Nature Of The Blue-Phase-III-Isotropic Critical Point: An Analogy With The Liquid-Gas Transition

The analogy with the liquid-gas critical point is analyzed to clarify the nature of the pretransitional behavior of physical properties in the vicinity of the Blue-Phase-m-isotropic transition in chiral liquid crystalline systems. The analogy is unusual: temperature serves as the ordering field and entropy plays the role of the order parameter. Both mean field and parametric equations of state are formulated in terms of scaling fields. The scaling fields are linear combinations of the physical fields, which are temperature and chirality. It is shown that mixing of the physical field variables naturally leads to a strong asymmetry with respect to the transition temperature in the behavior of the physical properties that cannot be described by simple power laws. While the mean field theory gives a good description of the experimental data, the scaling theory, if one incorporates mixing of the field variables, gives even better agreement with the experimental data, placing this transition in the same universality class as the three-dimensional Ising model

Optical and Magnetic Manipulation of Hybrid Micro and Nanoparticle Sensors.

Microparticles and nanoparticles have been used in a wide variety of applications ranging from biomedical to optical and electronic technologies. The microscopic and mesoscopic size scale of single particles makes them ideal tools for probing the local environments of biological cells, sensing the viscous properties of fluids and surfaces on the microscale, and interacting with photonic and magnetic fields. But the effectiveness of these particle systems is limited by the ability to manipulate and control them in predictable ways. In this work, two methods of microparticle and nanoparticle manipulation are investigated, namely optical tweezers (OT) and magnetic rotation. OT provide a mechanically non-invasive means of grasping microparticles and nanoparticles, utilizing focused laser light. Moreover, particles driven by magnetic rotation in viscous media exhibit nonlinear dynamical motion and are a subclass of systems known as nonuniform oscillators. Both the individual and combined synergistic use of these control schemes is studied, in particular, on hybrid particles systems comprised of several materials, including both dielectric microspheres and metallic or magnetic colloids. Classical electromagnetic theory was developed to describe the wavelength dependence of OT forces acting on a trapped, resonantly absorptive particle. Enhancements in the trapping strength could be obtained via near-resonance tuning of the laser wavelength. Experimental observation of this phenomenon on our hybrid particles was inhibited by increased destabilizing forces at the micron scale and the emergence of heating effects at high laser intensities often used in OT. Using reduced laser intensities in conjunction with magnetic rotation, hybrid particles could be two-dimensionally trapped and rolled at a substrate surface. Changes in the nonlinear dynamical motion of the particles were measured to distinguish particle roughness and surface friction. The response of rigid dimers of hybrid particles to optical and magnetic manipulation was studied. Observed changes in the dynamical motion with increased optical perturbation strength, using both numerical modeling and experiment, were investigated in terms of scattering forces, magnetization and heat generation from absorptive interactions. Finally, the escape into the third-dimension of a magnetic dimer of hybrid particles undergoing nonuniform rotation was studied experimentally and compared to both theory and numerical simulation.Ph.D.Applied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/58498/1/ragayan_1.pd

Slipping friction of an optically and magnetically manipulated microsphere rolling at a glass-water interface

The motion of submerged magnetic microspheres rolling at a glass-water interface has been studied using magnetic rotation and optical tweezers combined with bright-field microscopy particle tracking techniques. Individual microspheres of varying surface roughness were magnetically rotated both in and out of an optical trap to induce rolling, along either plain glass cover slides or glass cover slides functionalized with polyethylene glycol. It has been observed that the manipulated microspheres exhibited nonlinear dynamic rolling-while-slipping motion characterized by two motional regimes: At low rotational frequencies, the speed of microspheres free-rolling along the surface increased proportionately with magnetic rotation rate; however, a further increase in the rotation frequency beyond a certain threshold revealed a sharp transition to a motion in which the microspheres slipped with respect to the external magnetic field resulting in decreased rolling speeds. The effects of surface-microsphere interactions on the position of this threshold frequency are posed and investigated. Similar experiments with microspheres rolling while slipping in an optical trap showed congruent results.Comment: submitted to Journal of Applied Physics, 11 figure

