Adhesion in soft contacts is minimum beyond a critical shear displacement

The most direct measurement of adhesion is the pull-off force, i.e. the tensile force necessary to separate two solids in contact. For a given interface, it depends on various experimental parameters, including separation speed, contact age and maximum loading force. Here, using smooth contacts between elastomer spheres and rigid plates, we show that the pull-off force also varies if the contact is sheared prior to separation. For shear displacements below a critical valueabout 10% of that necessary to yield gross sliding, the pull-off force steadily decreases as shear increases. For larger shear, the pull-off force remains constant, at a residual value 10%--15% of its initial value. Combining force measurements and in situ imaging, we show how the unloading path leading to contact separation is modified by the initial shear. In particular, we find that the residual pull-off force prevails if the contact reaches full sliding during unloading. Based on those observations, a first modeling attempt of the critical shear displacement is proposed,involving a competition between jump instability and transition to sliding. Overall, those results offer new insights into the interplay between adhesion and friction, provide new constraints on adhesion measurements and challenge existing adhesive models. They will be useful wherever soft contacts undergo both normal and shear stresses, including tire grip, soft robotics, haptics and animal locomotion

Understanding fast macroscale fracture from microcrack post mortem patterns

Dynamic crack propagation drives catastrophic solid failures. In many amorphous brittle materials, sufficiently fast crack growth involves small-scale, high-frequency microcracking damage localized near the crack tip. The ultra-fast dynamics of microcrack nucleation, growth and coalescence is inaccessible experimentally and fast crack propagation was therefore studied only as a macroscale average. Here, we overcome this limitation in polymethylmethacrylate, the archetype of brittle amorphous materials: We reconstruct the complete spatio-temporal microcracking dynamics, with micrometer / nanosecond resolution, through post mortem analysis of the fracture surfaces. We find that all individual microcracks propagate at the same low, load-independent, velocity. Collectively, the main effect of microcracks is not to slow down fracture by increasing the energy required for crack propagation, as commonly believed, but on the contrary to boost the macroscale velocity through an acceleration factor selected on geometric grounds. Our results emphasize the key role of damage-related internal variables in the selection of macroscale fracture dynamics.Comment: 9 pages, 5 figures + supporting information (15 pages

Zero field muon spin lattice relaxation rate in a Heisenberg ferromagnet at low temperature

We provide a theoretical framework to compute the zero field muon spin relaxation rate of a Heisenberg ferromagnet at low temperature. We use the linear spin wave approximation. The rate, which is a measure of the spin lattice relaxation induced by the magnetic fluctuations along the easy axis, allows one to estimate the magnon stiffness constant.Comment: REVTeX 3.0 manuscript, 5 pages, no figure. Published in Phys. Rev. B 52, 9155 (1995

Determination of the zero-field magnetic structure of the helimagnet MnSi at low temperature

Below a temperature of approximately 29 K the manganese magnetic moments of the cubic binary compound MnSi order to a long-range incommensurate helical magnetic structure. Here, we quantitatively analyze a high-statistic zero-field muon spin rotation spectrum recorded in the magnetically ordered phase of MnSi by exploiting the result of representation theory as applied to the determination of magnetic structures. Instead of a gradual rotation of the magnetic moments when moving along a axis, we find that the angle of rotation between the moments of certain subsequent planes is essentially quenched. It is the magnetization of pairs of planes which rotates when moving along a axis, thus preserving the overall helical structure.Comment: 10 pages, 4 figure

Low temperature crystal structure and local magnetometry for the geometrically frustrated pyrochlore Tb2Ti2O7

We report synchrotron radiation diffraction and muon spin rotation (muSR) measurements on the frustrated pyrochlore magnet Tb2Ti2O7. The powder diffraction study of a crushed crystal fragment does not reveal any structural change down to 4 K. The muSR measurements performed at 20 mK on a mosaic of single crystals with an external magnetic field applied along a three-fold axis are consistent with published a.c. magnetic-susceptibility measurements at 16 mK. While an inflection point could be present around an internal field intensity slightly above 0.3 T, the data barely support the presence of a magnetization plateau.Comment: To appear in the proceedings of the 13th International Conference on Muon Spin Rotation, Relaxation and Resonance, Grindelwald, Switzerland, 1-6 June 201

Understanding the μ\muSR spectra of MnSi without magnetic polarons

Transverse-field muon-spin rotation ($\mu$SR) experiments were performed on a single crystal sample of the non-centrosymmetric system MnSi. The observed angular dependence of the muon precession frequencies matches perfectly the one of the Mn-dipolar fields acting on the muons stopping at a 4a position of the crystallographic structure. The data provide a precise determination of the magnetic dipolar tensor. In addition, we have calculated the shape of the field distribution expected below the magnetic transition temperature $T_C$ at the 4a muon-site when no external magnetic field is applied. We show that this field distribution is consistent with the one reported by zero-field $\mu$SR studies. Finally, we present ab initio calculations based on the density-functional theory which confirm the position of the muon stopping site inferred from transverse-field $\mu$SR. In view of the presented evidence we conclude that the $\mu$SR response of MnSi can be perfectly and fully understood without invoking a hypothetical magnetic polaron state.Comment: 10 pages, 12 figure