Crystal-liquid Interface Research Articles

Dynamic Complex Liquid Crystal Emulsions.

We report a new class of dynamically reconfigurable complex colloids comprising immiscible liquid crystals (LCs) and fluorocarbon oils. Producing stable spherical droplets requires the utilization of appropriately designed surfactants to reduce the high intrinsic surface tension between the LCs and the fluorocarbon oils that initially lead to nonspherical, "snowman-shaped" Janus droplets. After stabilizing the interfaces via surfactants, the LC droplet morphology can be dynamically switched between LC-in-fluorocarbon-in-water double emulsions (LC/F/W), spherical Janus emulsions, and inverted double emulsions (fluorocarbon-in-LC-in-water, F/LC/W) in response to changes in the surrounding surfactants. These stabilization methods can be extended to smectic LCs to create droplets with more complex internal arrangements and expand the range of LC emulsions that can be prepared. In addition, by using new mesogenic surfactants that control the LC director field at each LC interface, we prepare LC complex colloids exhibiting different internal configurations. The ability to control the LC anchoring conditions made it possible to create topological singularities as powerful templates for the precision assembly of antibodies at the droplets' interface. These dynamic LC complex colloids of controllable morphology and LC orientation are rich soft material platforms that will find utility in a variety of sensing applications.

ReaxFF reactive force field for molecular dynamics simulations of liquid Cu and Zr metals.

We develop a ReaxFF reactive force field used for the molecular dynamics simulations of thermophysical properties of liquid Cu and Zr metals. The ReaxFF parameters are optimized by fitting to the first-principles density-functional calculations on the equations of state for bulk crystal structures and surface energies. To validate the force field, we compare the ReaxFF results with those from experiments and embedded-atom-method (EAM) potentials. We demonstrate that the present ReaxFF force field well represents structural characteristics and diffusion behaviors of elemental Cu and Zr up to high-temperature liquid regions. It reasonably reproduces the thermodynamic processes associated with crystal-liquid interface. In particular, the equilibrium melting temperatures show better agreement with experimental measurements than the results from EAM potentials. The ReaxFF reactive force field method exhibits a good transferability to the nonreactive processes of liquid systems.

Thermophysical Properties of Stable and Supercooled Liquid Carbon

Temperature dependences of the isobaric heat capacity of stable and supercooled liquid carbon, which were obtained by processing thermograms of spontaneous cooling of spherical samples (levitation experiment), were determined. Using the thermodynamic analogy of the properties of the elements of the carbon subgroup of the Periodic Table for carbon, surface tension at the crystal–liquid interface, the size and the work of formation of the critical nucleus of the solid phase in a supercooled liquid, and the heat of fusion were found.

Detection of Biomarkers in Blood Using Liquid Crystals Assisted with Aptamer-Target Recognition Triggered in Situ Rolling Circle Amplification on Magnetic Beads.

Detection of biomarkers in body fluids is critical to both diagnosing the life-threatening diseases and optimizing therapeutic interventions. We herein report use of liquid crystals (LCs) to detect biomarkers in blood with high sensitivity and specificity by employing in situ rolling circle amplification (RCA) on magnetic beads (MBs). Specific recognition of cancer biomarkers, such as platelet derived growth factor BB (PDGF-BB) and adenosine, by aptamers leads to formation of a nucleic acid circle on MBs preassembled with ligation DNA, linear padlock DNA, and aptamers, thereby triggering in situ RCA. LCs change from dark to bright appearance after the in situ RCA products being transferred onto the LC interface decorated with octadecy trimethylammonium bromide (OTAB), which is particularly sensitive to the amplified DNA on MBs. Overall, this label-free approach takes advantages of high specificity of aptamer-based assay, efficient enrichment of signaling molecules on MBs, remarkable DNA elongation performance of the RCA reaction, and high sensitivity of LC-based assay. It successfully eliminates the matrix interference on the LC-based sensors and thus achieves at least 4 orders of magnitude improvement in sensitivity for detection of biomarkers compared to other LC-based sensors. In addition, performance of the developed sensor is comparable to that of the commercial ones. Thus, this study provides a simple, powerful, and promising approach to facilitate highly sensitive, specific, and label-free detection of biomarkers in body fluids.

