Liquid Transition Research Articles

Evidence for multiple liquid–liquid phase transitions in carbon, and the Friedel ordering of its liquid state

Carbon, the fourth most abundant element in the universe, forms a metallic fluid with transient covalent bonds on melting. Its liquid–liquid phase transitions, intensely sought using simulations, had been elusive. Here, we use density functional theory (DFT) simulations with up to 108 atoms using molecular dynamics, as well as one-atom DFT as implemented in the neutral pseudo-atom method where multi-atom effects are treated by ion–ion correlation functionals. Both methods use electron–electron exchange correlation functionals for electron many-body effects. Here, we show using both methods that liquid carbon displays multiple liquid–liquid transitions linked to changes in coordination number in the density range 3–6 g/cm3 when a coordination number of 12 is reached. The transitions disappear by 4 eV in temperature. The calculated pressures and transition densities are shown to be sensitive to the exchange-correlation functionals used. Significantly, we find that a simple metallic model yields the structure factors and thermodynamics with quantitative accuracy, without invoking any covalent-bonding features. The ion–ion structure factor for these densities and temperatures is found to have a subpeak tied to twice the Fermi wavevector, constraining the fluid in momentum space. The dominant Friedel oscillations forming the pair interactions correlate the ions and drive the multiple liquid–liquid phase transitions. Our results suggest that liquid carbon typifies a class of fluids whose structure is ordered by the long-ranged Friedel oscillations in the pair-potentials. These results are critical to terrestrial and astrophysical studies, inertial fusion using carbon drivers, refined shock experiments, and in seeking new carbon-based materials.

Flexible Temperature Sensor Utilizing MWCNT Doped PEG-PU Copolymer Nanocomposites.

In this study, polyethylene glycol (PEG) and polyurethane (PU)-based shape-stabilized copolymer nanocomposites were synthesized and utilized for developing low-cost and flexible temperature sensors. PU was utilized as a flexible structural material for loading a thermosensitive phase change PEG polymer by means of physical mixing and chemical crosslinking. Furthermore, the introduction of multi-walled carbon nanotubes (MWCNT) as a conductive filler in the PEG-PU copolymer resulted in a nanocomposite with thermoresistive properties. MWCNT loading concentrations from 2 wt.% to 10 wt.% were investigated, to attain the optimum conductivity of the nanocomposite. Additionally, the effect of MWCNT loading concentration on the thermosensitive behavior of the nanocomposite was analyzed in the temperature range 25 °C to 50 °C. The thermosensitive properties of the physically mixed and crosslinked polymeric nanocomposites were compared by spin coating the respective nanocomposites on screen printed interdigitated (IDT) electrodes, to fabricate the temperature sensor. The chemically crosslinked MWCNT-PEG-PU polymeric nanocomposite showed an improved thermosensitive behavior in the range 25 °C to 50 °C, compared to the physically mixed nanocomposite. The detailed structural, morphological, thermal, and phase transition properties of the nanocomposites were investigated using XRD, FTIR, and DSC analysis. XRD and FTIR were used to analyze the crystallinity and PEG-PU bonding of the copolymer nanocomposite, respectively; while the dual phase (solid–liquid) transition of PEG was analyzed using DSC. The proposed nanocomposite-based flexible temperature sensor demonstrated excellent sensitivity, reliability and shows promise for a wide range of bio-robotic and healthcare applications.

The Classical-Quantum Passage: A van der Waals Description.

We undertake a van der Waals inquiry at very low temperatures so as to find signs of a classical–quantum frontier. We investigate the relation of such signs with the celebrated van der Waals gas–liquid transition. We specialize the discussion with respect to the noble gases. For such purpose, we use rather novel thermal statistical quantifiers such as the disequilibrium, the statistical complexity, and the thermal efficiency. Fruitful insights are thereby gained.

Direct observation of reversible liquid–liquid transition in a trehalose aqueous solution

Water forms two glassy waters, low-density and high-density amorphs, which undergo a reversible polyamorphic transition with the change in pressure. The two glassy waters transform into the different liquids, low-density liquid (LDL) and high-density liquid (HDL), at high temperatures. It is predicted that the two liquid waters also undergo a liquid-liquid transition (LLT). However, the reversible LLT, particularly the LDL-to-HDL transition, has not been observed directly due to rapid crystallization. Here, I prepared a glassy dilute trehalose aqueous solution (0.020 molar fraction) without segregation and measured the isothermal volume change at 0.01 to 1.00 GPa below 160 K. The polyamorphic transition and the glass-to-liquid transition for the high-density and low-density solutions were examined, and the liquid region where both LDL and HDL existed was determined. The results show that the reversible polyamorphic transition induced by the pressure change above 140 K is the LLT. That is, the transition from LDL to HDL is observed. Moreover, the pressure hysteresis of LLT suggests strongly that the LLT has a first-order nature. The direct observation of the reversible LLT in the trehalose aqueous solution has implications for understanding not only the liquid-liquid critical point hypothesis of pure water but also the relation between aqueous solution and water polyamorphism.

