A study of adhesion forces by atomic force microscopy

An atomic force microscope (AFM) was used to investigate Si₃N₄ tip interactions with various materials in four different liquid media (water, ethanol, ethylene glycol, and formamide). The adhesion forces calculated using surface energies and the values measured experimentally were compared. For all materials, the calculated adhesion force closely correlated with AFM measurements, except in water. In the case of water, the AFM experiments showed strong adhesion, whereas theoretically (van Oss-Chaudhury-Good model) repulsion is predicted. The difference observed is discussed in terms of the chemical interactions between Si₃N₄ and water.     

原子力显微镜在聚氨酯材料性能分析中的应用

简述了原子力显微镜（AFM）探测物体表面形貌的工作原理和操作模式，介绍了AFM在观察聚氨酯（PU）材料微相分离、复合树脂的相容性的应用情况，综述了AFM应用于PU材料研究的新进展。     

Cohesive forces between particles of fluid-bed catalysts

The importance of van der Waals and capillary interparticle forces with respect to particle weight is discussed. Evaluation of solid to solid interaction on the basis of particle diameter leads to exceedingly high values of cohesive forces. The need for detailed analysis of particle surfaces is shown.Electron scanning microscopy is used to investigate particle surface characteristics of a number of catalytic powders. There are surface irregularities even in the case of apparently perfect microspheres. For some of the materials tested the investigation provides local radii of curvature of particle surface required for the evaluation of particle to particle interactions.     

Sample rotation angle dependence of graphene thickness measured using atomic force microscope

A precise measurement of graphene thickness is required for the design and development of nano-devices based on the material. Many factors affect this measurement when using scanning tunneling microscope (STM) and atomic force microscope (AFM), including the interaction between the scanning tip and ripples on graphene; such effects have not previously been explored. To investigate this, we measure the sample rotation angle dependence of graphene thickness as determined by contact mode and tapping mode AFM. The graphene thickness as determined by contact mode AFM follows a cosine modulus function of sample rotation angle, while tapping mode AFM reveals a constant graphene thickness, independent of sample rotation angle. For comparison, the AFM torsion signal is measured and follows a sine function of the sample rotation angle. All the measured sample rotation angle dependences can be explained by the interaction between linearly aligned ripples on graphene and the AFM tip in contact with the graphene.     

Dynamic response of a cracked atomic force microscope cantilever used for nanomachining

ABSTRACT: The vibration behavior of an atomic force microscope [AFM] cantilever with a crack during the nanomachining process is studied. The cantilever is divided into two segments by the crack, and a rotational spring is used to simulate the crack. The two individual governing equations of transverse vibration for the cracked cantilever can be expressed. However, the corresponding boundary conditions are coupled because of the crack interaction. Analytical expressions for the vibration displacement and natural frequency of the cracked cantilever are obtained. In addition, the effects of crack flexibility, crack location, and tip length on the vibration displacement of the cantilever are analyzed. Results show that the crack occurs in the AFM cantilever that can significantly affect its vibration response.PACS: 07.79.Lh; 62.20.mt; 62.25.Jk.     

Surface morphology of Nafion 117 membrane by tapping mode atomic force microscope

Surface morphology of Nafion 117 membrane was studied by tapping mode atomic force microscopy. Three different samples were analyzed and correspond respectively to dry membrane and wet membrane equilibrated either with water or with tributylphosphate. These studies show the supermolecular structure of the membrane, which is made of nodules or spherical grains of a mean diameter of 11 nm, and are surrounded by interstitial regions of a mean thickness of 50 Å. Roughness analysis of the samples shows the influence of the swelling properties of the membrane on its surface morphology. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 503–508, 1998     

