Effect of surface roughness and softness on water capillary adhesion in apolar media

The roughness and softness of interacting surfaces are both important parameters affecting the capillary condensation of water in apolar media, yet are poorly understood at present. We studied the water capillary adhesion between a cellulose surface and a silica colloidal probe in hexane by AFM force measurements. Nanomechanical measurements show that the Young's modulus of the cellulose layer in water is significantly less (~7 MPa) than in hexane (~7 GPa). In addition, the cellulose surface in both water and hexane is rather rough (6-10 nm) and the silica probe has a comparable roughness. The adhesion force between cellulose and silica in water-saturated hexane shows a time-dependent increase up to a waiting time of 200 s and is much (2 orders of magnitude) lower than that expected for a capillary bridge spanning the whole silica probe surface. This suggests the formation of one or more smaller bridges between asperities on both surfaces, which is confirmed by a theoretical analysis. The overall growth rate of the condensate cannot be explained from diffusion mediated capillary condensation alone; thin film flow due to the presence of a wetting layer of water at both the surfaces seems to be the dominant contribution. The logarithmic time dependence of the force can also be explained from the model of the formation of multiple capillary bridges with a distribution of activation times. Finally, the force-distance curves upon retraction show oscillations. Capillary condensation between an atomically smooth mica surface and the silica particle show less significant oscillations and the adhesion force is independent of waiting time. The oscillations in the force-distance curves between cellulose and silica may stem from multiple bridge formation between the asperities present on both surfaces. The softness of the cellulose surface can bring in additional complexities during retraction of the silica particle, also resulting in oscillations in the force-distance curves.     

Probing the viscoelastic response of glassy polymer films using atomic force microscopy

The mechanical properties of glassy films and glass surfaces have been studied using an atomic force microscope (AFM) through various imaging modes and measuring methods. In this paper, we discuss the viscoelastic response of a glassy surface probed using an AFM. We analyzed the force-distance curves measured on a glassy film or a glassy surface at temperatures near the glass transition temperature, Tg, using a Burgers model. We found that the material's characteristics of reversible anelastic response and viscous creep can be extracted from a force-distance curve. Anelastic response shifts the repulsive force-distance curve while viscous creep strongly affects the slope of the repulsive force-distance curve. When coupled with capillary force, due to the condensation of a thin layer of liquid film at the tip-surface joint, the anelasticity and viscous creep can alter the curve significantly in the attractive region.     

表面力仪的改进及溶液中DLVO理论的验证

改进了现有表面力仪,使之具有更低的造价和更高的实验精度.利用改进后的表面力仪测量了0.1mol/L的NaCl溶液中两云母表面间的作用力-距离曲线;通过与计算得到的曲线对比验证了DLVO理论.结果表明:在0.1mol/L的NaCl溶液中,两云母表面间在距离较大时的作用力测量值与DLVO理论值相符合,但在距离较小时测量值出现附加的短程斥力,且此斥力呈现指数衰减规律.     

Under pressure: predicting pressurized metered dose inhaler interactions using the atomic force microscope

Drug particulate interactions in pressurized metered dose inhalers (pMDI) may lead to a decrease in aerosolization efficiency and subsequent efficacy in patient treatment. The interactions between salbutamol sulfate (commonly used in Ventolin pMDIs) and a series of pMDI canister materials were investigated using the atomic force microscope (AFM) colloid probe technique. Approximately 4000 individual force-distance curves were determined for a drug probe and three surfaces (10 x 10 mum areas) in situ, in a model propellant. The area under each force-distance curve was integrated to obtain separation energy values. Median separation energy values followed the rank order borosilicate glass > aluminum > PTFE, suggesting PTFE to be the most suitable canister coating.     

Viscoelasticity of gelatin surfaces probed by AFM noise analysis

The viscoelastic properties of surfaces of swollen gelatin were investigated by analyzing the Brownian motion of an atomic force microscopy (AFM) cantilever in contact with the gel surface. A micron-sized glass sphere attached to the AFM cantilever is used as the dynamic probe. When the sphere approaches the gelatin surface, there is a static repulsive force without a jump into contact. The cantilever's Brownian movement is monitored in parallel, providing access to the dynamic sphere-surface interaction as quantified by the dynamic spring constant, kappa, and the drag coefficient, xi. Gelatin is used as a model substance for a variety of other soft surfaces, where the stiffness of the gel can be varied via the solvent quality, the bloom number, and the pH. The modulus derived from the static force-distance curve is in the kPa range, consistent with the literature. However, the dynamic spring constant as derived from the Brownian motion is much larger than the static differential spring constant dF/dz. On retraction, one observes a rather strong adhesion hysteresis. The strength of the bridge (as given by the dynamic spring constant and the drag coefficient) is very small.     

