Adhesion and friction in gecko toe attachment and detachment.

The load effect on the anisotropic friction property of gecko seta arrays have been experimentally investigated on a homemade micro adhesive and friction force detect device with isolated seta arrays. The result shows that when sliding against the direction of the seta curvature to grip out, the friction force is proportional to the normal load with a friction coefficient of about 0.6. When sliding along the direction of the seta curvature to grip in, the normal preload force could be fully or partly transformed into adhesion force and repulsive force while friction force rapidly rose with the increment of the applied preload. Under the same normal preload, the lateral friction force in grip in is more than twice of that in grip out. These properties of anisotropy in friction and adhesion are determined by the micro and nano-structures of setae.

Self-assembled monolayer (SAM) films have attracted immense attention for both fundamental and applied research. A SAM is composed of a large number of molecules with a head group that chemisorbs onto a substrate, a tail group that interacts with the outer surface of the film, and a spacer (backbone) chain group that connects the head and tail groups resulting in a coating. Interactions between spacer groups of different molecules, such as van der Waals forces and/or hydrogen bonding, hasten SAM film formation and contribute to its stability. In this dissertation, SAM and thin films have been formed onto copper and aluminum oxide surfaces by reaction with 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS), 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS), 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP), octylphosphonic acid (OP), decylphosphonic acid (DP), and octadecylphosphonic acid (ODP). The properties and stability of the films were investigated employing complementary surface analysis techniques: X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), friction force microscopy (FFM), a derivative of AFM, contact angle measurements (CAMs), and Fourier transform infrared reflection/absorption spectroscopy (FT-IRRAS). The perfluoroalkylsilane SAM on Cu is found to be extremely hydrophobic typically having sessile drop static contact angles of more than 130° for pure water and a surface energy of 14 mJ/m2 (mN/m). FFM showed a significant reduction in the adhesive force and friction coefficient of PFMS modified Cu (PFMS/Cu) compared to unmodified Cu. Treatment by exposure to harsh conditions showed that PFMS/Cu SAM can withstand boiling nitric acid (pH=1.8), boiling water, and warm sodium hydroxide (pH=12, 60 °C) solutions for at least 30 minutes. Furthermore, no SAM degradation was observed when PFMS/Cu was exposed to warm nitric acid solution for up to 70 min at 60 °C or 50 min at 80 °C. XPS and FT-IRRAS data reveal a coordination of the PFMS silicon (Si) atom with a cuprate (CuO) molecule present on the oxidized copper substrate. The data give good evidence that the stability of the SAM film on the PFMS modified oxidized Cu surface is largely due to the formation of a siloxy-copper (-Si-O-Cu-) bond via a condensation reaction between silanol (-Si-OH) and copper hydroxide (CuOH). For a PFTS modified Cu surface (PFTS/Cu), the sessile drop static contact angle of pure water has been measured to be more than 125° and the surface energy to be typically less than 16 mJ/m2. Stability tests show that the PFTS/Cu film can survive in boiling pure water for one hour, boiling nitric acid (pH 1.5 or 1.8) for 30 minutes, sodium hydroxide solution (pH 12, 70 °C) for 30 minutes, and autoclave conditions (steam at 134 °C and 3 atmospheres) for 15 minutes. The more commonly used self-assembled monolayer (SAM) modifications of Cu surfaces, e.g. thiol compounds, are significantly less stable than PFTS/Cu. Extremely hydrophobic (low surface energy) and stable PFMS/Cu SAMs and PFTS/Cu films could be useful as corrosion inhibitors in micro/nanoelectronic devices and/or as promoters for anti-wetting, low adhesion surfaces or drop-wise condensation on heat exchange surfaces. XPS analysis confirmed the presence of perfluorinated and non-perfluorinated alkylphosphonate molecules on the PFDP, DP, and ODP SAMs deposited at the aluminum oxide coated silicon (Al/Si) surfaces. The sessile drop static contact angle of pure water on PFDP SAMs was typically more than 130° and on DP and ODP typically more than 125° indicating that the phosphonic acid SAMs reacted with Al samples were very hydrophobic. The surface roughness for PFDP/Al, DP/Al, ODP/Al, and bare Al was approximately 35 nm, as determined by AFM. The surface energy for PFDP/Al was determined to be approximately 11 mJ/m2 by the Zisman plot method compared to 21 mJ/m2 and 20 mJ/m2 for DP/Al and ODP/Al, respectively. PFDP/Al gave the lowest adhesion and friction force while bare Al gave the highest. The adhesion and friction forces for ODP/Al and DP/Al SAMs were in between. ODP, DP, and OP SAMs have been studied in detail on relatively flat aluminum oxide surfaces. The rms surface roughness for ODP/Al, DP/Al, OP/Al, and bare Al was less than 15 nm, as determined by AFM. The sessile drop static contact angle of pure water on ODP/Al and DP/Al was typically more than 115° and on OP/Al typically less than 105°. The surface energy for ODP/Al and DP/Al was determined to be approximately 21 mJ/m2 and 22 mJ/m2, respectively, compared to 26 mJ/m2 for OP/Al. ODP/Al and OP/Al were studied by FFM to better understand their micro-/nano-tribological properties. ODP/Al gave the lowest coefficient of friction values while bare Al gave the highest. The adhesion forces for ODP/Al and OP/Al were comparable. The chemical stability of ODP/Al, PFDP/Al, DP/Al, OP/Al, and PFMS/Al SAMs has been inspected by exposure to warm nitric acid (pH 1.8, 30 min, 60-95 °C). The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid (PA) or silane molecule and the oxidized Al surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60-95 °C) show ODP/Al SAMs to be most stable followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al SAMs. For PFTS/Al, stability tests demonstrate that modified aluminum is able to survive exposure to warm nitric acid (pH = 1.8, 60 °C, 30 min) indicating some degree of robustness. Hydrophobic, low adhesion, and robust aluminum surfaces have useful applications for micro/nano-electromechanical systems (MEMS/NEMS), such as digital micro-mirror devices (DMDs). These studies are expected to aid in the design and selection of proper lubricants for MEMS/NEMS.

