Equilibrium and out-of-equilibrium dynamics in a molecular layer of azopolymer floating on water studied by Interfacial Shear Rheology

We report the details of the construction and calibration of an ultra sensitive surface rheometer, inspired by the setup described in [C.F. Brooks et al Langmuir 15, 2450 (1999)], which makes use of high resolution video tracking of the motion of a floating magnetized needle and is capable of measuring the viscoelastic response of a Langmuir monolayer with an accuracy of 10^-5 N/m. This instrument is then employed for the rheological characterization of a Langmuir monolayer of a photosensitive azobenzene polymer, which can be brought out of equilibrium by a suitable photoperturbation. The complex dynamic shear modulus G= G' + i G" is measured as a function of temperature and illumination power and wavelength. The reversible rheological ch anges induced in the film by photo-perturbation are monitored during time, observing a transition from a predominantly elastic (G' > G'') to a viscoelastic (G' \approx G'') regime. These results are confirmed by comparison with independent measurements performed by us using other rheological techniques. Finally a discussion is made, taking into account the results of a recent x-ray photon correlation spectroscopy experiment on the same polymer in equilibrium and out of equilibrium.Comment: Proceedings of the International Discussion Meeting on Relaxation in Complex Systems, Rome, 2009 12 pages, 7 figure

Particle sizing in non-dilute dispersions using diffusing wave spectroscopy with multiple optical path lengths

Non-dilute dispersed phase systems, such as foams, emulsions, and suspensions, are an important class of final formulations and chemical process intermediates in a variety of industries. The utility of these systems hinges on their stability over the lifetime of use, and therefore an accurate assessment of chemical and physical dynamics, asformulated, is required. We describe a unified treatment of diffusing wave spectroscopy (DWS) data using a range of optical path length with a goniometric instrument. DWS correlation data from multiple angles and robust Monte Carlo simulations are used to determine accurate values of the photon transport mean free path length. The variance on each correlation function is used to determine the physical time range that the mean squared displacement can be analyzed. Using standard solid particle suspensions of polystyrene and SiO2, we determine the average particle size with accuracy comparable to dynamic light scattering

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The strain distribution in the lumbar anterior longitudinal ligament is affected by the loading condition and bony features: An in vitro full-field analysis

The role of the ligaments is fundamental in determining the spine biomechanics in physiological and pathological conditions. The anterior longitudinal ligament (ALL) is fundamental in constraining motions especially in the sagittal plane. The ALL also confines the intervertebral discs, preventing herniation. The specific contribution of the ALL has indirectly been investigated in the past as a part of whole spine segments where the structural flexibility was measured. The mechanical properties of isolated ALL have been measured as well. The strain distribution in the ALL has never been measured under pseudo-physiological conditions, as part of multi-vertebra spine segments. This would help elucidate the biomechanical function of the ALL. The aim of this study was to investigate in depth the biomechanical function of the ALL in front of the lumbar vertebrae and of the intervertebral disc. Five lumbar cadaveric spine specimens were subjected to different loading scenarios (flexion-extension, lateral bending, axial torsion) using a state-of-the-art spine tester. The full-field strain distribution on the anterior surface was measured using digital image correlation (DIC) adapted and validated for application to spine segments. The measured strain maps were highly inhomogeneous: the ALL was generally more strained in front of the discs than in front of the vertebrae, with some locally higher strains both imputable to ligament fibers and related to local bony defects. The strain distributions were significantly different among the loading configurations, but also between opposite directions of loading (flexion vs. extension, right vs. left lateral bending, clockwise vs. counterclockwise torsion). This study allowed for the first time to assess the biomechanical behaviour of the anterior longitudinal ligament for the different loading of the spine. We were able to identify both the average trends, and the local effects related to osteophytes, a key feature indicative of spine degeneration

Synchrotron X-ray techniques for the investigation of structures and dynamics in interfacial systems

This review focuses on recent results obtained by synchrotron X-ray techniques applied to the characterization of interfacial systems, with main emphasis on flat interfaces and on colloidal systems. The techniques covered are, for structural determinations: X-ray reflectivity (XRR), grazing incidence X-ray diffraction (GIXRD) and grazing incidence X-ray excited fluorescence (GIXF), while dynamics are investigated by X-ray photon correlation spectroscopy (XPCS) mainly in the grazing-incidence geometry (GIXPCS). The systems reviewed are, in order of growing complexity, floating Langmuir monolayers, supported films of lipids and proteins, polymeric films, buried interfaces, colloidal systems and gels formed by colloids either in 3D or in the form of 2D interfacial layers. Recent results are critically discussed, and some interesting directions of development are outlined, having also in mind new technical developments such as X-ray free electron laser sources and micro-focused synchrotron beamlines

Soft pinning of liquid domains on topographical hemispherical caps

The role of lipid composition as a regulator or mediator of processes that take place in biological membranes is a very topical question, and important insights can be gained by studying in-vitro model lipid mixture systems. A particular question is the coupling of local curvature to the local phases in membranes of mixed composition. Working with an experimental system of giant unilamellar vesicles of ternary composition, the curvature is imposed by approaching the membrane to a topographically (on the micron scale) patterned surface. Performing experiments, we show that domains of the more disordered phase localise preferentially to regions of higher curvature. We characterise and discuss the strength of this "caging" behaviour. In future, the setup we discuss here could prove useful as a platform to localise domains rich in membrane proteins, or to promote the onset of biochemical processes at specific locations. Finally, we note that the methods developed here could have also applications in bio-sensing, as a similar -but metal coated- topography can sustain plasmonic resonances

Scintillating and magnetic stimuli-responsive nanostructures as adjuvants in cancer therapy

We present an overview of recent advances in stimuli-responsive nanostructures for medicine and drug delivery aiming at improving the efficacy and minimizing side effects of cancer therapies. We focus on two complementary system: Self-Lighted Photodynamic Therapy (SLPDT), and magnetic systems for hyperthermia and for controlled drug release. SPLDT, also known as X-ray triggered PDT, bases on hybrid nanostructures such as functionalized SiC/SiOx nanowires and CeF3/ZnO nanostructures. We emphasize the importance of discriminating the various Reactive Oxygen Species (ROS) produced, i.e., singlet oxygen, peroxide, superoxide, and hydroxyl radicals, which have different effects on cells and different therapeutic efficacy. Singlet oxygen is quantified by an ad hoc NIR fluorimeter. SLPDT can be usefully complemented by smart systems responsive to magnetic fields. As examples, we discuss the use of superparamagnetic iron oxide nanoparticles for magnetic hyperthermia and magnetic nanocapsules for triggered drug release. Finally, we highlight promising directions for future developments