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PubMed1991

[The effect of physiological tooth mobility on the friction between the bracket and the arch].

This study compared frictional forces which occurred in vivo and in vitro. A testing device was used which allowed reproduction of friction measurements carried out on upper central incisors of volunteers under laboratory conditions with the bracket fixed immovable. By this means changes of friction due to physiological tooth mobility and occlusal load of the bonded teeth could be investigated. While the friction measured in vitro with immovable brackets and in vivo without occlusal load did not differ significantly, additional tooth movement by occlusal load resulted in significant reduction of friction magnitude. It should be kept in mind that the mobility of those teeth investigated was absolutely normal, while it is usually increased during orthodontic treatment. Due to this effect and influences resulting from chewing various kinds of food, it can be estimated that the frictional forces occurring with orthodontic treatment are even smaller in comparison to in vitro experiments with immovable brackets.

Biophysical Phenomena↗
PubMed1990

Effects of surface roughness on the coefficients of friction in model orthodontic systems.

Orthodontists, like others (Engel, P.A. (1976) Impact Wear of Materials. Elsevier Scientific, New York.), often equate the smoothness of surfaces with the absence of friction. To investigate whether the surface roughness of opposing materials influence the coefficients of friction and ultimately the movement of teeth, arch wires were slid between contact flats to simulate orthodontic arch wire-bracket appliances. From laser specular reflectance measurements, the RMS surface roughness of these arch wires varied from 0.04 microns for stainless steel to 0.23 microns for nickel titanium. Using the same technique, the roughnesses of the contact flats varied from 0.03 microns for the 1 micron lapped stainless steel, to 0.26 microns for the as-received alumina. After each of the arch wire-contact flat couples was placed in a friction tester, fifteen normal forces were systemically applied at 34 degrees C. From plots of the static and kinetic frictional forces vs the normal forces, dry coefficients of friction was obtained that were greater than those reported in the dental literature. The all-stainless steel couples had lower kinetic coefficients (0.120-0.148) than the stainless steel-polycrystalline alumina couple (0.187). When pressed against the various flats, the beta-titanium arch wire (RMS = 0.14 microns) had the highest coefficients of friction (0.445-0.658), although the nickel titanium arch wire was the roughest (RMS = 0.23 microns). Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) verified that mass transfer of the beta-titanium arch wire occurred by adhesion onto the stainless steel flats or by abrasion from the sharply faceted polycrystalline alumina flats.

Aluminum Oxide↗
PubMed1989

Frictional forces between bracket and arch wire.

Guiding a tooth along an arch wire results in a counteracting frictional force. Clinically, a mesiodistally applied force must exceed the frictional force to produce a tooth movement. A friction-testing assembly simulating three-dimensional tooth rotations was constructed to study factors affecting friction magnitude. Five wire alloys (standard stainless steel, Hi-T stainless steel, Elgiloy blue, nitinol, and TMA) in five wire sizes (0.016, 0.016 x 0.022, 0.017 x 0.025, 0.018, and 0.018 x 0.025 inch) were examined with respect to three bracket widths (2.2, 3.3, and 4.2 mm) at four levels of retarding force (0, 1, 2, and 3 N). The following factors affected friction in decreasing order: retarding force (biologic resistance), surface roughness of wire, wire size (vertical dimension), bracket width, and elastic properties of wire. The study recommends the application of 0.016 x 0.022 inch stainless steel wire combined with a medium (3.3 mm) or wide (4.2 mm) bracket for an arch-guided mechanism with an 0.018 inch slot. The effective force of this arrangement has to increase twofold to overcome the friction. For TMA wire, however, the effective force must increase sixfold, resulting in a hazardous overload of the anchorage units.

Alloys↗
PubMed1990

Evaluation of friction between ceramic brackets and orthodontic wires of four alloys.