原子力显微镜技术研究重组人内皮抑素对内皮细胞的作用

目的应用原子力显微镜(atomic force microscope,AFM)技术研究重组人内皮抑素(recombinant human endostatin,rhES)对内皮细胞的作用。方法采用MTT法检测不同浓度的rhES(0.05~2.4μg/ml)对人脐静脉内皮细胞ECV304增殖活力的影响;分别用0.8和2μg/ml的rhES处理ECV304细胞,应用AFM观察内皮细胞整体形貌的变化,SPI 3800 New DFM动力显微镜观察ECV304细胞表面局部形貌的变化。结果 rhES可明显抑制ECV304细胞增殖,且呈剂量效应(P0.001);rhES可降低贴壁的ECV304细胞的厚度,且呈剂量依赖效应,使较光滑的细胞表面变粗糙,产生了一些微小的突起;经rhES处理的ECV304细胞表面结构呈现不规则的变化。结论 AFM技术具有样品制备简便和分辨率较高等优点,适合贴壁培养细胞的原位观察。     

On the use of the atomic force microscope to monitor physical degradation of polymeric coating surfaces

The atomic force microscope (AFM) was used to monitor changes in surface features of an acrylic melamine coating that was exposed to a variety of conditions. Exposure to ultraviolet (UV) radiation and high relative humidity caused general roughening of the surface and the formation of pits. Further, the damage of the coating surface was much more substantial for exposure to high relative humidity compared to exposure to dry environments. This difference in degradation rates correlated with measurements of chemical degradation determined using infrared spectra that were acquired along with the AFM images. Building Materials Division, 100 Bureau Dr., Stop 8621, Gaithersburg, MD 20899-8621.     

Segregation in granular materials and the direct measurement of surface forces using atomic force microscopy

It is known that one of the dominant forces controlling the macroscopic motion of particles is the cohesive force due to the presence of liquid bridges between particles. In a mixing process, this force directly impacts the degree of homogeneity achievable by the system. The work presented here provides a quantitative analysis of this relationship through concurrent direct measurements of surface forces due to moisture and blending/segregation experiments. Atomic force microscopy (AFM) was employed to measure the force required to remove the AFM's cantilever from the surface of a glass bead with varying degrees of surface moisture. Corresponding blending/segregation experiments were performed using the same materials and conditions to develop a correlation between the interparticle forces due to the liquid layer and the final state of a mixing process. The extent to which greater moisture content increased the interparticle surface forces was quantified, and it was observed that segregation decreases proportionately to increases in surface forces.     

Apparent contrast reversal in tapping mode atomic force microscope images on films of polystyrene-b-polyisoprene-b-polystyrene

Summary Thin films of phase separated polystyrene-b-polyisoprene-b-polystyrene block copolymers were studied by tapping mode atomic force microscopy. The relative contrast in height and phase mode images of the phase separated regions was found to be very sensitive to changes in the operating conditions of the microscope. Contrast variations and reversals were observed for height and phase mode images as a function of the set-point amplitude ratio and drive frequency. No unique height or phase contrast was observed for the the tri-block copolymer system examined in this study. Received: 30 December 1997/Revised version: 14 January 1998/Accepted: 15 January 1998     

Mechanical properties of porous methyl silsesquioxane and nanoclustering silica films using atomic force microscope

Mechanical properties of porous methyl silsesquioxane samples with dielectric constant 2.4 and 2.0 and a recently developed nanoclustering silica film samples with dielectric constants 2.3 and 2.0 were evaluated using an atomic force microscope based nanoindentation. It was found that the Young’s modulus and the hardness decrease while the fracture toughness increases with a decrease in the dielectric constant in the same type of material. Moreover, the Young’s modulus and the hardness of the nanoclustering silica films were observed to be at least twice and fracture toughness values ~1.3–1.5 higher than those for methyl silsesquioxane films with similar dielectric constants. The high resolution topographic imaging capability of atomic force microscope was shown to be particularly useful in the measurement of cracks generated by the ultra-low indentation loads, and the evaluation of the fracture toughness of the nanoscale volumes of materials.     

