Solid-like Structure Research Articles

Direct evidence for fluid–solid transition of nanoconfined fluids

Atomistically detailed simulations provide direct and reliable evidence that sufficiently nanoconfined fluids undergo a rapid and abrupt first-order transition to an ordered solid-like structure.

The Role of Polymer Polydispersity in Phase Separation and Gelation in Colloid−Polymer Mixtures

Mixtures of nonadsorbing polymer and colloidal particles exhibit a range of different morphologies depending on the particle and polymer concentrations and their relative size ratios. These can be very important for technological applications, where gelation can produce a weak solidlike structure that can help reduce phase separation, extending product shelf life. However, industrial products are typically formulated with polydisperse polymers, and the consequences of this on the phase behavior of the mixture are not known. We investigate the role of polymer polydispersity and show that a small amount of larger polymer in a distribution of nominally much smaller polymer can drastically modify the behavior. It can induce formation of a solidlike gel structure, abetted by the small polymer, but still allow further evolution of the phase separation process, as is seen with a monodisperse distribution of larger polymer. This coarsening ultimately leads to gravitational collapse. We describe the full phase behavior for polydisperse polymer mixtures and account for the origin of the behavior through measurements of the structure and dynamics and by comparing to the behavior with monodisperse polymers.

Temperature Dependent Electron Binding in (H2O)8

We combine classical molecular dynamics simulations and quantum density functional theory calculations to study the temperature effects on the electron affinity of the water octamer. The atomistic simulations provide a sample of the cluster's conformations as a function of the temperature, on which the density functional calculations are carried on. As the temperature increases, the cluster undergoes its characteristic phase change from a cubic, solidlike structure to a liquidlike state. This phase change is also reflected by an increase on the total dipole moment of the cluster. The quantum calculations indicate that the large dipole moment conformations have a positive electron affinity. Relaxing the high temperature conformations of the cluster anion to its local minimum, the average vertical detachment energy is calculated and shows a clear tendency to increase as the temperature increases. The analysis of the high temperatures conformations reveals that origin of higher values of the vertical detachment energy is not the stability of the negative octamer but the high energy of the corresponding neutral cluster.

Electrorheology of suspensions of elongated goethite particles

We describe the rheology of moderately concentrated suspensions of elongated goethite (β-FeOOH) particles with axial ratio around 8, both in the absence and presence of high-strength DC electric fields (up to 4.3 kV/mm). The selected liquid medium was a silicone oil with 1 Pa s nominal viscosity. The aim of this work is the evaluation of the electrorheological (ER) effect of suspensions containing highly anisotropic particles and the comparison with that exhibited by samples made of less anisotropic particles of similar chemical composition (hematite, α-Fe 2O 3). Under the application of large electric fields, goethite suspensions changed their rheological behavior, as expected, from Newtonian – at zero field – to shear thinning, thus displaying electrorheological response. A well defined yield stress ( σ y ) was observed in the electrified suspensions, that increases with both the field strength and particle concentration ϕ, although following different trends to those predicted by the classical chain models. σ y was found to depend on ϕ in a parabolic fashion, as a consequence of the fact that field-induced structures in the suspensions do not consist of individual chains as the classical models consider, but of much more complex particle aggregates. The yield stress was found to be almost linearly dependent on the field strength, contrary to the predictions of the polarization model ( σ y ∝ E 2). The field-induced enhancement of the conductivity of the host oil, leading to saturation of the electrical forces among polarized particles, is required to explain this deviation. The goethite suspensions were also analyzed under oscillating shear stresses for investigation of their viscoelastic properties as well. The results indicate that the ER effect was only noticeable for sufficiently high field strength and particle concentrations, typically >1 kV/mm and >4% in volume fraction, respectively. In such conditions the elastic modulus G′ was independent of the shear frequency as corresponds to an elastic solid-like structure. Suspensions of goethite particles display an ER effect with the same characteristics as hematite dispersions (same tendencies of σ y with both E and ϕ 2), indicating that the physical mechanism responsible of such effects is the same in both cases. However, suspensions containing elongated particles produce a more efficient response to the electric field than those made of irregularly shaped solids, since the former give rise to higher yield stress for the same field strength, and exhibit a lower viscosity in absence of external excitation.