The purpose of this investigation was to determine the frictional resistance offered by ceramic brackets used in combination with wires of different alloys and sizes during in vitro translatory displacement of brackets. Findings with ceramic brackets were also compared with outcomes of treatment with stainless steel brackets. Stainless steel, cobalt-chromium, beta-titanium, and nickel-titanium wires of different cross-sectional sizes were tested in medium-twin monocrystalline ceramic brackets with both 0.018-inch and 0.022-inch slot sizes. The wires were ligated into the brackets with elastomeric modules. Brackets were moved along the wire by means of an Instron universal testing machine, and frictional force was measured by a compression cell and recorded graphically on an xy recorder. Wire friction in the ceramic brackets increased as wire size increased, and rectangular wires produced greater friction than round wires. Beta-titanium and nickel-titanium wires were associated with higher frictional forces than stainless steel or cobalt-chromium wires. These findings follow the same general trends as those found with stainless steel brackets; however, wires in ceramic brackets generated significantly stronger frictional force than did wires in stainless steel brackets.

Analysis of Variance↗
PubMed2006

Nonlinear friction characteristics between silica surfaces in high pH solution.

Molecular-scale characteristics of friction forces between silica particles and silica wafers in aqueous solutions of the normal (pH 5.6) and high pH (pH 10.6) are investigated, using the lateral force measuring procedure of the atomic force microscope (AFM). Various significant differences of friction characteristics between solutions of normal and high pH's are found. In the case of solutions of normal pH, the friction force increases linearly with increasing loading force, as the Amonton's law for solid bodies indicates. However, in the case of high pH solutions, the increasing rate with the loading force is considerably reduced in the low loading region, but the value increases abruptly above a critical loading force to overcome the magnitude of friction force of normal pH above the region of very high loading. It is very interesting to know that this nonlinear force curve at high pH is independent of the atomic-scale roughness of surfaces, although the magnitude of friction is greatly influenced by the roughness in the case of normal pH. The reason why the friction at high pH is independent of the surface roughness is postulated to be due to the hairy-like layer formed on the silica surface. The existence of hairy-like layers at high pH is proven directly by the dynamic method of normal force measurements with AFM and the thickness is estimated to be at least ca. 1.3 nm.

Journal Article↗
PubMed2003

Adhesion and friction of PDMS networks: molecular weight effects.

The objective of this work is to find relations between adherence and friction behaviors of elastomer networks. The chosen approach is based on the parallel study of the initial molecular weight (i.e., the degree of cross-linking) dependence of both adherence and friction. The polymers used are cross-linked polydimethylsiloxane (PDMS) and the substrate is a smooth glass plate. The experimental procedure uses both friction (pin on disk tribometer) and adhesion (tack test) measurements, associated with surface analysis and mechanical and rheological characterizations. Tack results show that high molecular weight PDMS exhibits the greater adherence energy. This can be explained by the role of both chain length and free and pendant chains: more numerous and longer free chains favor the substrate wetting (at a molecular scale) and increases the energy dissipation during separation (extraction and reptation mechanisms). However, friction results indicate a higher friction resistance for low molecular weight PDMS. This result could be quite surprising. An explanation based on interfacial sliding properties of free and pendant chains can be proposed. Elsewhere, for the lower molecular weight polymer, elastic contact present during friction is able to act as a forced wetting, constraining the network and consequently leading to a greater energy dissipation.

Journal Article↗
PubMed1997

Increased friction of animal joints by experimental degeneration and recovery by addition of hyaluronic acid.

