Self-diffusion in plastic crystalline perfluorocyclohexane studied by nuclear magnetic resonance

Polarized Mandelstam-Brillouin scattering data in isomeric pentanols (normal-pentanol and 2-methyl-2-butanol) are presented. Hypersonic velocity ν_s and absorption α/f ² are measured as a function of temperature in the liquid phase. The experimental results indicate, for both the alcohols, a shear relaxation phenomenon in the GHz region. The shear relaxation times τ_s and the shear moduli G _∞, evaluated in the framework of visco-elastic theories, show a temperature dependence characteristic of strongly associated liquids in n-pentanol, whereas a more molecular behaviour is evidenced for 2-methyl-2-butanol.     

Shear elasticity of fluids at low-frequent shear influence

The visco-elastic properties of liquids have been investigated using acoustical resonance method. Piezoquatrz performed tangential oscillations on the main resonance frequency of 74 kHz contacts by the one end of horizontal surface with the studied liquid layer covered by quartz cover-plate. So the stagnant shear waves are installed in layer. The solution of interaction of piezoquartz-liquid layer-cover-plate gives three methods of determination of the real shear modulus (G) and the tangent of mechanical loss angle (tan theta) of liquid. The first method is realized at smaller thickness of liquid layer then the length of shear wave. Liquids of different classes have been studied using this method: polymer liquids, oils, glycols and alcohols. The second method is connected with the propagation of shear wave in liquid layer, parameters of which are determined the G and tan theta. And the third method is based on the determination of limit shift of resonance frequencies at completes damping of shear wave in thick layer of liquid. All these three methods give satisfactory agreement of results.     

Universal crossover of liquid dynamics in supercritical region

We demonstrate that all liquids in supercritical region may exist in two qualitatively different states: solid-like and gas-like. Solid-like to gas-like crossover corresponds to the condition τ ≈ τ₀, where τ is liquid relaxation time and τ₀ is the minimum period of transverse waves. This condition corresponds to the loss of shear stiffness of a liquid at all frequencies and defines a new narrow crossover zone on the phase diagram. We show that the intersection of this zone corresponds to the disappearance of high-frequency sound, qualitative changes of diffusion and viscous flow, increase in particle thermal speed to half of the speed of sound and reduction of the specific heat at constant volume to 2k _B per particle. The new crossover is universal: it separates two liquid states at arbitrarily high pressure and temperature, and even exists in systems where liquid-gas transition and the critical point are absent overall.     

Applications of Acoustic Wave Devices for Sensing in Liquid Environments

Abstract

Acoustic wave devices such as thickness shear mode (TSM) resonators and shear horizontal surface acoustic wave (SH‐SAW) devices can be utilized for characterizing physical properties of liquids and for chemical sensor applications. Basic device configurations are reviewed and the relationships between experimental observables (frequency shifts and attenuation) and physical properties of liquids are presented. Examples of physical property (density and viscosity) determination and also of chemical sensing are presented for a variety of liquid phase applications. Applications of TSMs and polymer‐coated guided SH‐SAWs for chemical sensing and uncoated SH‐SAWs for “electronic tongue” applications are also discussed.     

Impedance method for measuring shear elasticity of liquids

Experimental results of studying low-frequency (74 kHz) shear elasticity of polymer liquids by the impedance method (analogous to the Mason method) are presented. A free-volume thick liquid layer is placed on the horizontal surface of a piezoelectric quartz crystal with dimensions 34.7 × 12 × 5.5 cm. The latter performs tangential vibrations at resonance frequency. The liquid layer experiences shear strain, and shear waves should propagate in it. From the theory of the method, it follows that, with an increase in the layer thickness, both real and imaginary resonance frequency shifts should exhibit damped oscillations and tend to limiting values. For the liquids under study, the imaginary frequency shift far exceeds the real one, which testifies to the presence of bulk shear elasticity.     

On the polarization of chiral main-chain liquid-crystalline elastomers

A mechanism of developing a polarization in a chiral main-chain liquid-crystalline polymer by aligning the dipoles of the monomers is explored. It is shown that the polarization of a pure liquid crystal elastomer is zero in equilibrium due to rotation of the director in the elastomer. A constraint or specific non-ideality in the elastomer is required to prevent this relaxation of the director in order to realize a non-zero polarization. Three methods that circumvent this result are explored. We consider the effect of an oscillating shear, the pinning effect of the layers in a smectic-A composed of chiral smectogens and a binary mixture of chiral main chains and non-chiral side chains. Each of these methods is shown to produce a polarization, which is much larger than that produced in a piezoelectric -quartz crystal per unit stress.Received: 26 May 2004, Published online: 3 August 2004PACS: 61.30.Vx Polymer liquid crystals - 77.65.-j Piezoelectricity and electromechanical effects     

