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PubMed2000

Lateral Force Microscopy Study of the Friction between Silica Surfaces.

The friction between a single silica glass sphere and a flat silica glass surface has been investigated using lateral force microscopy. Two types of silica surface were investigated, one fully hydroxylated and the other partially dehydroxlated. These samples have markedly different wetting properties and are known as hydrophilic and hydrophobic silica, respectively. The lateral friction force as a function of applied normal load has been collected for five different-sized spheres for both surface types. Analysis of the friction versus load data indicates that the hydrophilic surfaces have a molecularly smooth contact. For the hydrophobic surfaces, it is not clear whether the friction-load response is caused by a smooth or an asperity-dominated contact zone. Copyright 2000 Academic Press.

Journal Article↗
PubMed1993

Friction at the digit-object interface scales the sensorimotor transformation for grip responses to pulling loads.

When restraining a mechanically "active" object (one that exerts unpredictable changes in loading forces) with a precision grip of the digits, we maintain a stable grasp by modulating our grip force using somatosensory information related to the loading forces. The response to ramp load increases consists of an initial fast rise in grip force ("catch-up") followed by a secondary response that steadily increases the grip force in parallel with the load force ("tracking"). The sizes of these response components scale in proportion to the loading rate. However, maintaining a stable grasp without employing an exceedingly large grip force may require further scaling of this load-to-grip sensorimotor transformation based on two additional factors: (1) the friction at the digit-object interface and (2) the grip force present at the start of the load increase. The present experiments sought to determine whether such scaling occurs and to characterize its control. Subjects restrained a manipulandum held between the tips of the thumb and index finger. At unpredictable times a pulling force appeared, directed away from the subject's hand. Each pull had a trapezoidal load profile beginning and ending at 0 N with 4-N/s ramps; each ramp was 1 s in duration. The texture of the gripped surfaces varied among sandpaper, suede, and rayon, which represented increasingly slippery surfaces. The grip force at the start of the load ramp (intertrial grip force), and the amplitudes of the catch-up and secondary grip responses scaled in proportion to the inverse friction. We interpret these results to indicate a uniform scaling of the transformations controlling the intertrial grip force, the catch-up response, and the secondary response. Initial-state information from tactile cues available upon object contact appeared to update the frictional scaling value. This conclusion is based on observations of immediate changes in the intertrial grip force upon contact with a new surface, and because differences in force-rate profiles appeared virtually by the onset of the catch-up response. Similarly, the intertrial grip force also constituted initial-state information. The size of the catch-up and secondary grip force responses varied inversely with the size of the intertrial grip force. These scalings of the load-to-grip-force sensorimotor transformation for friction and intertrial grip force level appear to be functionally adaptive, because they contribute to a stable grasp (prevent object slips) while avoiding exceedingly large safety margins.

Adolescent↗
PubMed1996

Muscle function during brief maximal exercise: accurate measurements on a friction-loaded cycle ergometer.

A friction loaded cycle ergometer was instrumented with a strain gauge and an incremental encoder to obtain accurate measurement of human mechanical work output during the acceleration phase of a cycling sprint. This device was used to characterise muscle function in a group of 15 well-trained male subjects, asked to perform six short maximal sprints on the cycle against a constant friction load. Friction loads were successively set at 0.25, 0.35, 0.45, 0.55, 0.65 and 0.75 N.kg-1 body mass. Since the sprints were performed from a standing start, and since the acceleration was not restricted, the greatest attention was paid to the measurement of the acceleration balancing load due to flywheel inertia. Instantaneous pedalling velocity (v) and power output (P) were calculated each 5 ms and then averaged over each downstroke period so that each pedal downstroke provided a combination of v, force and P. Since an 8-s acceleration phase was composed of about 21 to 34 pedal downstrokes, this many v-P combinations were obtained amounting to 137-180 v-P combinations for all six friction loads in one individual, over the widest functional range of pedalling velocities (17-214 rpm). Thus, the individual's muscle function was characterised by the v-P relationships obtained during the six acceleration phases of the six sprints. An important finding of the present study was a strong linear relationship between individual optimal velocity (vopt) and individual maximal power output (Pmax) (n = 15, r = 0.95, P < 0.001) which has never been observed before. Since vopt has been demonstrated to be related to human fibre type composition both vopt, Pmax and their inter-relationship could represent a major feature in characterising muscle function in maximal unrestricted exercise. It is suggested that the present method is well suited to such analyses.