A local field emission study of partially aligned carbon-nanotubes by atomic force microscope probe

Atomic force microscopy (AFM) was used to study the field emission (FE) properties of a dense array of long and vertically quasi-aligned multi-walled carbon nanotubes grown by catalytic chemical vapor deposition on a silicon substrate. The use of nanometric probes enables local field emission measurements to be made allowing the investigation of effects that are not detectable with a conventional parallel plate setup, where the emission current is averaged over a large sample area. The micrometric inter-electrode distance allows one to achieve high electric fields with a modest voltage. These features made us able to characterize field emission for macroscopic electric fields up to 250 V/μm and attain current densities larger than 10⁵ A/cm². FE behaviour is analyzed in the framework of the Fowler–Nordheim theory. A field enhancement factor γ ≈ 40–50 and a turn-on field E_turn-on 15 V/μm at an inter-electrode distance of 1 μm are estimated. Current saturation observed at high voltages in the I-V characteristics is explained in terms of a series resistance of the order of MΩ. Additional effects, such as electrical conditioning, CNT degradation, response to laser irradiation and time stability are investigated and discussed.     

Continuous electrospinning of polymer nanofibers of Nylon-6 using an atomic force microscope tip

An atomic force microscopy (AFM) probe is successfully utilized as an electrospinning tip for fabricating Nylon-6 nanofibers. The nanometre-size tip enabled controlled deposition of uniform polymeric nanofibers within a 1 cm diameter area. Nylon-6 nanofibers were continuously electrospun at a solution concentration as low as 1 wt% Nylon-6 in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). Wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) results of the AFM electrospun fibers indicated that the nanofibers predominantly display the meta-stable γ crystalline form suggesting rapid crystallization rate during the process. In addition to precise control over fiber deposition and diameter, some of the drawbacks of conventional electrospinning such as large volume of solutions and clogging of needles can be overcome using this AFM based electrospinning technique. Lastly, a comparison of electrospun fibers from syringe-needle based electrospinning and AFM probe-tip based electrospinning indicated significant morphological and microstructural differences in the case of AFM based electrospinning.     

The structure and morphology of the skin of polyethersulfone ultrafiltration membranes: A comparative atomic force microscope and scanning electron microscope study

Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were used to investigate the surface structure and morphology of 10,000, 30,000, and 100,000 dalton molecular weight cutoff (MWCO) polyethersulfone (PES) ultrafiltration membranes, and the results are compared. Although both approaches reveal the pore structure in the 30,000 and 100,000 MWCO membranes, the pore diameters derived from SEM are smaller than those measured by AFM. This discrepancy is a result of the diminution in pore dimensions during the sample preparation for SEM, that is, the solvent exchange procedure needed to remove the water from the membrane prior to the high vacuum gold coating deposition step. In contrast to SEM, which requires a high vacuum both during heavy metal coating and during examination, AFM can be performed on wet ultrafiltration membranes. Consequently, the potential of altering the membranes' pore structures during sample preparation is eliminated. Therefore, the pore diameters obtained from AFM are more accurate than those derived from SEM.     

Investigation of oxidation profile in PMR-15 polyimide using atomic force microscope (AFM)

Nanoindentation measurements are made on thermosetting materials using cantilever deflection vs. piezoelectric scanner position behavior determined by atomic force microscope (AFM). The spring model is used to determine mechanical properties of materials. The generalized Sneddon's equation is utilized to calculate Young's moduli for thermosetting materials at ambient conditions. Our investigations show that the force-penetration depth curves during unloading in these materials can be described accurately by a power law relationship. The results show that the accuracy of the measurements can be controlled within 7%. The above method is used to study oxidation profiles in PMR-15 polyimide. The thermo-mechanical profiles of PMR-15 indicate that the elastic modulus at the surface portion of the specimen is different from that at the interior of the material. It is also shown that there are two zones within the oxidized portion of the samples. Results confirm that the surface layer and the core material have substantially different properties.     

