Liquid-like State Research Articles

Thermodynamics of a Liquid-like Spin State in Molecule-based Magnets with Geometric Frustrations

Abstract We review novel thermodynamic properties of κ-(BEDT-TTF)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2, where BEDT-TTF stands for bisethylenedithiotetrathiafulvalene and dmit stands for 1,3-dithiole-2-thione-4,5-dithiolate unveiled via single-crystal calorimetry. These compounds are organic dimer-based Mott insulators with a two-dimensional triangular lattice, where electron correlations produce localized radical spins on each molecular dimer. Néel-type magnetic orderings are prohibited by geometric frustrations and strong quantum mechanical fluctuations. The spin orientation of localized π electron fluctuates like a liquid even at zero energy, and the so-called spin-liquid (SL) ground state appears in them. We have performed heat capacity measurements down to extremely low temperatures and observed that the low-temperature heat capacities show a gapless character, evidenced by the finite electronic heat capacity coefficient, γ. Although the charge-transport properties of these compounds are insulating, the thermodynamic parameters, which reflect the low-energy excitations from the liquid ground state, resemble those of typical metallic compounds with Fermi-liquid characteristics. The magnitude of the γ was scaled with the magnetic susceptibility extrapolated down to T = 0. The realization of the unusual magnetic state coupled with the charge degree of freedom is suggested by several other experiments in addition to the heat capacity measurements. Herein we review thermodynamic discussions based on the experiments reported up to now to clarify the novelty of this magnetic ground state.

Positron annihilation studies of 4-n-butyl-4′-isothiocyanato-1,1′-biphenyl

Positron annihilation lifetime spectroscopy (PALS) measurements were performed between 93 and 293 K in order to study the supercooled smectic-E (Sm-E) phase of 4-n-butyl-4'-isothiocyanato-1,1'-biphenyl (4TCB), the ordered molecular crystal of 4TCB, and the phase transition between the Sm-E phase and the ordered molecular crystal of 4TCB. The phase transition was well reflected in the abrupt increase of the ortho-positronium (o-Ps) lifetime and intensity. The value of the o-Ps lifetime in the Sm-E liquid crystalline phase of 4TCB, i.e., 2.21 ns at room temperature, was explained by the formation of bubbles induced by Ps atoms, which are created due to a liquidlike state of the butyl chains of 4TCB molecules in the Sm-E phase. The temperature dependence of the o-Ps intensity for the supercooled Sm-E phase can be explained by thermal generation of sites where bubbles are formed; an activation energy equal to 0.30±0.02 eV was estimated. This value was compared with the activation energies of molecular motions. The o-Ps lifetime in the ordered molecular crystal was interpreted as originating from the annihilation of o-Ps confined in molecular vacancy-type imperfections in the crystal lattice. The value of the o-Ps pickoff annihilation between 1.8 and 1.9 ns is in accordance with the size of the molecular vacancy for the 4TCB crystal lattice. Its intensity is lower than 5%. The isothermal crystallization of the 4TCB Sm-E phase was observed by PALS. The low-dimensional crystal growth was concluded from the Avrami equation fitted to the time dependence of the o-Ps intensity, which resulted in an Avrami exponent equal to 1.73.

Homogeneous connectivity of potential energy network in a solidlike state of water cluster

A novel route to the exponential trapping-time distribution within a solidlike state in water clusters is described. We propose a simple homogeneous network (SHN) model to investigate dynamics on the potential energy networks of water clusters. In this model, it is shown that the trapping-time distribution in a solidlike state follows the exponential distribution, whereas the trapping-time distribution in local potential minima within the solidlike state is not exponential. To confirm the exponential trapping-time distribution in a solidlike state, we investigate water clusters, (H2O)6 and (H2O)12, by molecular dynamics simulations. These clusters change dynamically from solidlike to liquidlike state and vice versa. We find that the probability density functions of trapping times in a solidlike state are described by the exponential distribution whereas those of interevent times of large fluctuations in potential energy within the solidlike state follow the Weibull distributions. The results provide a clear evidence that transition dynamics between solidlike and liquidlike states in water clusters are well described by the SHN model, suggesting that the exponential trapping-time distribution within a solidlike state originates from the homogeneous connectivity in the potential energy network.