Acceleration↗
PubMed1990

[The loss of force by friction in arch-guided tooth movement].

Employing arch-guided tooth movement always results in a loss of force by friction. In order to quantify the loss of force, an apparatus was designed featuring a simulated tooth with full three dimensional mobility. Five different wire alloys in five wire dimensions were combined with three brackets of different widths. Under optimal circumstances the ratio of applied force to orthodontically effective force was 2.3. In respect to the wire dimension the generated friction primarily depends on the vertical diameter. Wide brackets generate less friction than narrow ones. There are significant differences between the examined wire alloys. To minimize friction, a low surface roughness is of high importance.

Biomechanical Phenomena↗
PubMed1990

[The friction behavior of the ceramic bracket in arch wire-guided tooth movement].

In spite of their wellknown disadvantages ceramic brackets are in great demand by orthodontic patients because of their aesthetic features. The amount of friction produced during tooth movement when a ceramic bracket slides along an arch wire, is still unknown. Using a custom-made friction test device the current study was undertaken to measure friction values of brackets of different manufactures with an 0.018 inch slot sliding along an 0.016 x 0.022 inch stainless steel arch wire. The results of this study showed that stainless steel brackets had slightly lower values than polycrystalline ceramic brackets. The monocrystalline bracket however showed significant greater friction values than both stainless steel and polycrystalline brackets even though its surface, as revealed with the scanning electron microscopy, proved to be quite smooth.

Ceramics↗
PubMedDate not supplied

Calculation and experimental verification of the frictional ratio of hagfish proteinase inhibitor.

A major structural feature of the alpha-2-macroglobulin-like inhibitor of hagfish described in Osada, Nishigai, and Ikai [(1987) J. Ultrastruct. Mol. Struct. Res. 96, 00-00] was its highly open quaternary structure observed under an electron microscope. We drew a qualitative conclusion that the high frictional ratio obtained from the result of sedimentation study and the large Stokes radius obtained in gel chromatographic experiment were the reflection of such an open quaternary structure. In this paper I present several structural models of hagfish inhibitor based on its electron micrographs and calculate expected frictional ratios for such models according to the method developed by Bloomfield and his co-workers. Their method allows the calculation of frictional coefficient of a body of an arbitrary shape by approximating it with a collection of small spheres. To test the validity of such a method, macroscopic models were built from plastic spheres or cylindrical capsules and their translational frictional coefficients were measured by the free-falling method under experimental conditions where the Reynolds number was between 10(-3) and 10(-4).

Animals↗
PubMed1991

A comparative study of frictional forces between orthodontic brackets and arch wires.

An in vitro study of simulated canine retraction was undertaken to evaluate the difference in frictional resistance between stainless steel arch wires and steel and ceramic brackets with elastomeric, steel, and self-ligation. Each bracket slot was 0.018 x 0.025 inch. The arch wires used were 0.014-inch, 0.016-inch, 0.018-inch, 0.016 x 0.016-inch, and 0.016 x 0.22-inch stainless steel. A testing apparatus was designed to attempt to simulate the clinical situation in which teeth tip slightly while they slide along the arch wire. Under these testing conditions, the self-ligating steel bracket did not demonstrate less friction than the elastic or steel-ligated stainless steel brackets. For most wire sizes, elastomer-ligated ceramic brackets demonstrated the greatest friction when compared with other bracket/ligation technique combinations. The clinical significance of this study becomes apparent when stainless steel brackets are used on the posterior teeth and ceramic brackets are used on the anterior teeth. If sliding mechanics are used, the anterior teeth may be more resistant to movement than the posterior teeth because of the greater friction of the ceramic brackets. This could result in more posterior anchorage loss than would be expected if only one type of bracket were used.

