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In ultra-precision equipment, nanoscale rough surface contact widely exists, and the adhesion between interfaces has become a critical component in researching the friction phenomenon. However, adhesion is not considered in traditional friction models. In this study, the adhesion model between fractal contact surfaces is established by proposing the critical truncated area of adhesion, a c u t ′ , and a new friction model considering adhesion is established by combining with the traditional model. The present model shows that adhesion increases as the fractal dimension (D) increases or the surface roughness coefficient (G) decreases when contact occurs between nanoscale rough surfaces. The friction coefficient calculated based on the new model is larger than that of the traditional model, especially when G is small. In addition, a static friction force measurement system was built by using the reticle and vacuum chuck in the lithography system. The experimental system measured the maximum static friction between the chuck and the reticle under different normal loads, and the experimental results show that the new model proposed in this paper is more accurate in characterizing the static friction of nanoscale rough surfaces. [ABSTRACT FROM AUTHOR]

Detecting mechanical fatigue of metallic components is always a challenge in industries. In this work, we proposed to monitor the low-cycle fatigue of a 6061 aluminum alloy based on internal friction (IF) measurement, which is realized by a quantitative electromechanical impedance (Q-EMI) method using a small piezoelectric wafer bonded on the specimen. Large stress amplitude (230 MPa) was employed in the testing; thus, the fatigue life can always be below 105 cycles. It was found that except for the initial testing stage, the IF always increases steadily with the increasing fatigue cycles. Before the fatigue failure, the IF can reach 2.5–3.4 times the initial value, which is thought to be caused by the micro-cracks forming and growing. In comparison, the resonance frequency of the specimen just drops by about 2% compared with the initial value. Finally, a fatigue criterion based on IF measurement is suggested for aluminum alloys and it can also be extended to other metals. [ABSTRACT FROM AUTHOR]

To characterize liquid-solid friction using molecular dynamics simulations, Bocquet and Barrat (BB) [Phys. Rev. E 49, 3079–3092 (1994)] proposed to use the plateau value of a Green-Kubo (GK) integral of the friction force. The BB method is delicate to apply in finite-size simulations, where the GK integral vanishes at long times. Here, we derive an expression for the GK integral in finite-size systems, based on a Langevin description of a coarse-grained system effectively involving a certain thickness of liquid close to the wall. Fitting this expression to GK integrals obtained from simulations of a liquid slab provides the friction coefficient and the effective thickness of the coarse-grained system. We show that the coarse-grained system for a Lennard-Jones fluid between flat and smooth solid surfaces is 2–3 molecules thick, which provides a criterion for measuring the friction coefficient independently of confinement. As compared to nonequilibrium simulations, the new approach is more accurate and removes some ambiguities of nonequilibrium measurements. Overall, we hope that this new method can be used to characterize efficiently liquid-solid friction in a variety of systems of interest, e.g., for nanofluidic applications. [ABSTRACT FROM AUTHOR]

The aim of the work was to identify the physical parameters relevant to different perceptual attributes by comparing a range of commercial skin creams with various compositions. We investigated the possibility of predicting sensory properties using rheological and tribological methods. Rheological evaluations of the skin creams were carried out using continuous shear, creep, creep recovery, rheodestruction and recovery measurements, oscillatory strain sweep measurements [small amplitude oscillatory shear (SAOS) and large amplitude oscillatory shear (LAOS)], and frequency sweep measurements. Friction measurements were performed on the nonbiological skin model to investigate how surface properties are influenced by the application of different topical formulations. Furthermore, the formulations were characterized by a broad range of instrumental texture measurements. In vivo sensory analysis based on the spectrum descriptive analysis method was performed to discriminate the skin creams during pickup, rub out, and after feel. Using principal component analysis meaningful correlation relating rheotribological properties and sensory attributes during the entire process of product application was carried out. Rheological parameters deduced from the nonlinear regime were found to be important parameters affecting the frictional response of skin creams. In addition, friction data were correlated with slipperiness and stickiness—subjective attributes used for the sensory evaluation of after feel. Furthermore, a number of key textural parameters and sensorial data showed good correlation with results obtained from linear and nonlinear rheological measurement, indicating rheological analysis can be sufficiently used as a precise and valid tool for sensorial mapping of topical formulations. Our study further suggests that objective evaluation based on the flow curve, oscillatory strain sweep (SAOS and LAOS), and friction measurements can be used for sensorial screening of large number of prototype formulations, which otherwise may be time consuming and costly using a sensory panel. [ABSTRACT FROM AUTHOR]

