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1 Record 1 of 17 Strepp W; Sengupta S; Nielaba P Phase transitions of soft disks in external periodic potentials: A Monte Carlo study Source: PHYSICAL REVIEW E 2002, Vol 66, Iss 5, art. no The nature of freezing and melting transitions for a system of model colloids interacting via the Derjaguin, Landau, Verwey, and Overbeek potential in a spatially periodic external potential is studied using extensive Monte Carlo simulations. Detailed finite size scaling analyses of various thermodynamic quantities, such as the order parameter, its cumulants, etc., are used to map the phase diagram of the system for various values of the reduced screening length kappaa(s) and the amplitude of the external potential. We find clear indication of a reentrant liquid phase over a significant region of the parameter space. Our simulations therefore show that the system of soft disks behaves in a fashion similar to charge stabilized colloids, which are known to undergo an initial freezing, followed by a remelting transition as the amplitude of the imposed, modulating field produced by crossed laser beams is steadily increased. The detailed analysis of our data shows several features consistent with a recent dislocation unbinding theory of laser induced melting. Source item page count: 13 Publication Date: NOV 622DG 29-char source abbrev: PHYS REV E Record 2 of 17 Stiakakis E; Vlassopoulos D; Likos CN; Roovers J; Meier G Polymer-mediated melting in ultrasoft colloidal gels Source: PHYSICAL REVIEW LETTERS 2002, Vol 89, Iss 20, art. no Star polymers with a high number of arms, f = 263, become kinetically trapped when dispersed in an athermal solvent at concentrations above the overlapping one, forming physical gels. We show that the addition of linear chains at different concentrations and molecular weights reduces the modulus of the gel, eventually melting it. We explain this linear polymer-induced gel-liquid transition in terms of effective interactions and star depletion. In the limit of very high linear-chain molecular weight a "reentrant gelation" is detected and attributed to bridging flocculation, analogous to that observed in colloidal dispersions. Publication Date: NOV AL 29-char source abbrev: PHYS REV LETT Page 1 of 11

2 Record 3 of 17 Strepp W; Sengupta S; Lohrer A; Nielaba P Phase transitions of hard and soft disks in external periodic potentials: A Monte Carlo study Source: COMPUTER PHYSICS COMMUNICATIONS 2002, Vol 147, Iss 1-2, pp The nature of the melting transition for a system of hard disks with translational degrees of freedom in two spatial dimensions has been analyzed by a novel finite size scaling technique. The behavior of the system is consistent with the predictions of the KTHNY theory. Hard and soft disks in external periodic potentials show rich phase diagrams including freezing and melting transitions when the density of the system is varied. Here we report on extensive Monte Carlo simulations for detailed finite size scaling analyses of various thermodynamic quantities like the order parameter, its cumulants, etc. in order to map the phase diagram of the system for various values of the density and the amplitude of the external potential. For hard disks we find clear indication of a reentrant liquid phase over a significant region of the parameter space. Our simulations therefore show that the system of hard disks behaves in a fashion similar to charge-stabilized colloids which are known to undergo an initial freezing, followed by a re-melting transition as the amplitude of the imposed, modulating field produced by crossed laser beams is steadily increased. Detailed analysis of our data shows several features consistent with a recent dislocation unbinding theory of laser-induced melting. The differences and similarities of systems with soft potentials (DLVO, 1/r(12)) are discussed. (C) 2002 Elsevier Science B.V. All rights reserved. Publication Date: AUG 1 590LP 29-char source abbrev: COMPUT PHYS COMMUN Page 2 of 11

3 Record 4 of 17 Dzubiella J; Likos CN; Lowen H Phase behavior and structure of star-polymer-colloid mixtures Source: JOURNAL OF CHEMICAL PHYSICS 2002, Vol 116, Iss 21, pp We calculate the phase diagrams of mixtures between hard-sphere colloids and star-polymers of arm numbers f=2,6,32 for different star-polymer-colloid size ratios 0.2less than or equal toqless than or equal to0.6 using an effective one-component description for the colloids in the presence of the stars. We map the full twocomponent system onto an effective one-component system by inverting numerically the Ornstein-Zernike equation for binary mixtures, supplemented by the Rogers-Young closure, in the low-colloid density limit. The free energy for the fluid and crystalline phase is calculated by using both hard-sphere perturbation theory and thermodynamic integration of simulation data. We find stable fluid-fluid demixing transitions for low arm numbers f=2,6 above a critical value of the size ratio q(c) below preempted by a fcc-solid. For the linear polymer limit, f=2, the critical size ratio is found to be q(c)approximate to0.4, in agreement with other approaches to colloid-polymer mixtures. Increasing the arm number, the region of stability of the demixing transition with respect to crystallization of the colloids shrinks, and q(c) grows. A comparison between the one- and two-component descriptions that demonstrates the consistency between the two routes is also carried out. (C) 2002 American Institute of Physics. Source item page count: 13 Publication Date: JUN 1 550DA 29-char source abbrev: J CHEM PHYS Page 3 of 11