Analysis of Variance↗
PubMed2004

[Effect of laparoscopic trocar model on the quality of the movement: experimental study of friction].

OBJECTIVE: To keep the pneumoperitoneum and the tightness of the abdominal and pelvic cavity during a laparoscopy, the sheaths of trocar are provided with a device (valvule, membranes etc.) inducing a friction during the handling of the instrument. The objective of this article is to analyse friction from different types of trocar's port. MATERIALS AND METHODS: We present here the experimental results obtained during a testing bench of three different trocar sheaths. The mechanism of tightness of the first trocar sheath is made of a fine membrane associated to a valve, of a thick membrane for the second trocar sheath, whereas the third trocar sheath is made of two membranes, a fine and a thick, associated to a valve. After reporting the experimental device and the analytic model adopted to describe the relationship between the measured physical parameters, we lay out our results. The identification of parameters of this model makes it possible to objectively compare the three trocars. RESULTS: Our results revealed that, under experimental conditions, the amplitude of friction was significantly lower with trocar's port provided with a valve and a fine membrane than with both other ports (fivefold weaker). DISCUSSION AND CONCLUSION: We evoke the importance of the possible disruption brought by these frictions while referring these values to those of other measurements concerning interactions between instrument and organs. These different behaviours of the material could have some consequences in choosing the tools for the performance of precise gestures.

Laparoscopy↗
PubMed2005

pH dependence of friction forces between silica surfaces in solutions.

The pH dependence of the friction between a silica particle and a silica wafer was investigated using lateral force microscopy. Measurements were done in the range of 3.6 < or = pH < or = 10.6 and the effect of high loading force was also examined. It is found that the friction is independent of the pH of solutions and increases linearly with the applied load, when the pH is between 3.6 and 8.6. On the other hand, once the pH is above 9.0, the friction becomes extremely small and the dependence on the applied load becomes nonlinear. It is postulated that this transition is due to the development of a gel layer composed of polymer-like segments of silicilic acid anchored on the surface; at the lower applied load, this layer acts as a boundary lubricant between the surfaces, but, at the higher applied load, the entanglements of these segments and more direct contact between two solid surfaces leads to the increase of the friction. The effects found here are expected to play an important role in elucidating the basic mechanism of the planarization process of silica wafers.

Journal Article↗
PubMed2005

Toothbrush abrasion, simulated tongue friction and attrition of eroded bovine enamel in vitro.

OBJECTIVES: Enamel erosion results in the formation of a softened layer that is susceptible to disruption by mechanical factors such as brushing abrasion, tongue friction and attrition. The aim of this study was to investigate the individual contribution of those mechanical insults to the enamel loss caused by dental erosion. METHODS: Forty two bovine enamel samples were randomly divided into seven groups (n=6 per group) that were submitted to 3cycles of one of the following regimes: erosion and remineralization (er/remin); toothbrush abrasion and remineralization (abr/remin); erosion, toothbrush abrasion and remineralization (er/abr/remin); attrition and remineralization (at/remin); erosion, attrition and remineralization (er/at/remin); simulated tongue friction and remineralization (tg/remin); erosion, simulated tongue friction and remineralization (er/tg/remin). Erosion took place in a demineralization solution (50mM citric acid, pH 3) for 10min under agitation. Brushing abrasion, tongue friction and attrition were simulated for 1min using a home-made wear device. Remineralization was carried out in artificial saliva for 2h. Enamel loss was quantified using optical profilometry. RESULTS: One-way ANOVA indicated a significant difference between the amounts of enamel lost due to the different wear regimes (p<or=0.001). Multiple comparisons with Bonferroni procedure showed that the wear depths found for the er/at/remin (p<or=0.001) and er/tg/remin (p<or=0.001) were significantly higher than the amount found for the er/remin group (4.4+/-0.7microm). This was not the case for er/abr/remin group (p=0.075). CONCLUSIONS: The results suggest that the three forms of mechanical insults remove to varying extent the softened layer formed by the erosion challenges.

