Bicontinuous Microstructure Research Articles

In-Situ Manufacturing of NiAl-TiC Composites with Three Dimensional (3D) Discrete Particular Network and Bi-Continuous Microstructures

NiAl-TiC intermetallic-ceramic composite system is a candidate for structural applications. Thus, in this research, near fully dense NiAl–xTiC composites [x = 0, 28 and 48 vol fraction percent (vol.%)] were produced through in-situ manufacturing process using exothermic synthesis reactions of Al, Ni, Ti and C precursors and their consolidation via hot press apparatus. NiAl-28 vol.% TiC composite showed a simple three dimensional (3D) discrete networks of TiC particular fillers in the continuous polycrystalline monolithic NiAl intermetallic matrix as a thermal management strategy for elevated temperature applications while NiAl-48 vol.% TiC composite indicated interconnectivity of TiC particles in certain regions of NiAl matrix known as bi-continuous microstructure. Reasons for the higher mechanical properties of NiAl-28 vol.% TiC in comparison monolithic NiAl were also discussed.

Self-similarity and coarsening rate of a convecting bicontinuous phase separating mixture: Effect of the viscosity contrast

We present a computational study of the hydrodynamic coarsening in 3D of a critical mixture using the Cahn-Hilliard/Navier-Stokes model. The topology of the resulting intricate bicontinuous microstructure is analyzed through the principal curvatures to prove self-similar morphological evolution. We find that the self similarity exists for both systems: iso-viscous and with variable viscosity. However the two system have distinct topological character. Our simulations confirm that the predicted viscous growth regime exists in both cases. Moreover the coarsening rate is inversely proportional to an \textit{effective viscosity} that is the geometrical average of the viscosities of the two phases.

Nanostructure-Dependent Marcus-Type Correlation of the Shape Recovery Rate and the Young's Modulus in Shape Memory Polymer Aerogels.

Thermodynamic-kinetic relationships are not uncommon, but rigorous correlations are rare. On the basis of the parabolic free-energy profiles of elastic deformation, a generalized Marcus-type thermodynamic-kinetic relationship was identified between the shape recovery rate, Rt( N), and the elastic modulus, E, in poly(isocyanurate-urethane) shape memory aerogels. The latter were prepared with mixtures of diethylene, triethylene, and tetraethylene glycol and an aliphatic triisocyanate. Synthetic conditions were selected using a statistical design of experiments method. Microstructures obtained in each formulation could be put into two groups, one consisting of micron-size particles connected with large necks and a second one classified as bicontinuous. The two types of microstructures could be explained consistently by spinodal decomposition involving early versus late phase separation relative to the gel point. Irrespective of microstructure, all samples showed a shape memory effect with shape fixity and shape recovery ratios close to 100%. Larger variations (0.35-0.71) in the overall figure of merit, the fill factor, were traced to a variability in the shape recovery rates, Rt( N), which in turn were related to the microstructure. Materials with bicontinuous microstructures were stiffer and showed slower recovery rates. Thereby, using the elastic modulus, E, as a proxy for microstructure, the correlation of Rt( N) with E was traced to a relationship between the activation barrier for shape recovery, Δ A#, and the specific energy of deformation, (reorganization energy, λ), which in turn is proportional to the elastic modulus. Data were fitted well ( R2 = 0.92) by the derived equations. The inverse correlation between Rt( N) and the elastic modulus, E, provides a means for qualitative predictability of the shape recovery rates, the fill factors, and the overall quality of the shape memory effect.

3D stochastic bicontinuous microstructures: Generation, topology and elasticity

Motivated by recent experimental investigations of the mechanical behavior of nanoporous metal we explore an efficient and robust method for generating 3D representative volume elements (RVEs) with strikingly similar behavior. Our approach adopts Cahn's method of generating a Gaussian random field by taking a superposition of standing sinusoidal waves of fixed wavelength but random in direction and phase. In its theory part, our study describes closed-form expressions for how the solid volume fraction affects the binarization level, mean structure size, specific surface area, averages of mean and Gaussian curvature, and the scaled topological genus. Based on numerical studies we report on criteria for achieving representative realizations of the structure by proper choice of the number of waves and element size. We also show that periodic structures are readily created. We analyze the mechanical properties considering linear and infinitesimal elasticity and evaluate the residual anisotropy (which can be made small) and the effective values of the Young's modulus and Poisson's ratio. The numerical results are in excellent agreement with experimental findings for the variation of stiffness with solid fraction of nanoporous gold made by dealloying. We propose scaling relations that achieve naturally a perfect agreement with the numerical and experimental data. The scaling relation for the stiffness accounts for a percolation-to-cluster transition in the random field microstructure at a finite solid fraction. We propose that this transition is the origin of the previously reported anomalous compliance of nanoporous gold.