Liquid–liquid transition in elemental liquids investigated by sound velocity measurements: trends in the periodic table

Abstract The velocity of sound has been measured as a function of temperature to investigate structural changes in the molten As–Sb alloy. The overall profile of temperature dependence of the sound velocity changes smoothly from Sb to As. The results indicate that a tendency to change structures in the supercooled state develops with increasing As fraction, and confirm the previous conclusion that molten As changes rapidly its structure in the undercooled state and is already in the high-temperature side of an assumed liquid – liquid transition at the melting point. By confirming the structural transition in molten As, it is possible to mark the elements in the periodic table that undergo a crossover liquid –liquid transition around the melting point under the saturating vapor pressure. Observed trends for liquid – liquid transitions are discussed on the basis of the Peierls distortion mechanism.

Textural, electro-optical, dielectric and fluorescence studies of citrate buffer stabilized gold nanoparticles doped in polymer-dispersed liquid crystals composites

ABSTRACT This work demonstrates the effect of citrate buffer stabilised gold nanoparticles (AuNPs) on the morphological, optical and dielectric properties of polymer-dispersed liquid crystal (PDLC) composites prepared by polymerisation-induced phase separation method. The effect of voltage and temperature on liquid crystal (LC) droplet configurations, transmission, permittivity, conductivity and transition temperatures are studied. The relative transmission and contrast were improved by 22% and 54%, respectively, after the dispersion of AuNPs in PDLC composites as compared to undoped PDLC. The threshold voltage (Vth) of the doped composite was reduced by 13.34% as compared to the undoped composite. The increasing concentration of AuNPs has a minor effect on the (Vth) and transmission as well. Moreover, the low concentration of AuNPs also has better optical responses. The size of LC droplets was found in the range of 35–53 µm and almost independent of voltage and temperature. Beyond threshold voltage, maltese type LC droplet occurred. The photoluminescence spectra reveal that the intensity of AuNPs (1.5 µl) doped sample is higher and a blue shift is seen at 3.0 µl AuNPs-doped sample than others. With increasing AuNPs concentration, the energy bandgap also increases and is reported in the range of 3.4 eV at a higher concentration of AuNPs.

On the devitrification of Cu–Zr–Al alloys: Solving the apparent contradiction between polymorphic liquid-liquid transition and phase separation

In this work, the crystallization pathways and kinetics of the Cu47.5Zr45.1Al7.4 bulk metallic glass (BMG) are studied systematically using a broad collection of laboratory and synchrotron-based techniques, such as differential scanning calorimetry, transmission electron microscopy, in situ high-energy synchrotron X-ray diffraction (HEXRD), simultaneous small- and wide-angle X-ray scattering, and atom probe tomography. The crystallization sequence during heating at conventional slow rates and isothermal annealing near the glass transition temperature was determined to consist in preferential formation of (Cu, Al)10Zr7 followed by the formation of CuZr2 and AlCu2Zr induced by long-range atomic diffusion. In situ HEXRD shows that the formation of the latter two phases is accompanied by the partial dissolution of the primary Cu-rich phase. Microstructural characterizations of annealed samples suggest that a new liquid phase with enhanced local order and obscure compositional changes has formed from the homogeneous supercooled liquid before crystallization. A concept analogous to the disorder-to-order transition in crystalline materials is applied here to describe the observed transition in liquid, resolving the longstanding apparent contradiction between observing a nanoscale phase separation versus a liquid–liquid phase transition during devitrification. Furthermore, we suggest that the formation of an intermediate liquid phase prior to crystallization is the structural origin of the rapid devitrification kinetics during the primary crystallization. On the basis of our combined experimental results, a scheme is proposed to generalize the complex multistep devitrification mechanisms of the Cu47.5Zr45.1Al7.4 BMG during isothermal annealing or slow heating. This work may provide new insights into the crystallization behavior and thermal stability of Cu–Zr–Al BMGs in terms of thermodynamics and kinetics and might lead to deeper understanding of the underlying mechanisms of the often observed rapid crystallization processes of deeply undercooled metallic liquids.

