Surface Adhesion Force Research Articles

Indentation Measurements to Validate Dynamic Elasticity Imaging Methods

We describe macro-indentation techniques for estimating the elastic modulus of soft hydrogels. Our study describes (a) conditions under which quasi-static indentation can validate dynamic shear-wave imaging estimates and (b) how each of these techniques uniquely biases modulus estimates as they couple to the sample geometry. Harmonic shear waves between 25 and 400 Hz were imaged using ultrasonic Doppler and optical coherence tomography methods to estimate shear dispersion. From the shear-wave speed of sound, average elastic moduli of homogeneous samples were estimated. These results are compared directly with macroscopic indentation measurements measured two ways. One set of measurements applied Hertzian theory to the loading phase of the force-displacement curves using samples treated to minimize surface adhesion forces. A second set of measurements applied Johnson-Kendall-Roberts theory to the unloading phase of the force-displacement curve when surface adhesions were significant. All measurements were made using gelatin hydrogel samples of different sizes and concentrations. Agreement within 5% among elastic modulus estimates was achieved for a range of experimental conditions. Consequently, a simple quasi-static indentation measurement using a common gel can provide elastic modulus measurements that help validate dynamic shear-wave imaging estimates.

In situ activity recovery of aging biofilm in biological aerated filter: Surfactants treatment and mechanisms study

In situ activity recovery of aging biofilm in the biological aerated filter (BAF) is an important but underappreciated problem. Lab-scaled BAFs were established in this study and three kinds of surfactants containing sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS) and rhamnolipid were employed. Multiple indicators including effluent qualities, dissolved organic matters, biofilm physiology and morphology characteristics were investigated to explore the mechanisms. Results showed that removal rates of effluent COD in test groups significantly recovered to the level before aging. Compared with the control, effluent in SDBS and rhamnolipid-treated groups obtained more protein-like and humic-like substances, respectively. Furthermore, great live cell ratio, smooth surface and low adhesion force of biofilm were observed after rhamnolipid treatment, which was in consistent with good effluent qualities in the same group. This is the first report of applying rhamnolipid for in situ activity recovery of aging biofilm in bioreactors.

Adaptation of Campylobacter jejuni to biocides used in the food industry affects biofilm structure, adhesion strength, and cross-resistance to clinical antimicrobial compounds

The emergence of biocide-adapted Campylobacter jejuni strains that developed into biofilms and their potential to develop clinical resistance to antimicrobial compounds was studied. C. jejuni was grown in sub-lethal concentrations of five biocides used in the food industry. C. jejuni exhibited adaptation to these biocides with increased minimum inhibitory concentrations. The 3-D structures of the biofilms produced by the biocide-adapted cells were investigated by atomic force microscopy (AFM). The results revealed marked variability in biofilm architecture, including ice-crystal-like structures. Adaptation to the biocides enhanced biofilm formation, with significant increases in biovolume, surface coverage, roughness, and the surface adhesion force of the biofilms. Adaptation to commercial biocides induced resistance to kanamycin and streptomycin. This study suggests that the inappropriate use of biocides may lead to cells being exposed to them at sub-lethal concentrations, which can result in adaptation of the pathogens to the biocides and a subsequent risk to public health.

Development of in situ soft colloidal probe atomic force microscopy for probing the adhesion between wood extractives and model surfaces

A new method to attach colloidal particles in situ directly onto atomic force microscopy (AFM) cantilevers was developed and used as colloidal probes. In this method, cantilever surfaces were hydrophobically functionalised and the colloids adhered without glue in an aqueous environment. Experiments measuring the interaction forces between colloidal probes, composed of either wood extractives or a range of model compounds normally present in wood extractives, and glass, cellulose, and chromium surfaces were carried out. Wood extractive colloids exhibited the strongest adhesion to glass control surface (7.0 mN m−1), followed in decreasing order by triolein, abietic acid and oleic acid colloids (3.9 mN m−1, 2.2 mN m−1 and 1.2 mN m−1 respectively). Adhesion onto a cellulose surface was strongest for triolein colloids (8.1 mN m−1) followed by oleic acid colloids (6.1 mN m−1), while adhesion forces for abietic acid and wood extractives colloids were similar (4.9 mN m−1 and 4.3 mN m−1, respectively). When attached to a chromium surface adhesion forces of abietic acid and wood extractives were strongest and similar (3.6 mN m−1 and 3.9 mN m−1, respectively); the weakest interaction at this surface was recorded for oleic acid colloids (0.8 mN m−1). Similar adhesion forces for wood extractive colloids and abietic acid colloids (onto cellulose and chromium surfaces) reinforces the proposal that abietic acid occupies the outer layer of wood extractive colloids. The differences in the interaction between the colloids and the cellulose and chromium surface helps to explain the transfer of resin acid exclusively over triolein or oleic acid from a cellulose paper surface onto chromium plated printing cylinders.