Effects of Surface Chemistry on Structure and Thermodynamics of Water Layers at Solid−Vapor Interfaces

The effects of surface chemistry on the isotherm thickness and structure of the adsorbed water layer as well as the isosteric heat of adsorption and entropy of adsorption were studied using attenuated total reflection infrared spectroscopy. The degree of hydrophilicity seems to distinctively change the structure and thermodynamic properties of the water layers adsorbed on silicon oxide surfaces. On the highly hydrophilic silicon oxide surface covered with silanol groups, the water layer adsorbed at low humidities exhibits the OH stretching peak at 3230 cm−1 (characteristic of a solid-like water structure), and the isosteric heat of adsorption is much higher than the latent heat of ice sublimation. As the concentration of surface silanol groups decreases, both the initial isosteric heat of adsorption of water and the amount of solid-like water decrease. The water layer adsorbed on the hydrophilic surface at low humidities seems to have much lower entropies than bulk water, while the entropy of the water la...

Micro-extrusion of organic inks for direct-write assembly

Direct-write assembly is a highly versatile microfabrication technique used to create microfluidic networks by the robotic deposition of a fugitive ink onto a moving stage. To optimize the resulting shape of the microchannel, the translational speed of the moving stage has to closely match the linear velocity of the fugitive ink at the micro-nozzle exit. In this work, we have performed a comprehensive characterization of the micro-extrusion process of organic fugitive inks through a nozzle and characterized the rheological properties of petroleum jelly-based organic inks with various microcrystalline wax contents (10 to 40 wt%). The local microcrystal concentration has been probed using polarized optical microscopy and Raman spectroscopy. Small amplitude oscillatory shear tests in a vane geometry have revealed a solid-like structure of the organic inks, and a strong shear-thinning behavior of the complex viscosity. Particle tracking velocimetry (PTV) experiments performed in a glass microchannel have suggested the occurrence of apparent slip, showing a microcrystal depletion layer near the nozzle wall and a plug flow in the remainder of the micro-nozzle. From Raman spectroscopy and polarized microscopy performed on extruded samples, a crystal free layer was observed and estimated to be approximately 10–20 µm thick (or 2–4% of the microcapillary diameter), explaining the strong apparent wall slip behavior.

Unusual Polystyrene Nanocomposite Structure Using Emulsifier-Modified Layered Double Hydroxide as Nanofiller

A new hybrid organic inorganic assembly between an emulsifier, 4[12-(methacryloylamino)dodecanoylamino] benzenesulfonate acid (MADABS) and layered double hydroxide (LDH) of cation composition Zn2Al is described, as well as its subsequent dispersion into styrene. The characterization of the new hybrid LDH assembly by 1D electron density pictures the interleaved emulsifier well ensconced between LDH sheets, while showing the impossibility for the molecules to polymerize themselves. Indeed, the strong interdigitation abates the in situ polymerization. LDH/emulsifier:polystyrene nanocomposites, Zn2Al/MADABS:PS, are prepared by bulk polymerization, and “apparent immiscible” and intercalated PS nanocomposite structures are alternatively observed as a function of the thermal pretreatment of the filler. The rheological data are indicative of microstructural changes between the samples. For the precalcined filler, the PS nanocomposite presents a shear thinning exponent of −0.55 in the low frequency domain, associated to a hump shape for the damping coefficient δ, both curvatures consistent with the increase of glass transition temperature Tg, whereas the other sample exhibits a shear thinning exponent of 0 and a general behavior closely related to PS in spite of an increase in the elastic modulus G′ and the loss modulus G′′ of small amplitude. Such differences in the nanocomposite structure are further confirmed by TEM direct observations, with the presence of a tactoid network or of particle agglomerates when the filler is precalcined or not, respectively. Contrary to the general idea that pervades that intercalated polymer structure means an increase in the interaction between polymer chain and filler and consequently an enhancement of the mechanical properties, it is here demonstrated that such a behavior may be in conjunction with agglomerates while an opposite tendency is observed for the non-intercalated structure with the observation of an unusual solid-like structure for PS nanocomposite arising from friction welding effects between LDH tethered MADABS molecules and PS polymer chains.

