Suppressing Crack Formation in Particulate Systems by Utilizing Capillary Forces

Synthesis and Characterization of Core-Double-Shell-Structured PVDF-grafted-BaTiO3/P(VDF-co-HFP) Nanocomposite Films

Core-double-shell-structured nanocomposite films consisting of polyvinylidene fluoride-grafted-barium titanate (PVDF-g-BT) incorporated into a P(VDF-co-hexafluoropropylene (HFP)) copolymer matrix were produced via a solution mixing method for energy storage applications. The resulting films were thoroughly investigated via spectroscopic, thermal, and morphological analyses. Thermogravimetric data provided an enhancement of the thermal stability, while differential scanning calorimetry indicated an increase in the crystallinity of the films after the addition of PVDF-g-BT. Moreover, broadband dielectric spectroscopy revealed three dielectric processes, namely, glass-rubber relaxation (α_a), relaxation associated with the polymer crystalline phase (α_c), and slower relaxation in the nanocomposites resulting from the accumulation of charge on the interface between the PVDF-g-BT filler and the P(VDF-co-HFP) matrix. The dependence of the dielectric constant from the composition was analyzed, and we found that the highest permittivity enhancement was obtained by the highest concentration filler added to the largest concentration of P(VDF-co-HFP). Mechanical analysis revealed an improvement in Young's modulus for all nanocomposites versus pristine P(VDF-co-HFP), confirming the uniformity of the distribution of the PVDF-g-BT nanocomposite with a strong interaction with the copolymer matrix, as also evidenced via scanning electron microscopy. The suggested system is promising for use in high-energy-density storage devices as supercapacitors.

Crack-Free Structural Color Materials Prepared without Disrupting the Particle Arrangement by Controlling the Internal Stress Relaxation and Interactions of the Melanin Particles

In fabricating structural color materials with assembled colloidal particles, there is a trade-off between the internal stresses acting on the particles and the interactions between the particles during solvent volatilization. It is crucial to fabricate crack-free materials that maintain the periodic arrangements of the particles by understanding the mechanism for crack initiation. Here, we focused on the composition and additives of melanin particle dispersions to obtain crack-free structural color materials without disturbing the particle arrangements. The use of a water/ethanol mixture as a dispersant effectively reduced the internal stresses of the particles during solvent evaporation. Furthermore, the addition of low-molecular-weight, low-volatility ionic liquids ensured that the arrangement and interactions of the particles were maintained after solvent volatilization. Optimization of the composition and additives of the dispersion made it possible to achieve crack-free melanin-based structural color materials while maintaining vivid, angular-dependent color tones.

Printing Crack-Free Microporous Structures by Combining Additive Manufacturing with Colloidal Assembly

To date high printing resolution and scalability, i.e., macroscale component dimensions and fast printing, are incompatible characteristics for additive manufacturing (AM) processes. It is hereby demonstrated that the combination of direct writing as an AM process with colloidal assembly enables the breaching of this processing barrier. By tailoring printing parameters for polystyrene (PS) microparticle-templates, how to avoid coffee ring formation is demonstrated, thus printing uniform single lines and macroscale areas. Moreover, a novel "comb"-strategy is introduced to print macroscale, crack-free colloidal coatings with low viscous colloidal suspensions. The printed templates are transformed into ceramic microporous channels as well as photonic coatings via atomic layer deposition (ALD) and calcination. The obtained structures reveal promising wicking capabilities and broadband reflection in the near-infrared, respectively. This work provides guidelines for printing low viscous colloidal suspensions and highlights the advancements that this printing process offers toward novel applications of colloidal-based printed structures.