The friction force between nanoparticles and a silicon wafer is a crucial parameter for cleaning processes in the semiconductor industry. However, little is known about the pH-dependency of the friction forces and the shear strength at the interface. Here, we push polystyrene nanoparticles, 100 nm in diameter, with the tip of an atomic force microscope and measure the pH-dependency of the friction, adhesion, and normal forces on a silicon substrate covered with a native silicon dioxide layer. The peak force tapping mode was applied to control the vertical force on these particles. We successively increased the applied load until the particles started to move. The main advantage of this technique over single manipulation processes is the achievement of a large number of manipulation events in short time and in a straightforward manner. Geometrical considerations of the interaction forces at the tip-particle interface allowed us to calculate the friction force and shear strength from the applied normal force depending on the pH of an aqueous solution. The results clearly demonstrated that particle removal should be performed with a basic solution at pH 9 because of the low interaction forces between particle and substrate.

The centrifuge technique, which has been previously used in adhesion experiments, has been modified for use in single particle friction studies. Both flat compacted surfaces and large single particles were used as substrate surfaces to allow assessment of drug-drug, drug-drug carrier and drug carrier-drug carrier friction forces. Particle size, particle shape and surface roughness were identified as main factors influencing the change from a static into a dynamic friction process and the division between friction due to adhesion and ploughing. The forces of adhesion and friction were found to be proportional to the reversible energy of adhesion. The ratio between the force of adhesion and the press-on force applied and the ratio between the force of friction and the press-on force can be related to the yield stress and the reduced Young's modulus of the materials in contact.

In this study, the friction forces between a slider and disk surface with different lubricant thicknesses were measured at touchdown. Touchdown measurements of the friction and adhesion forces on a disk were made using a pin-on-disk tribotester. The friction force at touchdown could not be explained by a monotonic behavior, where the friction force gradually decreased with the lubricant thickness. Based on a comparison of the results measured using the pin-on-disk tribotester, it was estimated that the optimal adhesion force and a lower friction force are needed for the surfing regime at touchdown.