Linear and nonlinear viscoelasticity of water-in-oil emulsions: Effect of droplet elasticity

The effect of droplet elasticity on the linear and nonlinear viscoelasticity of water-in-oil emulsions was studied by comparing two kinds of emulsions: one with pure water as droplets, the other with a suspension of hectorite clay as droplets. The clay-filled emulsion had high droplet elasticity due to the gelation of the clay suspension. As the water-to-oil volume ratio ϕ increased, the emulsions changed from liquid-like state to solid-like state as the droplets formed a jammed state. Effect of droplet elasticity was most significant in the jammed state, where the clay-filled emulsion showed a much higher plateau modulus than the clay-free one. Nonlinear viscoelasticity was observed by the large amplitude oscillatory shear (LAOS) rheology. The Lissajous curve was dissected into the cage-yielding and cage-restructuring parts, and the cage modulus Gk in the nonlinear viscoelastic regime was higher for clay-filled emulsion than the clay-free one in the jammed state due to the contribution of the droplet elasticity. The droplet elasticity was found to contribute to the nonlinear viscoelasticity of the emulsions.

Stress-versus temperature-induced structural evolution in metallic glasses

Structure evolution induced by shear deformation was investigated via molecular dynamic simulation on CuZr metallic glass system and compared with that induced by temperature. Voronoi tessellation analysis found that the local structures evolve to a liquid-like state as shear stress increases, similar to the temperature-induced structure evolution. However, shear stress induces little change to the radial distribution functions (RDFs) compared to temperature, indicating that the global glassy state still sustains. This is in contrast to the opinion that shear deformation may be similar to temperature increase and induce a metallic glass system to experience glass-liquid transition. The analysis of anisotropic part of RDFs reveals that shear deformation can induce significant anisotropic structural evolution, while pure increase of temperature cannot. Our findings demonstrate that shear deformation may induce different structural evolution in metallic glasses compared to temperature.

Phase transition in an out-of-equilibrium monolayer of dipolar vibrated grains

We report an experimental study on the transition between a disordered liquidlike state and an ordered solidlike one, in a collection of magnetically interacting macroscopic grains. A monolayer of magnetized particles is vibrated vertically at a moderate density. At high excitation a disordered, liquidlike state is observed. When the driving dimensionless acceleration Γ is quasistatically reduced, clusters of ordered grains grow below a critical value, Γ(c). These clusters have a well-defined hexagonal and compact structure. If the driving is subsequently increased, these clusters remain stable up to a higher critical value, Γ(l). Thus, the solid-liquid transition exhibits a hysteresis cycle. However, the lower onset Γ(c) is not well defined as it depends strongly on the acceleration ramp speed and also on the magnetic interaction strength. Metastability is observed when the driving is rapidly quenched from high acceleration, Γ>Γ(l), to a low final excitation, Γ(q). After this quench, solid clusters nucleate after a time lag, τ(o), either immediately (τ(o)=0) or after some time lag (τ(o)>0) that can vary from seconds up to several hundreds of seconds. The immediate growth occurs below a particular acceleration value, Γ(s) (~/<Γ(c)). In all cases, for t≥τ(o) a solid cluster's temporal growth can be phenomenologically described by a stretched exponential law. The evolution of the parameters of this law as a function of Γ(q) is presented and the values of fitted parameters are discussed.

Micro- and nanophase separations in hierarchical self-assembly of strongly amphiphilic block copolymer-based ionic supramolecules

By a selective complexation between different alkyltrimethylammonium amphiphiles (C8, C12 and C16) and three different diblock copolymer systems of poly(styrene)-b-poly(methacrylic acid) at various grafting densities X (X = number of alkyl chains per acidic group of the poly(methacrylic acid) PMAA block), a class of ionic supramolecules are successfully synthesized whose molecular architecture consists of a poly(styrene) PS block (Linear block) covalently connected to a strongly amphiphilic comb-like block (AmphComb block), i.e. Linear-b-AmphComb. In the melt state, these ionic supramolecules can show simultaneous microphase (between Linear and AmphComb blocks) and nanophase (within the AmphComb blocks) separations. This leads to the formation of various structure-in-structure two-scale hierarchical self-assemblies, including S-in-SLL, S-in-SBCC, S-in-C, S-in-L and C-in-L, where S, SLL, SBCC, C and L stand for spherical, spherical in liquid-like state, spherical in body-centered-cubic arrangement, cylindrical and lamellar, respectively. Synchrotron small angle X-ray scattering (SAXS) and crossed polarizers, together with SAXS modelling analysis, were used for a detailed structural study of the samples. The morphology of the microphase separated state was mapped out on ‘master’ microdomain morphology diagrams as a function of the volume fraction of the AmphComb blocks (ΦAmphComb) and the compositional window of each microdomain morphology was determined. It was observed that, (i) within samples based on the same parent diblock copolymer system, the occurrence of a specific microdomain morphology is only dependent on the value of ΦAmphComb, regardless of the grafting density X and the length of the alkyl side-chains, and (ii) microdomain morphological boundaries occur at ΦAmphComb values of ∼0.20 (SLL/C and SBCC/C) and ∼0.28 (C/L). Finally, the specific influences of the strongly amphiphilic nature of the AmphComb blocks on the observed morphological and hierarchical behaviours of our system are discussed. For reference, stoichiometric strongly amphiphilic comb-like (AmphComb) ionic supramolecules, based on complexation between a homopolymer of PMAA and the various alkyltrimethylammonium amphiphiles, were prepared, which nanophase separated into S (C8) or C (C12 and C16) domains.