Animals↗
PubMed2004

Velocity dependent friction laws in contact mode atomic force microscopy.

Friction forces in the tip-sample contact govern the dynamics of contact mode atomic force microscopy. In ambient conditions typical contact radii between tip and sample are in the order of a few nanometers. In order to account for the large interaction area the dynamics of contact mode atomic force microscope (AFM) is investigated under the assumption of a multi-asperity contact interface between tip and sample. Thus, the kinetic friction force between tip and sample is the product of the real contact area between both solids and the interfacial shear strength. The velocity strengthening of the lateral force is modeled assuming a logarithmic relationship between shear-strength and velocity. Numerical simulations of the system dynamics with this empirical model show the existence of two different regimes in contact mode AFM: steady sliding and stick-slip where the tip undergoes periodically stiction and kinetic friction. The state of the system depends on the scan velocity as well as on the velocity dependence of the interfacial friction force between tip and sample. Already small viscous damping contributions in the tip-sample contact are sufficient to suppress stick-slip oscillations.

Journal Article↗
PubMed1997

The friction cost method: a comment.

The friction cost method has been proposed as an alternative to the human-capital approach of estimating indirect costs. We argue that the friction cost method is based on implausible assumptions not supported by neoclassical economic theory. Furthermore consistently applying the friction cost method would mean that the method should also be applied in the estimation of direct costs, which would mean that the costs of health care programmes are substantially decreased. It is concluded that the friction cost method does not seem to be a useful alternative to the human-capital approach in the estimation of indirect costs.

Absenteeism↗
PubMed2002

Effect of temperature on friction observed between a Si3N4 tip and a dodecanethiol self-assembled monolayer on Au(1 1 1).

Recent computational studies (Phys. Rev. Lett. 70 (1993) 1960; Phys. Rev. B 62 (2000) 17055) predicted that friction of ordered organic monolayer had characteristic dependence on temperature, where the maximum friction was observed around rotator transition point of the monolayer. This remained to be confirmed experimentally. Using a friction force microscope (FFM) combined with a temperature regulation module, we attempted to investigate such dependence on temperature (130 K-room temperature) on a self-assembled monolayer of dodecanethiol prepared on Au(1 1 1). The observed friction showed strong dependence on temperature and good agreement with the computational prediction.

Journal Article↗
PubMed2002

Dynamic simulations of adhesion and friction in chemical force microscopy.

A hybrid molecular simulation approach has been applied to investigate dynamic adhesion and friction between a chemical force microscope (CFM) tip and a substrate, both modified by self-assembled monolayers (SAMs) with hydrophobic methyl (CH(3)) or hydrophilic hydroxyl (OH) terminal groups. The method combines a dynamic model for the CFM tip-cantilever system and a molecular dynamics (MD) relaxation technique for SAMs on Au(111) at room temperature. The hybrid simulation method allows one to simulate force-distance curves (or adhesion) and friction loops (or friction coefficient) in the CFM on the experimental time scale for the first time. The simulation results also provide valuable molecular information at the interface that is not accessible in CFM experiments, such as the actual tip position with respect to the cantilever support position, molecular and hydrogen-bonding structures at the interface, and load distributions among different molecular chains (or single-molecule forces). Results show that the adhesion force and friction coefficient for the OH/OH contact pair are much larger than those for the CH(3)/CH(3) pair due to the formation of hydrogen bonds. During the retraction of a CFM tip from a surface, the CFM tip is away from the sample surface slightly while the spring undergoes dramatic elongation in the normal direction before rupture occurs. Single-molecule forces are distributed unevenly at the contact area. Surface energies calculated for functionalized surfaces compare well with those determined by experiments.