Evaporative Deposition of Surfactant-Laden Nanofluid Droplets over a Silicon Surface

Morphologies of evaporative deposition, which has been widely applied in potential fields, were induced by the competition between internal flows inside evaporating droplets. Controlling the pattern of deposition and suppressing the coffee-ring effect are essential issues of intense interest in the aspects of industrial technologies and scientific applications. Here, evaporative deposition of surfactant-laden nanofluid droplets over silicon was experimentally investigated. A ring-like deposition was formed after complete evaporation of sodium dodecyl sulfate (SDS)-laden nanofluid droplets with an initial SDS concentration ranging from 0 to 1.5 CMC. In the case of initial SDS concentrations above 1.3 CMC, no cracks were observed in the ring-like deposition, indicating that the deposition patterns of nanofluid droplets could be completely changed and cracks could be eliminated by sufficient addition of SDS. With the increase of the initial concentration of hexadecyl trimethylammonium bromide (CTAB), the width of the deposition ring gradually decreased until no ring-like structure was formed. On the contrary, with the increase of the initial Triton X-100 (TX-100) concentration, the width of the deposition ring gradually increased until a uniform deposition was generated. Moreover, when the initial TX-100 concentration was high, a "tree-ring-like" pattern was discovered. Besides, morphologies of evaporative pattern due to the addition of surfacants were qualitatively analyzed.

Giant Functional Properties in Porous Electroceramics through Additive Manufacturing of Capillary Suspensions

Dedicated hierarchical structuring of functional ceramics can be used to shift the limits of functionality. This work presents the manufacturing of highly open porous, hierarchically structured barium titanate ceramics with 3-3 connectivity via direct ink writing of capillary suspension-type inks. The pore size of the printed struts (∼1 μm) is combined with a printed mesostructure (∼100 μm). The self-organized particle network, driven by strong capillary forces in the ternary solid/fluid/fluid ink, results in a high strut porosity, and the distinct flow properties of the ink allow for printing high strut size to pore size ratios, resulting in total porosities >60%. These unique and highly porous additive manufactured log-pile structures with closed bottom and top layers enable tailored dielectric and electromechanical coupling, resulting in an energy harvesting figure of merit FOM₃₃ more than four times higher than any documented data for barium titanate. This clearly demonstrates that combining additive manufacturing of capillary suspensions in combination with appropriate sintering allows for creation of complex architected 3D structures with unprecedented properties. This opens up opportunities in a broad variety of applications, including electromechanical energy harvesting, electrode materials for batteries or fuel cells, thermoelectrics, or bone tissue engineering with piezoelectrically stimulated cell growth.

Magnetic-field-induced self-assembly of FeCo/CoFe2O4 core/shell nanoparticles with tunable collective magnetic properties

Self-assembly of nanoparticles into ordered patterns is a novel approach to build up new consolidated materials with desired collective physical properties. Herein, nanoparticle assemblies of composition-modulated bimagnetic nanoparticles have been produced via slow evaporation of their colloidal suspension in the absence or presence of magnetic fields. The assemblies obtained in the presence of the magnetic fields exhibit oriented nanoparticle chains in face-centered cubic superlattice structures, compared with the hexagonal closed-packed superlattice obtained without the magnetic field. The oriented structure has an alignment of the easy magnetization axis along the chains. This alignment leads to enhanced intra-superlattice interactions. As a result, the field-induced assembly displays collective magnetic properties with significantly enhanced magnetic anisotropy, remanent magnetization and coercivity. It is also found that the bimagnetic FeCo/CoFe2O4 core/shell nanostructure enhances the intra-particle interaction and thus is beneficial for the growth of oriented assembly of nanoparticles. Furthermore, the collective magnetic behavior is evidenced by the observation of a superferromagnetic-like magnetization relaxation in the ac-susceptibility curves.

Using an added liquid to suppress drying defects in hard particle coatings

HYPOTHESIS

Lateral accumulation and film defects during drying of hard particle coatings is a common problem, typically solved using polymeric additives and surface active ingredients, which require further processing of the dried film. Capillary suspensions with their tunable physical properties, devoid of polymers, offer new pathways in producing uniform and defect free particulate coatings.

EXPERIMENTS

We investigated the effect of small amounts of secondary liquid on the coating's drying behavior. Stress build-up and weight loss in a temperature and humidity controlled drying chamber were simultaneously measured. Changes in the coating's reflectance and height profile over time were related with the weight loss and stress curve.