High-precision nuclear magnetic resonance probe suitable for in situ studies of high-temperature metallic melts

High-temperature nuclear magnetic resonance (NMR) has proven to be very useful for detecting the temperature-induced structural evolution and dynamics in melts. However, the sensitivity and precision of high-temperature NMR probes are limited. Here we report a sensitive and stable high-temperature NMR probe based on laser-heating, suitable for in situ studies of metallic melts, which can work stably at the temperature of up to 2000 K. In our design, a well-designed optical path and the use of a water-cooled copper radio-frequency (RF) coil significantly optimize the signal-to-noise ratio (S/NR) at high temperatures. Additionally, a precise temperature controlling system with an error of less than ± 1 K has been designed. After temperature calibration, the temperature measurement error is controlled within ± 2 K. As a performance testing, 27Al NMR spectra are measured in Zr-based metallic glass-forming liquid in situ. Results show that the S/NR reaches 45 within 90 s even when the sample’s temperature is up to 1500 K and that the isothermal signal drift is better than 0.001 ppm per hour. This high-temperature NMR probe can be used to clarify some highly debated issues about metallic liquids, such as glass transition and liquid–liquid transition.

Folded network and structural transition in molten tin

The fundamental relationships between the structure and properties of liquids are far from being well understood. For instance, the structural origins of many liquid anomalies still remain unclear, but liquid-liquid transitions (LLT) are believed to hold a key. However, experimental demonstrations of LLTs have been rather challenging. Here, we report experimental and theoretical evidence of a second-order-like LLT in molten tin, one which favors a percolating covalent bond network at high temperatures. The observed structural transition originates from the fluctuating metallic/covalent behavior of atomic bonding, and consequently a new paradigm of liquid structure emerges. The liquid structure, described in the form of a folded network, bridges two well-established structural models for disordered systems, i.e., the random packing of hard-spheres and a continuous random network, offering a large structural midground for liquids and glasses. Our findings provide an unparalleled physical picture of the atomic arrangement for a plethora of liquids, shedding light on the thermodynamic and dynamic anomalies of liquids but also entailing far-reaching implications for studying liquid polyamorphism and dynamical transitions in liquids.

Disorder-driven phase transitions in bosonic fractional quantum Hall liquids

We investigate the disorder-driven phase transitions in bosonic fractional quantum Hall liquids at filling factors $f=1/2$ and $f=1$ in the lowest Landau level. We use the evolution of ground-state entanglement entropy, fidelity susceptibility, and Hall conductance with increasing disorder strength to identify the underlying phase transitions. The critical disorder strengths obtained from these different quantities are consistent with each other, validating the reliability of our numerical calculations based on exact diagonalization. At $f=1/2$, we observe a clear transition from the bosonic Laughlin state to a trivial insulating phase. At $f=1$, we identify a direct phase transition from the non-Abelian bosonic Moore-Read state to a trivial insulating phase, although some signs of a disorder-induced intermediate fractional quantum Hall phase were recently reported for the $f=5/2$ fermionic cousin.

Phase transition of quantum spin liquid in La-=SUB=-0.15-=/SUB=-Sm-=SUB=-0.85-=/SUB=-MnO-=SUB=-3+delta-=/SUB=- in weak magnetic field tochiral state with topological order during sample magnetization reversal

In this work, a detailed study of the nature of phase transitions in the Z2 quantum spin liquid in La La0.15Sm0.85MnO3+delta manganites induced at a temperature of 4.2 K by a weak external dc magnetic field in the ZFC and FC regimesof measuring the field dependences of the magnetization in the field range of ± 5 kOe is carried out. It is shown that during the magnetization reversal of the sample in the ZFC-regime near the external magnetic field H=0, a second-order phase transition Z2 quantum spin liquid occurs in the state of a chiral gap quantum spin liquid with a topological order. The transition is accompanied by an unusual hump-like increase in the number of magnetic excitations in the form of Majorana fermions and local gauge Z2 magnetic fluxes in the range of fields ± 500 Oe near zero field. A characteristic feature of the magnetization features is their strong dependence on the direction of growth of the external magnetic field, which is accompanied by the breakingof the mirror symmetry of the sample magnetization upon reversal of the sign of the external magnetic field, which is typical for the Z2 chiral state of a quantum spin liquid. In the FC-measurementregime, characteristic signs of excitation-destruction of 1D-fragments of gapless charge/spin density waves were found near H=0 during sample magnetization reversal due to confinement-deconfinement of spinon pairs in a system of spin chains. Keywords: chiral quantum spin liquid, Majorana fermions, confinement-deconfinement of spinon pairs, 1D-charge/spin density waves.