4 Record 5 of 17 Dzubiella J; Harreis HM; Likos CN; Lowen H Sedimentation profiles of systems with reentrant melting behavior Source: PHYSICAL REVIEW E 2001, Vol 6401, Iss 1, art. no We examine sedimentation density profiles of star polymer solutions as an example of colloidal systems in sedimentation equilibrium that exhibit reentrant melting in their bulk phase diagram. Phase transitions between a fluid and a fluid with an intercalated solid are observed below a critical gravitational strength alpha*. Characteristics of the two fluid-solid interfaces in the density profiles occurring in Monte Carlo simulations for alpha<<alpha>* are in agreement with scaling laws put forth in the framework of a phenomenological theory. Furthermore, we detect density oscillations at the fluid-gas interface at high altitudes for high-gravitational fields, which are verified with density-functional theory and should be observable in surface scattering experiments. Source item page count: 9 Publication Date: JUL Part number: 1 453GA 29-char source abbrev: PHYS REV E Record 6 of 17 Strepp W; Sengupta S; Nielaba P Phase transitions of hard disks in external periodic potentials: A Monte Carlo study Source: PHYSICAL REVIEW E 2001, Vol 6304, Iss 4, art. no The nature of freezing and melting transitions for a system of hard disks in a spatially periodic external potential is studied using extensive Monte Carlo simulations. Detailed finite size scaling analysis of various thermodynamic quantities like the order parameter, its cumulants, etc., are used to map the phase diagram of the system for various values of the density and the amplitude of the external potential. We find clear indication of a reentrant liquid phase over a significant region of the parameter space. Our simulations therefore show that the system of hard disks behaves in a fashion similar to charge-stabilized colloids that are known to undergo an initial freezing, followed by a remelting transition as the amplitude of the imposed, modulating field produced by crossed laser beams is steadily increased. Detailed analysis of our data shows several features consistent with a recent dislocation unbinding theory of laser induced melting. Source item page count: 10 Publication Date: APR Part number: 2 424CT 29-char source abbrev: PHYS REV E Page 4 of 11

5 Record 7 of 17 Likos CN; Lang A; Watzlawek M; Lowen H Criterion for determining clustering versus reentrant melting behavior for bounded interaction potentials Source: PHYSICAL REVIEW E 2001, Vol 6303, Iss 3, art. no We examine in full generality the phase behavior of systems whose constituent particles interact by means of potentials that do not diverge at the origin, are free of attractive parts, and decay fast enough to zero as the interparticle separation r goes to infinity. By employing a mean field-density functional theory which is shown to become exact at high temperatures and/or densities, we establish a criterion that determines whether a given system will freeze at all temperatures or it will display reentrant melting and an upper freezing temperature. Source item page count: 9 Publication Date: MAR Part number: 1 413QU 29-char source abbrev: PHYS REV E Record 8 of 17 Radzihovsky L; Frey E; Nelson DR Novel phases and reentrant melting of two-dimensional colloidal crystals Source: PHYSICAL REVIEW E 2001, Vol 6303, Iss 3, art. no We investigate two-dimensional (2D) melting in the presence of a one-dimensional (1D) periodic potential as, for example, realized in recent experiments on 2D colloids subjected to two interfering laser beams. The topology of the phase diagram is found to depend primarily on two factors: the relative orientation of the 2D crystal and the periodic potential troughs, which selects a set of Bragg planes running parallel to the troughs, and the commensurability ratio p = a'/d of the spacing a' between these Bragg planes to the period a' of the periodic potential. The complexity of the phase diagram increases with the magnitude of the commensurabilty ratio p. Rich phase diagrams, with "modulated liquid," "floating," and "locked floating" solid and smectic phases are found. Phase transitions between these phases fall into two broad universality classes, roughening and melting, driven by the proliferation of discommensuration walls and dislocations, respectively. We discuss correlation functions and the static structure factor in these phases, and make detailed predictions about the universal features close to the phase boundaries. We predict that for charged systems with highly screened short-range interactions, these melting transitions are generically reentrant as a function of the strength of the periodic potential, a prediction that is in accord with recent 2D colloid experiments. Implications of our results for future experiments are also discussed. Source item page count: 34 Publication Date: MAR Part number: 1 Page 5 of 11