Soft glassy rheology of supercooled molecular liquids

We probe the mechanical response of two supercooled liquids, glycerol and ortho-terphenyl, by conducting rheological experiments at very weak stresses. We find a complex fluid behavior suggesting the gradual emergence of an extended, delicate solid-like network in both materials in the supercooled state-i.e., above the glass transition. This network stiffens as it ages, and very early in this process it already extends over macroscopic distances, conferring all well known features of soft glassy rheology (yield-stress, shear thinning, aging) to the supercooled liquids. Such viscoelastic behavior of supercooled molecular glass formers is difficult to observe because the large stresses in conventional rheology can easily shear-melt the solid-like structure. The work presented here, combined with evidence for long-lived heterogeneity from previous single-molecule studies [Zondervan R, Kulzer F, Berkhout GCG, Orrit M (2007) Local viscosity of supercooled glycerol near T(g) probed by rotational diffusion of ensembles and single dye molecules. Proc Natl Acad Sci USA 104:12628-12633], has a profound impact on the understanding of the glass transition because it casts doubt on the widely accepted assumption of the preservation of ergodicity in the supercooled state.

Average molecular orientations in the adsorbed water layers on silicon oxide in ambient conditions

The average molecular orientation in the adsorbed water layers formed on amorphous SiO(2) in ambient conditions was determined as a function of relative humidity using polarization attenuated total reflectance infrared spectroscopy (ATR-IR). The silicon oxide surface was prepared by chemically cleaning in aqueous solution, washing with water, and drying with argon. After drying, this produced a SiO(2) surface with hydroxyl groups, giving rise to a water contact angle < 5 degrees. Primarily two types of vibrational peaks that correspond to liquid water and solid-like water were observed in the adsorbed water layers formed on this surface at room temperature. The average orientation of the water molecules was determined from the dichroic ratio of s- to p-polarization absorbances. At low relative humidities, the highly hydrogen bonded solid-like structure exhibits a dichroic ratio as low as approximately 0.4, while the liquid water structure exhibits a dichroic ratio close to approximately 1.0. As the relative humidity increases, the dichroic ratio of both water structures approaches a dichroic ratio of 0.7 approximately 0.8, which is consistent with the random orientation of molecules of bulk water and ice.

Ground-state path integral Monte Carlo simulations of positive ions in He4 clusters: Bubbles or snowballs?

The local order around alkali (Li(+) and Na(+)) and alkaline-earth (Be(+), Mg(+), and Ca(+)) ions in (4)He clusters has been studied using ground-state path integral Monte Carlo calculations. The authors apply a criterion based on multipole dynamical correlations to discriminate between solidlike and liquidlike behaviors of the (4)He shells coating the ions. As it was earlier suggested by experimental measurements in bulk (4)He, their findings indicate that Be(+) produces a solidlike ("snowball") structure, similar to alkali ions and in contrast to the more liquidlike (4)He structure embedding heavier alkaline-earth ions.

Size dependence of freezing temperature and structure instability in simulated Lennard-Jones clusters

Liquid Lennard-Jones clusters of 14 different sizes from N=55 to 923 particles were cooled down to find their temperature of liquid-solid transition and the internal structure of the solidified clusters. The decrease of the cluster temperature was attained by a gradual change of the system temperature in Monte Carlo simulations. The liquid-to-solid transition was found by analysis of the specific heat as well as by detection of the structural units of face-centred cubic, hexagonal close-packed and decahedral type. It was observed that near the detected transition temperature the solid-like cluster structure is not always stable and fluctuates between solid and liquid states. The fluctuations of the state were observed frequently for small clusters with N ≤147, where the temporary solid structure is created by a large part of internal atoms. Manual inspection of cluster structural data and the 10%N condition for minimal number of atoms as centres of solid-like units enable detection of stable cluster solidification at freezing temperature. It was found that the freezing temperature of all clusters, with the exception of N=55, decreases linearly with N-1/3. The extrapolated freezing temperature of the bulk LJ system is 13% lower than the experimental value of argon. After freezing, the solid phase remains but some atoms close to the cluster surface are not firmly included into the structure and oscillate mainly between solid structure and disordered one.

Surface-Enhanced Raman Spectroscopy of Dodecanethiol-Bound Silver Nanoparticles at the Liquid/Liquid Interface

Adsorption and domain formation of dodecanethiol (DT)-bound silver nanoparticles (SNPs) at the cyclohexane/water interface were studied by means of total internal reflection (TIR) light scattering microscopy and TIR surface-enhanced Raman scattering (SERS). By the TIR light scattering microscopy, the extent of the interfacial adsorption and domain formation of SNP was observed, which was produced by the reaction between citrate-reduced SNPs in the aqueous phase and DT in the cyclohexane phase. The Raman spectra of DT on SNP showed that the relative intensity ratios of gauche to trans conformers in the nu(C-S) band region decreased with the increase of the initial concentration of DT, suggesting the change from the liquidlike structure to the solidlike structure of the DT. The residue of the negative charges on the SNPs at the interface was detected by the resonance SERS (SERRS) peaks of the adsorbed cationic porphyrin, 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)-21H,23H-porphine (TMPyP). The efficiency of the interfacial SNPs domains as a SERS substrate for TMPyP strongly depended on the adsorption state of the DT.