Geometrical force constraint method for vessel and x-ray angiogram simulation.

This study proposes a novel geometrical force constraint method for 3-D vasculature modeling and angiographic image simulation. For this method, space filling force, gravitational force, and topological preserving force are proposed and combined for the optimization of the topology of the vascular structure. The surface covering force and surface adhesion force are constructed to drive the growth of the vasculature on any surface. According to the combination effects of the topological and surface adhering forces, a realistic vasculature can be effectively simulated on any surface. The image projection of the generated 3-D vascular structures is simulated according to the perspective projection and energy attenuation principles of X-rays. Finally, the simulated projection vasculature is fused with a predefined angiographic mask image to generate a realistic angiogram. The proposed method is evaluated on a CT image and three generally utilized surfaces. The results fully demonstrate the effectiveness and robustness of the proposed method.

Assessing the Local Nanomechanical Properties of Self-Assembled Block Copolymer Thin Films by Peak Force Tapping.

The mechanical properties of several types of block copolymer (BCP) thin films have been investigated using PeakForce quantitative nanomechanical mapping. The samples consisted of polystyrene/poly(methylmethacrylate) (PS/PMMA)-based BCP thin films with different pitches both randomly oriented and self-assembled. The measured films have a critical thickness below 50 nm and present features to be resolved of less than 22 nm. Beyond measuring and discriminate surface elastic modulus and adhesion forces of the different phases, we tuned the peak force parameters in order to reliably image those samples, avoiding plastic deformation. The method is able to detect the changes in mechanical response associated with the orientation of the PMMA cylinders with respect to the substrate (parallel versus vertical). The nanomechanical investigation is also capable of recognizing local stiffening due to the preferential growth of alumina deposited by atomic layer deposition on BCP samples, opening up new possibilities in the field of hard mask materials characterization.

Nanomechanical Switch Designs to Overcome the Surface Adhesion Energy Limit

An analytical model is developed to assess the switching energy of a nanomechanical switch. Alternative switch designs are proposed to leverage spring-restoring force to counterbalance surface adhesion force, reducing the depth of the potential energy well created by contact adhesion and thereby overcoming the surface adhesion energy limit.

Tunneling Nanoelectromechanical Switches Based on Compressible Molecular Thin Films.

Abrupt switching behavior and near-zero leakage current of nanoelectromechanical (NEM) switches are advantageous properties through which NEMs can outperform conventional semiconductor electrical switches. To date, however, typical NEMs structures require high actuation voltages and can prematurely fail through permanent adhesion (defined as stiction) of device components. To overcome these challenges, in the present work we propose a NEM switch, termed a "squitch," which is designed to electromechanically modulate the tunneling current through a nanometer-scale gap defined by an organic molecular film sandwiched between two electrodes. When voltage is applied across the electrodes, the generated electrostatic force compresses the sandwiched molecular layer, thereby reducing the tunneling gap and causing an exponential increase in the current through the device. The presence of the molecular layer avoids direct contact of the electrodes during the switching process. Furthermore, as the layer is compressed, the increasing surface adhesion forces are balanced by the elastic restoring force of the deformed molecules which can promote zero net stiction and recoverable switching. Through numerical analysis, we demonstrate the potential of optimizing squitch design to enable large on-off ratios beyond 6 orders of magnitude with operation in the sub-1 V regime and with nanoseconds switching times. Our preliminary experimental results based on metal-molecule-graphene devices suggest the feasibility of the proposed tunneling switching mechanism. With optimization of device design and material engineering, squitches can give rise to a broad range of low-power electronic applications.

