Suppression of cracking in drying colloidal suspensions with chain-like particles.

The prevention of drying-induced cracking is crucial in maintaining the mechanical integrity and functionality of colloidal deposits and coatings. Despite exploring various approaches, controlling drying-induced cracking remains a subject of great scientific interest and practical importance. By introducing chain-like particles composed of the same material and with comparable size into commonly used colloidal suspensions of spherical silica nanoparticles, we can significantly reduce the cracks formed in dried particle deposits and achieve a fivefold increase in the critical cracking thickness of colloidal silica coatings. The mechanism underlying the crack suppression is attributed to the increased porosity and pore sizes in dried particle deposits containing chain-like particle, which essentially leads to reduction in internal stresses developed during the drying process. Meanwhile, the nanoindentation measurements reveal that colloidal deposits with chain-like particles exhibit a smaller reduction in hardness compared to those reported using other cracking suppression approaches. This work demonstrates a promising technique for preparing colloidal coatings with enhanced crack resistance while maintaining desirable mechanical properties.

Varying and unchanging whiteness on the wings of dusk-active and shade-inhabiting Carystoides escalantei butterflies.

Whiteness, although frequently apparent on the wings, legs, antennae, or bodies of many species of moths and butterflies, along with other colors and shades, has often escaped our attention. Here, we investigate the nanostructure and microstructure of white spots on the wings of Carystoides escalantei, a dusk-active and shade-inhabiting Costa Rican rain forest butterfly (Hesperiidae). On both males and females, two types of whiteness occur: angle dependent (dull or bright) and angle independent, which differ in the microstructure, orientation, and associated properties of their scales. Some spots on the male wings are absent from the female wings. Whether the angle-dependent whiteness is bright or dull depends on the observation directions. The angle-dependent scales also show enhanced retro-reflection. We speculate that the biological functions and evolution of Carystoides spot patterns, scale structures, and their varying whiteness are adaptations to butterfly's low light habitat and to airflow experienced on the wing base vs. wing tip.

Bidirectional-Reinforced Carbon Fiber/Polyether-Ether-Ketone Composite Thin-Walled Pipes via Pultrusion-Winding for On-Orbit Additive Manufacturing.

Benefitting from lightweight, high strength, long life, and green recyclability, continuous fiber reinforced thermoplastic composite (CFTPC) pipes have attracted extensive interest, especially in the on-orbit additive manufacturing of structural components. However, the preparation of CFTPC pipes remains challenging due to the on-orbit limited space and high processing temperature of thermoplastic resin. Here, we report an effective approach for high performance carbon fiber/polyether-ether-ketone (CF/PEEK) thin-walled pipes via bidirectional reinforcement using the pultrusion-winding technique. The continuous fabrication of thin-walled pipes can be achieved, but the limitation by the size of core mold is also broken. The compressive and shear performance of CF/PEEK pipes with different layer designs have been studied based on experiments and simulations. With the increase in axial prepreg tape layer, the resultant CF/PEEK pipes exhibit greatly improved axial compression strength. The finite element analysis indicates that the maximum axial stress is decreased due to the axial enhancement. The flexural strength is greatly proved with pultrusion-winding cycles. The simulation confirms that the circumferential strain is effectively reduced. The high performance of bidirectional reinforced CF/PEEK pipes and the facile controllability of this approach highlight their suitability for utilization in on-orbit manufacturing of large-scale structures.

Two-dimensional Si(x)Ge(1-x) films with variable composition made via multilayer colloidal template-guided ionic liquid electrodeposition.

The binary alloy system Si(x)Ge(1-x) provides a continuous series of materials with gradually varying properties. In this paper, we report on a fundamental basis a method to make large-area macroporous Si(x)Ge(1-x) films with variable Ge content by electrodeposition in an ionic liquid, with SiCl(4) and GeCl(4) as precursors. The chemical composition of the products can be modified by changing the molar ratio of the precursors. Periodical macroporous Si(x)Ge(1-x) was made by a multilayer polystyrene (PS) template assembled as face-centered cubic lattice. Two-dimensional (2-D) Si(x)Ge(1-x) bowl-like and fishing-net structures can be obtained by applying different deposition temperatures. The results highlight the potential applications, including photonic bandgap and battery materials, as well as ultra-thin gratings, due to the effect of modification of light and improved tunability of composition, although Si(x)Ge(1-x) made by our method is sensitive to oxidation by air.

