It is often postulated that quasistatic cracks propagate along the direction allowing fracture for lowest load. Nevertheless, this statement debated, in particular anisotropic materials. We performed tearing experiments brittle thin sheets validate principle case of weak anisotropy. also predict existence forbidden directions and facets strongly materials, through an analogy with description equilibrium shapes crystals. However, we observe do not necessarily follow easiest but can select a...

Two mechanisms for vortex drift in an advective field are described. The velocity of tip movement is periodically modulated. It results the periodical modulations core size vortex. While changes (mechanism 1) result parallel to electric field, 2) opposite direction. Mechanism 1 dominates sparse vortices, mechanism 2 dense vortices. Arguments used predict effect suggest its generic nature.

A vertically hanging chain is released from rest and falls due to gravity on a scale pan. We discuss the various experimental theoretical aspects of this classic problem. Careful time-resolved force measurements allow us determine differences between idealized its implementation in laboratory observe that, spite upward exerted by pan chain, free end at top faster than freely falling body. Because real exhibits finite minimum radius curvature, contact bottom results tensional which pulls part...

We experimentally investigate the flow field in plane geometry around a slowly moving rigid finger dry, randomly packed granular medium. The enters medium vertically from its free surface, analogy with indentation tests on ductile materials. By developing particle imaging velocimetry technique, we identify localized finger, limited by two nearly symmetric shear bands that nucleate near fingertip and reach surface of compact. Evolution is discontinuous, exhibiting nucleation-relaxation...

Developing polymeric membranes capable of modulating the release compounds by controlling their surface properties and structure represents an important challenge for efficient drug active agent delivery systems. Herein, we combine two fibers, on one hand, core–shell and, other control wettability, to manufacture with dual delayed capacity agents. Fibers consisted a shell derivates poly(styrene-co-maleic anhydride) functionalized amino acids different hydropathic indexes loaded silver...

Controlling the fracture propagation in a film is an important element design of better packaging seals that must resist handling and yet provide tearing mechanism for easy opening. Here we show under simple initial setup conditions divergent tear can be obtained follows path logarithmic spiral over two decades length scale. We study general rules leading to "spiral growth" connect its geometry specific material properties film.

Granular ensembles subjected to confinement forces can reach metastable states that often break down via formation of shear bands for sufficiently high deviatoric stress. In this article we investigate the flow induced in a granular ensemble is perturbed by vertically moving finger quasiplanar geometry. The exhibits spiral-like and evolves discontinuously time, concert with an oscillating penetration force. We characterize nature nucleation-relaxation type process loose dense packing...

The ideal treatment for chronic wounds is based on the use of bioactive dressings capable releasing active agents. However, control rate at which these agents are released still a challenge. Bioactive polymeric fiber mats poly(styrene-co-maleic anhydride) [PSMA] functionalized with amino acids different hydropathic indices and L-glutamine, L-phenylalanine L-tyrosine levels allowed obtaining derivatives copolymers named PSMA@Gln, PSMA@Phe PSMA@Tyr, respectively, aim modulating wettability...

The factors governing bacterial adhesion to substrates with different topographies are still not fully identified. present work seeks elucidate for the first time and quantitative data roles of elasticity shape substrate topography in adhesion. With this aim, populations three species, P. aeruginosa DSM 22644, B. subtilis 10, S. aureus 20231 adhered on flat covered electrospun polycaprolactone fibers diameters ranging from 0.4 5.5 μm counted. Populations cells classified according preferred...

We study multiple tearing of a thin, elastic, brittle sheet indented with rigid cone. The n cracks initially prepared symmetrically propagate radially for n≥4. However, if n<4 the radial symmetry is broken and fractures spontaneously intertwine along logarithmic spiral paths, respecting order rotational symmetry. In limit very thin sheets, we find that fracture mechanics reduced to geometrical model correctly predicts maximum number spirals be strictly 4, together their growth rate...

We identify and study a persistent structure characteristic of the post-buckling regime thin cylindrical shell subjected to axial torsion. It consists pair developable cones ($d$ cones) joined by an S-shaped ridge, having size order radius cylinder. its formation applying concentrated load at center shell, which creates isolated $d$ cones, straight ridge that progressively tilts when torsion angle is imposed. interpret this response as equilibrium state interacting in presence in-plane shear...

