Rubber friction: role of the flash temperature

When a rubber block is sliding on a hard rough substrate, the substrate asperities will exert time-dependent deformations of the rubber surface resulting in viscoelastic energy dissipation in the rubber, which gives a contribution to the sliding friction. Most surfaces of solids have roughness on many different length scales, and when calculating the friction force it is necessary to include the viscoelastic deformations on all length scales. The energy dissipation will result in local heating of the rubber. Since the viscoelastic properties of rubber-like materials are extremely strongly temperature dependent, it is necessary to include the local temperature increase in the analysis. At very low sliding velocity the temperature increase is negligible because of heat diffusion, but already for velocities of order 0.01 m/s the local heating may be very important. Here I study the influence of the local heating on the rubber friction, and I show that in a typical case the temperature increase results in a decrease in rubber friction with increasing sliding velocity for v > 0.01 m/s. This may result in stick-slip instabilities, and is of crucial importance in many practical applications, e.g., for the tire-road friction, and in particular for ABS-breaking systems.Comment: 22 pages, 27 figure

Optimal speed of response of a roller follower cam driven system

This dissertation analyzes the problem of determination of the maximal speed of response of a roller follower plate cam driven system. The solution to the problem of the lift of a roller from lower to upper dwelling conditions under the dual constraints of (1) a maximal allowed stress, and (2) no undercutting of the cam, was obtained using the analogy of the Bang Bang Problem ; this problem has a very simple analytical solution, but its implications can be very usefully applied to the problem of cam lift. After applying this analogy in the present study, a technique of nonlinear programming was used to deal with the process of minimizing the angle of rotation of the cam under the constraint of a maximum allowable stress. Using the stress constraint at its limiting value (i.e., at the border of the domain) an initial value problem of a second order non-linear differential equation is defined, which can be solved numerically. The study of the singular solution of this problem and the study of this ODE in the phase plane provide important information about the existence of restrictions in the phase plane as well as its singular points. The maximum stress constraint is found to provide a maximum rate of deceleration for a low angular velocity problem. Also for low angular velocity it was necessary to add an additional constraint of a maximum acceleration rate, because the dual constraint did not bound the acceleration. In the region of high angular velocity it was found that the maximal stress constraint provides a maximum acceleration rate but a maximum deceleration rate is obtained using a low stress. The geometrical constraint of no undercutting determines the area of the phase plane for a feasible solution

On the nature of surface roughness with application to contact mechanics, sealing, rubber friction and adhesion

Surface roughness has a huge impact on many important phenomena. The most important property of rough surfaces is the surface roughness power spectrum C(q). We present surface roughness power spectra of many surfaces of practical importance, obtained from the surface height profile measured using optical methods and the Atomic Force Microscope. We show how the power spectrum determines the contact area between two solids. We also present applications to sealing, rubber friction and adhesion for rough surfaces, where the power spectrum enters as an important input.Comment: Topical review; 82 pages, 61 figures; Format: Latex (iopart). Some figures are in Postscript Level

The bashful and the boastful : prestigious leaders and social change in Mesolithic Societies

The creation and maintenance of influential leaders and authorities is one of the key themes of archaeological and historical enquiry. However the social dynamics of authorities and leaders in the Mesolithic remains a largely unexplored area of study. The role and influence of authorities can be remarkably different in different situations yet they exist in all societies and in almost all social contexts from playgrounds to parliaments. Here we explore the literature on the dynamics of authority creation, maintenance and contestation in egalitarian societies, and discuss the implications for our interpretation and understanding of the formation of authorities and leaders and changing social relationships within the Mesolithic

Kimmeridgian-Tithonian sea-level fluctuations in the Uljanovsk-Saratov Basin (Russian Platform)

Abstract The Uljanovsk-Saratov Basin, located in the southeast of the Russian Platform, presents an intriguing record of the Kimmeridgian-Tithonian sea-level fluctuations. In the Late Jurassic, this basin was a trough within the Interior Russian Sea. The data available from both outcrops and boreholes have permitted outlining a number of lithostratigraphic units and regional hiatuses in the northeastern segment of the Uljanovsk-Saratov Basin, thus permitting a precise reconstruction of transgressions/regressions and deepenings/shallowings. In total, three transgressive-regressive cycles and two deepening pulses have been established. These regionally documented changes were both related in part to global eustatic changes, and they also corresponded in part to the regional sea-level changes in some basins of Western Europe and Northern Africa, but not to those of the Arabian Platform. Differences observed between the global and regional curves as well as rapid Tithonian sea-level oscillations are explained by the influences of tectonic activity. It is hypothesized that the regional Tithonian oxygen depletion might have been a consequence from the rapid flooding of a densely vegetated land