An effective temperature approach for molecular dynamics simulations of quantum solids

We examine two standard methods to incorporate quantum effects in classical molecular dynamics simulations of quantum solids. It is shown that thermal harmonic quantum correction fails to generate a reliable structure of solid neon, while augmenting the classical equations of motion with a quantum Langevin force is no longer appropriate for heavier noble gases. An alternative approach is proposed which accounts as simply as possible for the characteristic frequency and the anharmonicity of lattice vibrations. Simulations carried out within this approach in the canonical ensemble show that normal solid-like structure could be achieved while lattice symmetry is preserved.

Structure change of Pb melt

Using viscometer, densitometer, magnetic susceptibility device, DSC and X-ray diffractometer for high temperature (XRD), the Pb melt has been investigated between 800–1200 K. Anomalous changes in the physical properties were found near the same temperature. X-ray diffraction shows that the size of clusters in the low temperature zone is much larger than that in the high temperature zone. It is presumed that some solid-like structure occurred at the anomalous point of the physical properties during the cooling process.

Corrected thermodynamic description of adsorption via formalism of the theory of volume filling of micropores

Based on the series of benzene adsorption and related enthalpy of adsorption data measured on porous carbons that possess various porous structures, we show that the creation of a solidlike structure in pores depends on the average pore diameter of an adsorbent. Taking into account the solidlike adsorbed phase in the thermodynamic description of the adsorption process via the formalism of the theory of volume filling of micropores (TVFM) leads to very good agreement between the data measured experimentally and those calculated from TVFM. Finally we show that the boundary between solidlike and liquidlike structures of benzene molecules in carbon pores is located around the average pore diameter, close to ca. 2.1–2.4 nm.

Preparation and rheological characterization of poly( n-butyl methacrylate)/montmorillonite composites

Intercalated nanocomposites constituted of poly(butyl methacrylate) (PBMA) as the matrix and an organically modified montmorillonite as the nanosize filler were prepared and rheologically characterized in detail. The rheological behavior of the composites showed dependence on both temperature and clay content. For composites of low clay contents, the steady shear viscosity showed a Newtonian plateau in the low shear rate region at low temperatures and the plateau was replaced by a shear-thinning curve when the temperature was raised. For composites of higher clay contents, strong shear-thinning behavior were observed at all shear rates and all temperatures. The viscoelastic data of the composites showed unusual terminal behavior of a decreasing terminal slope at low frequencies with increasing temperature and clay loading. X-ray diffraction spectra showed that annealing process at higher temperatures shifted the Bragg reflection peaks to a lower angle and broadened the peaks, which provided the evidence for the existence of a temperature-induced solid-like structure that was responsible for the shear thinning and the unusual terminal viscoelastic behavior.

OC5.07: Comparison of 3D power Doppler ultrasound (3D‐PDU) versus 3D with Inversion mode rendering in 3D fetal echocardiography

3D-PDU is an established technique in the assessment of the fetal cardiovascular system. The new “inversion mode (IM) rendering” enables without the use of Doppler to “invert” the hypoechoic informations in the volume box to a solid-like structure. We aimed to compared the accuracy of both techniques in 3D fetal echocardiography. 25 normal fetuses and 15 fetuses with various cardiac anomalies were examined between 20–30 weeks with both techniques. Presets were adapted individually according to the visualization conditions. Volumes were acquired in 3D static or in STIC (off-line 4D) technique. (GE-Voluson 730 Expert System). The evaluation was performed on an using the 4D-View software. Basic image quality was better with gray scale with a high image frame rate (80–110 Hz), vs PDU with slow rate (12–32 Hz). Time for presets optimisation was shorter for gray-scale in comparison to PDU image. Although on normal and abnormal hearts both techniques were similar in visualizing the heart and great vessels, this was easier and more accurately achieved using IM. IM was furthermore superior in visualizing tiny cardiac vessels in malformations, especially in unfavourable position. Power Doppler was superior in the demonstration of very small vessels and flow not seen on 2D as e.g. pulmonary veins, aberrant aortic arch vessels, muscular VSD or tricuspid regurgitation. Limitation of PDU was the overlapping of flow information, whereas the limitation of IM were the artefacts caused from shadowing (ribs, spine). The relationship to other thoracic structures was only possible in 3D-PDU glass body mode. 3D-IM technique is an easy accessible tool in 3D fetal echo. In regions with tiny vessels or in cases with hemodynamic events, power Doppler can be a complementary technique. The difficult preset optimisation and the low frame rates are still the main limitations of 3D-PDU.