Two-Step Preparation of CCF/PEEK Wrapped Yarn for 3D Printing Composites with Enhanced Mechanical Properties.

Continuous fiber reinforced thermoplastic composites (CFTPCs) have shown advantages such as high strength, long life, corrosion resistance, and green recyclability. Three-dimensional printing of CFTPCs opened up a new strategy for the fabrication of composites with complicated structures, low cost, and short production cycles. However, a traditional 3D printing process usually causes poor impregnation of the fiber or surface damage of the fiber due to the short impregnation time or high viscosity of the thermoplastic resin. Here, continuous carbon fiber/poly(ether-ether-ketones) (CCF/PEEK) wrapped yarn was fabricated via powder impregnation and using double spinning technology for the 3D printing. The concentration of PEEK powder suspension and wire speed were optimized as 15% and 2.0 m/min. The twist of wrapped yarn was optimized as 1037 T/m. Mechanical testing showed that the 3D-printed composite wire had excellent tensile and bending strength, which was about 1.6~4.2 times larger than those without the powder pre-impregnation process. It is mainly attributed to the improved impregnation of the CF which took place during the powder pre-impregnation process. We believe that our research on wrapped yarn for 3D-printed composites provides an effective strategy for the 3D printing of composites with enhanced mechanical properties.

Review on Mechanoresponsive Smart Windows: Structures and Driving Modes.

The growing awareness about the global energy crisis and extreme weather from global warming drives the development of smart windows market. Compared to conventional electrochromic, photochromic, or thermochromic smart windows, mechanoresponsive smart windows present advantages of simple construction, low cost, and excellent stability. In this review, we summarize recent developments in mechanoresponsive smart windows with a focus on the structures and properties. We outline the categories and discuss the advantages and disadvantages. Especially, we also summarize six unconventional driving modes to generate mechanical strain, including pneumatic, optical, thermal, electric, magnetic, and humidity modes. Lastly, we provide practical recommendations in prospects for future development. This review aims to provide a useful reference for the design of novel mechanoresponsive smart windows and accelerate their practical applications.

Mechano-Optical Response Behavior of Polymer-Dispersed Cholesteric Liquid Crystals for Reversible and Highly Sensitive Force Recorders.

Force recording (mode, intensity, and orientation) is of great importance in medical rehabilitation, military reconnaissance, space exploration, etc. However, sensors with both reversibility and memorability are still challenging. Here, a reversible sensor based on polymer-dispersed cholesteric liquid crystals (CLC) is developed as a force recorder. Based on the microarea mechano-optical response and finite element analysis, it is confirmed that the mechanochromic response is mediated by the shear deformation of the polymer network and neighboring CLC. There is an obvious quantitative relationship between force intensity, mode, orientation, and the microarea optical response. Moreover, the sensing layer with a lower modulus or thickness is advantageous for a more sensitive device with lower starting pressure. Additionally, the excellent sensitivity and accuracy also highlight the potential applications in force analysis, path tracking, or pattern detection.

Underwater Highly Pressure-Sensitive Fabric Based on Electric-Induced Alignment of Graphene.

Wearable pressure sensors have received widespread attention owing to their potential applications in areas such as medical diagnosis and human-computer interaction. However, current sensors cannot adapt to extreme environments (e.g., wet and underwater) or show moderate sensitivity. Herein, a highly sensitive and superhydrophobic fabric sensor is reported based on graphene/PDMS coating. This wearable sensor exhibits great superhydrophobicity (water contact angle of 153.9°) due to the hydrophobic alkyl long chains and rough structure introduced by the Ar plasma. Owing to the network structure created by the electric-induced alignment of graphene sheets, an enhanced sensitivity (ΔI/I0 of 55) and fast response time (~100 ms) are observed. Due to its superhydrophobicity and sensitivity, this wearable sensor demonstrates efficient and stable monitoring of various underwater activities, including pressure, blowing, and tapping. Our approach provides an alternative idea for highly sensitive wearable sensors while broadening the practical application scope.