Six-axis force/torque sensors are increasingly needed in mechanical engineering. Here, we introduce a flexure-based design for such sensors, which solves some of the drawbacks of the existing designs. In particular, it is backlash-free, it can be wirelessly monitored, it exactly enforces 90° angles between axes, and it enables visual inspection of the monitored system, thanks to its hollow structure. We first describe the generic design, implementation, and calibration procedure. We then demonstrate its capabilities through three illustration examples relevant to the field of tribology: low friction measurements under ultra-high vacuum, multi-directional friction measurements of elastomer contacts, and force/torque-based contact position monitoring. [ABSTRACT FROM AUTHOR]

Rheological experiments on model and industrial colloidal dispersions that exhibit reversible shear thickening are compared to a recent model that correlates simulation results for the shear rheology of bidisperse suspensions of particles with varying levels of contact friction. Comparisons are presented with rheological results for a variety of colloidal dispersions varying from model systems to industrially relevant, commercial dispersions where both shear and first normal stress differences are available. Particle-scale measurements of particle friction coefficients are provided through a survey of the literature. The comparisons show under which conditions the model can quantitatively describe the measured rheology and how the model can be used to correlate data for systems with more complex interactions when allowances are made for effective volume fractions and adjustments in friction coefficients. Discrepancies evident in comparisons for the first normal stress differences highlight the importance of enhanced hydrodynamic friction in many systems, especially for continuous shear thickening, in qualitative agreement with recent simulations that incorporate enhanced lubrication hydrodynamics due to elastohydrodynamics or particle roughness. Finally, a literature survey of measurements of jamming volume fractions and particle friction coefficients indicates that the model overestimates the experimentally observed jamming volume fraction. The comparisons presented in this work provide quantitative information valuable for those modeling suspension processing as well as suggestions for further model improvement. [ABSTRACT FROM AUTHOR]

A friction setup combining real-time ice-rubber contact visualization, force measurement, and a compact controlled cold environment system was developed in order to investigate ice-rubber contact complex tribological response and the various contributions to friction, such as viscoelastic deformation, ice surface melting, adhesion, ice creep, or quasi-liquid layer effect. The cold system was based on a cryogenic bath circulator, an air convection circuit, and several thermal insulation combinations such as silica aerogel and expanded polystyrene. The KŌRI tribometer allows one to reach negative temperatures until −20 °C and to perform tribological experiments for velocity from 50 μm s−1 to 1 m s−1 under load up to 50 N and to simultaneously measure resultant forces until 30 N and visualize the contact in real-time. In parallel, an ice manufacturing unit and a specific protocol were developed to grow a transparent ice disc with a controlled initial roughness and surface state. Real-time and simultaneous visualization of the ice-rubber contact provides additional data, such as the apparent contact area and the mean size of a real contact spot during friction, after adequate and dedicated image processing. To illustrate the capability of the KŌRI tribometer, rubber-ice friction measurements were performed at −10 °C and the results are presented here, as a function of time and velocity. [ABSTRACT FROM AUTHOR]

Students often struggle to understand the independence of velocity vectors in two-dimensional motion. Despite seeing classic projectile demonstrations, it is probably not obvious to most new students why the path of motion is parabolic. Likewise, first-year students might not predict that the path of motion is linear in two-dimensional vector problems such as a boat traveling at constant velocity across a river moving at constant velocity. There do exist excellent computer simulations, but students are more likely to believe the results if they can create the motion themselves. Fortunately, students can directly explore such motion using blocks of wood that are commonly used in the classroom to measure friction. The instructor only needs to make a few inexpensive modifications to the blocks to not just teach vector motion, but with a little imagination teach many more concepts in physics. In this article I will elaborate on those modifications and uses. [ABSTRACT FROM AUTHOR]