FINDINGS

Capillary suspensions dry uniformly without defects. Lateral drying is inhibited by the high yield stress, causing the coating to shrink to an even height. The bridges between particles prevent air invasion and extend the constant drying period. The liquid in the lower layers is transported to the interface via corner flow within surface pores, leading to a partially dry layer near the substrate while the pores above are still saturated. Using capillary suspensions for hard particle coatings results in more uniform, defect free films with better printing characteristics, rendering high additive content obsolete.

Visualizing Heterogeneity of Monodisperse CdSe Nanocrystals by Their Assembly into Three-Dimensional Supercrystals

We show that the self-assembly of monodisperse CdSe nanocrystals synthesized at lower temperature (∼310 °C) into three-dimensional supercrystals results in the formation of separate regions within the supercrystals that display photoluminescence at two distinctly different wavelengths. Specifically, the central portions of the supercrystals display photoluminescence and absorption in the orange region of the spectrum, around 585 nm, compared to the 575 nm photoluminescence maximum for the nanocrystals dispersed in toluene. Distinct domains on the surfaces and edges of the supercrystals, by contrast, display photoluminescence and absorption in the green region of the spectrum, around 570 nm. We attribute the different-colored domains to two subpopulations of NCs in the monodisperse ensemble: the nanocrystals in the "orange" regions are chemically stable, whereas the nanocrystals in the "green" regions are partially oxidized. The susceptibility of the "green" nanocrystals to oxidation indicates a lower coverage of capping molecules on these nanocrystals. We propose that the two subpopulations correspond to nanocrystals with different surfaces that we attribute to the polytypism of CdSe.

Thick Free-Standing Metallic Inverse Opals Enabled by New Insights into the Fracture of Drying Particle Films

Metallic inverse opals are porous materials with enhanced mechanical, chemical, thermal, and photonic properties used to improve the performance of many technologies, such as battery electrodes, photonic devices, and heat exchangers. Cracking in the drying opal templates used to fabricate inverse opals, however, is a major hindrance to the use of these materials for practical and fundamental studies. In this work, we conduct desiccation experiments on polystyrene particle opals self-assembled on indium-tin oxide coated substrates to study their fracture mechanisms, which we describe using an energy-conservation fracture model. The model incorporates film yielding, particle order, and interfacial friction to explain several experimental observations, including thickness-dependent crack spacings, cracking stresses, and order-dependent crack behavior. Guided by this model, we are the first to fabricate 120 μm thick free-standing metallic inverse opals, which are 4 times thicker than previously reported non-free-standing metallic inverse opals. Moreover, by controlling cracks, we achieve a crack-free single-crystal domain up to 1.35 mm², the largest ever reported in metallic inverse opals. This work improves our understanding of fracture mechanics in drying particle films, provides guidelines to reduce crack formation in opal templates, and enables the fabrication of free-standing large-area single-crystal inverse opals.

Bismaleimide bridged silsesquioxane aerogels with excellent heat resistance: effect of sol-gel solvent polarity

Due to the poor heat-resistance and intrinsic weakness of the bridging moieties in aerogel matrixes, it remains greatly challenging to fabricate highly thermostable and toughened silsesquioxane aerogels. By utilizing bismaleimide as the bridging part and optimizing the solvent polarity, lightweight (ρ < 0.09 g cm-3), compressible (80% strain) and superhydrophobic (CA ≈ 150°) bismaleimide bridged silsesquioxane aerogels (BMIT-BSAs) are constructed. The microstructure and compressive modulus of BMIT-BSAs can be tuned by the sol-gel solvents with different polarities. Moreover, stable low-temperature wettability at -196 °C and a significantly increased initial deposition temperature of 336 °C for both N2 and O2 atmospheres were measured, demonstrating the wide temperature tolerance of BMIT-BSAs.