OBJECTIVE: To investigate the lubricating ability in osteoarthritic (OA) stifle joints. DESIGN: An experimental study in rabbit stifles in vitro. BACKGROUND: The lubricating ability in OA joints, and the effects of the application of hyaluronic acid (HA), a drug for OA, to the osteoarthritic joints have not yet been reported. METHODS: The friction in OA stifles induced by two different methods, the intra-articular injection of papain, and transection of the anterior cruciate ligament (ACL) was measured using a robotic arm under force control. RESULTS: The frictional coefficient in the papain-injected model, that caused slight degeneration in the articular cartilage, was 1.3 to 1.8 times as large as that in the contralateral uninjected stifles. The elevated values decreased significantly by addition of HA to the articular surface (P < 0.05). In another group (n = 4), the frictional coefficient in the stifle at 9 months after ACL transection that caused severe degeneration in the articular cartilage was 1.6 to 7.8 times as large as that in the contralateral unoperated stifle. The addition of HA decreased the elevated values in three of the four ACL-transected joints. CONCLUSION: Friction was higher in the OA joints than in the healthy joints. The addition of HA was effective in the early stage of OA represented by papain injected case, while its effect seems not to be evident for advanced OA produced ACL transection which accompanies macroscopic shape changes. RELEVANCE: The present findings are relevant to the current research investigating the case of human osteoarthritis. We measured the frictional coefficients of experimentally degenerated rabbit stifles. The frictional coefficients can be used as an index of joint degeneration. We also assessed the recovery of the lubricating ability by addition of hyaluronic acid, a drug used for human osteoarthritis.

Journal Article↗
PubMed1999

Indirect costs of back pain in the Netherlands: a comparison of the human capital method with the friction cost method.

In this study we estimated the indirect costs of back pain in 1991 in The Netherlands on the basis of two approaches: the traditionally used human capital method and the more recently developed friction cost method. The indirect costs of illness were defined as the value of production losses of paid labour and related costs to society due to back pain. The results of this study in 1991 in The Netherlands show that the short-term indirect costs estimated by the human capital method were more than three times as high as the indirect costs estimated by the friction cost method (US$ 4.6 billion vs. USS 1.5 billion, respectively). The lower estimate of indirect costs when using the friction cost method is mainly due to the fact that in this method actual production losses are estimated during a relatively short friction period, which is defined as the period needed to restore the initial production level. In contrast with the human capital method, long-term absenteeism and disability do not induce additional costs when applying the friction cost method. Since the friction cost method takes into account that employees can be replaced, we believe that this method produces a more accurate estimate of indirect costs than the human capital method. Notwithstanding the resulting decrease in indirect costs of back pain, these costs are still impressive, representing 0.28% of the GNP in The Netherlands in 1991. As a consequence, but particularly stimulated by structural changes in the Dutch social security system, policies aimed at reducing indirect costs of back pain, increasingly concentrate on the development and evaluation of interventions early after the onset of disease. This is complemented, on the one hand, by the development of clinical guidelines for the management of back pain in primary care and, on the other hand, by governmental policies aimed at reintegration of chronically ill in the labour force.

Adult↗
PubMed2002

Influence of teardrop studs on rotating frictional base plate on spheroid quality in rotary spheronization.

The effects of teardrop-shaped studs on the quality of rotary processed spheroids were investigated. The spheroids were produced under similar conditions using three rotating frictional base plates with teardrop studs of different height, volume, cross-sectional area and surface area. Spheroid properties were rated by size, size distribution, shape, friability and density. The amounts of lumps and fines produced, and the adhesion of material on the rotating frictional base plates was also looked into. The dimension of the teardrop studs on the rotating frictional base plate affected spheroid quality. The resultant shear forces and energy input during rotary spheronization differed depending on the different height, volume, cross-sectional area and surface area of studs. With the increase in height, volume, cross-sectional area and surface area of studs on the frictional base plate, the mass median diameter, e(R) and circularity of spheroids increased with corresponding decrease in span, lumps and fines. Although the frictional base plate with shortest studs had little adhesion, it may not supply enough shear force and energy input for the spheronization process, resulting in a less stable process. A balance between energy input and adhesion on the rotating frictional base plate was needed in order to optimize the production of spheroids by rotary processing.

Adhesiveness↗
PubMed1990

Frictional resistance of ceramic and stainless steel orthodontic brackets.