The effects of surface roughness on nanotribology of confined two-dimensional films

A molecular dynamics simulation is used to investigate the effect of surface roughness on the nano-rheology of ultra-thin films confined between two solid walls. The results show that friction is sensitive to the confining surface morphology and the fluid molecules near the two confining surfaces are in a layering solid-like structure. At a high shear rate, the interfacial layers of the film stick to the walls, resulting in partial slip inside the film and the development of shear stress in the viscous molecular fluid. The frictional resisting force is due to the shear in the viscous molecular fluid but not dependent on the critical yield stress of the solid-like structure as in the case of smooth boundaries.

Physically Cross-Linked Alkylacrylamide Hydrogels: A SANS Analysis of the Microstructure

The microstructure of copolymer hydrogels composed of either N,N-dimethylacrylamide (DMA) or N-isopropylacrylamide(NIPA) and 2-(N-ethylfluorooctanesulfonamido)ethyl acrylate, FOSA, was studied by small-angle neutron scattering (SANS). The FOSA/NIPA gels exhibited a thermally reversible volume phase transition and a temperature-sensitive microstructure, while the FOSA/DMA gels did not. A modified interacting core-shell microstructure model was used to explain the SANS data for both gels. The cross-link junctions of the gel consisted of nanophase-separated FOSA domains composed of a FOSA core surrounded by a water-poor layer of the alkylacrylamide. The average spherical core radius ranged from 1 to 3 nm, and the average shell thickness ranged from 0 to 1.5 nm; the FOSA aggregation number ranged from 10 to 180. The hydrogels exhibited chain-sized heterogeneities due to solutionlike thermal fluctuations and larger sized heterogeneities due to a solidlike cluster structure. The chain correlation and the large heterogeneities were similar to those observed in covalent DMA and NIPA hydrogels. The chain correlation length ranged from 0.5 to 3.5 nm, and the large heterogeneities ranged from 50 to 90 nm. A previous paper 1 described the phase behavior and small-angle scattering characterization of the micro- structure of hydrophobically modified hydrogels com- posed of 2-(N-ethylperfluorooctanesulfonamido)ethyl acry- late (FOSA) and N-isopropylacrylamide (NIPA) or N,N- dimethylacrylamide (DMA). Aggregation of the FOSA moieties produced a physically cross-linked microstruc- ture. The FOSA/NIPA copolymer hydrogels exhibited a thermally reversible volume phase transition (VPT) 2,3 between a swollen and a collapsed phase, similar to that observed for covalently cross-linked NIPA gels. Increas- ing the FOSA content decreased the temperature of the VPT (TVPT) and broadened the transition. In contrast, covalently cross-linked DMA hydrogels did not exhibit a VPT, and the introduction of FOSA did not produce a VPT for those copolymers. This paper describes the

Alkali and alkali-earth ions inHe4systems

We study with variational Monte Carlo simulations a single ion impurity in nanodroplets and in liquid $^{4}\mathrm{He}$. Within the shadow wave function (SWF) approach we have studied alkali-earth ion doped $^{4}\mathrm{He}$ systems (${\mathrm{Be}}^{+}$ and ${\mathrm{Mg}}^{+}$) and we have also improved our previous variational results for alkali ion impurities (${\mathrm{Na}}^{+}$, ${\mathrm{K}}^{+}$, and ${\mathrm{Cs}}^{+}$). The new $\mathrm{SWF}$ allows for anisotropic correlations between $^{4}\mathrm{He}$ atoms and the ion. The first shell of $^{4}\mathrm{He}$ atoms around the ions has always a well defined solidlike structure, which is remarkably different for each ion but it does not depend on the $^{4}\mathrm{He}$ system (bulk liquid or cluster). We also give results for the chemical potential, the single particle excitation spectrum, and the effective mass of the ions in the bulk.