Three-dimensionally ordered macroporous silicon films made by electrodeposition from an ionic liquid.

Three dimensionally ordered macroporous (3DOM) silicon films have been made via ordered polystyrene (PS) templates by electrodeposition from an ionic liquid (IL). For this purpose, the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py(1,4)]Tf(2)N) with SiCl(4) dissolved in it was used as an electrolyte and the electrodeposition of macroporous silicon could be achieved at room temperature (~20 °C). Self-assembled PS colloidal crystals with different diameters were used as templates. Scanning electron microscopy and X-ray photoelectron spectroscopy confirm the quality of the samples, and the optical transmission measurement demonstrates that the 3DOM silicon film has a bandgap in the near infrared regime. Such a material has the potential to make 3DOM silicon feasible for electrical and optical applications.

Enhanced IR Radiative Cooling of Silver Coated PA Textile.

Smart textile with IR radiative cooling is of paramount importance for reducing energy consumption and improving the thermal comfort of individuals. However, wearable textile via facile methods for indoor/outdoor thermal management is still challenging. Here we present a novel simple, yet effective method for versatile thermal management via silver-coated polyamide (PA) textile. Infrared transmittance of coated fabric is greatly enhanced by 150% due to the multi-order reflection of silver coating. Based on their IR radiative cooling, indoors and outdoors, the skin surface temperature is lower by 1.1 and 0.9 °C than normal PA cloth, allowing the textile to be used in multiple environments. Moreover, the coated fabric is capable of active warming up under low voltage, which can be used in low-temperature conditions. These promising results exemplify the practicability of using silver-coated textile as a personal thermal management cloth in versatile environments.

Recoverable underwater superhydrophobicity from a fully wetted state via dynamic air spreading.

Maintaining the superhydrophobicity underwater offers drag resistance reduction, antifouling, anti-corrosion, noise reduction, and gas collection for boat hulls and submarine vehicles. However, superhydrophobicity typically does not last long underwater since the Cassie state is metastable. Here, we report a reversible and localized recovery of superhydrophobicity from the fully wetted state via air bubble spreading. Composed of sparse fluorinated chained nanoparticles, the submerged surface shows super-low energy barrier for bubble attachment. Especially the recovered plastron exhibits excellent longevity. Based on a simplified, truncated nanocone model, the dynamic spreading of bubbles is analyzed considering two basic parameters, i.e., surface geometric structure and surface energy (which appeared as intrinsic water contact angle). Numerical simulation results via COMSOL confirms the effect of geometric structure on bubble spreading. This investigation will not only offer new insights for the design of robust recoverable superhydrophobic surfaces but also broaden the applications of superhydrophobic coatings.

Earthworm-Inspired Ultradurable Superhydrophobic Fabrics from Adaptive Wrinkled Skin.

Wrapped by periodically wrinkled skin, soft earthworm shows excellent robustness against sticky soil. Mimicking this deformation adaptability, here, we report an ultradurable superhydrophobic fabric by exploiting the formation of adaptive, soft wrinkled poly(dimethylsiloxane) (PDMS) skins. Uniform wrinkles are created on woven fabric fibers due to the surface instability of PDMS coating with a cross-linking gradient induced by Ar plasma treatment. Both the surface topography of wrinkles and the viscoelasticity of the underlying compliant layer to release stress endow the treated superhydrophobic fabrics with extraordinary durability, withstanding 800 standard laundries or 1000 rubbing cycles under 44.8 kPa. Additionally, superhydrophobic fabrics are self-healable after heating or plasma treatment. This insight of engineering soft skins with periodic submicron surface topography and gradient modulus provides a pathway for the design of ultradurable, multifunctional wearables.