Ultrafine-grained W–25 wt-%Cu composite with superior high-temperature characteristics

ABSTRACTIn the present work, several ultrafine-grained W–Cu composites with bi-continuous microstructure have been prepared through combined processes of high-energy ball-milling, liquid-phase sintering and infiltration. The microstructure, relative density, hardness and electrical conductivity of the W–Cu composites with copper content ranging from 20 to 35 wt-% were investigated to determine the optimal composition. Then, the high-temperature characteristics such as thermo-mechanical stability, high-temperature compressive performance and high-temperature wear behaviour of the optimal W–25 wt-%Cu composite were systematically assessed in comparison with those of the micron-sized W–25 wt-%Cu composite. The results showed that the ultrafine-grained W–Cu composite exhibited distinctly superior thermo-mechanical stability, as well as high-temperature compressive and wear-resistant properties, indicating its great potential in enduring repeated contact-loading at high temperature.

Emulsions: Simple shaking yields bicontinuity.

A macroscopically homogeneous emulsion with bicontinuous microstructure is created by simple shaking of a mixture containing polymers, nanoparticles and two immiscible fluids.

Modelling the elastic properties of bi-continuous composite microstructures captured with TriBeam serial-sectioning

The elastic and physical properties of a tungsten-copper (W-Cu) bi-continuous composite were predicted from microstructural data captured using TriBeam serial sectioning. The reconstructed 3D volume was converted into a Finite Element (FE) mesh. The minimum representative volume elements (RVEs) required for calculating phase volume fraction, Young's modulus and Poisson’s ratio were determined. The predicted volume fraction of Cu and Young's modulus were found to be within 2.6%, and 3.6% of the respective experimentally determined values. The minimum RVE size is found to be dependent on the material property. The variability in the required RVE size must be considered for material properties of W-Cu and similar bi-continuous composite microstructures.

Facile synthesis, characterization and catalytic activity of nanoporous supports loaded with monometallic and bimetallic nanoparticles

Abstract When an aqueous platinum chloride or mixed rhodium and platinum chloride solutions are added to a mixture of lecithin, AOT and titanium isopropoxide (TIP) in isooctane, the surfactants partition the aqueous and oil phases into a nanoscale bicontinuous microstructure. Hydrolysis and condensation of the TIP generates a highly connected porous anatase support. Reduction of the metal salt/s in the confined aqueous nanodomains of the bicontinuous template results in monometallic or bimetallic alloy NPs. The NPs are well-distributed throughout the interior surfaces of the support. This route, where monometallic and bimetallic nanoparticles are synthesized together with the support material, can be exploited for generating diverse catalytically active materials for various applications.

In Vitro Spoilation of Silicone-Hydrogel Soft Contact Lenses in a Model-Blink Cell

We developed an in vitro model-blink cell that reproduces the mechanism of in vivo fouling of soft contact lenses. In the model-blink cell, model tear lipid directly contacts the lens surface after forced aqueous rupture, mirroring the pre-lens tear-film breakup during interblink. Soft contact lenses are attached to a Teflon holder and immersed in artificial tear solution with protein, salts, and mucins. Artificial tear-lipid solution is spread over the air/tear interface as a duplex lipid layer. The aqueous tear film is periodically ruptured and reformed by withdrawing and reinjecting tear solution into the cell, mimicking the blink-rupture process. Fouled deposits appear on the lenses after cycling, and their compositions and spatial distributions are subsequently analyzed by optical microscopy, laser ablation electrospray ionization mass spectrometry, and two-photon fluorescence confocal scanning laser microscopy. Discrete deposit (white) spots with an average size of 20 to 300 μm are observed on the studied lenses, confirming what is seen in vivo and validating the in vitro model-blink cell. Targeted lipids (cholesterol) and proteins (albumin from bovine serum) are identified in the discrete surface deposits. Both lipid and protein occur simultaneously in the surface deposits and overlap with the white spots observed by optical microscopy. Additionally, lipid and protein penetrate into the bulk of tested silicone-hydrogel lenses, likely attributed to the bicontinuous microstructure of oleophilic silicone and hydrophilic polymer phases of the lens. In vitro spoilation of soft contact lenses is successfully achieved by the model-blink cell confirming the tear rupture/deposition mechanism of lens fouling. The model-blink cell provides a reliable laboratory tool for screening new antifouling lens materials, surface coatings, and care solutions.

