the interaction forces between silica surfaces have been measured directly at different pH values and salt concentrations. The results confirm the presence of the short-range repulsion at any experimental condition (even at high salt concentration). A comparison between the experimental data and theoretical fits obtained from different theories has been performed in order to elucidate the nature of this non-DLVO repulsive force.     

Effect of Surface Depressions on Wetting and Interactions between Hydrophobic Pore Array Surfaces

The surface structure is known to significantly affect the long-range capillary forces between hydrophobic surfaces in aqueous solutions. It is, however, not clear how small depressions in the surface will affect the interaction. To clarify this, we have used the AFM colloidal probe technique to measure interactions between hydrophobic microstructured pore array surfaces and a hydrophobic colloidal probe. The pore array surfaces were designed to display two different pore spacings, 1.4 and 4.0 μm, each with four different pore depths ranging from 0.2 to 12.0 μm. Water contact angles measured on the pore array surfaces are lower than expected from the Cassie-Baxter and Wenzel models and not affected by the pore depth. This suggests that the position of the three-phase contact line, and not the interactions underneath the droplet, determines the contact angle. Confocal Raman microscopy was used to investigate whether water penetrates into the pores. This is of importance for capillary forces where both the movement of the three-phase contact line and the situation at the solid/liquid interface influence the stability of bridging cavities. By analyzing the shape of the force curves, we distinguish whether the cavity between the probe and the surfaces was formed on a flat part of the surface or in close proximity to a pore. The pore depth and pore spacing were both found to statistically influence the distance at which cavities form as surfaces approach each other and the distance at which cavities rupture during retraction.     

AFM probing in aqueous environment of Staphylococcus epidermidis cells naturally immobilised on glass: physico-chemistry behind the successful immobilisation

AFM probing of microbial cells in liquid environments usually requires them to be physically or chemically attached to a solid surface. The fixation mechanisms may influence the nanomechanical characterization done by force curve mapping using an AFM. To study the response of a microbial cell surface to this kind of local measurement this study attempts to overcome the problem associated to the uncertainties introduced by the different fixation treatments by analysing the surface of Staphylococcus epidermidis cells naturally (non-artificially mediated) immobilised on a glass support surface. The particularities of this natural bacterial fixation process for AFM surface analysis are discussed in terms of theoretical predictions of the XDLVO model applied to the systems bacteria/support substratum and bacteria/AFM tip immersed in water. In this sense, in the first part of this study the conditions for adequate natural fixation of three S. epidermidis strains have been analyzed by taking into account the geometries of the bacterium, substrate and tip. In the second part, bacteria are probed without the risk of any possible artefacts due to the mechanical or chemical fixation procedures. Forces measured over the successfully adhered cells have (directly) shown that the untreated bacterial surface suffers from a combination of both reversible and non-reversible deformations during acquisition of force curves all taken under the same operational conditions. This is revealed directly through high-resolution tapping-mode imaging of the bacterial surface immediately following force curve mapping. The results agree with the two different types of force curves that were repeatedly obtained. Interestingly, one type of these force curves suggests that the AFM tip is breaking (rather than pushing) the cell surface during acquisition of the force curve. In this case, adhesive peaks were always observed, suggesting a mechanical origin of the measured pull-off forces. The other type of force curves shows no adhesive peaks and exhibits juxtaposing of approaching and retraction curves, reflecting elastic deformations.     

非支撑磷脂双层膜制备及温度对其力学性质的影响

结合聚苯乙烯球刻蚀和微机电系统技术加工氮化硅纳米多孔膜, 并在其上用囊泡法制备非支撑磷脂双层膜, 通过温控原子力显微术(AFM)的成像模式和力曲线模式对非支撑磷脂双层膜的形貌和力学性质进行研究. 实验结果表明, 该方法制备的非支撑磷脂双层膜具有流动性, 能进行自我修复, 该特点有利于提供足够的非支撑磷脂双层膜区域用于其性质研究; 非支撑磷脂双层膜的膜破力和粘滞力均随着温度的升高而减小, 即膜的机械稳定性随着温度的升高而降低. 非支撑磷脂双层膜膜破力小于支撑磷脂双层膜的膜破力, 并且非支撑磷脂双层膜粘滞力随温度的变化趋势与支撑磷脂双层膜的变化趋势相反.     