Ferroelectric ordering at interface of paraelectric phase of liquid crystal and solid substrate in confined geometry

The interfacial molecular dynamics at the interface of solid substrate and paraelectric phase liquid crystal is investigated by dielectric spectroscopy in surface stabilized geometry of strongly and weakly rubbed cells. The material under study is ferroelectric liquid crystal (FLC) below the paraelectric phase. The studies are intended to explore the well-known surface stabilized memory effect in thin cells. The memory effect in weakly rubbed cells is long lasting than strongly rubbed cell. The studies near transition temperature (Tc) of ferroelectric (chiral smectic C) to paraelectric (chiral smectic A) phase is carried out. The molecular interface of FLC with substrate causes the phason mode to appear abnormally in paraelectric phase. The phason mode is the character of ferroelectric phase in liquid crystals. In strongly rubbed cell, the amplitude of phason mode is appeared just in ferroelectric phase only whereas in the weakly rubbed cells, the phason mode and soft mode are seen simultaneously at separate frequencies in paraelectric phase near Tc. The appearance of phason mode at the interface of FLC and solid substrate indicates towards the unusual ferroelectric molecular interface of FLC with substrate which is responsible for the longer duration memory effect in weakly rubbed cell.

Liquid Crystal based Detection of Pb(II) Ions Using Spinach RNA as Recognition Probe.

We report a new method for label-free, sensitive, and facile detection of lead(II) ions (Pb2+) based on an aptamer-target binding event, which is recognized by orientations of liquid crystals (LCs) at aqueous interfaces. The LC film suspended in the aqueous phase demonstrated a homeotropic orientation in contact with a cationic surfactant cetyltrimethylammonium bromide (CTAB) due to self-assembly of CTAB molecules at the aqueous-LC interface. The ordering of LC subsequently changed to planar in the presence of the spinach RNA aptamer (SRNA) due to interactions between CTAB and SRNA. In the presence of the Pb2+ ion, the ordering of LC changed to homeotropic caused by reorganization of CTAB at the LC-aqueous interface. This is due to formation of more stable quadruplex structures of SRNA with Pb2+ ions in comparison to the CTAB-SRNA complex. The sensor exhibited a detection limit of 3 nM, which is well below the permissible limit of Pb2+ in drinking water. Our experiments establish that addition of Pb2+ leads to (i) the formation of Pb2+-SRNA complexes and (ii) a decrease in density of SRNA on the LC interface, but additional studies are required to determine which of these processes underlie the response of the LCs to the Pb2+. We have also demonstrated the potential application of the LC sensor for detection of Pb2+ in tap water. Unlike current laboratory-based heavy-metal-ion assays, this method is comparatively simple in terms of instrumentation, operation, and optical readout.

Shock growth of ice crystal near equilibrium melting pressure under dynamic compression

Crystal growth is governed by an interplay between macroscopic driving force and microscopic interface kinetics at the crystal-liquid interface. Unlike the local equilibrium growth condition, the interplay becomes blurred under local nonequilibrium, which raises many questions about the nature of diverse crystal growth and morphological transitions. Here, we systematically control the growth condition from local equilibrium to local nonequilibrium by using an advanced dynamic diamond anvil cell (dDAC) and generate anomalously fast growth of ice VI phase with a morphological transition from three- to two-dimension (3D to 2D), which is called a shock crystal growth. Unlike expected, the shock growth occurs from the edges of 3D crystal along the (112) crystal plane rather than its corners, which implies that the fast compression yields effectively large overpressure at the crystal-liquid interface, manifesting the local nonequilibrium condition. Molecular dynamics (MD) simulation reproduces the faster growth of the (112) plane than other planes upon applying large overpressure. Moreover, the MD study reveals that the 2D shock crystal growth originates from the similarity of the interface structure between water and the (112) crystal plane under the large overpressure. This study provides insight into crystal growth under dynamic compressions, which makes a bridge for the unknown behaviors of crystal growth between under static and dynamic pressure conditions.

The Effect of the Interphase Kinetics on the Motion of a Quantum Crystal in Superfluid Liquid

The fall of a quantum crystal in the state of "burst-like growth" in a superfluid liquid is considered. The experimental data of the pressure variation in the container during the fall of a crystal are discussed. The model of the motion of the crystal is suggested taking the interface dynamics into account. The results for the numerical simulation of the fall of a crystal are consistent with the experimental data. We find a significant effect of the liquid-crystal interface kinetics on the hydrodynamics of the liquid flow encircling a crystal.

Impact of Polymer Enrichment at the Crystal-Liquid Interface on Crystallization Kinetics of Amorphous Solid Dispersions.