Coupling process of phase separation and gelation in konjac glucomannan and gelatin system

The influence of konjac glucomannan (KGM) on the cool-induced gelation of gelatin was investigated using dynamic oscillatory rheological measurements at low strain amplitude and microstructural analysis by fluorescence microscopy. Rheological tests were measured during gel formation, including isothermal cooling and heating/cooling at a constant rate. The transition of phase compatibility to segregative phase separation with increase of KGM concentration was observed on KGM/gelatin mixture systems at 40 °C. The structural and mechanical of these hydrogels are dominated by the interplay between phase separation and gelation. In compatible phase status, the presence of KGM prolonged the gelation to occur for disordering the gelatin coil-helix transition, decreased the gel strength. The consequence of phase separation was dependent on KGM concentrations, resulting in bicontinuous microstructures. In the case of phase separation, the gelation was accelerated. It was demonstrated that by using different protein-polysaccharide mixtures at phase compatibility or separation concentrations could be feasible ways to control micro-networks and apply structural design in food formulation.

Composition as a Means To Control Morphology and Properties of Epoxy Based Dual-Phase Structural Electrolytes

Structural electrolytes were prepared using a fully formulated commercially available high performance epoxy resin (MTM57) and an ionic liquid based electrolyte: lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) dissolved in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI). Through a systematic study, the composition of the formulations was found to have a greater effect than the curing temperature on the morphology and properties of the resulting structural electrolytes. The presence of lithium salt is essential to form a structurally homogeneous electrolyte. Bicontinuous morphologies containing continuous (coarse) epoxy networks surrounded by connected spherical epoxy nodules were obtained with different length scales upon varying the lithium salt concentration. Increasing the LiTFSI concentration improved the miscibility of MTM57 with the electrolyte and decreased the characteristic length scale of the resulting bicontinuous microstructure. The properties of the structural electrolytes correlated with the morphology, showing increased Youngs modulus and decreased ionic conductivity with increasing lithium salt concentration. The miscibility of the epoxy system with the electrolyte was also improved by substitution of EMIM-TFSI with an equal weight of an aprotic organic solvent, propylene carbonate (PC); however, the window of PC concentrations which resulted in structural electrolytes with bicontinuous microstructures was very narrow; at PC concentrations above 1 wt %, gel-like polymers with no permanent mesoporosity were obtained.

Application of Quantitative 19F Nuclear Magnetic Resonance Spectroscopy in Tape-Stripping Experiments with Natural Microemulsions

The skin penetration of flufenamic acid (Fluf) and fluconazole (Fluc) from innovative natural microemulsions was investigated in tape-stripping experiments on pig ears. The formulations were based on the eudermic surfactants lecithin, sucrose laurate, alkylpolyglycoside or a mixture thereof. The quantification of the penetrated drug amounts was executed by (19) F nuclear magnetic resonance (NMR) in comparison with high-performance liquid chromatography (HPLC). The data obtained by the (19) F NMR method were confirmed by additional quantitative studies using HPLC. An excellent linear correlation was found for Fluf as well as for Fluc between (19) F NMR and HPLC data. This work presents a strategy outlining the use of (19) F NMR to selectively monitor the skin penetration routes of fluorinated compounds. Fluc penetrated generally well into the stratum corneum with the significantly highest amounts from the sucrose laurate microemulsion on the tape strips 1-5. Similarly, the highest amounts of penetrated Fluf could be observed from the formulation based on sucrose laurate. In addition, NMR self-diffusion studies were conducted and revealed a bicontinuous microstructure of the investigated microemulsions. The skin penetration results are in good agreement with the obtained (19) F NMR self-diffusion coefficients of the active compounds in the microemulsion systems.