Flow-driven formation of solid-like microsphere heaps

We observe the formation of heaps of repulsive microspheres, created by flowing a colloidal microsphere suspension towards a flat-topped ridge placed within a quasi two-dimensional microfluidic channel. This configuration allows for both shear and normal forces on the microspheres in contact with the ridge. The heaps, which are formed against the ridge, are characterized by two distinct phases: a solid-like bulk phase in the interior and a highly fluctuating, liquid-like state which exists along its leading edge. We observe that heaps only form above a critical flow velocity, vc, and that they are destroyed by thermal rearrangements when the flow ceases. We monitor the dynamics of heap formation using fluorescence video microscopy, measuring the heap volume and the angle of repose in response to microsphere deposition and erosion processes. We find that the steady state angle of repose, θf, increases as a function of inflow velocity, v∞, with a functional form .

Frustrated crystallisation and melting in two-dimensional pentagonal confinement

The crystallisation and melting behaviour of superparamagnetic colloidal particles confined within a two-dimensional pentagonal environment is studied by systematically varying either the external magnetic field or the number of particles. A system containing 16 confined particles melts monotonically from a commensurate crystalline configuration to a confined liquid-like state as the magnetic field is reduced. When the magnetic field is kept constant and the number of confined particles is increased from 11 to 21, highly non-monotonic behaviour is observed involving re-entrant structural ordering and dynamically frustrated states.

DNA Hydrogel Has Good Memory

A new DNA-based hydrogel collapses into a liquidlike puddle when taken out of water and regains its original shape when plunged back in, according to a report in Nature Nanotechnology (DOI: 10.1038/nnano.2012.211). Although the researchers who discovered the unique material aren’t yet sure why it’s capable of such a feat, they think it could eventually be used for drug delivery and in electrical switches and robotics. The research team, led by Dan Luo of Cornell University, has a lot of experience with DNA hydrogels. Previously, the scientists fabricated one type of hydro­gel using branched DNA and an enzyme called a T4 ligase that cross-linked the DNA together chemically (Nat. Mater., DOI: 10.1038/nmat1741). Although those gels could be molded into a variety of shapes, they couldn’t convert between solidlike and liquidlike states as the new gels do. To make the shape-remembering material, Luo, grad students Jong Bum Lee and Songming Peng, ...

Optical spectroscopy shows that the normal state of URu 2 Si 2 is an anomalous Fermi liquid

Fermi showed that, as a result of their quantum nature, electrons form a gas of particles whose temperature and density follow the so-called Fermi distribution. As shown by Landau, in a metal the electrons continue to act like free quantum mechanical particles with enhanced masses, despite their strong Coulomb interaction with each other and the positive background ions. This state of matter, the Landau-Fermi liquid, is recognized experimentally by an electrical resistivity that is proportional to the square of the absolute temperature plus a term proportional to the square of the frequency of the applied field. Calculations show that, if electron-electron scattering dominates the resistivity in a Landau-Fermi liquid, the ratio of the two terms, b, has the universal value of b = 4. We find that in the normal state of the heavy Fermion metal URu(2)Si(2), instead of the Fermi liquid value of 4, the coefficient b = 1 ± 0.1. This unexpected result implies that the electrons in this material are experiencing a unique scattering process. This scattering is intrinsic and we suggest that the uranium f electrons do not hybridize to form a coherent Fermi liquid but instead act like a dense array of elastic impurities, interacting incoherently with the charge carriers. This behavior is not restricted to URu(2)Si(2). Fermi liquid-like states with b ≠ 4 have been observed in a number of disparate systems, but the significance of this result has not been recognized.

Melting Transition of Vortex Lattice in Point Vortex Systems

Point vortices take a triangular lattice structure in a rotating system as a minimum energy state. We perform a numerical simulation of point vortex systems using initial conditions indicating that the triangular lattice is randomly perturbed. The total energy increases with the magnitude of the perturbation. When the energy is increased, the vortex lattice becomes irregular and a layered structure appears. When the energy is further increased, the layered structure disappears and a liquidlike state appears. We interpret the melting transition with a mean-field approximation for layered structures.

