Patterns from drying drops

The objective of this review is to investigate different deposition patterns from dried droplets of a range of fluids: paints, polymers and biological fluids. This includes looking at mechanisms controlling the patterns and how they can be manipulated for use in certain applications such as medical diagnostics and nanotechnology.     

Interactions and phase transitions in protein solutions

Understanding interactions and phase transitions in protein solutions is essential in order to develop systematic protein crystallisation strategies. Recent studies show that proteins near crystallisation behave as particles interacting via a very short-range attractive potential. The microscopic origin of this attractive term, and its strong dependence on the nature of the crystallisation agent, is, however, still obscure. The interplay of crystal nucleation with formation of weakly bonded amorphous aggregates also deserves further analysis.     

Shape and buckling transitions in solid-stabilized drops

We study shape and buckling transitions of particle-laden sessile and pendant droplets that are forced to shrink in size. Monodisperse polystyrene particles were placed at the interface between water and decane at conditions that are known to produce hexagonal, crystalline arrangements on flat interfaces. As the volumes of the drops are reduced, the surface areas are likewise diminished. This effectively compresses the particle monolayer coating and induces a transition from a fluid film to a solid film. Since the particles are firmly attached to the interface by capillary forces, the shape transitions are reversible and shape/volume curves are the same for drainage and inflation. Measurements of the internal pressure of the drops reveal a strong transition in this variable as the buckling transition is approached.     

Salt-induced phase inversion in aqueous cationic/anionic surfactant two-phase systems

Phase inversion of aqueous two-phase systems with excess cationic surfactant (abbreviated as ATPS-C) formed by aqueous mixtures of 1,3-propanediyl bis(dodecyl dimethylammonium bromide) (abbreviated as 12-3-12) and sodium dodecyl sulfonate (abbreviated as AS) at 318.15 K was investigated. The experimental results indicate that addition of NaF, NaCl, NaHCO 3, or NaNO 3 can result in phase inversion of ATPS-C formed by 12-3-12/AS systems; however, addition of NaBr cannot lead to phase inversion. TEM micrographic experiments illustrate that there is no direct relationship between the microstructures of the concentrated phase in ATPS-C and phase inversion. To interpret the phase-inversion phenomena of ATPS-C, the phase composition, phase density, and phase volume ratio between the dilute phase and the concentrated phase in ATPS-C were investigated. Phase composition analysis results illustrate that for the ATPS-C formed by 0.10 mol.kg (-1) 12-3-12/AS mixed system, the concentration of Br (-) counterions in the dilute phase of ATPS-C increases with addition of NaF, NaCl, NaHCO 3, or NaNO 3. At the same time, the molar ratio between the F (-) (Cl (-), HCO 3 (-), or NO 3 (-)) counterions and Br (-) counterions in the concentrated phase of ATPS-C increases also. It illustrates that part of the bromide counterions which are the natural counterions of the surfactant 12-3-12 in excess are exchanged by other anionic counterions when an additional salt is added to the system. The investigation indicates that the common ground of the added F (-), Cl (-), HCO 3 (-), or NO 3 (-) counterions is that they all make a smaller density contribution than that of Br (-) counterions, although they have a weaker or stronger counterion binding ability with the mixed positively charged aggregates in ATPS-C than that of Br (-) counterion. Density experiments illustrate that the density increase of the dilute phase is larger than that of the concentrated phase in the ATPS-C with addition of NaF, NaCl, NaHCO 3, or NaNO 3; thus, phase inversion occurs. The densities of the added inorganic sodium salt aqueous solution and the order of the Hofmeister series for the added inorganic anions with respect to the chaotropic headgroup of 12-3-12 play important roles in the phase inversion of ATPS-C.     

Model for phase coexistence in phase transitions

We propose a general model for describing the phenomena of phase coexistence in relation to pressure induced phase transformations by means of the T–P distribution in statistical thermodynamics. Using the well‐known B1–B2 transition in NaCl as a prototype, we demonstrate how phase coexistence gives rise to the changes in the bulk modulus and the equation‐of‐state across the transition. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Frozen state transitions in relation to freeze drying

Freeze-drying is used as a gentle dehydration method for heat sensitive materials especially in food and pharmaceutical industries. Most materials, including dissolved sugars in water, do not crystallise during freezing prior to freeze-drying. Supersaturated, freeze-concentrated solutions are amorphous materials and they solidify into a glassy state when their temperature is depressed to below the glass transition temperature,T _g. Differential scanning calorimetry has been used to show that maximally freeze-concentrated sugar solutions, when properly frozen, show during heating a glass transition,T _g , which is followed by ice melting endotherm with onset atT _m . Low molecular weight materials are difficult to freeze-dry as they have lowT _g , andT _m slightly above T _g . High molecular weight materials, such as carbohydrate polymers, exhibit improved dehydration characteristics and they have _g and _m at about the same temperature close to the melting point of pure water. The amorphous, glassy structure typical of freeze-dried materials is formed during prefreezing and retained after removal of ice and the unfrozen water from the freeze-concentrated material. Dehydration temperatures belowT _g allow removal of ice within the solid, glassy solutes, but temperatures aboveT _m result in collapse. The frozen state transitions and properties of freeze-dried materials can be shown in state diagrams which are used to derive proper freeze-drying conditions and storage requirements for various materials.     