The strong deformation of a thin sheet or shell is characterized by the formation sharp structures. In that forced to collapse, these features develop folds make process more efficient. On other hand, in crumpling membrane, same tend stiffen structure. We experimentally investigated mechanism relevant collapse shells and early stages crumpling, namely mutual annihilation vertex singularities. show two vertices displacement-controlled experiment accompanied decrease forces at equilibrium....

A laser speckle correlation method is developed to measure slow macroscopic flow fields in a randomly packed granular medium. The useful for planar geometry configurations like the one considered here, namely sample that subjected an indentation type test. main feature revealed by this system, formation of shear bands well-defined shapes, proper Coulomb-like material. We characterize these structures showing they concentrate vorticity and deformation. In addition, provides high-precision...

Isometric deformations in thin elastic films easily form ridges to connect large flat regions or facets. Depending on the forces applied boundary conditions imposed, these can be isometric, with no stretching "stretching ridges" when bending and are required relax energy. Here we study a simple configuration observe transition between an isometric ridge well-known observed crumpled films, obtain parameters that determine type. Specifically, show transversal size of acts as cutoff length...

We investigate the normal modes of a developable cone singularity as observed in circular sheet supported by rigid frame and pushed at its center. When center is addition submitted to sinusoidal forcing, two types bending modes, named here rolling tilt are parametrically excited. The mode an angular oscillation concave sector structure. If amplitude vibration high enough, increases dramatically giving rise both continuous rotation material displacement sheet, similar that produced moving...

Elimination of spiral waves prevents transition to chaotic state in excitable media different physicochemical nature. In cardiac muscle, prevent death, the are usually removed together with all propagating by a strong electric shock: 5 kV, 20 A ("defibrillation"). We have found an approach extinguish without destroying normally waves. Chemical excitation spatial open reactor Belousov−Zhabotinsky reaction controlled light were used as experimental model. three scenarios, depending on rate...

Abstract Experiments reveal that structural transitions in thin sheets are mediated by the passage of transient and stable mobile localized elastic excitations. These “crumples” or “d-cones” nucleate, propagate, interact, annihilate, escape. Much dynamics occurs on millisecond time scales. Nucleation sites correspond to regions where generators ideal unstretched surface converge. Additional intermediate states illustrate two forms quasistatic inter-crumple interaction through ridges valleys....

Developing polymeric membranes capable of modulating the release compounds by controlling their surface properties and structure represents an important challenge for efficient drug active agent delivery systems. Herein, we combine two fibers, on one hand, core-shell structure, other, control wettability, to manufacture with dual delayed capacity agents. Fibers consisted a shell derivates poly(styrene-co-maleic anhydride) functionalized amino acids different hydropathic indexes loaded silver...

Cutting a brittle thin sheet with blunt object leaves an oscillating crack that seemingly violates the principle of local symmetry for fracture. We experimentally find at critical value well chosen control parameter straight propagation is unstable and leads to oscillatory pattern whose amplitude wavelength grow by increasing parameter. propose simple model unifies this instability related problem, namely perforated sheet, where through similar bifurcation series radial cracks spontaneously...

Indentation is a standard, widely used technique in mechanical assays and theoretical analysis. It unveils the fundamental modes of deformation predicts response material under more complex loads. Here we present an experimental setup for testing thin-film materials by studying lateral indentation narrow opening cut into film, triggering cascade buckling events. The force F dominated bending stretching effects small displacements slowly varies with indenter displacement ∼ d2/5, to finally...

An advective field induces drift of a vortex in excitable media. The component the velocity C( perpendicular ) to is known change its sign with chirality vortex. In an experiment vortices electric chemical medium, we have found unexpectedly that changes also independently changing composition medium. We did not succeed explain this phenomenon by using existing mathematical models described calls for more realistic models.(c) 1999 American Institute Physics.

Magnetotactic bacteria (MTB) are endowed with an exquisite orientation mechanism allowing them to swim along the geomagnetic field lines [1-3]. This consists of a chain bio-synthesized magnetic nano-crystals [1,4,5]. Although physics behind minimum size this biological compass is well understood [6], it yet unclear what sets its maximum [6-8]. Here, by using macroscopic experiment inspired in these microorganisms, we show that larger moments will drive collective behavior MTB into phase...