Quantization of the spectrum of spinon pairs in a magnetic field and formation of bosons in the form of fragments of 1D-waves of charge/spin density in SmMnO-=SUB=-3+delta-=/SUB=-

Quantum oscillations of the temperature dependences of magnetization of SmMnO3+delta in magnetic fields H=100 and 350 Oe, 1 and 3.5 kOe are studied within a temperature interval of 4.2-12 K. The continuum of thermal excitations of spinon pairs in magnetic field H=100 Oe is divided into three overlapping Landau bands with energies En (n=1,2,3) and fractional band filling factors ν. The symmetric intense "supermagnetization" doublet consisting of two overlapping peaks around mean excitation temperature Tspinon=~8 K produces the primary contribution to magnetization. As the field increases to H=1 kOe, the spinon excitation spectrum transforms into a broad sinusoid peak with its apex around Tspinon=~8 K, which is typical of the excitation continuum of spinon pairs with strong dispersion in the regime of their confinement. The thermal excitation spectrum of spinons changes significantly in magnetic field H=3.5 kOe: a new type of quantization of the spinon spectrum in the form of magnetization features shaped like narrow steps (plateaus), which correspond to integer filling with spinons of three Landau bands with a finite gap, emerges in Landau bands with n=1,2, and 3. The evolution of the boson spectrum in the form of thermal excitations of gapless quasi-one-dimensional waves of charge/spin density in the Luttinger liquid at temperatures below 60 K is examined. A continuous quantum phase transition of the quantum spin liquid into the Luttinger liquid state induced by an increase in the magnetic field intensity is found at temperatures around T=0. This transition has features typical of the formation of 1D-waves of charge/spin density induced by the confinement of spinon pairs and is accompanied by strong field hysteresis. Keywords: quantum spin liquid, Luttinger liquid, confinement of spinon pairs, spinon --- gauge field system, landau quantization, 1D-waves of charge/spin density, quantum oscillations.

Annealing and melting of active two-dimensional soliton lattices in chiral nematic films.

In this work, thousands of electrically driven dissipative solitons, called directrons, are generated in a chiral nematic liquid crystal. The directrons start with random motions but soon synchronize their motions and self-organize into a two-dimensional hexagonal lattice. The directron lattice moves collectively and forms a hexatic phase. By increasing the applied voltage, the lattice exhibits a first-order hexatic-to-liquid phase transition.

Spontaneous generation of stable CO2 emulsions via the dissociation of nanoparticle-aided CO2 hydrate

This study finds that CO2 hydrate dissociation spontaneously generates fine-textured emulsions or foams, and that the phase state of CO2 which was used to form hydrate determines the stability of the emulsion or foam when hydrate is dissociated in an aqueous dispersion of hydrophilic silica nanoparticles. This process suggests an energy-efficient method of generating stable emulsions for high-pressure applications without a need for mechanical energy input. We proved experimentally that the CO2 hydrate phase could be used to generate foams and emulsions because the hydrate formation process naturally disconnects a hydrate guest molecule phase from the bulk water phase. During dissociation, easy adsorption of nanoparticles at CO2-water interfaces hinders the coalescence of bubbles. As a result, CO2 emulsions or foams were generated upon the completion of hydrate dissociation. The CO2 emulsions generated remained fairly stable, while the CO2 foams generated became unstable and the buoyant force of the CO2 bubbles led to their coalescence. The concepts experimentally proven here could be applicable to any suitable clathrate compound undergoing a solid to a liquid phase transition.

Structure of the analytical theories of heat

We examine the analytical theories of heat developed by Laplace, Poisson and Carnot, and the thermodynamic theories based on the energy conservation and the principle of the increase of entropy formulated by Clausius. We present an analysis of the approaches developed by Maxwell, Gibbs, Planck, Duhem, Nernst, and De Donder as well as the irreversible thermodynamics of Prigogine, and the theory of liquid gas transition of van der Waals. We show that the thermodynamic potentials dominated the development of the theory of thermodynamics since Gibbs. They were introduced by Massieu under the name of characteristic functions and were demonstrated by Gibbs to have the suitable properties to characterize equilibrium of systems with variable composition, and to hold the convexity properties. We also show that the thermodynamic potentials were the relevant quantity in the Duhem approach to thermodynamics and to thermochemistry.

Stability of the high-density Jagla liquid in 2D: sensitivity to parameterisation.