Photoinduced phase transitions

Employing actinic light to alter/stabilise a particular thermodynamic phase via the photo-isomerisation of the constituent molecules is an interesting tool to investigate soft matter from a new dimension. This article focuses on our recent results on several aspects of these non-equilibrium phase transitions, which are isothermal in nature. We specifically discuss (i) the influence of different parameters, such as confinement, applied electric field, pressure etc., on the dynamics associated with both the photochemical transition driving the equilibrium nematic to the non-equilibrium isotropic phase and the thermal back relaxation recovering the nematic phase, (ii) unique light-driven disorder–order transition in a reentrant system, (iii) dynamic self-assembly of the smectic A phase, which is stabilised only in the presence of actinic light, (iv) novel temperature-intensity phase diagrams and an example of primary and secondary photo-ferroelectric effects in an antiferroelectric smectic C system. These results highlight the fact that the actinic light can be used as a new tool to study phase transitions and the associated critical phenomena that could also bring about effects that are not seen in equilibrium situations.     

Tethered polymer layers: phase transitions and reduction of protein adsorption

The structural and thermodynamic properties of tethered polymer layers formed by spreading diblock copolymers at a solid surface or at a fluid‐fluid interface are studied using a molecular mean‐field theory. The role of the anchoring block in determining the properties of the tethered polymer layer is studied in detail. It is found that both the anchoring and the tethered blocks are very important in determining the phase behavior of the polymer layer. The structures of the coexisting phases, the phase boundaries and the stability of the layer are found to depend on the ratio of molecular weight between the two blocks, the polymer‐interface (surface) interactions and the strength of the interactions between the two blocks. The different phase transitions found are related to experimental observations. The properties of the polymer layers at coexistence reflect the block that is the dominant driving force for phase separation. The ability of the tethered polymer layers, under different conditions, to control protein adsorption to surfaces is also studied. It is found that the most important factors determining the ability of a polymer layer to reduce the equilibrium amount of proteins adsorbed to a surface are the surface coverage of polymer and the surface‐polymer interactions. The polymer chain length plays only a secondary role. For the kinetic control, however, it is found that the potential of mean‐force, and thus the early stages of adsorption, depends strongly on polymer molecular weight. Further, it is found that the molecular factors determining the ability of the tethered polymer layer to reduce the equilibrium amount of protein adsorption are different than those that control the kinetic behavior. Comparisons with experimental observations are presented. The predictions of the theory are in very good agreement with the measured adsorption isotherms. Guidelines for building optimal surface protection for protein adsorption, both kinetic and thermodynamic, are discussed.     

Toluene-induced phase transitions in blue phase liquid crystals

ABSTRACT

We report phase transitions in blue phase-forming liquid crystals (LCs) that are triggered by exposure to toluene vapours. Specifically, we reveal that room-temperature cholesteric phase mixtures of MLC-2142 and S-811 form blue phases (BP I, II and III) with increasing vapour pressure of toluene. To probe the mechanism underlying this observation, we investigated the phase behaviour of mixtures of BP-forming LCs containing a range of non-volatile aromatic compounds (e.g. pyrene). We interpret our observations to indicate that the principal effect of small aromatic compounds is to decrease the energy penalty associated with the formation of disclination lines in BPs. We also conclude that the absorption of toluene into the BP-forming LCs lowers the energy required for the formation of disclination cores in the BP phase, thus allowing the elastically favoured double-twist cylinders to form at lower temperatures. We demonstrate that BP-forming LCs containing pyrene can be used to detect toluene at concentrations below 200 ppm at room temperature. Overall, these results guide the design of LC-based materials that respond to VOCs at concentrations relevant to occupational settings.     

High-pressure phase transitions in liquid crystals

Simultaneous measurements of the rate of heat evolution and changes of the mechanical variable of a transformation such as volume or pressure, performed in a p-V-T controlled scanning calorimeter have been applied to investigations of phase transitions in liquid crystals. In the instrument, the phase transitions can be induced by a controlled change of pressure, volume or temperature under isothermal, isobaric or isochoric conditions respectively. The present investigations have ben performed on 4-n-penthyl-penthylthiol-4-decycloxybenzoate which demonstrates in the liquid crystal state a nematic and three smectic phases

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Zusammenfassung In einem p-V-T-kontrollierten Scanning Kalorimeter wurden zur Untersuchung der Phasenumwandlungen in Flüssigkeitskristallen simultane Messungen der Geschwindigkeit der Wärmeentwicklung und der Veränderungen von mechanischen Größen von Umwandlungen, wie z.B. von Volumen oder Druck durchgeführt. In diesem Gerät kann die Phasenumwandlung durch eine kontrollierte Veränderung von Druck, Volumen oder Temperatur unter isothermen, isobaren oder isochoren Bedingungen ausgelöst werden. Vorliegende Untersuchungen wurden an 4-n-Pentyl-phenylthiol-4-dezyloxybenzoat durchgeführt, welches im Flüssigkristallzustand eine nematische und drei smektische Phasen aufweist.

