Imaging the stick-slip peeling of an adhesive tape under a constant load

Using a high speed camera, we study the peeling dynamics of an adhesive tape under a constant load with a special focus on the so-called stick-slip regime of the peeling. It is the first time that the very fast motion of the peeling point is imaged. The speed of the camera, up to 16000 fps, allows us to observe and quantify the details of the peeling point motion during the stick and slip phases: stick and slip velocities, durations and amplitudes. First, in contrast with previous observations, the stick-slip regime appears to be only transient in the force controlled peeling. Additionally, we discover that the stick and slip phases have similar durations and that at high mean peeling velocity, the slip phase actually lasts longer than the stick phase. Depending on the mean peeling velocity, we also observe that the velocity change between stick and slip phase ranges from a rather sudden to a smooth transition. These new observations can help to discriminate between the various assumptions used in theoretical models for describing the complex peeling of an adhesive tape. The present imaging technique opens the door for an extensive study of the velocity controlled stick-slip peeling of an adhesive tape that will allow to understand the statistical complexity of the stick-slip in a stationary case

Microstructural Characterization of Graphite Spheroids in Ductile Iron

The present work brings new insights by transmission electron microscopy allowing disregarding or supporting some of the models proposed for spheroidal growth of graphite in cast irons. Nodules consist of sectors made of graphite plates elongated along a hai direction and stack on each other with their c axis aligned with the radial direction. These plates are the elementary units for spheroidal growth and a calculation supports the idea that new units continuously nucleate at the ledge between sectors

On the growth and form of spherulites

Many structural materials (metal alloys, polymers, minerals, etc.) are formed by quenching liquids into crystalline solids. This highly non-equilibrium process often leads to polycrystalline growth patterns that are broadly termed "spherulites" because of their large-scale average spherical shape. Despite the prevalence and practical importance of spherulite formation, only rather qualitative concepts of this phenomenon exist. The present work explains the growth and form of these fundamental condensed matter structures on the basis of a unified field theoretic approach. Our phase field model is the first to incorporate the essential ingredients for this type crystal growth: anisotropies in both the surface energy and interface mobilities that are responsible for needle-like growth, trapping of local orientational order due to either static heterogeneities (impurities) or dynamic heterogeneities in highly supercooled liquids, and a preferred relative grain orientation induced by a misorientation-dependent grain boundary energy. Our calculations indicate that the diversity of spherulite growth forms arises from a competition between the ordering effect of discrete local crystallographic symmetries and the randomization of the local crystallographic orientation that accompanies crystal grain nucleation at the growth front (growth front nucleation or GFN). The large-scale isotropy of spherulitic growth arises from the predominance of GFN.Comment: 14 pages, 11 figure

Note sur une méthode d'étude des défauts de surface des lames nématiques orientées par un support directionnel

The use of birefringence colors of thin crystal plates in conjunction with microdensitometric studies of the transmitted intensities allows visualization, with a high degree of sensitivity, of the superficial molecular orientations of a thin layer of nematic liquid crystal contained between two directional supports, when the director remains in a planar orientation. In particular, this method enables us to measure the angular variation of the director at the level of superficial twisted loops more accurately than by simple observation between polariser and analyser. We are able to distinguish a variation in orientation of a few degrees per micron.L'utilisation des couleurs de biréfringence des lames cristallines minces permet de mettre en évidence optiquement, avec une bonne sensibilité, les orientations moléculaires superficielles des cristaux liquides nématiques placés en couche mince entre deux supports directionnels. Cette méthode permet, en particulier, de connaître la variation du directeur au niveau des boucles superficielles de torsion de façon plus précise que la simple observation entre polariseur et analyseur

Spherulitic branching in the crystallization of liquid selenium

Liquid selenium is a spherulite-forming liquid. In a previous study G. Ryschenkow and G. Faivre (1988), several spherulitic modes of crystallization have been observed to coexist, at a given undercooling of the liquid, with the growth of single crystals. The spherulitic modes were thought to be basically due to a mechanism inducing a regular polygonization of crystal during growth (small-angle branching). A morphological investigation is presented of the spherulites of selenium, by optical and electron microscopy, which substantiate this conjecture. At medium undercooling of the liquid, the small-angle branching periodically triggers a homoepitaxial large-angle branching. This gives rise to nonringed spherulites. At higher undercooling the spherulites are ringed, which is attributed to the ineffectiveness of the large-angle branching. The existence of several spherulitic modes signifies that several stable regimes of the small-angle branching exis

Non-linear elastic behavior of light fibrous materials

PACS. 62.20.-x Mechanical properties of solids[:AND:]62.20.Dc Elasticity, elastic constants,

On new type of electrohydrodynamics instability in tilted nematic layers

We have predicted and observed in a nematic phase of a liquid crystal (MBBA) a new kind of instability formed of rolls perpendicular to the Williams domains. In contrast to the latter, the spatial period of this new instability is a function of the voltage. This instability is due to the fact that the director points out of the substrate plane.On prévoit et on observe une nouvelle instabilité dans les nématiques : les striations sont orientées perpendiculairement à celles que l'on observe dans le cas des domaines de Williams, et leur période spatiale dépend du voltage appliqué. Ce type d'instabilité est essentiellement dû à l'ancrage oblique imposé par les surfaces limitant l'échantillon