Frictional resistance of orthodontic appliances is recognized by most clinicians to be detrimental to tooth movement. The purpose of this study was to compare planar, static frictional forces among stainless steel and ceramic brackets. Both nitinol and stainless steel rectangular arch wires were passed freely through the slots of a pair of brackets from each type. Tests were carried out in air and in artificial saliva. A 300-gm load was suspended from the arch wire to simulate the normal force, and an incremental horizontal force was applied until movement of the arch wire was initiated. Under all conditions, the stainless steel brackets had lower coefficients of friction than the ceramic brackets. The stainless steel wire generated less friction than nitinol, and friction increased in the presence of artificial saliva in comparison with air alone. These results show that, under experimental conditions, ceramic brackets, nitinol arch wires, and saliva all increase static frictional resistance.

Air↗
PubMed2004

Modeling of dry sliding friction dynamics: from heuristic models to physically motivated models and back.

After giving an overview of the different approaches found in the literature to model dry friction force dynamics, this paper presents a generic friction model based on physical mechanisms involved in the interaction of a large population of surface asperities and discusses the resulting macroscopic friction behavior. The latter includes the hysteretic characteristic of friction in the presliding regime, the velocity weakening and strengthening in gross-sliding regime, the frictional lag and the stick-slip behavior. Out of the generic model, which is shown to be a good, but rather computationally intensive, simulation tool, a simpler heuristic model, which we call the generalized Maxwell-slip friction, is deduced. This model is appropriate for quick simulation and control purposes being easy to implement and to identify. Both of the generic and heuristic model structures are compared, through simulations, with each other and with experimental data.

Journal Article↗
PubMed2005

Correlations between adhesion hysteresis and friction at molecular scales.

Correlations between adhesion hysteresis and local friction are theoretically and experimentally investigated. The model is based on the classical theory of adhesional friction, contact mechanics, capillary hysteresis, and nanoscale roughness. Adhesion hysteresis was found to scale with friction through the scaling factor containing a varying ratio of adhesion energy over the reduced Young's modulus. Capillary forces can offset the relationship between adhesion hysteresis and friction. Measurements on a wide range of engineering samples with varying adhesive and elastic properties confirm the model. Adhesion hysteresis is investigated under controlled, low humidity atmosphere via ultrasonic force microscopy. Friction is measured by the friction force microscopy.

Journal Article↗
PubMed2005

Local friction in polyolefin blends.

Processes on different length scales affect the dynamics of chain molecules. The friction experienced by a short chain segment depends on both small-scale chain properties and on the local environment of the segment. As a consequence, the (monomeric) friction coefficients of the two components of a binary polymer blend will, in general, differ from each other and from the friction coefficients of the corresponding melts. In this work, we investigate local friction in polyolefin blends with the aid of a small-scale simulation approach. The polymer chains, in united atom representation, are assumed to occupy the sites of a partially filled simple cubic lattice. The simulation focuses on short chain sections with straight backbones and enumerates all possible binary contacts and relative movements of such sections. By evaluating the exact enumeration results in conjunction with equations of state for the blends, we are able to make predictions about the variation of the friction coefficients with local chain architecture and thermodynamic state (temperature, pressure, and composition). We calculate relative values of friction coefficients at temperatures well above the glass transition for blends of PEP, an alternating copolymer of polyethylene and polypropylene, with polyethylene and polyisobutylene and for blends of polyethylene and atactic polypropylene. We also investigate a blend of PEP with head-to-head polypropylene and compare our results with experimental data.

Journal Article↗
PubMed2006

Friction laws for lubricated nanocontacts.

We have used friction force microscopy to probe friction laws for nanoasperities sliding on atomically flat substrates under controlled atmosphere and liquid environment, respectively. A power law relates friction force and normal load in dry air, whereas a linear relationship, i.e., Amontons' law, is observed for junctions fully immersed in model lubricants, namely, octamethylciclotetrasiloxane and squalane. Lubricated contacts display a remarkable friction reduction, with liquid and substrate specific friction coefficients. Comparison with molecular dynamics simulations suggests that load-bearing boundary layers at junction entrance cause the appearance of Amontons' law and impart atomic-scale character to the sliding process; continuum friction models are on the contrary of limited predictive power when applied to lubrication effects. An attempt is done to define general working conditions leading to the manifestation of nanoscale lubricity due to adsorbed boundary layers.