On the adhesion between fine particles and nanocontacts: an atomic force microscope study

In this study we measured the adhesion forces between atomic force microscope (AFM) tips or particles attached to AFM cantilevers and different solid samples. Smooth and homogeneous surfaces such as mica, silicon wafers, or highly oriented pyrolytic graphite, and more rough and heterogeneous surfaces such as iron particles or patterns of TiO2 nanoparticles on silicon were used. In the first part, we addressed the well-known issue that AFM adhesion experiments show wide distributions of adhesion forces rather than a single value. Our experiments show that variations in adhesion forces comprise fast (i.e., from one force curve to the next) random fluctuations and slower fluctuations, which occur over tens or hundreds of consecutive measurements. Slow fluctuations are not likely to be the result of variations in external factors such as lateral position, temperature, humidity, and so forth because those were kept constant. Even if two solid bodies are brought into contact under precisely the same conditions (same place, load, direction, etc.) the result of such a measurement will often not be the same as that of the previous contact. The measurement itself will induce structural changes in the contact region, which can change the value for the next adhesion force measurement. In the second part, we studied the influence of humidity on the adhesion of nanocontacts. Humidity was adjusted relatively fast to minimize tip wear during one experiment. For hydrophobic surfaces, no signification change in adhesion force with humidity was observed. Adhesion force versus humidity curves recorded with hydrophilic surfaces either showed a maximum or continuously increased. We demonstrate that the results can be interpreted with simple continuum theory of the meniscus force. The meniscus force is calculated based on a model that includes surface roughness and takes into account different AFM tip (or particle) shapes by a two-sphere model. Experimental and theoretical results show that the precise contact geometry has a critical influence on the humidity dependence of the adhesion force. Changes in tip geometry on the sub-10-nm length scale can completely change adhesion force versus humidity curves. Our model can also explain the differences between earlier AFM studies, where different dependencies of the adhesion force on humidity were observed.     

Influence of surface topography on the interactions between nanostructured hydrophobic surfaces

Nanostructured particle coated surfaces, with hydrophobized particles arranged in close to hexagonal order and of specific diameters ranging from 30 nm up to 800 nm, were prepared by Langmuir-Blodgett deposition followed by silanization. These surfaces have been used to study interactions between hydrophobic surfaces and a hydrophobic probe using the AFM colloidal probe technique. The different particle coated surfaces exhibit similar water contact angles, independent of particle size, which facilitates studies of how the roughness length scale affects capillary forces (previously often referred to as "hydrophobic interactions") in aqueous solutions. For surfaces with smaller particles (diameter < 200 nm), an increase in roughness length scale is accompanied by a decrease in adhesion force and bubble rupture distance. It is suggested that this is caused by energy barriers that prevent the motion of the three-phase (vapor/liquid/solid) line over the surface features, which counteracts capillary growth. Some of the measured force curves display extremely long-range interaction behavior with rupture distances of several micrometers and capillary growth with an increase in volume during retraction. This is thought to be a consequence of nanobubbles resting on top of the surface features and an influx of air from the crevices between the particles on the surface.     

Modification of Surface Forces by Metal Ion Adsorption

Abstract

Sorption of ions may lead to variations in interparticle forces and, thus, changes in the stability of colloidal particles. Chemical interactions between metal ions and colloidal particles modify the molecular structure of the surface, the surface charge, and the electrical potential between colloidal particles. These modifications to the surface and to the electrical double layer due to metal ion sorption are reflected in the interaction force between a particle and another surface, which is measured in this study by atomic force microscopy (AFM). Specifically, AFM is used to investigate the sorption of copper ions from aqueous solutions by silica particles. The influence of metal ion concentration and solution ionic strength on surface forces is studied under transient conditions. Results show that as the metal ion concentration is decreased, charge reversal occurs and a longer period of time is required for the system to reach equilibrium. The ionic strength has no significant effect on sorption kinetics. Furthermore, neither metal concentration nor ionic strength exhibits any effect on sorption equilibria, indicating that for the experimental conditions used in this study, the surface sites of the silica particle are fully occupied by copper ions.     

Measuring colloidal forces using evanescent wave scattering

The evanescent wave scattering technique total internal reflection microscopy has enabled the direct measurement of the mean potential energy of interaction between a Brownian particle and a flat surface. With a distance resolution of 1 nm and a force resolution of 10 fN, this technique has successfully measured a variety of colloidal forces. Recent measurements of van der Waals interactions have given rise to new theories for the effect of surface roughness on the interaction. In addition, recent measurements of depletion interactions have shown that energetic as well as entropic effects must be considered when computing the interaction potential.     

Effect of polymer architecture on the adsorption properties of a nonionic polymer

The adsorption of a linear- and bottle-brush poly(ethylene oxide (PEO))-based polymer, having comparable molecular weights, was studied by means of quartz crystal microbalance with dissipation monitoring ability (QCM-D) and AFM colloidal probe force measurements. The energy dissipation change monitored by QCM-D and the range of the steric forces obtained from force measurements demonstrated that linear PEO forms a more extended adsorption layer than the bottle-brush polymer, despite that the adsorbed mass is higher for the latter. Competitive adsorption studies revealed that linear PEO is readily displaced from the interface by the bottle-brush polymer. This was attributed to the higher surface affinity of the latter, which is governed by the number of contact points between the polymers and the interface, and the smaller loss of conformational entropy.