We computed the pressure-temperature phase diagram of the hard-core two-scale ramp potential in two-dimensions, with the parameterisation originally suggested by Jagla [E. A. Jagla, Phys. Rev. E, 63, 061501 (2001)], as well as with a series of systematically modified variants of the model to reveal the sensitivity of the stability of phases. The nested sampling method was used to explore the potential energy landscape, allowing the identification of thermodynamically relevant phases, such as low- and high-density liquids and various crystalline forms, some of which have not been reported before. We also proposed a smooth version of the potential, which is differentiable beyond the hard-core. This potential reproduces the density anomaly, but forms a dodecahedral quasi-crystal structure at high pressure. Our results allow to hypothesise on the necessary modifications of the original model in order to improve the stability of the metastable high-density liquid phase in 3D.

Molecular dynamics study of thermal hysteresis during melting-crystallization of noble metals

By constructing the thermal hysteresis of the enthalpy and density of the noble metals of gold (Au) and copper (Cu), non-equilibrium processes are investigated during the melting – crystallization phase transformations, i.e. during the solid–liquid transition. Thermal hysteresis is obtained from the atomistic modeling. The limiting temperatures of superheating of the solid phase during melting and undercooling of the liquid phase during crystallization of gold and copper are obtained. The possibility of the formation of highly superheated-undercooled metastable states of solid and liquid phases with rapid heating-cooling of the studied metals has been confirmed. The results obtained are compared with the results of alternative calculations.

STUDY ON PHASE TRANSITION FEATURES AND INFRARED SPECTROSCOPY OF 3OCB LIQUID CRYSTAL

In the study, some physical properties of the 3OCB liquid crystal belonging to the cyanobiphenyl group, which is frequently used in dimeric molecules and eutectic mixtures, were investigated. In this context, its optical properties in different temperature interval have been investigated. As a result of optical measurements, it was determined that 3OCB exhibited a monotropic mesophase and that the phase range of the mesophase was determined. In addition, it was observed that a heterophase region appeared in the liquid crystalline isotropic phase-nematic mesophase transition. Morphological analysis of the mesophase and heterophase region was also performed. Also, by performing calorimetric measurements, how the relevant liquid crystal phase interval changes in bulk state was compared with the results obtained from optical measurements. Information about the 3OCB's dominant vibration bands was obtained by performing infrared spectroscopic measurements. The infrared spectroscopy of 3OCB, which is frequently used in industry and scientific studies, has been performed to provide information about the functional groups and structure of the molecule.

Self-assembly of supramolecular complexes of charged conjugated polymers and imidazolium-based ionic liquid crystals

The use of supramolecular interactions to construct mesogens represents a versatile means of accessing new macromolecular architectures, and material properties in the resulting mesophases. Here, we demonstrate the formation of thermotropic liquid crystalline supramolecular conjugates of sulfonated poly(alkyl thiophenes) backbones coupled to biphenyl-containing 1-alkyl-3-methylimidazolium salts with bromide counter anions. Two types of liquid crystalline imidazolium salts are studied with chemical structures differing in the terminal group of the biphenyl mesogenic unit. Both ionic liquid crystals exhibit multiple phase transitions with lamellar structures, which correspond to crystal and smectic liquid crystalline transitions. Supramolecular complexes of sulfonated polythiophene and ionic liquid crystals display thermotropic liquid crystalline phases with lamellar-like nanostructures. Detailed analysis show that the lamellar periodicity increases in the supramolecular complexes, compared to those of the ionic liquid crystals. Aligned textures were produced in thin films of supramolecular complexes by shear and magnetic field alignment of the liquid crystalline mesophases. The incorporation of ionic liquid crystal units into a conjugated polymer opens opportunities for fabrication of aligned thin films of mixed electron-ion conducting polymers. Such molecular materials could be of interest in electrochemical devices.

Using Small Molecule Absorbers to Create a Photothermal Wax Motor.

Organic phase change materials are used in actuators like wax motors. The solid→liquid phase transition that drives expansion is commonly induced by resistive heating that requires an electrical connection. The use of light to generate a phase change provides a non-contact way to power wax motors. Here, it is demonstrated that small molecules can act as absorbers to enable a photoinduced solid→liquid melting transition in eicosane, a low molecular weight phase change material. Three different small molecule absorbers are utilized: (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), azobenzene (AZOB), and guaiazulene (GAZ). The GAZ/eicosane mixture is characterized in detail because its absorption extends out to 750nm, opening up the possibility of using near-infrared diodes as the photon source. The GAZ/eicosane composite is incorporated into a commercial wax motor assembly and 532nm laser light is used to lift up to 400g. The temporal response, work and force output, and efficiency are measured, and no loss of lifting capability or degradation is observed after ten cycles of irradiation. The incorporation of small aromatic molecules with low-energy absorption features into phase change materials can provide a general way to make light powered wax motors.