Journal Article↗
PubMed2002

Functional handgrip test to determine the coefficient of static friction at the hand/handle interface.

The aim was to devise a method of measuring friction at the hand/handle interface during a functional handgrip task. No descriptions of methods of this kind was found in the literature. An indirect technique of measuring normal grip force was employed to determine friction at the hand/handle interface while performing a functional handgrip action with a grabrail. The coefficient of static friction was calculated between palmar skin (dry, wet, and soapy hands) and five grabrail materials (stainless steel, powder-coated steel, chrome, textured aluminium and knurled steel). Thirty subjects participated (15 female, 15 male), who were aged from 17 to 45 years with a mean age of 30 years. Knurled steel produced a significantly larger mean coefficient of static friction than chrome, powder-coated steel and stainless steel, and textured aluminium had a significantly larger coefficient of static friction than stainless steel. Soapy hands produced the lowest mean coefficients (0.46+/-0.04), significantly less then dry (1.72+/-0.16, p <0.001) and wet hands (1.42+/-0.16, p <0.001). This study has demonstrated the influence of grabrail material and palmar skin treatments on static friction at the hand/handle interface. The use of a functional test that incorporates an indirect determination of normal handgrip force has provided a quantitative method of observing stability at the hand/handle interface.

Adolescent↗
PubMed1991

Effect of bracket and wire composition on frictional forces.

Previous work on friction has considered movement of single teeth along an archwire. The aim of this investigation was to consider friction in buccal segment attachments during overjet reduction involving sliding mechanics. A buccal segment model was constructed to compare friction in steel and ceramic brackets, using steel and nickel titanium wires of two sizes along with a new experimental polymeric wire. The results indicate that friction during overjet reduction is minimized by using larger dimension rectangular wires and by using steel rather than nickel titanium. Comparing steel with ceramic brackets in series, the latter show greater frictional resistance, but only when used with the smaller rectangular wires. The combined effect of environment, ligation, bracket, and archwire significantly reduced the difference. Clinically, there may, therefore, be little to choose between steel and ceramic brackets in the buccal segments, with wire choice as determined by tooth displacement being more important. Comparison of the results with those obtained using single brackets illustrates the problems of interpreting results from friction experiments. The polymeric archwire in its present form was found to be unsuitable for use in orthodontics.

Acetals↗
PubMed2005

Adsorbate-induced enhancement of electrostatic noncontact friction.

We study the noncontact friction between an atomic force microscope tip and a metal substrate in the presence of bias voltage. The friction is due to energy losses in the sample created by the electromagnetic field from the oscillating charges induced on the tip surface by the bias voltage. We show that the friction can be enhanced by many orders of magnitude if the adsorbate layer can support acoustic vibrations. The theory predicts the magnitude and the distance dependence of friction in good agreement with recent puzzling noncontact friction experiment [B. C. Stipe, H. J. Mamin, T. D. Stowe, T. W. Kenny, and D. Rugar, Phys. Rev. Lett. 87, 096801 (2001).]. We demonstrate that even an isolated adsorbate can produce high enough friction to be measured experimentally.

Journal Article↗
PubMed1983

Work of friction and net work during compaction.

Series of tablets were compressed in a reciprocating tablet machine with a gradually increasing die wall friction. The force needed on the upper punch to maintain the tablet dimensions constant increased with the die wall friction while the lower punch force decreased. The change in punch forces due to differences in die wall friction had no effect on the tablet strength. The net work of compaction should be constant under these circumstances. The net work calculated by subtracting the work of friction and the expansion work from the gross work input was constant when the frictional work was calculated according to one of the two equations proposed in the literature (Järvinen & Juslin 1974) while the other appears to give an overestimation of the work of friction.

Tablets↗