Effect of Body Biasing on the Energy-Delay Performance of Logic Relays

The effect of the body bias voltage on logic relay performance is investigated. The switching hysteresis voltage, which sets a lower limit for the relay operating voltage, is experimentally found to decrease with increasing body bias voltage, due to reduced surface adhesive force. It is demonstrated that the switching energy of a relay can be reduced by body biasing, at a tradeoff of increased mechanical turn-ON delay. Simulations of nanoscale relay designs indicate that body biasing can be used to mitigate relay manufacturing challenges, while enabling ultralow-voltage (sub-100 mV) operation with relatively fast switching speed.

Preparation and nanotribological properties of polyelectrolyte multilayers with in situ Au nanoparticles

Abstract Polyelectrolyte multilayers with in situ Au nanoparticles were prepared by alternate immersion of a substrate in poly(allylamine hydrochloride) (PAH)-AuCl4− complexes solution and poly(acrylic acid) solution for layer-by-layer self assemble process followed by reduction of the metal cations Au3+ through immersion into a fresh NaBH4 solution. UV–vis spectrum, atomic force microscopy (AFM), X-ray photoelectron spectra (XPS), and transmission electron microscope (TEM) were used to confirm the construction of multilayers and synthesis of the Au nanoparticles. The nanotribological behaviors also have been studied using AFM. The polyelectrolyte multilayers with in situ Au nanoparticles exhibited a lower surface adhesion and friction force than the pure polyelectrolyte multilayers due to the nanoparticles improved the mutilayers surface structure and possess a good load-carrying capacity in nanoscale.

Comparison of the macroscale and microscale tests for measuring elastic properties of polydimethylsiloxane

ABSTRACTPolydimethylsiloxane (PDMS) has many biomedical applications, since it is biocompatible, easy to fabricate and inexpensive. The response of biological cells and tissues is affected by the mechanical properties of the PDMS surface, which can be controlled by varying the crosslinking percentage. It is essential to find reliable ways to measure elastic properties of PDMS prepared with different surface conditions or stiffness gradients. In this paper, the elastic modulus of PDMS was measured at different scales as a function of the crosslinking percentage varied from 2% to 9%. Macroscopic compression and tension tests were used and compared with the nano‐JKR (Johnson‐Kendall‐Roberts) method applied to a microindentation test. Depending on the test, the PDMS elastic modulus increased from 10 to 85 fold with the crosslinking percentage change from 2% to 9%. The PDMS elastic modulus varied as a sigmoid function with the crosslinking percentage for each type of test. The compression macroscale test is the easiest way to estimate the elastic modulus of stiffer PDMS with higher crosslinking percentage. For the more compliant and tacky PDMS samples with lower crosslinking percentage the nano‐JKR test is more suitable, as it is sensitive and accounts for the surface adhesion forces. The samples with the lower crosslinking percentage are much less stiff in tension than in compression, resembling liquid‐like behavior. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42680.

The relationship between the electron work function and friction behavior of passive alloys under different conditions

Electron work function (EWF), adhesion and friction coefficient of three passive alloys were investigated. During dry sliding tests, surfaces having higher work functions showed larger coefficients of friction. While under oil-lubricated sliding condition, the trend was reversed. It appears that when the surface passive film was damaged, the surface broken atomic bonds became active, leading to larger friction. However, under the oil-lubricated sliding condition, the passive film was retained, resulting in a different trend. Surface adhesive forces of freshly polished surfaces and etched ones were measured using an atomic force microscope. Results of the adhesion measurement are consistent with the behaviors of the alloys observed during the friction tests.

Self-assembled monolayers of perfluoroalkylsilane on plasma-hydroxylated silicon substrates

