12/28/2007
In recent years many experimentalists have reported an anomalously enhanced thermal conductivity in liquid suspensions of nanoparticles. Despite the importance of this effect for heat transfer applications, no agreement has emerged about the mechanism of this phenomenon, or even about the experimentally observed magnitude of the enhancement. To address these issues, this paper presents a combined experimental and theoretical study of heat conduction and particle agglomeration in nanofluids. On the experimental side, nanofluids of alumina particles in water and ethylene glycol are characterized using thermal conductivity measurements, viscosity measurements, dynamic light scattering, and other techniques. The results show that the particles are agglomerated, with an agglomeration state that evolves in time. The data also show that the thermal conductivity enhancement is within the range predicted by effective medium theory. On the theoretical side, a model is developed for heat conduction through a fluid containing nanoparticles and agglomerates of various geometries. The calculations show that elongated and dendritic structures are more efficient in enhancing the thermal conductivity than compact spherical structures of the same volume fraction, and that surface (Kapitza) resistance is the major factor resulting in the lower than effective medium conductivities measured in our experiments. Together, these results imply that the geometry, agglomeration state, and surface resistance of nanoparticles are the main variables controlling thermal conductivity enhancement in nanofluids.
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03/29/2011
In many colloidal systems, an orientationally ordered nematic (N) phase emerges fromthe isotropic (I) melt in the form of spindlelike birefringent tactoids. In cases studied so far, the tactoids always reveal a mirrorsymmetric nonchiral structure, sometimes even when the building units are chiral. We report on chiral symmetry breaking in the nematic tactoids formed in molecularly nonchiral polymercrowded aqueous solutions of lowmolecular weight disodium cromoglycate. The parity is broken by twisted packing of selfassembled molecular aggregates within the tactoids as manifested by the observed optical activity. Fluorescent confocal microscopy reveals that the chiral N tactoids are located at the boundaries of cells. We explain the chirality induction as a replacement of energetically costly splay packing of the aggregates within the curved bipolar tactoidal shape with twisted packing. The effect represents a simple pathway of macroscopic chirality induction in an organic system with no molecular chirality, as the only requirements are orientational order and curved shape of confinement.
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02/26/2013
Complex morphologies in lipid membranes typically arise due to chemical heterogeneity, but in the tilted gel phase, complex shapes can form spontaneously even in a membrane containing only a single lipid component. We explore this phenomenon via experiments and coarsegrained simulations on giant unilamellar vesicles of 1,2dipalmitoylsnglycero3phosphocholine. When cooled from the untilted Lalpha liquidcrystalline phase into the Lbeta, tilted gel phase, vesicles deform from smooth spheres to disordered, highly crumpled shapes. We propose that this shape evolution is driven by nucleation of complex membrane microstructure with topological defects in the tilt orientation that induce nonuniform membrane curvature. Coarsegrained simulations demonstrate this mechanism and show that kinetic competition between curvature change and defect motion can trap vesicles in deeply metastable, defectrich structures.
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08/02/1990
H2 NMR of a liquid crystal confined to submicron cylindrical cavities, in a 17K interval above the nematicisotropic transition, reveals a weakly orientationally ordered molecular layer at the cavity wall governed by local interactions. The order (S0 = 0.02) is temperature independent so that a partial wetting is realized with no prewetting transition expected. A molecular exchange rate between the surface layer and the bulk is measured to be approximately 10(3) s1. The coherence length measuring the order penetrating further into the cavity is described by alpha = 0.4 and xi0 = 0.6 nm.
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12/15/2007
We report the measurement of Lambda and (Lambda) over bar yields and inverse slope parameters in d+Au collisions at root s(NN)=200 GeV at forward and backward rapidities (y=+/ 2.75), using data from the STAR forward time projection chambers. The contributions of different processes to baryon transport and particle production are probed exploiting the inherent asymmetry of the d+Au system. Comparisons to model calculations show that baryon transport on the deuteron side is consistent with multiple collisions of the deuteron nucleons with gold participants. On the gold side, HIJINGbased models without a hadronic rescattering phase do not describe the measured particle yields, while models that include target remnants or hadronic rescattering do. The multichain model can provide a good description of the net baryon density in d+Au collisions at energies currently available at the BNL Relativistic Heavy Ion Collider, and the derived parameters of the model agree with those from nuclear collisions at lower energies.
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12/01/1997
Monte Carlo (MC) simulations based exclusively on nearestneighbor intermolecular interactions reveal the existence of stable longrange deformations and topological defects in a thin nematic film confined between two surfaces with antagonistic (normal and tangential) molecular orientations. Thus the MC technique allows one to describe a delicate balance of bulk elasticity and surface energy usually treated only with macroscopic theories.
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04/01/1998
The nematic (N) to lamellar (Lα) phase transition in binary mixtures of cesiumperfluorooctanoate (CsPFO) and water has been studied by highresolution synchrotron xray scattering at 46.6 weight % CsPFO. The longitudinal correlation length ξ∥ and the susceptibility σ associated with the lamellar phase fluctuations in the N phase, measured over three decades of reduced temperature, diverge with critical exponents ν∥=0.86±0.04 and γ=1.37±0.11, respectively. These results show that the N to Lα phase transition is quantitatively similar to the N to smecticA phase transition of thermotropic liquid crystals with a wider nematic range.
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10/07/1991
Highresolution xray scattering experiments to study the critical smecticA fluctuations in the nematic phase have been performed on a mixture of two nonpolar, twobenzenering compounds. The ratio T(NA)/T(NI) (= 0.885) for this mixture is much smaller than for previously studied similar materials. The smectic correlation lengths parallel and perpendicular to the smecticlayer normal are found to diverge anisotropically with characteristic exponentsnu parallelto = 0.75 +/ 0.03, nu perpendicularto = 0.65 +/ 0.03, and the susceptibility with gamma = 1.24 +/ 0.05.
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07/15/2008
Recent experiments report that the longlookedfor thermotropic biaxial nematic phase has been finally detected in some thermotropic liquid crystalline systems. Inspired by these experimental observations, we concentrate on some elementary theoretical issues concerned with the classical sixthorder Landaude Gennes free energy expansion in terms of the symmetric and traceless tensor order parameter Q(alpha beta). In particular, we fully explore the stability of the biaxial nematic phase giving analytical solutions for all distinct classes of the phase diagrams that theory allows. This includes diagrams with triple, critical, and tricritical points and with multiple (reentrant) biaxial and uniaxial phase transitions. A brief comparison with predictions of existing molecular theories is also given.
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03/15/1991
Possible phases in a nematic liquid crystal confined to a spherical submicrometer droplet embedded in a solid polymer are analyzed in terms of a Landaude Gennes theory. For a droplet with a radial structure we show that the strength of the nematicpolymer interfacial interaction affects the nematicparanematic (partially ordered isotropic phase) phase transition and may in addition induce a boundarylayer nematic phase. This boundary layer phase exists only in a narrow (approximately 0.1 K) temperature interval above the nematic phase for a restricted range of interfacial interactions. Also in the radial structure the degree of ordering is suppressed close to the center of the droplet where a defect is located. As the size of the droplet decreases, the relative size of this region of suppressed ordering increases. Below a critical radius R(c) (0.22mum for 4npentyl4'cyanobiphenyl), if the surface interaction is above a critical value (q(max) = 1.85 x 10(3)), the transition between the nematic phase and the paranematic phase no longer occurs. A threedimensional phase diagram is presented to demonstrate the effect of the surface interaction strength, droplet radius, and sample temperature on the stability of phases within a droplet.
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