Wang-Landau Monte Carlo simulation of capillary forces at low relative humidity in atomic force microscopy

A lattice gas model is used with Wang-Landau Monte Carlo sampling to predict the capillary force between a model of an atomic force microscopy (AFM) probe and a smooth surface as a function of separation, relative humidity (RH), and tip hydrophilicity. Completely wetting AFM tips exhibit a maximum in the capillary force as the RH increases, while the magnitude of the capillary force in the presence of partially wetting and partially drying tips is relatively independent of the RH. Capillary forces can also be significant in low RH environments and should not be discounted in AFM studies involving hydrophilic surfaces.     

Assessment of adhesion and autoadhesion forces between particles and surfaces

A centrifuge technique has been used to investigate the autoadhesion force between particles and a plane surface of the same material compacted into a disk, which is resistant to the centrifugal force. When measured by profilemetry, these compact surfaces are rougher than metal or plastic surfaces used in previous studies. This results in a change of the detachment force distribution from a log-normal to a right-shifted distribution. The relationship between press-on force and median autoadhesion force depends on particle size, shape and particle surface morphology of the powder particles autoadhered. The lower the autoadhesion force, the greater the possibility that the substance can be used in a single-component powder application. The bulk properties of the powders such as cohesiveness or free powder flow were found to be related to the autoadhesion force. Estimation of the distance between the particles and surfaces has been made based on the Lifshitz-van der Waals constant derived from low frequency dielectric measurements. There was a decrease in distance of separation with increase in press-on force for both sets of particles. The contact between angular lactose monohydrate particles and a lactose monohydrate surface can be increased by more than that between irregular or spherical salmeterol xinafoate particles and their equivalent compact surface.     

Structures and interactions between two colloidal particles in adsorptive polymer solutions

The role of weak adsorptive polymer chains in the colloidal particles solution is studied by self-consistent field theory (SCFT). The numerical results show the potential between colloids are attractive interaction. Besides the depletion effects the chain conformations such as loop, tail and bridge between two spherical colloidal particles play important roles. The quantitative polymer concentration dependent chain conformations and then the effective potential are also addressed.     

Effect of material composition on nano-adhesive characteristics of styrene-butadiene-styrene copolymer-modified bitumen using atomic force microscope technology

To investigate the effect of the raw material composition on the nano-adhesive characteristics of styrene-butadiene-styrene (SBS) copolymer-modified bitumen, force-distance curves were obtained to examine the nano-adhesive forces of SBS-modified bitumen samples prepared using different virgin bitumen types, SBS modifiers, an extraction oil, and a stabilizer using atomic force microscopy (AFM). The influence of the composition of the raw materials, such as the virgin bitumen, SBS modifier, extracting oil, and stabilizing agent contents, on the cohesive characteristics were discussed for SBS-modified bitumen. The results show that force-distance curves obtained using AFM can be used to gain insight into nano-adhesive forces of SBS-modified bitumen samples. The force-spectrum technique using AFM can be used to obtain the nano-adhesive forces of the SBS-modified bitumen samples. Neat bitumen has good compatibility with a SBS modifier, which is suitable for forming SBS-modified bitumen with an increased nano-adhesive force. The nano-adhesive force of the branched-shaped SBS-modified bitumen was larger than that of linear-shaped SBS-modified bitumen. The nano-adhesive force of SBS-modified bitumen increased as the SBS modifier and stabilizer contents increased. The nano-adhesive force of the SBS-modified bitumen decreased when the extraction oil content increased.     

Surface force interactions between micrometer-size polystyrene spheres and silicon substrates using atomic force techniques

The surface force interactions between a single micrometer-size polystyrene sphere and a p-type silicon substrate were investigated using atomic force microscope techniques. The force of removal and the degree of deformation of the particle determined as a function of the applied loading force. The work of removal, estimated assuming a perfectly spherical particle and a smooth substrate, was also determined. The influence of surface contamination and the implications of the short contact times used in these experiments are discussed.