Novel phase-transition behavior in an aqueous electrolyte solution

We have investigated the near-critical behavior of the susceptibility of a ternary liquid mixture of 3-methylpyridine, water, and sodium bromide as a function of the salt concentration. The susceptibility was determined from light-scattering measurements performed at a scattering angle of 90° in the one-phase region near the locus of lower consolute points. A sharp crossover from asymptotic Ising behavior to mean-field behavior has been observed at concentrations ranging from 8 to 16.5 mass% NaBr. The range of asymptotic Ising behavior shrinks with increasing salt concentration and vanishes at a NaBr concentration of about 17 mass%, where complete mean-field-like behavior of the susceptibility is observed. A simultaneous pronounced increase in the background scattering at concentrations above 15 mass%, as well as a dip in the critical locus at 17 mass% NaBr, suggests that this phenomenon can be interpreted as mean-field tricritical behavior associated with the formation of a microheterogeneous phase due to clustering of the molecules and ions. An analogy with tricritical behavior observed in polymer solutions as well as the possibility of a charge-density-wave phase is also discussed. In addition, we, have observed a third soap-like phase on the liquid-liquid interface in several binary and ternary liquid mixtures

Shape of crossover between mean-field and asymptotic critical behavior in a three-dimensional Ising lattice

Recent numerical studies of the susceptibility of the three-dimensional Ising model with various interaction ranges have been analyzed with a crossover model based on renormalization-group matching theory. It is shown that the model yields an accurate description of the crossover function for the susceptibility.Comment: 4 pages RevTeX + 3 PostScript figures. Uses epsf.sty and rotate.sty. Final version; accepted for publication in Physics Letters

Application of artificial intelligence for Euler solutions clustering

International audienceResults of Euler deconvolution strongly depend on the selection of viable solutions. Synthetic calculations using multiple causative sources show that Euler solutions cluster in the vicinity of causative bodies even when they do not group densely about the perimeter of the bodies. We have developed a clustering technique to serve as a tool for selecting appropriate solutions. The clustering technique uses a methodology based on artificial intelligence, and it was originally designed to classify large data sets. It is based on a geometrical approach to study object concentration in a finite metric space of any dimension. The method uses a formal definition of cluster and includes free parameters that search for clusters of given properties. Tests on synthetic and real data showed that the clustering technique successfully outlines causative bodies more accurately than other methods used to discriminate Euler solutions. In complex field cases, such as the magnetic field in the Gulf of Saint Malo region (Brittany, France), the method provides dense clusters, which more clearly outline possible causative sources. In particular, it allows one to trace offshore the main inland tectonic structures and to study their interrelationships in the Gulf of Saint Malo. The clusters provide solutions associated with particular bodies, or parts of bodies, allowing the analysis of different clusters of Euler solutions separately. This may allow computation of average parameters for individual causative bodies. Those measurements of the anomalous field that yield clusters also form dense clusters themselves. Application of this clustering technique thus outlines areas where the influence of different causative sources is more prominent. This allows one to focus on these areas for more detailed study, using different window sizes, structural indices, etc

Application of Minimal Subtraction Renormalization to Crossover Behavior near the 3^3He Liquid-Vapor Critical Point

Parametric expressions are used to calculate the isothermal susceptibility, specific heat, order parameter, and correlation length along the critical isochore and coexistence curve from the asymptotic region to crossover region. These expressions are based on the minimal-subtraction renormalization scheme within the $\phi^4$ model. Using two adjustable parameters in these expressions, we fit the theory globally to recently obtained experimental measurements of isothermal susceptibility and specific heat along the critical isochore and coexistence curve, and early measurements of coexistence curve and light scattering intensity along the critical isochore of $^3$He near its liquid-vapor critical point. The theory provides good agreement with these experimental measurements within the reduced temperature range $|t| \le 2\times 10^{-2}$

Scaling and Crossover to Tricriticality in Polymer Solutions

We propose a scaling description of phase separation of polymer solutions. The scaling incorporates three universal limiting regimes: the Ising limit asymptotically close to the critical point of phase separation, the "ideal-gas" limit for the pure-solvent phase, and the tricritical limit for the polymer-rich phase asymptotically close to the theta point. We have also developed a phenomenological crossover theory based on the near-tricritical-point Landau expansion renormalized by fluctuations. This theory validates the proposed scaled representation of experimental data and crossover to tricriticality.Comment: 4 pages, 3 figure