     

Interfacial phase transitions in conducting fluids

We present a review, largely based on recent experimental work of our group, on phase transitions at interfaces of fluid metals, alloys and ionic liquids. After a brief analysis of possible experimental errors and limitations of surface sensitive methods, we first deal with first-order wetting transitions at the liquid/vapour and liquid/wall interface in systems such as Ga-based alloys, K-KCl melts, and fluid Hg. The following chapter refers to surface freezing or surface induced crystallization in different metal alloys. The respective surface phase diagrams are discussed in comparison with their bulk counterpart. In the last part we present very recent investigations of ionic liquid interfaces, including order-disorder transitions at the liquid/vapour interface and examples of two-dimensional phase transitions at the electrified ionic liquid/metal interface. Finally, a simple electrowetting experiment with an ionic liquid droplet under vacuum is described which gives new insight into the contact angle saturation problem. The article ends up with a few perspective remarks on open problems and potential impact of interfacial phenomena on applied research.     

Structural phase transitions in CaC2

Pure CaC2, free of CaO impurities, was obtained by the reaction of elemental calcium with graphite at 1,070 K. By means of laboratory X-ray and synchrotron powder diffraction experiments, the phase diagram was investigated in the temperature range from 10 K to 823 K; this confirmed the literature data that reported the partial coexistence of up to four modifications. Aside from a cubic high-temperature modification CaC2 IV (Fm3m, Z = 4) and the well-known tetragonal modification CaC2 I (I4/mmm, Z = 2), a low-temperature modification CaC2 II (C2/c, Z =4) that crystallizes in the ThC2 structure type and a metastable modification CaC2 III (C2/m, Z = 4) that crystallizes in a new structure type were found. It was shown that phase transition temperatures as well as the relative amounts of the various CaC2 modifications depend upon the size of the crystallites, the thermal treatment. and the purity of the sample, as a comparison with technical CaC2 confirmed.     

Pressure-induced phase transitions in LiNH2

In situ high-pressure Raman spectroscopy studies on LiNH2 (lithium amide) have been performed at pressures up to 25 GPa. The pressure-induced changes in the Raman spectra of LiNH2 indicates a phase transition that begins at approximately 12 GPa is complete at approximately 14 GPa from ambient-pressure alpha-LiNH2 (tetragonal, I) to a high-pressure phase denoted here as beta-LiNH2. This phase transition is reversible upon decompression with the recovery of the alpha-LiNH2 phase at approximately 8 GPa. The N-H internal stretching modes (nu([NH2]-)) display an increase in frequency with pressure, and a new stretching mode corresponding to high-pressure beta-LiNH2 phase appears at approximately 12.5 GPa. Beyond approximately 14 GPa, the N-H stretching modes settle into two shouldered peaks at lower frequencies. The lattice modes show rich pressure dependence exhibiting multiple splitting and become well-resolved at pressures above approximately 14 GPa. This is indicative of orientational ordering [NH2]- ions in the lattice of the high-pressure beta-LiNH2 phase.     

Temperature-induced phase transitions in BaTbO3

The crystal structures of BaTbO₃ have been investigated over a wide temperature range between 40 and 773 K using high-resolution time-of-flight neutron powder diffraction. Two-phase transitions were observed. Below about 280 K, BaTbO₃ adopts an orthorhombic perovskite structure (space group Ibmm), which is characterized by rotation of TbO₆ octahedra about the pseudocubic two-fold axis. Above 280 K, BaTbO₃ undergoes a first-order phase transition to a tetragonal symmetry (space group I4/mcm), in which the tilting of the octahedra is around the pseudocubic four-fold axis. As the temperature is further increased, BaTbO₃ adopts the primitive cubic aristotype at about 623 K. This later phase transformation is characterized by a gradual decrease of the rotation angle, indicating a continuous phase transition, which is described by a critical exponent β=0.35.     

Pressure induced phase transitions in hydroquinone

High pressure behavior of alpha-hydroquinone (1,4-dihydroxybenzene) has been studied using Raman spectroscopy up to pressures of 19 GPa. Evolution of Raman spectra suggests two transitions around 3.3 and 12.0 GPa. The first transition appears to be associated with the lowering of crystal symmetry. Above 12.0 GPa, Raman bands in the internal modes region exhibit continuous broadening suggesting that the system is progressively evolving into a disordered state. This disorder is understood as arising due to distortion of the hydrogen-bonded cage across the second transition around 12 GPa.     

Pressure-induced phase transitions in mercury chalcogenides

Polymorphic transitions of HgSe_xTe_{(1 ? x)} induced by pressure were investigated for 0 <- x <- 1. The transitions were followed by both volume and resistivity measurements on samples of different alloy compositions and of different textures. The transition pressure seems to be a linear function of the alloy composition.