In this study, a novel kind of fluoroalkylsilane monomers with different fluoroalkyl chain lengths was synthesized via three steps method and characterized by Fourier transform infrared (FT-IR) spectroscopy, 1H and 19F nuclear magnetic resonance (1H NMR and 19F NMR), and mass spectra (MS). Fluoroalkyl-terminated self-assembled monolayers (SAMs) on silanol-terminated silicon substrates (O2 plasma treatment) were chemically fabricated via –Si–O– covalent bonds using the liquid phase deposition method (LPD). The wetabilities of the SAMs were characterized by water contact angles (CA), surface free energies and adhesive force (AF) measurements. 3-(1H,1H,2H,2H-perfluorooctyloxycarbonyl)-propionamidepropyl-triethoxysilane (PFOPT) assembled monolayer was chosen for in-depth investigation as its CA was higher than the others. Attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS) were used to validate the attachment of PFOPT on the silicon substrate, together with the chemical composition and structure of the SAMs. The surface morphologies and roughness of the monolayers were obtained and calculated through atomic force microscopy (AFM). The migration of fluoroalkyl groups to the outermost surface of the SAMs was confirmed by AFM, XPS, CA, and AF measurements. It was found that the silicon substrate assembled with fluoroalkylsilane after heat treatment has higher contact angle, lower surface energy and adhesive force compared to that without heat treatment, and the outermost surface was rich in fluorine due to the surface microphase separation.

A Computational Study of Dry Static Friction Between Elastoplastic Surfaces Using a Statistically Homogenized Microasperity Model

Friction is a complex phenomenon that arises from the interaction of deforming surface microasperities and adhesive forces at very small length scales. In this work, we use a computational model to understand the effects of various physical parameters on the friction response between two similar linearly elastic-perfectly plastic surfaces. The main ingredients of the computational model are a statistical model of the surface based on a Gaussian autocorrelation function (ACF), a parametric representation of the normal and shear responses of a single microasperity, and a statistical homogenization procedure to compute the overall friction response. The surfaces are assumed to be isotropic in nature. We employ this computational model to develop constitutive relationships for the friction force and coefficient of friction for Aluminum 6061 and stainless steel surfaces. We study the effects of various quantities such as surface roughness, material properties, normal load, and adhesive forces on the overall friction response. Our results show that the model is able to capture a wide variety of friction responses. Our results also suggest that the root mean squared (RMS) roughness of the surface alone is insufficient to describe the friction characteristics of a surface, and that an additional parameter is needed. We propose one such parameter, the aspect ratio, which is the ratio of the RMS roughness to the correlation length.

Minimizing antibody surface density on liposomes while sustaining cytokine-activated EC targeting

Liposomes may be engineered to target inflamed endothelium by mimicking ligand–receptor interactions between leukocytes and cytokine-activated endothelial cells (ECs). The upregulation and assembly of vascular cell adhesion molecule-1 (VCAM1) and E-selectin on the cell membrane upon exposure to cytokines have shown potential for drug delivery vehicles to target sites of chronic endothelial inflammation, such as atherosclerosis and cancer. Herein, we characterized EC surfaces by measuring the E-selectin and VCAM1 surface densities and adhesion forces of aVCAM1 and aE-selectin to ECs. We quantified the antibody density, ratio, and diffusivity of liposomes to achieve significant binding and internalization. At 1 h, the 1:1 ratio of VCAM1:E-selectin antibodies was significantly higher than 1:0 and 0:1. Significant binding and uptake was achieved at aE-selectin densities as low as 400 molecules/μm2. The highest levels of binding and uptake were achieved when using a 1:1 ratio of VCAM1:E-selectin antibodies at a density of 1000 molecules/μm2; this density is 85% lower than previous reports. The binding and uptake of functionalized liposomes were reduced to levels comparable to IgG functionalized liposomes upon a 10-fold reduction in liposome membrane diffusivity. We conclude with a liposomal design that discriminates between healthy and inflamed endothelium while reducing antibody surface presentation.

Surface adhesive forces: a metric describing the drag-reducing effects of superhydrophobic coatings.

Nanomaterials with superhydrophobic properties are promising as drag-reducing coatings. However, debates regarding whether superhydrophobic surfaces are favorable for drag reduction require further clarification. A quantified water adhesive force measurement is proposed as a metric and its effectiveness demonstrated using three typical superhydrophobic coatings on model ships with in situ sailing tests.