Charge instabilities in strongly correlated bilayer systems

We investigate the charge-instabilities of the Hubbard-Holstein model with two coupled layers. In this system the scattering processes naturally separate into contributions which are either symmetric or antisymmetric combinations with respect to exchange of the layers. It turns out that the short-range strong correlations suppress finite wave-vector nesting instabilities for both symmetries but favor the occurrence of phase separation in the symmetric channel. Inclusion of a sizeable long-range Coulomb (LRC) interaction frustrates the q=0 instabilities and supports the formation of incommensurate charge-density waves (CDW). Upon reducing doping from half-filling and for small electron-phonon coupling g the CDW instability first occurs in the antisymmetric channel but both instability lines merge with increasing g. While LRC forces always suppress the phase separation instability in the symmetric channel, the CDW period in the antisymmetric sector tends to infinity ( ) for sufficiently small Coulomb interaction. This feature allows for the possibility of singular scattering over the whole Fermi surface. We discuss possible implications of our results for the bilayer high-T _c cuprates.Received: 21 July 2003, Published online: 2 October 2003PACS: 71.27.+a Strongly correlated electron systems; heavy fermions - 74.72.-h Cuprate superconductors (high-T _c and insulating parent compounds) - 74.25.Kc Phonons     

Accumulation of particles in time-dependent thermocapillary flow in a liquid bridge. Modeling of experiments

The study addresses the phenomenon of accumulation of rigid tracer particles suspended in a time-dependent thermocapillary flow in a liquid bridge. We report the results of the three-dimensional numerical modeling of recent experiments [1,2] in a non-isothermal liquid column. Exact physical properties of both liquids and particles are used for the modeling. Two liquids are investigated: sodium nitrate (NaNO₃) and n-decane (C₁₀H₂₂). The particles are modeled as perfect spheres suspended in already well developed time-dependent thermocapillary flow. The particle dynamics is described by the Maxey-Riley equation. The results of our simulations are in excellent agreement with the experimental observations. For the first time we reproduced numerically formation of the particle accumulation structure (PAS) both under gravity and under weightlessness conditions. Our analysis confirms the experimental observations that the existence of PAS depends on the strength of the flow field, on the ratio between liquid and particle density, and on the particle size.     

Thin liquid layers in shear: non-Newtonian effects

When shear flow is generated in molecularly thin liquid films of simple liquids confined between two parallel plates, the effective viscosity of the liquid increases by many orders of magnitude compared to its bulk value. Non-Newtonian effects such as shear thinning with a universal power law exponent of are observed in experiments and computer simulations. We present a simple model of these phenomena based on shear melting of solid-like layers induced by the strong coupling with the crystalline walls.     

Equilibrium molecular dynamics simulation of shear viscosity of two-dimensional complex (dusty) plasmas

Shear viscosity is examined throughout the entire range of strongly coupled states of two-dimensional complex (dusty) plasma liquids (CDPLs). We have employed equilibrium molecular dynamics (EMD) simulation to compute the shear viscosity coefficients of CDPLs. In the strongly coupled liquid region, the values of valid viscosity coefficient can be estimated only in order of magnitude. The variations in the valid viscosity coefficients with screening strength (κ) and Coulomb coupling strengths (Γ) are observed. A systematic dependence of shear viscosity on κ is observed for an intermediate and higher Γ. The investigations showed that the position of the minimum viscosity coefficient shifts towards higher Γ as κ increases. The computational results for the entire range of liquid states of the strongly coupled dusty plasma obtained using the shear autocorrelation functions are in good agreement with the available simulation results and experimental data. It is shown that new simulations extended the range of plasma states (Γ, κ) used in our earlier simulation results for the existence of a finite minimum possible viscosity coefficient and it is also dependent on plasma states.     

Structures and autocorrelation functions of liquid Al and Mg modelled via Lennard-Jones potential from molecular dynamics simulation

The structures and autocorrelation functions of Al and Mg in the liquid state are investigated through the pair distribution functiong(r), the diffusion coefficients as well as the shear viscosity via the Green-Kubo and Einstein relations. From the structure and the Enskog relation we determined the frequency of collisions of atoms in the first shell ofg(r) in the systems. We also discovered that the packing fraction of Lennard-Jones liquids should be approximately half the reduced density value. This approximation is accurate to within 99%. The temperature dependence of the pair distribution function and the atomic mean square displacement are investigated by performing simulations at various experimental temperatures and corresponding densities. The structures of the systems are affected by temperature via movements of atoms in the first minimum ofg(r). The Lennard-Jones model shows that density dependence of the shear viscosity is in agreement with what is expected of simple liquids in the range of investigated temperatures and densities. In the gas limit, the Stoke-Einstein relationDη =K _BT /2πσ is grossly overestimated by Lennard-Jones model. This could not be attributed to deficiencies in the model, as other investigators using first principle method could not obtain the gas limit of the Stoke-Einstein relation.     