Additive Manufacturing of Optical Quality Germania-Silica Glasses

Direct ink writing (DIW) three-dimensional (3D) printing provides a revolutionary approach to fabricating components with gradients in material properties. Herein, we report a method for generating colloidal germania feedstock and germania-silica inks for the production of optical quality germania-silica (GeO₂-SiO₂) glasses by DIW, making available a new material composition for the development of multimaterial and functionally graded optical quality glasses and ceramics by additive manufacturing. Colloidal germania and silica particles are prepared by a base-catalyzed sol-gel method and converted to printable shear-thinning suspensions with desired viscoelastic properties for DIW. The volatile solvents are then evaporated, and the green bodies are calcined and sintered to produce transparent, crack-free glasses. Chemical and structural evolution of GeO₂-SiO₂ glasses is confirmed by nuclear magnetic resonance, X-ray diffraction, and Raman spectroscopy. UV-vis transmission and optical homogeneity measurements reveal comparable performance of the 3D printed GeO₂-SiO₂ glasses to glasses produced using conventional approaches and improved performance over 3D printed TiO₂-SiO₂ inks. Moreover, because GeO₂-SiO₂ inks are compatible with DIW technology, they offer exciting options for forming new materials with patterned compositions such as gradients in the refractive index that cannot be achieved with conventional manufacturing approaches.

Structure of capillary suspensions and their versatile applications in the creation of smart materials

In this article, we review recent research in the field of capillary suspensions and highlight a variety of applications in the field of smart materials. Capillary suspensions are liquid-liquid-solid ternary systems where one liquid is only present in a few percent and induces a strong, capillary-induced particle network. These suspensions have a large potential for exploitation, particularly in the production of porous materials since the paste itself and the properties of the final material can be adapted. We also discuss the rheological properties of the suspension and network structure to highlight the various ways these systems can be tuned.

Negative normal stress differences N1-N2 in a low concentration capillary suspension

Here, negative normal stress differences are reported in capillary suspensions, i.e. particle suspensions in a two-fluid system that creates strong capillary attractions, at a solid concentration of 25%, and a volume fraction that has heretofore been considered too low to show such normal stress differences. Such capillary suspensions have strong particle networks and are shear thinning for the entire range of shear rates studied. Capillary suspensions exist in two states: a pendular state when the secondary fluid preferentially wets the particles, and a capillary state when the bulk fluid is preferentially wetting. In the pendular state, the system undergoes a transition from a positive normal stress difference at high shear rates to a negative stress difference at low shear rates. These results are an indication of flexible flocs in the pendular state that are able to rotate to reorientate in the vorticity direction under shear. Analogous experiments were also conducted for the capillary state, where only a negative normal stress difference occurs. The capillary state system forms more network contacts due to droplet breakup at higher shear rates, which enhances the importance of hydrodynamic interactions in the non-colloidal suspension.

Fractal approaches to characterize the structure of capillary suspensions using rheology and confocal microscopy

The rheological properties of a particle suspension can be substantially altered by adding a small amount of a secondary fluid that is immiscible with the bulk phase. The drastic change in the strength of these capillary suspensions arises due to the capillary forces, induced by the added liquid, leading to a percolating particle network. Using rheological scaling models, fractal dimensions are deduced from the yield stress and from oscillatory strain amplitude sweep data as function of the solid volume fraction. Exponents obtained using aluminum-oxide-based capillary suspensions, with a preferentially wetting secondary fluid, indicate an increase in the particle gel's fractal dimension with increasing particle size. This may be explained by a corresponding relative reduction in the capillary force compared to other forces. Confocal images using a glass model system show the microstructure to consist of compact particle flocs interconnected by a sparse backbone. Thus, using the rheological models two different fractal dimensionalities are distinguished - a lower network backbone dimension (D = 1.86-2.05) and an intrafloc dimension (D = 2.57-2.74). The latter is higher due to the higher local solid volume fraction inside of the flocs compared to the sparse backbone. Both of these dimensions are compared with values obtained by analysis of spatial particle positions from 3D confocal microscopy images, where dimensions between 2.43 and 2.63 are computed, lying between the two dimension ranges obtained from rheology. The fractal dimensions determined via this method corroborate the increase in structural compactness with increasing particle size.