6 413QU 29-char source abbrev: PHYS REV E Record 9 of 17 Bechinger C; Wei QH; Leiderer P Reentrant melting of two-dimensional colloidal systems Source: JOURNAL OF PHYSICS-CONDENSED MATTER 2000, Vol 12, Iss 8A, pp A425-A430 When a two-dimensional colloidal suspension of highly charged particles is subjected to a periodic one-dimensional (1D) light field, a liquid-solid transition can be induced. This phase transition is well known as light-induced freezing. However, upon further increase of the intensity, the crystal is found to remelt (laser-induced melting) to a 1D liquid. This reentrance behaviour is in good agreement with predictions based on Monte Carlo simulations. We suggest explaining this intriguing phenomenon in terms of particle fluctuations which tend to stabilize the crystalline phase. Source item page count: 6 Publication Date: FEB WN 29-char source abbrev: J PHYS-CONDENS MATTER Record 10 of 17 Watzlawek M; Likos CN; Lowen H Phase diagram of star polymer solutions Source: PHYSICAL REVIEW LETTERS 1999, Vol 82, Iss 26, pp The phase diagram of star polymer solutions in a good solvent is obtained over a wide range of densities and arm numbers by Monte Carlo simulations. The effective interaction between the stars is modeled by an ultrasoft pair potential which is logarithmic in the core-core distance. Among the stable phases are a fluid as well as body-centered cubic, face-centered cubic, body-centered orthogonal, and diamond crystals. In a limited range of arm numbers, reentrant melting and reentrant freezing transitions occur for increasing density. Publication Date: JUN GN 29-char source abbrev: PHYS REV LETT Page 6 of 11

7 Record 11 of 17 Wei QH; Bechinger C; Rudhardt D; Leiderer P Experimental study of laser-induced melting in two-dimensional colloids Source: PHYSICAL REVIEW LETTERS 1998, Vol 81, Iss 12, pp We study the phase behavior of a charged two-dimensional colloidal system in the presence of a periodic light field composed of two interfering laser beams. Above a certain light intensity, the light field initiates a liquid-solid transition. This effect is known as light-induced freezing. When the light field is increased above a critical value, however, the induced crystal is predicted to melt into a modulated liquid again. In this paper we report the first direct experimental observation of this reentrance phenomenon. [S (98) ]. Publication Date: SEP LM 29-char source abbrev: PHYS REV LETT Record 12 of 17 Das C; Krishnamurthy HR Laser-induced quasicrystalline order in charge-stabilized colloidal systems Source: PHYSICAL REVIEW B 1998, Vol 58, Iss 10, pp R5889-R5892 We have studied the ordering of a two-dimensional charge-stabilized colloidal system in the presence of a stationary one-dimensionally modulated laser field formed by the superposition of two modulations with wave vectors q(0)tau and q(0)/tau, where q(0) is the wave vector corresponding to the first peak of the direct correlation function of the unperturbed liquid, and tau is the golden mean. In the framework of the Landau-Alexander-McTague theory we find that a decagonal quasicrystalline phase is stabler than the liquid or the triangular lattice in certain regions of the phase diagram. Our study also shows a reentrant melting phenomenon for larger laser field strengths. We find that the transition from the modulated liquid to the quasicrystalline phase is continuous in contrast to a first order transition from the modulated liquid to the triangular crystalline phase. Publication Date: SEP 1 121HN 29-char source abbrev: PHYS REV B Page 7 of 11

8 Record 13 of 17 McConnell GA; Gast AP Melting of ordered arrays and shape transitions in highly concentrated diblock copolymer solutions Source: MACROMOLECULES 1997, Vol 30, Iss 3, pp Polystyrene/polyisoprene (PS/PI) diblocks suspended in decane serve as a model system for the investigation of highly concentrated diblock copolymer solutions. Bulk melts of PS/PI with no solvent exhibit ordered morphologies including lamellae, close-packed cylinders, etc. that depend on the block asymmetry. These same diblocks self-assemble in decane at low concentrations to form monodisperse, spherical micelles with a dense polystyrene core and a diffuse polyisoprene corona. Strongly interacting polymeric micelles, observed at modest polymer concentrations, order into cubic arrays that include both face-centered and bodycentered cubic crystals depending on the length scale of the repulsion relative to the core dimension. These ordered morphologies of the melt and micellar crystals provide limiting reference states for the poorly understood high-concentration regime studied in this work. As we increase the polymer concentration, we observe a curious melting of the micellar crystals before the onset of anisotropy. Since the melting of the micellar crystal is not predicated upon shape transitions, we return to tethered-chain models of our spherical polymeric micelles to qualitatively describe the disordering process in terms of a loss of the osmotic pressure gradient between micelles. One system exhibits a reentrant disorder-order-disorder-order phase transition. Finally, the development of anisotropy in the scattering pattern is linked to shape transitions that develop as melt conditions are approached. We monitor the degree of anisotropy to estimate the concentration for the onset of these shape transitions. Source item page count: 10 Publication Date: FEB 10 WH char source abbrev: MACROMOLECULES Page 8 of 11