Amorphous solid dispersions have been widely used as an effective formulation strategy in the oral delivery of poorly soluble drugs. However, one of the main challenges in the development of amorphous drugs is to maintain their physical stability. The underlying mechanism of amorphous drugs crystallized in polymeric matrices is still poorly understood. Herein, we report the phenomenon of polymer enrichment at the crystal-liquid interface during the crystallization of griseofulvin (GSF) containing poly(ethylene oxide) (PEO). Confocal Raman microscopy, scanning electron microscope (SEM), and energy-dispersive X-ray spectroscopy (EDS) are employed to reveal the heterogeneous distribution of GSF and PEO at the crystal growth front. The concentration of PEO in the polymer-rich phase at the crystal-liquid interface is determined by Raman spectroscopic analysis. At a given temperature, the crystal growth rates of GSF initially increase with increasing the PEO loading in the bulk and then reach a plateau at high polymer loadings. We propose that the crystal growth rates of GSF are predominantly controlled by the local concentration of PEO at the growth front rather than the overall bulk concentration. This study provides the direct evidence of physical mechanisms that contributes to the local phase separation occurred at the crystal-liquid interface, which governs the kinetics of crystal growth in amorphous solid dispersions. These results are important for understanding the crystallization behavior of amorphous solid dispersions and beneficial for the rational design of robust amorphous formulations.

Site-specific attraction dynamics of surface colloids driven by gradients of liquid crystalline distortions.

The site-specific migration dynamics of small particles at the air-nematic liquid crystal (LC) interface is investigated, using a LC film with a unique gradient of LC distortions. This gradient has been identified as the direct origin of the site-specific migration, while the elastic multipole interaction between the pre-existing distortion and that induced around particles is negligible. The results reveal a basic behavior of small particles with weak anchoring strength in LC distortions, which is often hidden in prominent elastic multipole interactions typically found with larger particles. Moreover, the present mechanism of the site-specific attraction of particles is specifically relevant to directed patterning and manipulations of nano-particles in nematic LCs with distortions.

Gold nanoparticle-mediated signal amplification of liquid crystal biosensors for dopamine.

A unique design strategy was developed for the detection of dopamine using a newly synthesized amphiphile containing boronic acid head group at the aqueous-liquid crystal (LC) interface. The optical signal of LC for the detection of dopamine was highly amplified in the presence of functionalized gold nanoparticles.

A liquid crystal based method for detection of urease activity and heavy metal ions by using stimulus-responsive surfactant-encapsulated phosphotungstate clusters.

A liquid crystal (LC) based method is described for the sensitive determination of the activity of urease and of heavy metal ions which acts as inhibitors. Stimulus-responsive surfactant-encapsulated phosphotungstate clusters (SECs) were fabricated and deposited onto octadecyltrichlorosilane-coated glass. A copper TEM grid filled with LCs was placed on the substrate to construct the LC optical cell. Upon addition of water to the LC interface, the optical appearance of LCs on the glass undergoes a bright-to-dark shift due to an orientational transition of the LCs from a planar to a homeotropic state. However, the LCs display a bright appearance if they are pretreated with an aqueous solution containing urea and urease. This is caused by the disassemby of the SECs from the glass surface due to an increase of the pH value that is induced by the enzymatic hydrolysis of urea by urease. The method is highly sensitive and can detect urease activities as low as 0.03mU/mL. It can also be applied to the determination of heavy metal ions which exert an inhibitory effect on the activity of urease. For example, Cu(II) can be quantified via urease inhibition in 1nM concentration. Graphical abstract Schematic presentation of a liquid crystal-based sensor for detection of urease and heavy metal ions by using stimulus-responsive surfactant-encapsulated phosphotungstate clusters.

Analysis of crystal growth and viscosity in Ge-Sb-Se-Te undercooled melts

A comprehensive analysis on crystal growth and viscosity is presented for the Ge2Sb2Se5-xTex (x = 0.5 and 1) undercooled melts in this article. Temperature dependence of the measured viscosities (107.5–1012.5 Pa·s) can be described by a simple exponential Arrhenius type equation in the relatively narrow temperature region in which the measurements were performed. Fragility of the measured materials are all roughly equal to 43, which means that these materials belong to the so called “intermediate” liquids (lying in the middle between strong and fragile liquids) and cannot be described by simple Arrhenius type equation if the temperature region is expanded from glass transition to melting, which is important for description of crystal growth. Therefore, MYEGA equation is used to extrapolate the viscosity data into the temperature region of studied crystal growth. Nucleation and crystal growth started on a surface of studied samples. The isothermal growth rate of the formed surface crystalline layer was measured at various temperatures. Analysis of the growth data revealed a crystal growth driven by liquid-crystal interface kinetics. Therefore, the standard crystal growth models were used for description of crystal growth rates in the studied systems.

