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Search Results: 1 - 10 of 104 matches for " Jurij Kotar "
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CCD based phase resolved stroboscopic photometry of pulsars
Jurij Kotar,Simon Vidrih,Andrej Cadez
Physics , 2003, DOI: 10.1063/1.1571949
Abstract: A stroboscope designed to observe pulsars in the optical spectrum is presented. The absolute phase of the stroboscope is synchronized to better than 2.5 microseconds with the known radio ephemerides for a given pulsar. The absolute timing is provided by the GPS clock. With such a device phase resolved photometry of pulsars can be performed. We demonstrate the instrument's capabilities with the results of a set of observations of the Crab pulsar, the brightest of the known optical pulsars, with a visual magnitude of 16.5, and a rotational frequency of ~29Hz.
Non-linear mechanical response of the Red Blood Cell
Young-Zoon Yoon,Jurij Kotar,Gilwon Yoon,Pietro Cicuta
Physics , 2007, DOI: 10.1088/1478-3975/5/3/036007
Abstract: We measure the dynamical mechanical properties of human red blood cells. Single cell response is measured with optical tweezers. We investigate both the stress relaxation following a fast deformation, and the effect of varying the strain rate. We find a power law decay of the stress as a function of time, down to a plateau stress, and a power law increase of the cell's elasticity as a function of the strain rate. Interestingly, the exponents of these quantities violate the linear superposition principle, indicating a nonlinear response. We propose that this is due to breaking of a fraction of the crosslinks during the deformation process. The Soft Glassy Rheology Model accounts for the relation between the exponents we observe experimentally. This picture is consistent with recent models of bond remodeling in the red blood cell's molecular structure. Our results imply that the blood cell's mechanical behavior depends critically on the deformation process.
Real-time monitoring of complex moduli from micro-rheology
Taiki Yanagishima,Daan Frenkel,Jurij Kotar,Erika Eiser
Physics , 2010, DOI: 10.1088/0953-8984/23/19/194118
Abstract: We describe an approach to online analysis of micro-rheology data using a multi-scale time-correlation method. The method is particularly suited to process high-volume data streams and compress the relevant information in real time. Using this, we can obtain complex moduli of visco-elastic media without suffering from the high-frequency artefacts that are associated with the truncation errors in the most widely used versions of micro-rheology. Moreover, the present approach obviates the need to choose the time interval for data acquisition beforehand. We test our approach first on an artificial data set and then on experimental data obtained both for an optically trapped colloidal probe in water and a similar probe in poly-ethylene glycol solutions at various concentrations. In all cases, we obtain good agreement with the bulk rheology data in the region of overlap. We compare our method with the conventional Kramers-Kronig transform approach and find that the two methods agree over most of the frequency regime. For the same data set, the present approach is superior to Kramers-Kronig at high frequencies and can be made to perform at least comparable at low frequencies.
2D colloidal condensation driven by substrate elasticity
Sabrina Jahn,Taiki Yanagishima,Nienke Geerts,Jurij Kotar,Erika Eiser
Physics , 2010,
Abstract: This paper has been withdrawn by the authors.
Interactions between colloids induced by a soft cross-linked polymer substrate
Lorenzo Di Michele,Taiki Yanagishima,Anthony R. Brewer,Jurij Kotar,Seth Fraden,Erika Eiser
Physics , 2011, DOI: 10.1103/PhysRevLett.107.136101
Abstract: Using video-microscopy imaging we demonstrate the existence of a short-ranged equilibrium attraction between heavy silica colloids diffusing on soft surfaces of cross-linked polymer gels. The inter-colloid potential can be tuned by changing the gel stiffness or by coating the colloids with a polymer layer. On sufficiently soft substrates, the interaction induced by the polymer matrix leads to large-scale colloidal aggregation. We correlate the in-plane interaction with a colloid-surface attraction.
Multistep kinetic self-assembly of DNA-coated colloids
Lorenzo Di Michele,Francesco Varrato,Jurij Kotar,Simon H. Nathan,Giuseppe Foffi,Erika Eiser
Physics , 2013, DOI: 10.1038/ncomms3007
Abstract: Self-assembly is traditionally described as the process through which an initially disordered system relaxes towards an equilibrium ordered phase only driven by local interactions between its building blocks. However, This definition is too restrictive. Nature itself provides examples of amorphous, yet functional, materials assembled upon kinetically arresting the pathway towards the ground state. Kinetic self-assembly is intrinsically more flexible and reliable than its equilibrium counterpart, allowing control over the morphology of the final phase by tuning both the interactions and the thermodynamic pathway leading to kinetic arrest. Here we propose strategies to direct the gelation of two-component colloidal mixtures by sequentially activating selective interspecies and intra-species interactions. We investigate morphological changes in the structure of the arrested phases by means of event driven molecular dynamics (MD) simulations and experimentally using DNA-coated colloids (DNACCs). Our approach can be exploited to finely tune the morphology of multicomponent nano- or micro-porous materials with possible applications in hybrid photovoltaics, photonics and drug delivery.
