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Search Results: 1 - 10 of 440 matches for " Ilona Kretzschmar "
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Molecular dynamics simulations of the evaporation of particle-laden droplets
Weikang Chen,Joel Koplik,Ilona Kretzschmar
Physics , 2012, DOI: 10.1103/PhysRevE.87.052404
Abstract: We use molecular dynamics simulations to study the evaporation of particle-laden droplets on a heated surface. The droplets are composed of a Lennard-Jones fluid containing rigid particles which are spherical sections of an atomic lattice, and heating is controlled through the temperature of an atomistic substrate. We observe that sufficiently large (but still nano-sized) particle-laden drops exhibit contact line pinning, measure the outward fluid flow field which advects particle to the drop rim, and find that the structure of the resulting aggregate varies with inter-particle interactions. In addition, the profile of the evaporative fluid flux is measured with and without particles present, and is also found to be in qualitative agreement with earlier theory. The compatibility of simple nanoscale calculations and micron-scale experiments indicates that molecular simulation may be used to predict aggregate structure in evaporative growth processes.
Rotational Dynamics and Angular Locking of Nanoparticles at liquid Interfaces
Sepideh Razavi,Ilona Kretzschmar,Joel Koplik,Carlos E. Colosqui
Physics , 2013,
Abstract: Nanoparticles with different surface morphologies that straddle the interface between two immiscible liquids are studied via molecular dynamics simulations. The methodology employed allows us to compute the interfacial free energy at different angular orientations of the nanoparticle. Due to their atomistic nature, the studied nanoparticles present both microscale and macroscale geometrical features and cannot be accurately modeled as a perfectly smooth body (e.g., spheres, cylinders). Under certain physical conditions, microscale features can produce free energy barriers that are much larger than the thermal energy of the surrounding media. The presence of these energy barriers can effectively "lock" the particle at specific angular orientations with respect to the liquid-liquid interface. This work provides new insights on the rotational dynamics of Brownian particles at liquid interfaces and suggests possible strategies to exploit the effects of microscale features with given geometric characteristics.
Metamaterial based broadband engineering of quantum dot spontaneous emission
Harish N S Krishnamoorthy,Zubin Jacob,Evgenii Narimanov,Ilona Kretzschmar,Vinod M. Menon
Physics , 2009,
Abstract: We report the broadband (~ 25 nm) enhancement of radiative decay rate of colloidal quantum dots by exploiting the hyperbolic dispersion of a one-dimensional nonmagnetic metamaterial structure.
Topological Transitions in Metamaterials
Harish N. S. Krishnamoorthy,Zubin Jacob,Evgenii Narimanov,Ilona Kretzschmar,Vinod M. Menon
Physics , 2011, DOI: 10.1126/science.1219171
Abstract: The ideas of mathematical topology play an important role in many aspects of modern physics - from phase transitions to field theory to nonlinear dynamics (Nakahara M (2003) in Geometry, Topology and Physics, ed Brewer DF (IOP Publishing Ltd, Bristol and Philadelphia), Monastryskiy M (1987) in Riemann Topology and Physics, (Birkhauser Verlag AG)). An important example of this is the Lifshitz transition (Lifshitz IM (1960) Anomalies of electron characteristics of a metal in the high-pressure region, Sov Phys JETP 11: 1130-1135), where the transformation of the Fermi surface of a metal from a closed to an open geometry (due to e.g. external pressure) leads to a dramatic effect on the electron magneto-transport (Kosevich AM (2004) Topology and solid-state physics. Low Temp Phys 30: 97-118). Here, we present the optical equivalent of the Lifshitz transition in strongly anisotropic metamaterials. When one of the components of the dielectric permittivity tensor of such a composite changes sign, the corresponding iso-frequency surface transforms from an ellipsoid to a hyperboloid. Since the photonic density of states can be related to the volume enclosed by the iso-frequency surface, such a topological transition in a metamaterial leads to a dramatic change in the photonic density of states, with a resulting effect on every single physical parameter related to the metamaterial - from thermodynamic quantities such as its equilibrium electromagnetic energy to the nonlinear optical response to quantum-electrodynamic effects such as spontaneous emission. In the present paper, we demonstrate the modification of spontaneous light emission from quantum dots placed near the surface of the metamaterial undergoing the topological Lifshitz transition, and present the theoretical description of the effect.
Using the Discrete Dipole Approximation and Holographic Microscopy to Measure Rotational Dynamics of Non-spherical Colloidal Particles
Anna Wang,Thomas G. Dimiduk,Jerome Fung,Sepideh Razavi,Ilona Kretzschmar,Kundan Chaudhary,Vinothan N. Manoharan
Physics , 2013, DOI: 10.1016/j.jqsrt.2013.12.019
Abstract: We present a new, high-speed technique to track the three-dimensional translation and rotation of non-spherical colloidal particles. We capture digital holograms of micrometer-scale silica rods and sub-micrometer-scale Janus particles freely diffusing in water, and then fit numerical scattering models based on the discrete dipole approximation to the measured holograms. This inverse-scattering approach allows us to extract the the position and orientation of the particles as a function of time, along with static parameters including the size, shape, and refractive index. The best-fit sizes and refractive indices of both particles agree well with expected values. The technique is able to track the center of mass of the rod to a precision of 35 nm and its orientation to a precision of 1.5$^\circ$, comparable to or better than the precision of other 3D diffusion measurements on non-spherical particles. Furthermore, the measured translational and rotational diffusion coefficients for the silica rods agree with hydrodynamic predictions for a spherocylinder to within 0.3%. We also show that although the Janus particles have only weak optical asymmetry, the technique can track their 2D translation and azimuthal rotation over a depth of field of several micrometers, yielding independent measurements of the effective hydrodynamic radius that agree to within 0.2%. The internal and external consistency of these measurements validate the technique. Because the discrete dipole approximation can model scattering from arbitrarily shaped particles, our technique could be used in a range of applications, including particle tracking, microrheology, and fundamental studies of colloidal self-assembly or microbial motion.