Self-assembled Gemini surfactant film-mediated dispersion stability

The force-distance curves of 12-2-12 and 12-4-12 Gemini quaternary ammonium bromide surfactants on mica and silica surfaces obtained by atomic force microscopy (AFM) were correlated with the structure of the adsorption layer. The critical micelle concentration was measured in the presence or absence of electrolyte. The electrolyte effect (the decrease of CMC) is significantly more pronounced for Gemini than for single-chain surfactants. The maximum compressive force, F(max), of the adsorbed surfactant aggregates was determined. On the mica surface in the presence of 0.1 M NaCl, the Gemini micelles and strong repulsive barrier appear at surfactant concentrations 0.02-0.05 mM, which is significantly lower than that for the single C(12)TAB (5-10 mM). This difference between single and Gemini surfactants can be explained by a stronger adsorption energy of Gemini surfactants. The low concentration of Gemini at which this surfactant forms the strong micellar layer on the solid/solution interface proves that Gemini aggregates (micelles) potentially act as dispersing agent in processes such as chemical mechanical polishing or collector in flotation. The AFM force-distance results obtained for the Gemini surfactants were used along with turbidity measurements to determine how adsorption of Gemini surfactants affects dispersion stability. It has been shown that Gemini (or two-chain) surfactants are more effective dispersing agents, and that in the presence of electrolyte, the silica dispersion stability at pH 4.0 can also be achieved at very low surfactant concentrations ( approximately 0.02 mM).     

Optical observation of gas bridging between hydrophobic surfaces in water

The very strong and long-ranged interaction force between hydrophobic surfaces in water has been a debated issue for a long time. Recent studies suggest that the long-range attraction is attributable to the bridging of nanoscopic bubbles attached on the surfaces. However, it is still unclear at present whether such a bridging is able to exit stably or not. To clarify the existence of the gas bridge, we conducted the optical observation and the force measurement between the hydrophobic glass particle and plate in water simultaneously, using a combined apparatus of an atomic force microscope and an optical inverted microscope. It is found that (i) the image of the dark ring of ca. 1 mum in diameter appears in the region where the existence of the bridge is confirmed by force curves, but disappears when the separation between surfaces becomes shorter than the low limit of the wavelength of visible light, and (ii) the sudden disappearance of the image coincides well with the breakage of the bridge estimated from the separating force curve. The results obtained here are consistent with the above-mentioned mechanism for the long-range attraction between hydrophobic surfaces in water.     

Oriented purple membrane monolayers covalently attached to gold by multiple thiole linkages analyzed by single molecule force spectroscopy

Highly oriented monolayers of bacteriorhodopsin (BR) in purple membrane (PM) form are obtained by the reaction of BR-Q3C, where a cysteine was introduced into the N-terminal region, with a gold surface. Single molecule force spectroscopy was used to show that about 50% of the BRs are covalently bound to the surface. The linkage between the cysteine and the gold causes an additional characteristic peak in the force-distance curves to appear. Because several thousand cysteine-gold bonds exist between each PM patch and the surface, the PM is irreversibly bound. Such oriented PM monolayers may serve as an interface between metal surfaces and biomaterials, which may be linked to the PM surface chemically. Photoelectric applications of BR will benefit from the high degree of orientation obtained by this method.     