9 Record 14 of 17 Sood AK Some novel states of colloidal matter: Modulated liquid, modulated crystal and glass Source: PHYSICA A 1996, Vol 224, Iss 1-2, pp Easy tunability of interparticle interactions makes aqueous suspensions of charged polystyrene spheres ideal systems for understanding freezing of liquids to crystal and glass. This review will cover two aspects of our recent work: (i) Density functional theory and Monte-Carlo simulations of the phase diagram and the order of the modulated liquid to crystal transition in laser field modulated colloids. We show that for sufficient strength of the modulating potential beta V-e, the freezing transition changes from first order to a continuous transition. Our simulations reveal a novel reentrant laser-induced melting transition from the crystal to the modulating liquid phase with increasing beta V-e (ii) Brownian dynamics simulations on binary colloids to study structural evolution and dynamics as liquid freezes into a crystal (at low volume fraction phi) or into a glass (at high phi). The two- and fourpoint density autocorrelation functions show that there is no growing correlation length near the glass transition. The bond-orientational order is characterised in terms of quadratic rotational invariants and bond-orientational correlation functions. The temperature-dependence of average relaxation times extracted from the stretched exponential fit to correlation functions obey the Vogel-Tammann-Fulcher law. The van-hove correlation functions and the subdiffusive behaviour of the mean-squared displacements reveal strongly cooperative particle motion near the glassy state. Source item page count: 14 Publication Date: FEB 1 TV char source abbrev: PHYSICA A Page 9 of 11

10 Record 15 of 17 CHAKRABARTI J; KRISHNAMURTHY HR; SOOD AK; SENGUPTA S REENTRANT MELTING IN LASER FIELD MODULATED COLLOIDAL SUSPENSIONS Source: PHYSICAL REVIEW LETTERS 1995, Vol 75, Iss 11, pp We present results from a Monte Carlo simulation study of the phase diagram and the order of the modulated liquid to crystal transition in laser field modulated colloids. We find that for low values of beta V-e, the strength of the modulation potential, the transition is first order, but changes to a continuous transition for high values of beta V-e, in agreement with the conclusions from the density functional theory of laser induced freezing. However, we find in the simulation a novel reentrant laser induced melting transition from the crystal to the modulated liquid phase with increasing beta V-e, unlike the (mean held) density functional phase diagram. Publication Date: SEP 11 RT char source abbrev: PHYS REV LETT Record 16 of 17 LAHIRI R; RAMASWAMY S SHEAR-INDUCED MELTING AND REENTRANCE - A MODEL Source: PHYSICAL REVIEW LETTERS 1994, Vol 73, Iss 7, pp A simple two-variable model for the dynamics of sheared colloidal crystals gives shear-melting at low shear rates and suggests a novel mechanism for recrystallization to the solid phase at high shear rates. The nonequilibrium phase diagram agrees qualitatively with experiments and molecular dynamics simulations. Publication Date: AUG 15 PB char source abbrev: PHYS REV LETT Page 10 of 11

11 Record 17 of 17 CHUI ST; QIU X TOPOLOGICAL DEFECTS, MULTIPARTICLE EXCHANGE AND REENTRANT MELTING Source: EUROPHYSICS LETTERS 1994, Vol 26, Iss 3, pp Coherent quantum-mechanical climb motion of a dislocation pair involves exchanges of an array of particles. For electrons in a magnetic field this motion becomes possible near odd denominator filling factors, the pair is liberated and its energy is decreased, consistent with the lowering of the activation energy in recent transport measurements by Jiang et al. This decrease also provides a mechanism of re-entrant melting via spontaneous generation of dislocation pairs. Source item page count: 6 Publication Date: APR 20 NJ char source abbrev: EUROPHYS LETT Acceptable Use Policy Copyright 2003 Thomson ISI Page 11 of 11

Colloidal Suspension Rheology Chapter 1 Study Questions

Colloidal Suspension Rheology Chapter 1 Study Questions 1. What forces act on a single colloidal particle suspended in a flowing fluid? Discuss the dependence of these forces on particle radius. 2. What