Journal Article↗
PubMed2004

Reduction of friction at oxide interfaces upon polymer adsorption from aqueous solutions.

Reduction of the interfacial friction for the contact of a silicon oxide surface with sodium borosilicate in aqueous solutions has been accomplished through the adsorption of poly(L-lysine)-graft-poly(ethylene glycol) on one or both surfaces. Spontaneous polymer adsorption has been achieved via the electrostatic attraction of the cationic polylysine polymer backbone and a net negative surface charge, present for a specific range of solution pH values. Interfacial friction has been measured in aqueous solution, in the absence of wear, and on a microscopic scale with atomic force microscopy. The successful investigation of the polymer-coated interfaces has been aided by the use of sodium borosilicate microspheres (5.1 microm diameter) as the contacting probe tip. Measurements of interfacial friction as a function of applied load reveal a significant reduction in friction upon the adsorption of the polymer, as well as sensitivity to the coated nature of the interface (single-sided versus two-sided) and the composition of the adsorbed polymer. These measurements demonstrate the fundamental opportunity for lubrication in aqueous environments through the selective adsorption of polymer coatings.

Journal Article↗
PubMed2004

Frictional dynamics of fluorine-terminated alkanethiol self-assembled monolayers.

The frictional dynamics of fluorine-terminated alkanethiol (S(CH2)8CF3) self-assembled monolayers (SAMs) on gold are studied using molecular dynamics simulations. The simulations treat the interactions between two SAMs on flat surfaces. The structure and frictional behavior are investigated as a function of applied pressure (200 MPa to 1 GPa) for a shear velocity of 2 m/s and compared to methyl-terminated alkanethiol SAMs. The maximum adhesive pressure between the SAMs is 220 MPa for both end groups. In agreement with experiments on the molecular scale, the shear stress and the coefficient of friction for CF3-terminated alkanethiols are larger than for CH3-terminated alkanethiols. The main source for the difference is primarily the tighter packing of the fluorinated terminal group resulting in a higher degree of order. The molecular scale coefficient of friction is correlated with the degree of order among all the systems.

Journal Article↗
PubMed2005

Odd and even model self-assembled monolayers: links between friction and structure.

The friction between an amorphous carbon tip and two n-alkane monolayers has been examined using classical molecular dynamics simulations. The two monolayers have the same packing density, but the chains comprising each monolayer differ in length by one -CH2- unit. The simulations show that the monolayers composed of C13 chains have higher friction than those composed of C14 chains when sliding in the direction of chain cant; the difference in friction becomes more pronounced as the load is increased. Examination of the contact forces between the chains and the tip, along with conformational differences between the two chain types, lends insight into the friction differences.

Journal Article↗
PubMed2006

Influence of the solvent environment on the contact mechanics of tip-sample interactions in friction force microscopy of poly(ethylene terephthalate) films.

Friction force microscopy measurements have been carried out on free-standing films of poly(ethylene terephthalate) in a variety of different media. In ethanol, the adhesion force was small, and the friction-load relationship was linear. In perfluorodecalin, nonlinearity was observed in the friction-load relationship, and the data have been found to fit the Johnson-Kendall-Roberts model of contact mechanics. The behavior in hexadecane was also characterized by a single-asperity contact model, but in this case, the data were found to fit the Derjaguin-Müller-Toporov model. It is suggested that these differences are due to the different strengths of tip-sample adhesion, which arise from the differences in the dielectric constants of the media: in ethanol, which has a high dielectric constant, the friction force varies linearly with the load, whereas in media of low dielectric constant, adhesion-limited behavior is observed.

Journal Article↗