Shear effect on the phase behavior and morphology in pmma/san-29.5 blend

The effect of simple shear flow on the miscibility and morphology of blends of poly(methyl methacrylate) (PMMA) and a styrene-acrylonitrile random copoly-mer with 29.5 wt% acrylonitrile (SAN-29.5) has been investigated using shear apparatus and transmission electron microscopy (TEM). The obtained data showed that only shear-induced mixing was observed for all of the composition ratios. The increase of the cloud point (or homogenization temperature) ΔT(γdot; = T(γdot;) - T(0) was investigated as a function of shear rate γdot;; in addition, the normalized shift in the cloud point ΔT(γdot;)/T(0) versus γdot; was also studied and compared with that of simple liquid mixtures and polymer solutions. The results showed that the polymer blends were more sensitive to the shear rate than both simple-liquid mixtures and polymer solutions. The morphology of the PMMN SAN(= 75/25) blend (the critical composition) indicated that shear-induced phase mixing occurred at a critical shear rate value, below which the two phases were highly oriented and elongated in the flow direction. Three regimes, depending on the applied shear rate values, were detected that were in good agreement with the literature data for polymer solutions. The effect of relaxation times after shear cessation showed a decrease in the orientation of the elongated particles, but it did not completely vanish even for 10 min after the shear cessation.     

Scaling and instability analyses on flame spread over liquids

Stability and scaling analyses were applied to experimental data obtained by this group and other researchers on pulsating flame spread over liquids. Data to be analyzed include recent findings of cyclic appearance of a cold temperature valley at the liquid surface-created surface-wave ahead of the spreading flame, and main-pulsation of 0.5–2 Hz and sub-pulsation of 5–10 Hz. Our stability analysis is performed to understand the mechanism of instability on the liquid surface ahead of a flame’s leading edge, which is thought of as the major cause for pulsating flame spread. The scaling analysis is performed to explore the role of four independent (gravity, surface-tension, viscose, and inertia) forces on the mechanisms of flame spread. These four forces form three independent pi-numbers: Marangoni (Ma) number, Weber (We) number, and Froude (Fr) number, all of which include the critical length scale ratio: (height of sub-surface circulation)/(horizontal length of preheated liquid surface). We combined the wave equation obtained from the stability analysis, the three pi-numbers, and the critical length scale ratio, and used them as a universal formula to describe flame spread over liquids. Using this formula, flame spread mechanism over four different types of alcohols was divided into two separate regimes: the thin liquid pool and the thick-liquid pool. For the thin liquid pool, the flame spread rate was correlated with (Fr/Ma^0.5)^−1.0, while for the thick-liquid pool it was correlated with (Fr/Ma^0.5)^−1.5. Change of flame spread pattern from the uniform to the pulsating can be described with temperature difference between the flash point and bulk liquid temperature. For the thin liquid pool this temperature difference is correlated with Ma^−0.5, while for the thick-liquid pool it is correlated with Ma⁻¹. The frequency of pulsation is correlated with We^−1.0 for the thin liquid pool, while it is correlated with We^−1.5 for the thick-liquid pool.     

Deformable tetrahedratic phases: The effects of external fields and flows

We discuss changes in the symmetry and physical properties of an isotropic phase which has initially tetrahedral symmetry characterized by four unit vectors. In its undeformed state, these four vectors are at the tetrahedral angle ( ) to each other. We find that this optically isotropic phase becomes uniaxial under the influence of an external electric field, , resulting in a phase with C_3v symmetry. For an applied simple shear flow, the system becomes biaxial and a time-dependent state with C₁ symmetry arises. We discuss to what extent deformations induced by external forces and flows on this optically isotropic phase, which we call a "deformable tetrahedratic phase", are consistent with observations at the isotropic-B7 transition found recently in compounds composed of banana-shaped molecules and suggest a number of experiments to test the conclusions of this model.Received: 28 June 2002, Published online: 15 July 2003PACS: 61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order - 64.70.Md Transitions in liquid crystals - 05.70.Ln Nonequilibrium irreversible thermodynamics     

Study of dynamics of thin liquid layer breakdown under conditions of spot heating and formation of a droplet cluster

Breakdown dynamics was studied experimentally for the horizontal layers of various liquids (ethanol, water) with the thickness of 300 μm under the conditions of spot heating from the substrate. The main stages of the process of liquid layer breakdown were determined, and time of dry spot formation was measured. Time of dry spot formation for ethanol at the heat flux of 12.6 W/cm² was 7.85 s, and for water at the heat flux of 117 W/cm², it was 0.13 s. It was found that for both working liquids, a residual layer appears in the region of spot heating before liquid layer breakdown. It is shown that together with the thermocapillary effect, evaporation is one of the main factors affecting dynamics of liquid layer breakdown and dry spot formation.     