Kinetics of the Formation of a Disordered Crystal Structure during High-Speed Solidification

A phase–field description is given for the kinetic transition in the structure of a binary ordering crystal growing from a supercooled liquid. The results are interpreted on the basis of the solution of the phasefield dynamics equation and the relaxation equation for the long-range order parameter at the diffuse crystal–liquid interface and in the bulk of the crystal. The critical temperatures for the variation of the growth rate and the long-range order parameter of a growing A50B50 crystal are determined. A qualitative difference and analogies are found between the processes of nonequilibrium impurity trapping and the formation of a disordered structure of rapidly growing crystals. The results of the study are analyzed in comparison with the results of the theory of kinetic phase transitions [A.A. Chernov, Sov. Phys. JETP 26, 1182 (1967)] and the experimental data on the high-speed solidification of congruently melting samples [H. Hartmann et al., Europhys. Lett. 87, 40007 (2009)].

Dynamic optical properties of gold nanoparticles/cholesteric liquid crystal arrays

Abstract

Mesoscopic simulations of temperature-dependent anchoring and wetting behavior at aqueous-liquid crystal interfaces in the presence of a rod-coil amphiphilic monolayer.

Dissipative particle dynamics simulations have been applied to study the temperature dependent anchoring and wetting behavior of thermotropic liquid crystals (LCs) in the presence of a rod–coil amphiphilic monolayer at the aqueous–LC interface. Upon cooling in the nematic phase, a thermally-induced anchoring transition from homeotropic through tilted to planar has been observed. The growth and propagation of smectic order from the interfaces to the bulk nematic LCs are demonstrated to be mainly responsible for this novel transition sequence. In particular, when a complete smectic layer in the amphiphile monolayer is induced around the bulk transition of nematic–smectic-A, the propagation of homeotropic alignment fails instantly and a unique planar anchoring configuration is formed instead. While heating towards the isotropic phase, simulation results show that the nematic–isotropic transition of confined LCs is slightly shifted to a higher temperature, and a nematic wetting layer with homeotropic alignment appears in the rod block monolayer when the bulk LCs is isotropic. Our systematic simulations throughout the whole phase regimes of LCs provide important molecular-level insight into how the coupling between the ordering of LCs and adsorbents and their temperature dependencies affect the anchoring behavior in this complex system, which should be instrumental in the rational design and application of advanced LC-based biosensors with optimal operating temperature range.

Extended Study on Crystal Growth and Viscosity in Ge-Sb-Se Bulk Glasses and Thin Films.

Crystal growth rates in Ge18Sb28Se54 bulk glass and thin film were measured using optical and scanning electron microscopy under isothermal conditions. The studied temperature region was 255-346 °C and 254-286 °C for bulk glass and thin film, respectively. The compact crystalline layer growing from the surface into the amorphous core was formed in bulk glasses and no bulk crystallization was observed. In the case of thin films, needle-shape crystals were formed. The crystalline layer and needle-shape crystals grew linearly with time that corresponds to a crystal growth controlled by the crystal-liquid interface kinetics. In the narrow temperature range, crystal growth rates exhibit simple exponential behavior, so the activation energies of crystal growth for the studied temperature regions were estimated (EG = 294 ± 6 kJ/mol for bulk glass and EG = 224 ± 12 kJ/mol for thin film). Viscosity of Ge18Sb28Se54 material was measured in the region of the undercooled melt and glass. The extrapolation of viscosity data into the immeasurable, but important, temperature range is discussed. The experimental growth data were combined with melting and viscosity data and the appropriate growth models were proposed to describe crystal growth in a wide temperature region. The standard crystal growth models are based on a simple proportionality of the crystal growth rate to the viscosity (u ∝ η-1). This simple proportionality holds for the bulk material. Nevertheless, in the thin films the decoupling of the crystal growth rate from the inverse viscosity occurs, and the standard kinetic growth models need to be corrected. Such corrections provide better description of experimental data and more realistic value of the parameter describing the mean interatomic distance in the crystal-liquid interface layer, where the crystal growth takes place.