Electronic effects on the melting of small gallium clusters

Motivated by experimental reports of higher-than-bulk melting temperatures in small gallium clusters, we perform first-principles molecular dynamics simulations of Ga(20) and Ga(20)(+) using parallel tempering in the microcanonical ensemble. The respective specific heat (C(V)) curves, obtained using the multiple histogram method, exhibit a broad peak centered at approximately 740 and 610 K--well above the melting temperature of bulk gallium (303 K) and in reasonable agreement with experimental data for Ga(20)(+). Assessment of atomic mobility confirms the transition from solid-like to liquid-like states near the C(V) peak temperature. Parallel tempering molecular dynamics simulations yield low-energy isomers that are ~0.1 eV lower in energy than previously reported ground state structures, indicative of an energy landscape with multiple, competing low-energy morphologies. Electronic structure analysis shows no evidence of covalent bonding, yet both the neutral and charged clusters exhibit greater-than-bulk melting temperatures.

Polymeric Material with Metal-Like Conductivity for Next Generation Organic Electronic Devices

The reduced pressure synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) with sheet-like morphology has been achieved with the introduction of an amphiphilic triblock copolymer into the oxidant thin film. Addition of the copolymer not only results in an oxidant thin film which remains liquid-like under reduced pressure but also induces structured growth during film formation. PEDOT films were polymerized using the vacuum vapor phase polymerization (VPP) technique, in which we show that maintaining a liquid-like state for the oxidant is essential. The resulting conductivity is equivalent to commercially available indium tin oxide (ITO) with concomitant optical transmission values. PEDOT films can be produced with a variety of thicknesses across a range of substrate materials from plastics to metals to ceramics, with sheet resistances down to 45 Ω/□ (ca. 3400 S·cm–1), and transparency in the visible spectrum of >80% at 65 nm thickness. This compares favorably to ITO and its currently touted replacements.

Feasibility of using ash-free coal in a solid-oxide-electrolyte direct carbon fuel cell

The feasibility of using ash-free coal as the fuel in a direct carbon fuel cell (DCFC) was investigated. Tubular-electrolyte-supported single cells were operated at high temperatures with continuous fuel feeding equipment. A single cell consisted of an yttria-stabilized zirconia (YSZ) electrolyte tube support, a NiO-YSZ cermet anode, and an LSM-YSZ composite cathode. The solvent extraction method was used to remove mineral matter and ash from raw coal. From thermal gravimetric analysis, the ash content of the ash-free coal was found to be nearly zero, but its volatile matter content was 38.34 wt.%. Scanning electron microscopy (SEM) observations and visualization tests were carried out for characterization of the interface of the ash-free coal; the results showed that the ash-free coal was in a liquid-like phase at high temperatures because the volatile matter in the coal produced by solvent extraction could help maintain carbon in the liquid-like state on the anode at high temperatures. The cell performance was affected by this liquid-like state at high temperatures.

Thermodynamic Theory of Two Rough Surfaces Friction in the Boundary Lubrication Mode

The thermodynamic theory of thin lubricant film melting, confined between two hard rough solid surfaces, is built using the Landau phase transition approach. For the description of a melting condition, the order parameter is introduced which is a periodical part of microscopic medium density function. The shear modulus of lubricant is proportional to the order parameter squared. The thermodynamic and shear melting are described consistently. A mechanical analogue of a tribological system in the boundary friction mode is studied. The time dependencies of the friction force, the relative velocity of the interacting surfaces, and the elastic component of the shear stresses appearing in the lubricant are obtained. It is shown that the shear modulus of the lubricant and the elastic stresses become zero in the liquid-like state. The irregular stick-slip mode of melting is described, which is observed in experiments. It is shown that the frequency of stiction spikes in the irregular mode increases with growth of the shear velocity. Comparison of the obtained results with experimental data is carried out.