A lethality zone due to an ignited natural gas release is often used to characterize the consequences of a pipeline rupture. A 1% lethality zone defines a zone where the lethality to a human is greater than or equal to 1%. The boundary of the zone is defined by the distance (from the point of rupture) at which the probability of lethality is 1%. Currently in the gas pipeline industry, the most detailed and validated method for calculating this zone is embodied in the PIPESAFE software. PIPESAFE is a software tool developed by a joint industry group for undertaking quantitative risk assessments of natural gas pipelines. PIPESAFE consequence models have been verified in laboratory experiments, full scale tests, and actual failures, and have been extensively used over the past 10–15 years for quantitative risk calculations. The primary advantage of using PIPESAFE is it allows for accurate estimation of the likelihood of lethality inside the impacted zone (i.e. receptors such as structures closer to the failure are subject to appropriately higher lethality percentages). Potential Impact Radius (PIR) is defined as the zone in which the extent of property damage and serious or fatal injury would be expected to be significant. It corresponds to the 1% lethality zone for a natural gas pipeline of a certain diameter and pressure when thermal radiation and exposure are taken into account. PIR is one of the two methods used to identify HCAs in US (49 CFR 192.903). Since PIR is a widely used parameter and given that it can be interpreted to delineate a 1% lethality zone, it is important to understand how PIR compares to the more accurate estimation of the lethality zones for different diameters and operating pressures. In previous internal studies, it was found that PIR, when compared to the more detailed measures of the 1% lethality zone, could be highly conservative. This conservatism could be beneficial from a safety perspective, however it is adding additional costs and reducing the efficiency of the integrity management process. Therefore, the goal of this study is to determine when PIR is overly conservative and to determine a way to address this conservatism. In order to assess its accuracy, PIR was compared to a more accurate measure of the 1% lethality zone, calculated by PIPESAFE, for a range of different operating pressures and line diameters. Upon comparison of the distances calculated through the application of PIR and PIPESAFE, it was observed that for large diameters pipelines the distances calculated by PIR are slightly conservative, and that this conservativeness increases exponentially for smaller diameter lines. The explanation for the conservatism of the PIR for small diameter pipelines is the higher wall friction forces per volume transported in smaller diameter lines. When these higher friction forces are not accounted for it leads to overestimation of the effective outflow rate (a product of the initial flow rate and the decay factor) which subsequently leads to the overestimation of the impact radius. Since the effective outflow rate is a function of both line pressure and diameter, a simple relationship is proposed to make the decay factor a function of these two variables to correct the excess conservatism for small diameter pipelines.

In this paper, the adhesion and friction forces between a glass pin and disk surfaces coated by lubricants with different numbers of hydroxyl endgroups (Z-03, Z-dol, Z-tetraol, and Z-TMD) were measured using a pin-on-disk microtribotester. The adhesion force decreased in the order of Z-dol, Z-TMD, Z-tetraol, and Z-03. The friction force at the applied load force of 0 mN decreased in the order of Z-TMD, Z-tetraol, Z-dol, and Z-03. These trends could not be obviously explained by the number of hydroxyl endgroups. Next, we investigated the relationship between the adhesion energy and the total surface energy. The adhesion force was roughly proportional to the surface energy, and the friction force was proportional to the friction factor defined by the friction coefficients of the carbon overcoat and the lubricant film, the lubricant film coverage, and the total surface energy. Based on the results, we suggested a friction model of the contact between the pin and disk surfaces.

Synthesis, structure, and properties of the high Nb–TiAl alloy after Ni coatings by plasma surface alloying technique

Origins of friction and wear of the thin metallic layer of metal evaporated magnetic tape

In quasi-static tests of large-scale structural columns and/or columns under large axial loads, the lateral friction force between the column and the loading system can become a significant problem: they may cause considerable deviation between the measured lateral force and the actual reaction force of the column, especially under large axial compression load. Many researchers have come up with different methods to reduce or eliminate the influence of such friction force. In this article, previous treatments on the lateral friction force in quasi-static tests are first discussed. A shear force measurement device, for accurate measurement of the friction force, is then presented and calibrated. Based on the friction forces measured by the device in real tests, a simple model is proposed to predict the lateral friction force in quasi-static tests. Using the model, the measured lateral force in such tests can be corrected to obtain the actual reaction force of the column when a friction measurement device is absent. The proposed model and the correction method are then validated using results from several previous tests.