Crystal growth in fluid flow: Nonlinear response effects

We investigate crystal-growth kinetics in the presence of strong shear flow in the liquid, using molecular-dynamics simulations of a binary-alloy model. Close to the equilibrium melting point, shear flow always suppresses the growth of the crystal-liquid interface. For lower temperatures, we find that the growth velocity of the crystal depends nonmonotonically on the shear rate. Slow enough flow enhances the crystal growth, due to an increased particle mobility in the liquid. Stronger flow causes a growth regime that is nearly temperature-independent, in striking contrast to what one expects from the thermodynamic and equilibrium kinetic properties of the system, which both depend strongly on temperature. We rationalize these effects of flow on crystal growth as resulting from the nonlinear response of the fluid to strong shearing forces.

Strength of Alkane–Fluid Attraction Determines the Interfacial Orientation of Liquid Alkanes and Their Crystallization through Heterogeneous or Homogeneous Mechanisms

Alkanes are important building blocks of organics, polymers and biomolecules. The conditions that lead to ordering of alkanes at interfaces, and whether interfacial ordering of the molecules leads to heterogeneous crystal nucleation of alkanes or surface freezing, have not yet been elucidated. Here we use molecular simulations with the united-atom OPLS and PYS alkane models and the mW water model to determine what properties of the surface control the interfacial orientation of alkane molecules, and under which conditions interfacial ordering results in homogeneous or heterogeneous nucleation of alkane crystals, or surface freezing above the melting point. We find that liquid alkanes present a preference towards being perpendicular to the alkane–vapor interface and more parallel to the alkane–water interface. The orientational order in the liquid is short-ranged, decaying over ~1 nm of the surface, and can be reversed by tuning the strength of the attractions between alkane and the molecules in the other fluid. We show that the strength of the alkane–fluid interaction also controls the mechanism of crystallization and the face of the alkane crystal exposed to the fluid: fluids that interact weakly with alkanes promote heterogeneous crystallization and result in crystals in which the alkane molecules orient perpendicular to the interface, while crystallization of alkanes in the presence of fluids, such as water, that interact more strongly with alkanes is homogeneous and results in crystals with the molecules oriented parallel to the interface. We conclude that the orientation of the alkanes at the crystal interfaces mirrors that in the liquid, albeit more pronounced and long-ranged. We show that the sign of the binding free energy of the alkane crystal to the surface, ΔGbind, determines whether the crystal nucleation is homogeneous (ΔGbind ≥ 0) or heterogeneous (ΔGbind < 0). Our analysis indicates that water does not promote heterogeneous crystallization of the alkanes because water stabilizes more the liquid than the crystal phase of the alkane, resulting in ΔGbind > 0. While ΔGbind < 0 suffices to produce heterogeneous nucleation, the condition for surface freezing is more stringent, ΔGbind < −2 γxl, where γxl is the surface tension of the liquid–crystal interface of alkanes. Surface freezing of alkanes is favored by their small value of γxl. Our findings are of relevance to understanding surface freezing in alkanes and to develop strategies for controlling the assembly of chain-like molecules at fluid interfaces.

Transient nucleation and microstructural design in flash-annealed bulk metallic glasses

Due to their inherent metastability, metallic glasses crystallize upon thermal activation, which involves nucleation and growth. It is debatable, however, whether steady-state kinetics of crystal nucleation and growth apply when glasses are heated quickly. We partially devitrified a CuZr-based bulk metallic glass at heating rates in the range of 0.08–180 K s−1. Immediate quenching after heating preserves the partially transformed microstructure and allows direct observation of crystallite sizes and numbers. The crystallization reaction changes from eutectic to polymorphic at high heating rates and uniformly dispersed metastable shape-memory crystals precipitate in the glass. A beneficial side effect of this microstructure is a strongly enhanced room-temperature deformability. The analysis of crystal populations suggests that the nucleation rate on fast heating is effectively lower than in the steady state. At the later stage of crystallization by flash-annealing, the glass sample is traversed by a thermal front that migrates almost ten times faster than the crystal-liquid interface in levitation experiments. This phenomenon is related to the high population of crystals already present. Flash-annealing, thus, not only permits tailoring of BMG composites to overcome the intrinsic brittleness of monolithic BMGs, but also permits study of the fundamental crystallization processes in highly undercooled melts.