NMR-Based Liquid Explosives Detector

Liquid explosives pose a threat to security on airplanes and other public places, since they can easily be concealed as benign liquids. A detector, able to quickly identify liquids, would increase the possibility to detect such threats and speed up security checks. As a step towards a long-term goal to develop a liquid explosive detector, we have constructed an experimental setup based on a low-cost 1.1 T permanent magnet with huge static magnetic field gradient of 4.8 T/m, which allows us to measure proton relaxation times T ₁ and T ₂ and the self-diffusion coefficient D in liquid samples in a thin slice excited by radio-frequency pulses. We have developed a simple model in order to explain diffusion-enhanced non-exponential magnetization recovery in inversion recovery T ₁ experiment in this setup. Measuring a wide variety of liquid samples, we have demonstrated that it is possible to discriminate between the liquids based solely on these parameters. We discuss further improvements to the detection method, among those the choice of magnetic field, based on the fast field-cycling measurements.     

Enthalpy recovery of a glass-forming liquid constrained in a nanoporous matrix: Negative pressure effects

The T _g of organic liquids confined to nanoporous matrices and that of thin polymer films can decrease dramatically from the bulk value. One possible explanation for this phenomenon is the development of hydrostatic tension during vitrification under confinement that results in a concomitant increase in the free volume. Here we present experimental evidence and modeling results for ortho-terphenyl (o-TP) confined in pores as small as 11.6 nm that indicate that, although there is an important hydrostatic tension in the liquid in the pores, it does not develop until near the reduced T _g of the constrained material --well below the bulk T _g. Enthalpy recovery for the o-TP in the nanopores exhibits accelerated physical aging relative to the bulk, as well as a leveling off of the fictive temperature at equilibrium values greater than the aging temperature. An adaptation of the structural recovery model that incorporates vitrification under isochoric conditions is able to provide a quantitative explanation for the apparently anomalous aging observed in nanopore confined liquids and in thin polymeric films. The results strongly support the existence of an intrinsic size effect as the cause of the reduced T _g. Received 3 September 2001     

Impulse characteristic as a response of a liquid sensor based on shear-horizontal surface acoustic waves

The feasibility of using the impulse response of a liquid sensor based on shear-horizontal surface acoustic waves to identify liquids is justified. Identification of some liquid analytes with a single-channel sensor is experimentally demonstrated. The sensor is built around a delay line based on shear-horizontal surface acoustic waves that are excited in a 36°YX LiTaO₃-Al film-SiO₂ film-molecularly imprinted polymer film layered structure. By way of example, the concentration of the morpholine impurity (molecules of which are used as a template in synthesis of this molecularly imprinted polymer) in the liquids is measured. Impulse characteristics are derived by applying the fast inverse Fourier transformation to the amplitude-frequency responses of the sensor taken in a 60-MHz wide band at a center frequency of 105 MHz. The reproducibility of results obtained using such a technique and its application are discussed.     

Statistical mechanics of semiflexible polymers

We present the statistical-mechanical theory of semiflexible polymers based on the connection between the Kratky-Porod model and the quantum rigid rotator in an external homogeneous field, and treatment of the latter using the quantum mechanical propagator method. The expressions and relations existing for flexible polymers can be generalized to semiflexible ones, if one replaces the Fourier-Laplace transform of the end-to-end polymer distance, 1/(k ²/3 + p), through the matrix , where D and M are related to the spectrum of the quantum rigid rotator, and considers an appropriate matrix element of the expression under consideration. The present work provides also the framework to study polymers in external fields, and problems including the tangents of semiflexible polymers. We study the structure factor of the polymer, the transversal fluctuations of a free end of the polymer with fixed tangent of another end, and the localization of a semiflexible polymer onto an interface. We obtain the partition function of a semiflexible polymer in half space with Dirichlet boundary condition in terms of the end-to-end distribution function of the free semiflexible polymer, study the behaviour of a semiflexible polymer in the vicinity of a surface, and adsorption onto a surface.Received: 23 March 2004, Published online: 23 July 2004PACS: 36.20.-r Macromolecules and polymer molecules - 61.41. + e Polymers, elastomers, and plastics - 82.35.Gh Polymers on surfaces; adhesion     

Anchoring of Polymers by Traps Randomly Placed on a Line

We study the dynamics of a Rouse polymer chain which diffuses in a three-dimensional space under the constraint that one of its ends, referred to as the slip-link, may move only along a one-dimensional line containing randomly placed, immobile, perfect traps. Extensions of this model occur naturally in many fields, ranging from the spreading of polymer liquids on chemically active substrates to the binding of biomolecules by ligands. For our model we succeed in computing exactly the time evolution of the probability P _sl(t) that the chain slip-link will not encounter any of the traps until time t and, consequently, that until this time the chain will remain mobile.