Semiconductor devices have been achieved high performance by improving processing speed and integration degree due to downsize and multilayer wiring structure. Along with this tendency, the importance of CMP processing is increasing. In addition, due to many particles are generated after CMP process, post-CMP cleaning is also very important process. In the post-CMP cleaning, PVA (PolyVinyl Acetal) brush have been widely used. However, in this scrub cleaning, problems have been reported that many particles are generated from the PVA brush itself by adhesion of PVA brush onto surface and contaminated the wafer surface. Therefore, it is necessary to control adhesion between PVA brushes and semiconductor surfaces. In our early study, adhesion of PVA brush was evaluated by rotating torques or frictional forces. In this study, we tried to measure directly adhesion forces between PVA brush and several solid surfaces with a high response load cell. We believe that the measurement of the adhesion force can discriminate the material that is likely to adhere to the PVA brush. We compared the adhesion forces for two types of brushes with or without a skin layer. Moreover, we investigated whether adhesion force could be controlled by changing brush temperature. Because of the PVA brushes are porous polymer, mechanical properties change with its temperature. We consider that the temperature also affects adhesion force onto solid surfaces. In this study, we measured the adhesion forces between PVA brushes and solid surfaces in the atmosphere. The adhesion forces Fa were measured with a load cell. PVA brush was pressed against the test surfaces and releasing it at high speed after relaxing for a certain period of time tw . We used three types of test surfaces, Glass, Polymethyl methacrylate (PMMA) and Polytetrafluoroethylene (PTFE). The contact angles were 28°, 61° and 97°, respectively. The temperature of test surfaces were controlled with a Peltier device and were set to 20 degrees to 50 degrees. Attached figure shows an example of relationship between the adhesion force Fa of PVA brush with skin layer and relaxation time tw on the different surfaces. As shown in the figure, Fa increased with increase tw in the all surface cases. We consider that this cause is real contact area increased as the tw increased. In the dependence of test surfaces material, Glass and PTFE case showed similar adhesion force values. On the other hand, the forces of PMMA case showed larger values than other test surfaces. This result indicates that there are materials that strongly interacts with skin layer PVA brushes. In the experiments using without skin layer case, however, we could not measure the adhesion force in the all experimental conditions. This result suggests that without skin layer brush is maybe not contact onto surface and the adhesion force is strongly depended on the existence of skin layer. Next, we focus on temperature dependence of the adhesion forces. In the all test surface cases, adhesion force decreased with increase temperature. Particularly the adhesion force of the PMMA case was greatly decreased at 50 degrees. These results suggest that the temperature may control the adhesion force of PVA brush onto solid surface. Figure 1

We present a simple microscopic model describing the unique friction behavior of gecko setal arrays as they are dragged on smooth surfaces. Unlike other solids of high elastic modulus that do not stick under van der Waals forces alone, the gecko setal arrays do not require a compressive force to display a drag resistance but rather develop a tensile normal force when they are dragged (J. Experim. Biol. 2006, 209, 3569). We describe this unique behavior with a microscopic model involving curved beam structures at two length scales: at the spatula level, thousands of independent curved beams repeat detachment and reattachment, whereas at the seta level, the curved beam geometry of the seta induces a coupling between the frictional force and the adhesive force that depends on the angle of contact, therefore allowing easy release when the animal needs it. Our model accounts well for the dependence of the drag and adhesion forces on the drag velocity and can also explain macroscopic attachment/ detachment cycles of the setal array.

Surface and friction forces between grafted polysaccharide layers in the absence and presence of surfactant

Static friction between sidewall contact surfaces of polycrystalline silicon micromachines was investigated under different contact pressures, vacuum conditions, relative humidity levels, and temperatures. The static coefficient of friction exhibited a nonlinear dependence on the external contact pressure. A difference between in-contact and pull-out adhesion forces was observed due to the elastic recovery of the deformed asperities at the contact interface. The true static coefficient of friction was determined by considering the effects of the dominant adhesion forces (i.e., van der Waals and capillary forces) on the normal force applied at the sidewall contact interface. The roles of van der Waals and capillary forces in the sidewall friction behavior were analyzed in light of results for the interfacial shear strength and the adhesion force. The major benefits of the present friction micromachine and the developed experimental scheme are discussed in the context of static coefficient of friction and adhesion force results obtained under different environmental and loading conditions

Bacterial adhesion is problematic in many diverse applications. Coatings of hydrophilic polymer chains in a brush configuration reduce bacterial adhesion by orders of magnitude, but not to zero. Here, the mechanism by which polymer‐brush functionalized surfaces reduce bacterial adhesion from a flowing carrier fluid by relating bacterial adhesion with normally oriented adhesion and friction forces on polymer (PEG)‐brush coatings of different softness is studied. Softer brush coatings deform more than rigid ones, which yields extensive bond‐maturation and strong, normally oriented adhesion forces, accompanied by irreversible adhesion of bacteria. On rigid brushes, normally oriented adhesion forces remain small, allowing desorption and accordingly lower numbers of adhering bacteria result. Friction forces, generated by fluid flow and normally oriented adhesion forces, are required to oppose fluid shear forces and cause immobile adhesion. Summarizing, inclusion of friction forces and substratum softness provides a more complete mechanism of bacterial adhesion from flowing carrier fluids than available hitherto.