Mechanical properties and scaling laws of nanoporous gold

Nanoporous metals are a class of novel nanomaterials with potential applications in many fields such as sensing, catalysis, and fuel cells. The present paper is aimed to investigate atomic mechanisms associated with the uniaxial tensile deformation behavior of nanoporous gold. A phase field method is adopted to generate the bicontinuous open-cell porous microstructure of the material. Molecular dynamics simulations then reveal that the uniaxial tensile deformation in such porous materials is accompanied by an accumulation of stacking faults in ligaments along the loading direction and their junctions with neighboring ligaments, as well as the formation of Lomer–Cottrell locks at such junctions. The tensile strain leads to progressive necking and rupture of some ligaments, ultimately resulting in failure of the material. The simulation results also suggest scaling laws for the effective Young's modulus, yield stress, and ultimate strength as functions of the relative mass density and average ligament size in the material.

Effects of confinement on phase separation kinetics and final morphology of whey protein isolate–gellan gum mixtures

The final microstructure of a phase separating and gelling biopolymer mixture is affected by physical confinement, because the structure evolution is limited by the surrounding surfaces. Here, we used various confining geometries such as microdroplets, parallel cover glasses, and networks of cellulose fibers to analyze the structure evolution and final morphology of mixtures of whey protein isolate (WPI) and gellan gum. The results were evaluated by confocal laser scanning microscopy (CLSM) and 2D fast Fourier transform (FFT) image analysis. This showed that the final morphology within the restricted geometry is determined by the characteristic wavelength of the bicontinuous bulk microstructure in relation to the dimensions of the confinements. The growth of the characteristic wavelength in the WPI–gellan gum system has two different regimes, which are similar in the bulk phase and large microdroplets. The growth is then kinetically trapped by the gelation. In microdroplets with a smaller dimension than the characteristic bulk wavelength, the growth of the spinodal structure is observed to collapse as the characteristic wavelength approaches the microdroplet diameter. This results in a core–shell structure with WPI present at the interface. Furthermore, 3D reconstructions of CLSM images from the parallel glass confinement showed that the bicontinuous structure is converted to columnar structures interconnecting two wetting layers at the glass surfaces. Similar columnar structures are observed between cellulose fibers. The present findings demonstrate that it is important to understand the impact of confinement on phase separation and gelation in order to control the final microstructure.

Evaluation of Cu-Cr3C2 composite with interpenetrating network

A new copper based bi-continuous composite was fabricated by infiltrating a preformed chromium carbide foam with molten copper at elevated temperature. The preformed chromium carbide foam was prepared using a reactive sintering process in a carbonaceous environment. The infiltration was conducted in a tube furnace under a reduction atmosphere with an ambient pressure. The obtained composite exhibited a homogenous structure with pronounced interfacial bonding. Some mechanical properties (hardness, elastic modulus) of the new composite were evaluated and compared with theoretically calculated values as well as other published composites. This composite shows relatively higher elastic property, which can be attributed to the feature of the bi-continuous homogeneous microstructure and the good interfacial bonding between the matrix phase and the reinforcing phase.

Microfluidic production of monodisperse biopolymer particles with reproducible morphology by kinetic control

Microdroplets of phase separating and gelling gelatin-maltodextrin mixtures were produced by a microfluidic technique. The microstructures observed inside the gelled particles were highly reproducible. This resulted from both the controlled production of monodisperse droplets in a microfluidic device and the tuning of gelation and phase separation kinetics.We showed that the internal particle microstructure can be tailored by varying the cooling rate (90 °C/min, 55 °C/min, 5 °C/min), the biopolymer composition (4% gelatin and 6%–7.3% maltodextrin) and the gelatin type (lime hide, pig skin). The particles were analyzed using confocal scanning laser microscopy and image analysis. Microstructures with smaller domain sizes were formed at the fastest cooling rate (90 °C/min), and microstructures with large domain sizes were obtained at the slowest cooling rate (5 °C/min). Furthermore, differences in particle morphology were observed at this slowest cooling rate. In particles containing pig skin gelatin, maltodextrin was located in the core, whereas gelatin was present at the water–oil interface. The opposite was observed for particles consisting of lime hide gelatin where the maltodextrin was found toward the oil phase. The results also showed that a higher concentration of maltodextrin formed larger bicontinuous microstructures compared to the ones obtained with lower concentrations.

