Fabrication Of Microstructures Research Articles

Fabrication of high-aspect-ratio metallic microstructures by microelectroforming using silver-coated polydimethylsiloxane molds with controllable wettability

Abstract Microelectroforming is a specialized electroplating process to prepare functional metallic microstructures. However, the formability of microelectroforming is usually restricted by the limited mass transfer in high-aspect-ratio microcavities of molds. This paper presents a simple and reliable method utilizing silver (Ag)-coated polydimethylsiloxane (PDMS) molds with controllable wettability to enhance the formability of microelectroforming. The surfaces of these molds exhibited reversible water contact angles ranging from 4° to 151° realized through ultraviolet irradiation and heat treatment. The hydrophilicity of the PDMS molds facilitated liquid-phase mass transfer, contributing to the fabrication of complete and defect-free nickel microstructures with high aspect ratios. Subsequently, the hydrophobic PDMS molds reduced the interfacial adhesion between these molds and nickel microstructures, which was beneficial for perfect demolding. Nickel microstructures with an aspect ratio of 10 can be achieved by using the PDMS molds, which significantly enhance the formability of microelectroforming. This method provides a potential method to prepare high-aspect-ratio metallic microstructures.

Optically induced fabrication of three-dimensional photonic lattice microstructures by using cutting-combination lens

A convenient method for producing three-dimensional photonic lattice microstructures was experimentally demonstrated by using a cutting-combination lens. Three-dimensional photonic lattices are optically induced by (4 + 1) plane waves interference in an iron-doped lithium niobate photorefractive crystal. Induced three-dimensional lattice microstructures are observed and verified by planar guided wave intensity imaging and far-field diffraction pattern. This method is inexpensive and does not require complex adjustment devices. In addition, it also has flexibility and scalability. This method can be used to fabricate other more complex three-dimensional photonic lattice microstructures by designing the cutting-combination lens appropriately.

An integrated exposure and measurement tool for 5-DOF direct laser writing

Accurate and uniform fabrication of microstructures on highly curved substrates requires exposure with the waist of a focused laser beam at every point. In order to realize this, the exposure beam must be held perpendicular and focused onto the local substrate. Here we present an optical tool for our developed 5-axis nano-positioning and nano-measurement machine based on the chromatic differential confocal microscope.

Unidirectional Freezing of Polymer Solution Droplets.

Ice templating provides a means of generating textures with a well-defined topography. Recent applications involve the freezing of water droplets, with or without colloids, on flat or textured surfaces. An interesting feature of water droplets freezing on a substrate is the formation of a pointy tip at a constant angle, regardless of the substrate temperature, surface energy, or droplet volume. Here, by adding the polymer to water, we demonstrate how to manipulate and even prevent the formation of such an icy tip. We find that the sharpness of the tip decreases with increasing polymer concentration until completely disappearing above the overlap concentration, while the total freezing time increases concomitantly. Building on these observations, we combined simple geometrical arguments with heat flux measurements to model and connect the spatial and temporal evolution of polymer droplets under unidirectional freezing. Together our results provide new ways to control the shape of frozen droplets for ice templating or microstructure fabrication, with applications in tissue engineering, separation membranes, and soft robotics.

Sub-microscale 3D printing based on cold-constrained local electrodeposition

ABSTRACT Locally confined electrodeposition of precursors in liquid is one of the most effective approaches for additive manufacturing of metal microstructures. However, reported data were overwhelmingly obtained at the micrometer scale, whereas sub-microscale and nanoscale microstructures are of interest in electromagnetic or optical fields. In this paper, we propose a cold-constrained local electrodeposition (CCLE) technique to address the issue that locally confined electrodeposition of precursors in liquid is challenging in the fabrication of sub-microscale three-dimensional microstructures. Microstructure printing is conducted via locally confined electrodeposition by a pipette with a sub-microscale opening to deliver electrodeposition electrolyte in a supporting electrolyte. The electrolytes are cooled to 5 °C, which constrains the large-scale diffusion of the electrodeposition electrolyte and significantly reduces the electrodeposition area. It is found that the proposed CCLE technique can print sub-microscale (minimum size is low to 290 nm) structures, which have highly uniform deposits, smooth surfaces and high strength.