Highly conductive and transparent coatings from flow-aligned silver nanowires with large electrical and optical anisotropy.

Conductive and transparent coatings consisting of silver nanowires (AgNWs) are promising candidates for emerging flexible electronics applications. Coatings of aligned AgNWs offer unusual electronic and optical anisotropies, with potential for use in micro-circuits, antennas, and polarization sensors. Here we explore a microfluidics setup and flow-induced alignment mechanisms to create centimeter-scale highly conductive coatings of aligned AgNWs with order parameters reaching 0.84, leading to large electrical and optical anisotropies. By varying flow rates, we establish the relationship between the shear rate and the alignment and investigate possible alignment mechanisms. The angle-dependent sheet resistance of the aligned AgNW networks exhibits an electronic transport anisotropy of ∼10× while maintaining low resistivity (<50 Ω sq-1) in all directions. When illuminated, the aligned AgNW coatings exhibit angle- and polarization-dependent colors, and the polarized reflection anisotropy can be as large as 25. This large optical anisotropy is due to a combination of alignment, polarization response, and angle-dependent scattering of the aligned AgNWs.

Multistate and On-Demand Smart Windows.

Composite films consisting of wrinkles on top of the elastomeric poly(dimethylsiloxane) film and a thin layer of silica particles embedded at the bottom is prepared as on-demand mechanoresponsive smart windows. By carefully varying the wrinkle geometry, silica particle size, and stretching strain, different initial optical states and a large degree of optical transmittance change in the visible to near infrared range with a relatively small strain (as small as 10%) is achieved. The 10% pre-strain sample has shallow wrinkles with a low amplitude and shows moderate transmittance (60.5%) initially and the highest transmittance of 86.4% at 550 nm when stretched at the pre-strain level. Stretching beyond the pre-strain level leads to a drastic decrease of the transmittance at 550 nm, 39.7% and 70.8% with an additional 10% and 30% strain, respectively. The large drop of optical transmittance is the result of combined effects from the formation of secondary wrinkles and nanovoids generated around the particles. The 20% pre-strain sample has wrinkles with a moderate amplitude, showing 36.9% transmittance in the initial state, and the highest transmittance of 71.5% at 550 nm when stretched to the pre-strain level. Further stretching leads to increased opacity similar to that seen from the 10% pre-strain sample.

A bio-inspired homogeneous graphene oxide actuator driven by moisture gradients.

An actuator driven by moisture gradients has been developed from a homogeneous graphene oxide film, relying on the in situ formation of a bilayer structure induced by water adsorption. This actuator shows efficient and controllable bending motions, coupled with the capability of lifting objects 8 times heavier than itself.

Cuts Guided Deterministic Buckling in Arrays of Soft Parallel Plates for Multifunctionality.

Harnessing buckling instability in soft materials offers an effective strategy to achieve multifunctionality. Despite great efforts in controlling the wrinkling behaviors of film-based systems and buckling of periodic structures, the benefits of classical plate buckling in soft materials remain largely unexplored. The challenge lies in the intrinsic indeterminate characteristics of buckling, leading to geometric frustration and random orientations. Here, we report the controllable global order in constrained buckling of arrays of parallel plates made of hydrogels and elastomers on rigid substrates. By introducing patterned cuts on the plates, the randomly phase-shifted buckling in the array of parallel plates transits to a prescribed and ordered buckling with controllable phases. The design principle for cut-directed deterministic buckling in plates is validated by both mechanics model and finite element simulation. By controlling the contacts and interactions between the buckled parallel plates, we demonstrate on-demand reconfigurable electrical and optical pathways, and the potential application in design of mechanical logic gates. By varying the local stimulus within the plates, we demonstrate that microscopic pathways can be written, visualized, erased, and rewritten macroscopically into a completely new one for potential applications such as soft reconfigurable circuits and logic devices.

Colloidal inks from bumpy colloidal nanoparticles for the assembly of ultrasmooth and uniform structural colors.