The deformation and adhesion of randomly rough and patterned surfaces

Using a surface forces apparatus (SFA) and an atomic force microscope (AFM) we have studied the effects of surface roughness (root-mean-square (RMS) roughness between 0.3 and 220 nm) on the "contact mechanics", which describes the deformations and loading and unloading adhesion forces, of various polymeric surfaces. For randomly rough, moderately stiff, elastomeric surfaces, the force-distance curves on approach and separation are nearly reversible and almost perfectly exponentially repulsive, with an adhesion on separation that decreases only slightly with increasing RMS. Additionally, the magnitude of the preload force is seen to play a large role in determining the measured adhesion. The exponential repulsion likely arises from the local compressions (fine-grained nano- or submicron-scale deformations) of the surface asperities. The resulting characteristic decay lengths of the repulsion scale with the RMS roughness and correlate very well with a simple finite element method (FEM) analysis based on actual AFM topographical images of the surfaces. For "patterned" surfaces, with a nonrandom terraced structure, no similar exponential repulsion is observed, suggesting that asperity height variability or random roughness is required for the exponential behavior. However, the adhesion force or energy between two "patterned" surfaces fell off dramatically and roughly exponentially as the RMS increased, likely owing to a significant decrease in the contact area which in turn determines their adhesion. For both types of rough surfaces, random and patterned, the coarse-grained (global, meso- or macroscopic) deformations of the initially curved surfaces appear to be Hertzian.     

AFM interaction study of alpha-alumina particle and c-sapphire surfaces at high-ionic-strength electrolyte solutions

Ionic strength dependence of interaction and friction forces between hydrophilic alpha-alumina particles and c-sapphire surfaces (0001) were investigated under basic pH conditions using the colloidal probe method. The compression of the double layer could be seen from force-distance curves as the ionic strength of the solution increased. The forces were repulsive at all ionic strengths measured, even though the interaction distance changed drastically. No jump to contact occurred. The interaction distance decreased from about 20 nm in 10(-3) M KCl solution to about 7 nm in the 1 M KCl case. The lubricating effect of hydrated cations on the lateral friction force was demonstrated at high electrolyte concentrations. This was attributed to more hydrated cations being present in the solution. The friction behavior was closely related to the short-range repulsive forces between the alpha-alumina surfaces at pH 11.     

Antigen binding forces of single antilysozyme Fv fragments explored by atomic force microscopy

We used atomic force microscopy (AFM) to explore the antigen binding forces of individual Fv fragments of antilysozyme antibodies (Fv). To detect single molecular recognition events, genetically engineered histidine-tagged Fv fragments were coupled onto AFM tips modified with mixed self-assembled monolayers (SAMs) of nitrilotriacetic acid- and tri(ethylene glycol)-terminated alkanethiols while lysozyme (Lyso) was covalently immobilized onto mixed SAMs of carboxyl- and hydroxyl-terminated alkanethiols. The quality of the functionalization procedure was validated using X-ray photoelectron spectroscopy (surface chemical composition), AFM imaging (surface morphology in aqueous solution), and surface plasmon resonance (SPR, specific binding in aqueous solution). AFM force-distance curves recorded at a loading rate of 5000 pN/s between Fv- and Lyso-modified surfaces yielded a distribution of unbinding forces composed of integer multiples of an elementary force quantum of approximately 50 pN that we attribute to the rupture of a single antibody-antigen pair. Injection of a solution containing free Lyso caused a dramatic reduction of adhesion probability, indicating that the measured 50 pN unbinding forces are due to the specific antibody-antigen interaction. To investigate the dynamics of the interaction, force-distance curves were recorded at various loading rates. Plots of unbinding force vs log(loading rate) revealed two distinct linear regimes with ascending slopes, indicating multiple barriers were present in the energy landscape. The kinetic off-rate constant of dissociation (k(off) approximately = 1 x 10(-3) s(-1)) obtained by extrapolating the data of the low-strength regime to zero force was in the range of the k(off) estimated by SPR.     

Atomic force microscopy measurement of heterogeneity in bacterial surface hydrophobicity

The structure and physicochemical properties of microbial surfaces at the molecular level determine their adhesion to surfaces and interfaces. Here, we report the use of atomic force microscopy (AFM) to explore the morphology of soft, living cells in aqueous buffer, to map bacterial surface heterogeneities, and to directly correlate the results in the AFM force-distance curves to the macroscopic properties of the microbial surfaces. The surfaces of two bacterial species, Acinetobacter venetianus RAG-1 and Rhodococcus erythropolis 20S-E1-c, showing different macroscopic surface hydrophobicity were probed with chemically functionalized AFM tips, terminating in hydrophobic and hydrophilic groups. All force measurements were obtained in contact mode and made on a location of the bacterium selected from the alternating current mode image. AFM imaging revealed morphological details of the microbial-surface ultrastructures with about 20 nm resolution. The heterogeneous surface morphology was directly correlated with differences in adhesion forces as revealed by retraction force curves and also with the presence of external structures, either pili or capsules, as confirmed by transmission electron microscopy. The AFM force curves for both bacterial species showed differences in the interactions of extracellular structures with hydrophilic and hydrophobic tips. A. venetianus RAG-1 showed an irregular pattern with multiple adhesion peaks suggesting the presence of biopolymers with different lengths on its surface. R. erythropolis 20S-E1-c exhibited long-range attraction forces and single rupture events suggesting a more hydrophobic and smoother surface. The adhesion force measurements indicated a patchy surface distribution of interaction forces for both bacterial species, with the highest forces grouped at one pole of the cell for R. erythropolis 20S-E1-c and a random distribution of adhesion forces in the case of A. venetianus RAG-1. The magnitude of the adhesion forces was proportional to the three-phase contact angle between hexadecane and water on the bacterial surfaces.     