Fluidized States of Vibrated Granular Media Studied by Mechanical Spectroscopy

We investigate the jamming transition observed in vibrated granular systems composed of millimeter size glass beads. When a granular system is submitted to vibrations with decreasing intensity, it evolves in a way similar to glass-forming liquids: from a low viscosity, liquid-like state, it evolves into an amorphous jammed state. This evolution is observed by the means of an immersed oscillator acting as a torsion pendulum in forced mode. The complex susceptibility of the oscillator is measured as a function of the probe forcing frequency and of the vibration intensity. Focusing on the strongly vibrated states, we observe that there are two different dynamic regions. The first is a high fluidization regime, where the internal friction is found to be proportional to the ratio between the pulsation and the vibration intensity: . In this region, the system shows an apparent viscous friction . In the second, low fluidization, regime, we observe a more complex behavior, and the measured internal friction appears to be well described by a relation of the form: . In this second case, the key role is played by a critical breakaway stress, σcr, needed to break the network of chains of forces that form between the grains. Finally, if vibration intensities are still reduced, we also observe that onset of jamming is clearly distinguishable: we observe a sharp increase in the apparent dynamic modulus together with a peak in internal friction. This transition presents important similarities to those observed in glasses, and it leads to the second (low vibrations) regime, where the key role is played by the square root of the vibration intensity.

Shear-induced gelation of soft strawberry-like particles in the presence of polymeric P(BA-b-AA) surfactants

The effect of polymeric surfactants, copolymers of n-butyl acrylate (BA) and acrylic acid (AA), on shear-induced gelation of a colloidal system in the absence of additional electrolytes is studied. One random (PBA-co-PAA) and two block copolymer (PBA-b-PAA, with different PAA lengths) surfactants have been synthesized by ATRP and used in this work. The colloidal system is composed of strawberry-like particles with a rubbery core, partially covered by a few grafted plastic patches. In the absence of any surfactant, as the colloidal system passes through a microchannel at a Peclet number of 220 and a particle volume fraction of 0.15, shear-induced gelation occurs and the particles coalesce partially, due to the rubbery core, leading to a fractal dimension of the clusters constituting the gel equal to 2.78. On the other hand, in the presence of any of the three polymeric surfactants, shear-induced gelation occurs only in the range of low surfactant surface density. Meanwhile, the fractal dimension of the clusters decreases with adsorption of the two block surfactants, reaching a plateau value of about 2.58, while for the random surfactant it remains constant and equal to 2.78, like in the absence of any surfactant. This indicates that adsorption of the block surfactants can reduce the particle coalescence, while adsorption of the random surfactant cannot. Moreover, for all three surfactants, as their surface density increases progressively, a transition from solid-like gel to a liquid-like state occurs and finally no shear-induced gelation or even aggregation occurs. Since the three surfactants comprise carboxylic groups, considering also the results in the literature (Zaccone et al., J. Phys. Chem. B, 2008, 112, 1976; 6793), we can reach a general conclusion that carboxylic groups on the particle surface not only stabilize the particles through electrostatic repulsion, but also generate very short-range, strongly repulsive (e.g. hydration, steric) forces, which when high enough protect the particles from intense shear-activated aggregation.

Symmetric C–C Stretching Mode Splitting versus CH2-Chain Conformation Order in Sodium Montmorillonite Modified by Cetyltrimethylammonium Bromide

Exploiting Raman spectroscopy and computational modeling, for the first time, we report and explain an interesting phenomenon in clay modified by cetyltrimethylammonium bromide. A splitting of the CH(2)-chain's symmetric C-C stretching Raman mode found at ~1128 cm(-1) in cetyltrimethylammonium bromide into two bands at 1128 and 1139 cm(-1) in clay modified by cetyltrimethylammonium bromide is observed. We demonstrate that this splitting appears if two types of trans-segments with nonequivalent lengths and terminal groups coexist in the CH(2)-chain of the alkylammonium ion embedded into the clay interlayer space. We report Raman experimental evidence for a CH(2)-chain bending within the clay galleries, resulting in the symmetric C-C stretching band splitting, as was also suggested by computational modeling. Noteworthy, we postulate that this unique behavior based on CH(2)-chain bending provides a general understanding of conformation reorganization and switching within long CH(2)-chain molecules confined within modified clay interlayer galleries. For all modifier concentrations, we show that the intercalated cetyltrimethylammonium ions exist in a liquid-like state, consisting mainly of trans conformations (~86%) of two types in approximately equal proportions. Moreover, we demonstrate that the integral Raman intensity ratio I(1295)(CH(2))/I(705)(clay) provides a rapid nondestructive quantification of the relative content of alkylammonium ions in modified clays. These results demonstrate that a simple direct monitoring of specific modifier-dependent interlayer conformational states is possible, which is of great importance for a tunable fabrication of modified clays-based nanocomposites with desired properties.

Marginal matters

Most soft materials, such as sand, can be in either a solid-like or a liquid-like state. New experiments probe the surprisingly rich nonlinear physics that can occur in between these two states. See Letter p.355