Spin-polarized Voltages on a 2D Self-assembled Plasmonic Crystal
Nicholas V. Proscia,Matthew Moocarme,Roger Chang,Ilona Kretzschmar,Vinod M. Menon,Luat T. Vuong
Physics , 2015,
Abstract: The Photon Drag Effect (PDE) is a nonlinear process akin to optical rectification in which the momentum of light is transferred to charged carriers and converted to a DC voltage. Here, we experimentally demonstrate the spin-polarized voltage, associated with the transference of light's spin angular momenta to the linear momenta of charges, with visible-light illumination on a nanovoid self-assembled plasmonic crystal surface. Numerical calculations show that the gradient force, generally considered independent of polarization, is responsible for the majority of the momentum transfer. The PDE in this achiral system represents a distinct spin-orbit interaction that produces asymmetric hotspots whose locations change with circular polarization handedness. Our results significantly advance our understanding of the PDE and demonstrate realistic potential for scalable plasmonic materials that utilize PDE.
Transforming growth factor-beta and breast cancer: Transforming growth factor-β/SMAD signaling defects and cancer
Marcus Kretzschmar
Breast Cancer Research , 2000, DOI: 10.1186/bcr42
Abstract: The transforming growth factor-β (TGF-β) family of polypeptide growth factors regulates cellular processes, including cell division, differentiation, motility, adhesion, and death, in virtually all tissues ([1] and references therein). TGF-β is an important regulator of normal mammary gland development and function, as well as of the development and progression of breast tumors. TGF-β potently inhibits cell cycle progression of epithelial cells, including those of the lobules and ducts of the mammary gland, and it thereby controls epithelial cell proliferation and regression during mammary gland development, and during and after lactation in the adult gland [2].In breast cancer, TGF-β has been suggested to play a dual role [3]. It acts as a tumor suppressor in early stages of the disease when it inhibits the outgrowth of carcinomas in situ via its antiproliferative functions. This has been demonstrated in transgenic mouse models, in which over expression of TGF-β1 (one isoform of TGF-β) is targeted to the mammary gland, and tumor formation is induced by concomitant over-expression of TGF-β and administration of a chemical carcinogen [4]. In later stages of the disease, TGF-β is believed to promote tumor progression, in part by enhancing tumor cell motility and invasiveness [5,6] and the capacity to form metastases [6,7,8]. Tumor promoting functions of TGF-β correlate with increased secretion of TGF-β by the cancer cells during tumor progression [3].This apparent switch of the role of TGF-β in the regulation of tumorigenesis is reflected in changes of tumor cell responsiveness. Similar to other types of carcinomas, many malignant breast carcinoma cells have lost most or all sensitivity to TGF-β-induced growth inhibition, while tumor cells derived from early stages of the disease are usually inhibited [9]. This loss of antiproliferative responsiveness thereby predisposes to or causes cancer progression.TGF-β induces growth inhibition by arresting cells in the G1 phase
Towards a Christian Ethic of Work in South Africa
L Kretzschmar
Acta Theologica , 2012,
Abstract: This paper draws on the academic field of Christian ethics and focuses attention on an ethic of work within the South African context. Key terms such as ‘an ethic of work’, ‘a work ethic’ and ‘ethics at work’ are discussed in relation to varied experiences of work. The issues of why one ought to work and what constitutes ‘good’ work are discussed with reference to current ethical and economic challenges. I argue that a Christian worldview, or understanding of reality, provides a much more credible contribution to an ethic of work than either a materialist view of reality or a system of patronage.
Cultural pathways and pitfalls in South Africa: a reflection on moral agency and leadership from a Christian perspective
L. Kretzschmar
Koers : Bulletin for Christian Scholarship , 2010, DOI: 10.4102/koers.v75i3.97
Abstract: The nature and importance of moral agency for the transformation of persons and society, particularly from a Christian perspective, are discussed in this article. The focus is on cultural pathways and pitfalls with respect to the formation and exercise of moral agency on the part of individuals, leaders and communities. The six dimensions of cultural values as developed by Hampden-Turner and Trompenaars (2000) are used as a framework to describe and evaluate mainly western and African cultural values in order to identify ways of developing moral responsibility and genuine social transformation.
Integrity and consensus: A Christian perspective on ethical management and education in South Africa
L. Kretzschmar
Koers : Bulletin for Christian Scholarship , 2002, DOI: 10.4102/koers.v67i4.378
Abstract: In this article the compliance- and values-based approaches to ethical management are explained and the challenges of the contemporary business and societal contexts in South Africa noted. The need for both moral integrity and character (with respect to perception, intention and virtue) and ethical consensus is extensively discussed. The importance of ethical dialogue as a means of reaching moral consensus, and the contribution of public theology (particularly Christian theology) are assessed. Finally, vital issues relating to business ethics management and education are outlined and some practical possibilities suggested.
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