Long-Range Interaction Forces between Polymer-Supported Lipid Bilayer Membranes

Much of the short-range forces and structures of softly supported DMPC bilayers has been described previously. However, one interesting feature of the measured force-distance profile that remained unexplained is the presence of a long-range exponentially decaying repulsive force that is not observed between rigidly supported bilayers on solid mica substrate surfaces. This observation is discussed in detail here based on recent static and dynamic surface force experiments. The repulsive forces in the intermediate distance regime (mica-mica separations from 15 to 40 nm) are shown to be due not to an electrostatic force between the bilayers but to compression (deswelling) of the underlying soft polyelectrolyte layer, which may be thought of as a model cytoskeleton. The experimental data can be fit by simple theoretical models of polymer interactions from which the elastic properties of the polymer layer can be deduced.     

Nanomechanics of silicon surfaces with atomic force microscopy: an insight to the first stages of plastic deformation

The use of stiff cantilevers with diamond tips allows us to perform nanoindentations on hard covalent materials such as silicon with atomic force microscopy. Thanks to the high sensitivity in the force measurements together with the high resolution upon imaging the surface, we can study nanomechanical properties. At this scale, the surface deforms, following a simple non-Hertzian spring model. The plastic onset can be assessed from a discontinuity in the force-distance curves. Hardness measurements with penetration depths as small as 1 nm yield H= approximately 25 GPa, thus showing a drastic increase with penetration depths below 5 nm.     

Atomic force microscopy evaluation of aqueous interfaces of immobilized hyaluronan

Hyaluronan (HA) was immobilized on aminated glass surfaces in three different ways: by simple ionic interaction and by covalent linking at low density and at full density. In agreement with previous reports, in vitro experiments show that the outcome of fibroblast adhesion tests is markedly affected by the details of the coupling procedure, suggesting that different interfacial forces are operating at the aqueous/HA interface in the three cases investigated. The interfacial properties of the HA-coated surfaces were probed by force-distance curves obtained with the atomic force microscope (AFM). This approach readily shows significant differences among the tested samples, which are directly related to the coupling strategy and to results of cell adhesion tests. In particular, the range of interaction between the tip and the surface is much lower when HA is covalently linked than when it is ionically coupled, suggesting a more compact surface structure in the former case. Increasing HA surface density minimizes the interaction force between the surface and the AFM tip, likely reflecting more complete shielding by the HA chains of the underlying substrate. In summary, these measurements clearly show the different nature of the aqueous interfaces tested, and underline the role of this analytical approach in the development and control of finely tuned biomaterial surfaces.     

Nanomechanical properties of mechanical double-layers: a novel semiempirical analysis

Force-displacement curves have been acquired with a commercial atomic force microscope on a thin film of poly(n-butyl methacrylate) on glass substrates. The film thickness is nonuniform, ranging in the measured area from 0 to 30 nm, and gives the possibility to survey the so-called "mechanical double-layer" topic, i.e., the influence of the substrate on the mechanical properties of the film in dependence of the film thickness. The stiffness and the deformation for each force-distance curve were determined and related to the film thickness. We were able to estimate the resolution of the film thickness that can be achieved by means of force-distance curves. By exploiting the data acquired in the present and in a previous experiment, a novel semiempirical approach to describe the mechanical properties of a mechanical double-layer is introduced. The mathematical model, with which deformation-force curves can be described, permits to calculate the Young's moduli of film and substrate in agreement with literature values and to determine the film thickness in agreement with the topography.     