Making Organic‐Inorganic Nanocomposites via Selective Dispersion of PS‐Tethered SiO2 Particles in Polystyrene‐Block‐Polymethylmethacrylate Copolymer

SiO2 nanoparticles have been dispersed selectively in the polystyrene (PS) microdomain of polystyrene‐block‐polymethylmethacrylate (PS‐b‐PMMA) block copolymer via the blending of PS‐b‐PMMA with PS‐tethered SiO2. As observed by atomic force microscopy and scanning electron microscopy, the incorporation of SiO2 particles not only enlarges the PS microdomain but also reduces the surface energy of the PS microdomain and transforms the morphology from either lamellar layers or cylinders to islanded bicontinuous microstructures. Blending SiO2 particles with an excessive amount or with a particle size larger than that of the PS microdomain would pose an extreme constraint on the molecular rearrangement, unstabliize the microdomain separation, and even make the microdomain separation unobservable. The nanosize and the uniform distribution of the PS microdomain in the PS‐b‐PMMA polymer have thus enabled us to achieve a uniform distribution of the inorganic SiO2 particles in the organic polymeric matrix.

Release of fluorescent markers from phase‐separated gelatin‐maltodextrin hydrogels

AbstractGenipin‐crosslinked gelatin‐maltodextrin phase‐separated hydrogels consisting of gelatin‐continuous or bicontinuous microstructures were developed to regulate swelling and release behavior of four fluorescent markers of varying molecular weights [(fluorescein (332 Da) and FITC‐dextrans (FD) (4000–250,000 Da)]. Bicontinuous hydrogels showed significantly greater swelling than gelatin‐continuous hydrogels under all conditions (at pH 1.5 and 7.4 and three genipin/gelatin crosslinking ratios) (P < 0.05). With both microstructures, fluorescein showed the largest release rate and total release followed by FD 4000 Da, FD 40,000 Da, and FD 250,000 Da (P < 0.05). Marker molecular weight, pH, and crosslink ratio all affected the rate and amount of release. The mode of transport for the solvent and all markers was Fickian or slightly anomalous, with diffusional exponent (n) values ranging from 0.35 to 0.64. These results demonstrated that with the proper combination of crosslink density, solvent pH, and microstructure, hydrogels with a specified swelling behavior may be developed. This, coupled with a marker of appropriate size, can lead to controllable levels and rates of release. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Location of anti-TB drugs and microstructural changes in organized surfactant media using optical properties

Microstructure elucidation is a critical step in the understanding of organized surfactant media such as microemulsions. In this work, a systematic approach for revelation of microstructure changes of Tween 80 microemulsion has been carried out using photophysical properties. Microemulsion structural changes have been monitored using pyrene and different solvatochromic probes for the presence of oil-rich, water-rich, and bicontinuous microstructures. The hydrophobic optical probe, Nile red (NR), has been used to report from the side of the oil–surfactant interface and to complement this, a hydrophilic optical probe tris(2,2′-bipyridine)ruthenium(II) dichloride (RC) from the side of the water–surfactant interface. The results show the partitioning of probes into different microenvironments within the system. NR occupies the interface toward the apolar side and RC toward the polar. The analysis also indicates conspicuous dependence of the emission/absorbance behavior of optical probes on the water:surfactant molar ratio ( ω). Quantitative estimates have been obtained for the polarities (ET(30)) of solubilization sites of the probes in different regions of microemulsions and are well correlated with a Stokes shift. Lastly, these probes of different solubilities have been used to locate the position of anti-tuberculosis drugs, i.e., rifampicin (hydrophobic), pyrazinamide, and isoniazid (hydrophilic) of variable solubilities loaded in Tween 80 systems.

Spatial correlations in symmetric and asymmetric bicontinuous structures

Spatial correlations of interfacial curvature are compared for symmetric and asymmetric two-phase mixtures produced following spinodal decomposition as given by a numerical solution to the Cahn–Hilliard equation in three dimensions. By calculating radial distribution functions of the density of interfacial area as a function of the mean interfacial curvature of these bicontinuous microstructures, it is found that long-range diffusive interactions, in combination with the morphology of the system, yield a variety of correlations and anticorrelations over a range of length scales. The asymmetric mixtures show some similarities to the symmetric mixtures, as well as other unique features.