Understanding the bond-energy, hardness, and adhesive force from the phase diagram via the electron work function

Properties of metallic materials are intrinsically determined by their electron behavior. However, relevant theoretical treatment involving quantum mechanics is complicated and difficult to be applied in materials design. Electron work function (EWF) has been demonstrated to be a simple but fundamental parameter which well correlates properties of materials with their electron behavior and could thus be used to predict material properties from the aspect of electron activities in a relatively easy manner. In this article, we propose a method to extract the electron work functions of binary solid solutions or alloys from their phase diagrams and use this simple approach to predict their mechanical strength and surface properties, such as adhesion. Two alloys, Fe-Ni and Cu-Zn, are used as samples for the study. EWFs extracted from phase diagrams show same trends as experimentally observed ones, based on which hardness and surface adhesive force of the alloys are predicted. This new methodology provides an alternative approach to predict material properties based on the work function, which is extractable from the phase diagram. This work may also help maximize the power of phase diagram for materials design and development.

Evaluation of surface wettability by means of the measurement of the adhesive force between a microstructured hydrophobic surface and a water droplet

The pull-off forces between water and microstructured hydrophobic surfaces were calculated and measured in this study. A hydrophilically modified nickel disk was used to hold a drop of water. Regardless of the type of surface structure, as the solid fraction decreased, the pull-off force gradually decreased. Surfaces with pillar type structures had similar pull-off forces with different pillar sizes when the solid fractions were identical. This indicates that pillar size had no significant effect on the pull-off force. The pull-off force measured from the pore structures was higher than that measured from the pillar structures. As a result, the pillar-type microstructure provides a surface with a lower surface adhesive force than the pore type. The measurement technique used in this study allows the surface adhesive force to be quantified, making it very useful tool for evaluating the wettability of a solid surface.

Effects of sildenafil on nanostructural and nanomechanical changes in mitochondria in an ischaemia-reperfusion rat model.

Sildenafil exerts cardioprotective effects by activating the opening of mitochondrial ATP-sensitive potassium channels to attenuate ischaemia-reperfusion (IR) injury. In the present study, we used atomic force microscopy (AFM) to investigate changes in mitochondrial morphology and properties to assess sildenafil-mediated cardioprotection in a rat myocardial infarction model. To investigate the cardioprotective effects of sildenafil, we used an in vivo Sprague-Dawley rat model of IR. Rats were randomly divided into three groups: (i) sham-operated rats (control; n=5); (ii) IR-injured rats treated with vehicle (normal saline; IR; n=10); and (iii) IR-injured rats treated with 0.75mg/kg, i.p., sildenafil (IR+Sil; n=10). Morphological and mechanical changes to mitochondria were analysed by AFM. Infarct areas were significantly reduced in sildenafil-treated rats (7.8±3.9% vs 20.4±7.0% in the sildenafil-treated and untreated IR groups, respectively; relative reduction 62%; P<0.001). Analysis of mitochondria by AFM showed that IR injury significantly increased the areas of isolated mitochondria compared with control (24150±18289 vs 1495±1139nm(2) , respectively; P<0.001), indicative of mitochondrial swelling. Pretreatment with sildenafil before IR injury reduced the mitochondrial areas (7428±3682nm(2) ; P<0.001; relative reduction 69.2% compared with the IR group) and ameliorated the adhesion force of mitochondrial surfaces. Together, these results suggest that sildenafil has cardioprotective effects against IR injury in a rat model by improving the morphological and mechanical characteristics of mitochondria.

Micro- and Nano-Characteration of Interaction Between Asphalt and Filler

AbstractThis study explored the testing technologies characterizing the interaction between asphalt and filler. An atomic force microscope (AFM) was used to describe the surface morphology and surface adhesion force of asphalt mastic and analyze their effect factors. Fourier transform infrared spectroscopy (FTIR) was used to analyze the physicochemical interaction between asphalt and filler. Adsorption tests were used to study adsorption and desorption properties of asphalt on aggregate. The results show that andesite aggregate had a smoother surface and a weaker adhesion force than granite aggregate. The greater the asphalt penetration, the stronger the surface adhesion force. A small number of filler material increased the adhesion force of the asphalt mastic, but too much filler decreased the adhesion force. FTIR tests were employed to explore the various functional groups of asphalt and fillers. No obvious new stretching vibration peak or deformation vibration peak indicated that the interaction between asphalt and filler was mainly a physical action. Net absorption test revealed that the greater the equilibrium concentration, the greater the adsorption amount. Distilled water stripped asphalt from aggregate. The type of adsorption isotherm corresponded to multi-molecular layer adsorption that further indicated that the interaction between asphalt and fillers was mainly a physical action.