A membrane-inserted structural model of the yeast mitofusin Fzo1

Mitofusins are large transmembrane GTPases of the dynamin-related protein family, and are required for the tethering and fusion of mitochondrial outer membranes. Their full-length structures remain unknown, which is a limiting factor in the study of outer membrane fusion. We investigated the structure and dynamics of the yeast mitofusin Fzo1 through a hybrid computational and experimental approach, combining molecular modelling and all-atom molecular dynamics simulations in a lipid bilayer with site-directed mutagenesis and in vivo functional assays. The predicted architecture of Fzo1 improves upon the current domain annotation, with a precise description of the helical spans linked by flexible hinges, which are likely of functional significance. In vivo site-directed mutagenesis validates salient aspects of this model, notably, the long-distance contacts and residues participating in hinges. GDP is predicted to interact with Fzo1 through the G1 and G4 motifs of the GTPase domain. The model reveals structural determinants critical for protein function, including regions that may be involved in GTPase domain-dependent rearrangements

Inequality in the treatment of disc herniation. Socio-demographic characteristics of patients undergoing first-time, single-level, simple lumbar discectomy

BACKGROUND: In disc herniation, nonsurgical treatments are recommended prior to elective discectomy but are often associated with consultation fees, whereas the discectomy itself may be without user payment. This may affect individual preferences in the choice of treatment. This retrospective case-control study examined the association between socioeconomic characteristics and the event of undergoing a first-time, single-level, simple lumbar discectomy. METHODS: The consecutively formed study population comprised patients undergoing elective lumbar discectomy at a Danish public hospital between 2010 and 2013. A national authority identified three gender- and age-matched controls per case for comparison. Measures investigated in this study were marital status, ethnicity, socioeconomic classification, educational level, the extent of sick leave 52 weeks prior to surgery, personal income, and equivalized disposable household income. All measures were provided by national registries. The associations were examined using uni- and multivariate logistic regression analysis. RESULTS: In a study population of 888 operated patients (age (SD) 46 (14); ODI (SD) 47 (18); leg pain intensity (VAS) (iqr) 74 (33), EQ-5D (iqr) 0.26 (0.62)) compared to 2664 controls, the probability of undergoing lumbar discectomy was significantly associated with lower vs. higher educational levels ((OR 1.98-2.53), and with periods of sick leave exceeding two weeks within one year prior to surgery (OR 9.47 (95% CI 7.68-11.68)). In the multivariate analysis, the event of undergoing discectomy was insignificantly associated with any other socioeconomic characteristics, whereas the personal income was of significant importance in the univariate analysis. CONCLUSION: The event of undergoing free-of-fee elective first-time, single-level, simple lumbar discectomy is more common among individuals with low educational levels and unstable labor market attachment when examined in a case-control study. Being a multifactorial challenge, this calls upon the active engagement of several policy sectors

Effect of matrix-modulating enzymes on the cellular uptake of magnetic nanoparticles and on magnetic hyperthermia treatment of pancreatic cancer models in vivo

Magnetic hyperthermia can cause localized thermal eradication of several solid cancers. However, a localized and homogenous deposition of high concentrations of magnetic nanomaterials into the tumor stroma and tumor cells is mostly required. Poorly responsive cancers such as the pancreatic adenocarcinomas are hallmarked by a rigid stroma and poor perfusion to therapeutics and nanomaterials. Hence, approaches that enhance the infiltration of magnetic nanofluids into the tumor stroma convey potentials to improve thermal tumor therapy. We studied the influence of the matrix-modulating enzymes hyaluronidase and collagenase on the uptake of magnetic nanoparticles by pancreatic cancer cells and 3D spheroids thereof, and the overall impact on magnetic heating and cell death. Furthermore, we validated the effect of hyaluronidase on magnetic hyperthermia treatment of heterotopic pancreatic cancer models in mice. Treatment of cultured cells with the enzymes caused higher uptake of magnetic nanoparticles (MNP) as compared to nontreated cells. For example, hyaluronidase caused a 28% increase in iron deposits per cell. Consequently, the thermal doses (cumulative equivalent minutes at 43 ◦C, CEM43) increased by 15–23% as compared to heat dose achieved for cells treated with magnetic hyperthermia without using enzymes. Likewise, heatinduced cell death increased. In in vivo studies, hyaluronidase-enhanced infiltration and distribution of the nanoparticles in the tumors resulted in moderate heating levels (CEM43 of 128 min as compared to 479 min) and a slower, but persistent decrease in tumor volumes over time after treatment, as compared to comparable treatment without hyaluronidase. The results indicate that hyaluronidase, in particular, improves the infiltration of magnetic nanoparticles into pancreatic cancer models, impacts their thermal treatment and cell depletion, and hence, will contribute immensely in the fight against pancreatic and many other adenocarcinomas