Angle-independent structural colors obtained from colloidal nanoparticles (NPs) are of interest for potential applications in displays, color printing, 3D printing, and direct writing. However, it remains challenging to prepare uniform structural colors that can be directly written from colloidal inks that not only have no coffee-ring effect, but also have ultrasmooth film thickness, which will be important for layer-by-layer stacking. Herein, we synthesize polypyrrole (PPy) black coated silica NPs that have a low coverage (∼10.7 wt%) of bumpy PPy nanogranules (10-30 nm in diameter). When the PPy@silica NPs are drop-cast on a substrate, the surface roughness of the PPy@silica NPs effectively suppresses the coffee-ring effect and center aggregation during the drying of the colloidal ink, leading to ultrasmooth surfaces with sub-micron thickness and uniform structural colors with wide viewing angles. The color can be fine-tuned by the size of silica NPs, and the presence of PPy black significantly enhances the color contrast by suppressing incoherent and multiple light scattering. Moreover, we show that the bumpy colloidal ink is very versatile: the ink can be formulated from both low and high surface tension liquids as solvents and applied to a hydrophilic or hydrophobic substrate. We demonstrate direct writing of uniformly colored lines and three different color drops stacked on top of each other.

Production of Structural Colors with High Contrast and Wide Viewing Angles from Assemblies of Polypyrrole Black Coated Polystyrene Nanoparticles.

Structural color with wide viewing angles has enormous potential applications in pigment, ink formulation, displays, and sensors. However, colors obtained from colloidal assemblies with low refractive index contrast or without black additives typically appear pale. Here, we prepare polypyrrole (PPy) black coated polystyrene (PS) nanoparticles and demonstrate well-defined colors with high color contrast and wide viewing angles under ambient light. Depending on the loading of pyrrole during polymerization, PPy nanogranules of different sizes and coverages are grafted to the surface of PS nanoparticles. The bumpy particles can self-assemble into quasi-amorphous arrays, resulting in low angle dependent structure colors under ambient light. The color can be tuned by the size of the PS nanoparticles, and the presence of the PPy black on PS nanoparticles enhances the color contrast by suppressing incoherent and multiple scattering.

A high-efficiency superhydrophobic plasma separator.

To meet stringent limit-of-detection specifications for low abundance target molecules, a relatively large volume of plasma is needed for many blood-based clinical diagnostics. Conventional centrifugation methods for plasma separation are not suitable for on-site testing or bedside diagnostics. Here, we report a simple, yet high-efficiency, clamshell-style, superhydrophobic plasma separator that is capable of separating a relatively large volume of plasma from several hundred microliters of whole blood (finger-prick blood volume). The plasma separator consists of a superhydrophobic top cover with a separation membrane and a superhydrophobic bottom substrate. Unlike previously reported membrane-based plasma separators, the separation membrane in our device is positioned at the top of the sandwiched whole blood film to increase the membrane separation capacity and plasma yield. In addition, the device's superhydrophobic characteristics (i) facilitates the formation of well-defined, contracted, thin blood film with a high contact angle; (ii) minimizes biomolecular adhesion to surfaces; (iii) increases blood clotting time; and (iv) reduces blood cell hemolysis. The device demonstrated a "blood in-plasma out" capability, consistently extracting 65 ± 21.5 µL of plasma from 200 µL of whole blood in less than 10 min without electrical power. The device was used to separate plasma from Schistosoma mansoni genomic DNA-spiked whole blood with a recovery efficiency of >84.5 ± 25.8%. The S. mansoni genomic DNA in the separated plasma was successfully tested on our custom-made microfluidic chip by using loop mediated isothermal amplification (LAMP) method.

A robust smart window: reversibly switching from high transparency to angle-independent structural color display.

A smart window is fabricated from a composite consisting of elastomeric poly(dimethylsiloxane) embedded with a thin layer of quasi-amorphous silica nanoparticles. The smart window can be switched from the initial highly transparent state to opaqueness and displays angle-independent structural color via mechanical stretching. The switchable optical property can be fully recovered after 1000 stretching/releasing cycles.