Interaction between poly[(R)-3-hydroxybutyrate] depolymerase and biodegradable polyesters evaluated by atomic force microscopy

Adsorption of PHB depolymerase from Ralstonia pickettii T1 to biodegradable polyesters such as poly[(R)-3-hydroxybutyrate] (PHB) and poly(l-lactic acid) (PLLA) was investigated by atomic force microscopy (AFM). The substrate-binding domain (SBD) with histidines within the N-terminus was prepared and immobilized on the AFM tip surface via a self-assembled monolayer with a nitrilotriacetic acid group. Using the functionalized AFM tips, the force-distance measurements for polyesters were carried out at room temperature in a buffer solution. In the case of AFM tips with immobilized SBD and their interaction with polyesters, multiple pull-off events were frequently recognized in the retraction curves. The single rupture force was estimated at approximately 100 pN for both PLLA and PHB. The multiple pull-off events were recognized even in the presence of a surfactant, which will prevent nonspecific interactions, but reduced when using polyethylene instead of polyesters as a substrate. The present results provide that the PHB depolymerase adsorbs specifically to the surfaces of polyesters and that the single unbinding event evaluated here is mainly associated with the interaction between one molecule of SBD and the polymer surface.     

Surface adsorption and vesicle formation of dilauroylphosphatidylcholine in room temperature ionic liquids

In this work, we study the adhesion forces between atomic force microscopy (AFM) tips and superficial dentin etched with phosphoric acid. Initially, we quantitatively analyze the effect of acid etching on the surface heterogeneity and the surface roughness, two parameters that play a key role in the adhesion phenomenon. From a statistical study of the force-distance curves, we determine the average adhesion forces on the processed substrates. Our results show that the average adhesion forces, measured in water, increase linearly with the acid exposure time. The highest values of such forces are ascribed to the high density of collagen fibers on the etched surfaces. The individual contribution of exposed collagen fibrils to the adhesion force is highlighted. We also discuss in this paper the influence of the environmental medium (water/air) in the adhesion measurements. We show that the weak forces involved require working in the aqueous medium.     

Fabrication and characterization of hierarchical nanostructured smart adhesion surfaces

The mechanics of fibrillar adhesive surfaces of biological systems such as a Lotus leaf and a gecko are widely studied due to their unique surface properties. The Lotus leaf is a model for superhydrophobic surfaces, self-cleaning properties, and low adhesion. Gecko feet have high adhesion due to the high micro/nanofibrillar hierarchical structures. A nanostructured surface may exhibit low adhesion or high adhesion depending upon fibrillar density, and it presents the possibility of realizing eco-friendly surface structures with desirable adhesion. The current research, for the first time uses a patterning technique to fabricate smart adhesion surfaces: single- and two-level hierarchical synthetic adhesive structure surfaces with various fibrillar densities and diameters that allows the observation of either the Lotus or gecko adhesion effects. Contact angles of the fabricated structured samples were measured to characterize their wettability, and contamination experiments were performed to study for self-cleaning ability. A conventional and a glass ball attached to an atomic force microscope (AFM) tip were used to obtain the adhesive forces via force-distance curves to study scale effect. A further increase of the adhesive forces on the samples was achieved by applying an adhesive to the surfaces.     

Photoiniferter-based thermoresponsive graft architecture with albumin covalently fixed at growing graft chain end

The aim of this study was to develop a novel surface graft architecture in which albumin is covalently fixed at the growing chain end of the hydrophilic polymers: poly(N, N-dimethylacylamide), PDMAM, and poly(N-isopropylacrylamide), PNIPAM. Photoiniferter-based surface-grafted polymers were prepared using either an albuminated iniferter or a nonalbuminated iniferter, both of which were derivatized on glass surfaces, and ultraviolet (UV)-light-irradiated in the presence of a DMAM or NIPAM monomer. Surface chemical composition analysis by X-ray photoelectron spectroscopy, contact angle measurement, immunostaining using fluorescence labeled antibody and the measurement of graft thickness, as determined from force-distance curves obtained in water at 25 degrees C and 37 degrees C by atomic force microscopy, evidenced that the thickness of graft layer increased with photoirradiation time and albumin molecules exist at growing chain ends. For PNIPAM-grafted surfaces, the interconversion between swollen and collapsed graft chains was observed below and above the lower critical solution temperature of PNIPAM. The potential application of a thermoresponsive graft with albumin covalently fixed at its growing chain end was discussed in terms of "active" nonfouling surface design based on the temperature-dependent switching of phase transition.