Minimal two-sphere model of the generation of fluid flow at low Reynolds numbers
Marco Leoni,Bruno Bassetti,Jurij Kotar,Pietro Cicuta,Marco Cosentino Lagomarsino
Physics , 2009, DOI: 10.1103/PhysRevE.81.036304
Abstract: Locomotion and generation of flow at low Reynolds number are subject to severe limitations due to the irrelevance of inertia: the "scallop theorem" requires that the system have at least two degrees of freedom, which move in non-reciprocal fashion, i.e. breaking time-reversal symmetry. We show here that a minimal model consisting of just two spheres driven by harmonic potentials is capable of generating flow. In this pump system the two degrees of freedom are the mean and relative positions of the two spheres. We have performed and compared analytical predictions, numerical simulation and experiments, showing that a time-reversible drive is sufficient to induce flow.
Volume and porosity thermal regulation in lipid mesophases by coupling mobile ligands to soft membranes
Lucia Parolini,Bortolo M. Mognetti,Jurij Kotar,Erika Eiser,Pietro Cicuta,Lorenzo Di Michele
Physics , 2014, DOI: 10.1038/ncomms6948
Abstract: Short DNA linkers are increasingly being exploited for driving specific self-assembly of Brownian objects. DNA-functionalised colloids can assemble into ordered or amorphous materials with tailored morphology. Recently, the same approach has been applied to compliant units, including emulsion droplets and lipid vesicles. The liquid structure of these substrates introduces new degrees of freedom: the tethers can diffuse and rearrange, radically changing the physics of the interactions. Unlike droplets, vesicles are extremely deformable and DNA-mediated adhesion causes significant shape adjustments. We investigate experimentally the thermal response of pairs and networks of DNA-tethered liposomes and observe two intriguing and possibly useful collective properties: negative thermal expansion and tuneable porosity of the liposome networks. A model providing a thorough understanding of this unexpected phenomenon is developed, explaining the emergent properties out of the interplay between the temperature-dependent deformability of the vesicles and the DNA-mediated adhesive forces.
Hydrodynamically synchronized states in active colloidal arrays
Lo?c Damet,Giovanni M. Cicuta,Jurij Kotar,Marco Cosentino Lagomarsino,Pietro Cicuta
Physics , 2011,
Abstract: Colloidal particles moving in a fluid interact via the induced velocity field. The collective dynamic state for a class of actively forced colloids, driven by harmonic potentials via a rule that couples forces to configurations, to perform small oscillations around an average position, is shown by experiment, simulation and theoretical arguments to be determined by the eigenmode structure of the coupling matrix. It is remarkable that the dynamical state can therefore be predicted from the mean spatial configuration of the active colloids, or from an analysis of the fluctuations near equilibrium. This has the surprising consequence that while 2 particles, or polygonal arrays of 4 or more colloids, synchronize with the nearest neighbors in anti-phase, a system of 3 equally spaced colloids synchronizes in-phase. In the absence of thermal fluctuations, the stable dynamical state is predominantly formed by the eigenmode with longest relaxation time.
Presentation of the selected French libraries and the French library system
Moja Kotar
Knji?nica : Revija za Podro?je Bibliotekarstva in Informacijske Znanosti , 2007,
Abstract: The Selected French libraries (Bibliotheque nationale de France, Bibliotheque publique d’information, multimedia library of the Cité des sciences et de l’industrie as well as Paris public libraries) are presented in the article. France does not have a union catalogue at the national level, therefore libraries use different platforms for shared cataloguing and compile more union catalogues. According to their needs, French libraries join into consortia for the acquisition of electronic resources, which can be either geographically or thematically delimited or formed by the institutions of the same status. The author believes that the Slovenian library network works well considering much smaller budget for culture and higher education in comparison with France. To improve its performance, more funds would have to be allocated and higher reputation of the library profession achieved, comparable to the situation in France. Digitization of resources is the area where the Slovenian librarianship lags most behind the French one.
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