Fabrication of hollow fiber membrane microstructures and the investigation of antifouling mechanism for vegetable oil wastewater

In this study, four kinds of hollow fiber ultrafiltration membranes with different hydrophilic chains and surface microstructures were prepared via the “physical blending-surface grafting” method. The influence of surface microstructures for the formation process of the vegetable oil fouling layer on the surface of membranes and antifouling performance were investigated. By characterizing the changes in adhesion force on the surface of membranes and the surface charge of emulsified oil droplets before and after separation, the differences in the flocculation and coalescence processes for emulsified oil droplets on different membrane surfaces were revealed. The synergistic effect of “the flocculation of physical contact by polymer chains, the hydrophilic repulsion of hydration layer and electrostatic neutralization”, led to the formation of large-sized oil droplets on the surface of membranes with weakly charged surface and loose accumulation ultimately, which will in turn weaken the adsorption of pollutants and the irreversible fouling can be delayed. The synergistic antifouling mechanism of “hydrophilic repulsion of the hydration layer-grafting long chains barrier & coalescence” was proposed based on the membrane's antifouling performance, the adhesion and spreading process of pollutants on the membrane surface, as well as the differences of the surface hydration layer microstructure. The membrane modified by grafting PEI and PVA exhibits excellent retention performance (oil rejection rate ∼100 %) and resistance to oil-in-water emulsion fouling (flux recovery rate (FRR) = 99.12 %), weakest adhesion force (9.32 nN) and minimal adhesion mass (1.7 mg) of pollutants on the surface of membrane. This work offers new research insights into the theoretical study and fabrication of hollow fiber ultrafiltration membranes with high oil fouling resistance.

Dynamic visible-infrared transmittance regulation based on fluoroaluminate glass by femtosecond-laser-customized induced microstructure

Herein, we report significant microstructure-induced dynamic visible–infrared transmittance regulation on a novel fluoroaluminate glass (AlF3) via femtosecond-laser-customized fabrication technology. Fluoroaluminate glass, which has excellent optical transmittance in the visible–infrared wavelength range (400–5000 nm), is a candidate for visible and infrared optical devices. Based on its performance, three types of microstructures or material transitions (Snow-Layer Microstructure (SLS), Surface-Ablation Microstructure (SAS), and Interior-Modified Microstructure (IMS)) were successfully achieved using a femtosecond-laser-customized microstructure, including surface and interior fabrication routes with cylindrical lens focusing. Dynamic and reversible structural transition features were observed between the snow-layer and surface-ablation microstructures. These types of structures showed different transmittance regulation trends at visible and infrared wavelengths. The transmittance is decreased for IMS, SAS, and SLS in the visible region, while in the infrared region the transmittance increased for IMS and decreased for SAS and SLS. Based on Raman spectroscopy analyses, the chemical structures of the SAS before and after femtosecond-laser ablation appeared to remain constant, whereas the SLS showed a significant difference, giving rise to different transmittance regulations. This study provides a novel fabrication method and strategy for regulating visible and infrared transmission and promotes smart application of glass materials.

Liquid Crystalline Network Microstructures for Stimuli Responsive Labels with Multi-Level Encryption.

Two-photon direct laser writing enables the fabrication of shape-changing microstructures that can be exploited in stimuli responsive micro-robotics and photonics. The use of Liquid Crystalline Networks (LCN) allows to realize 3D micrometric objects that can contract along a specific direction in response to stimuli, such as temperature or light. In this paper, the fabrication of free-standing LCN microstructures is demonstrated as graphical units of a smart tag for simple physical and optical encryption. Using an array of identical pixels, information can be hidden to the observer and revealed only upon application of a specific stimulus. The reading mechanism is based on the shape-change of each pixel under stimuli and their color that combinetogether in a two-level encryption label. Once the stimulus is removed, the pixels recover their original shape and the message remains completely hidden. Therefore, an opto-mechanical equivalent of an "invisible ink" is realized. This new concept paves the way for introducing enhanced functionalities in smart micro-systems within a single lithography step, spanning from storage devices with physical encryption to complex motion actuators.

Two-Photon Direct Laser Writing of 3D Scaffolds through C, H-Insertion Crosslinking in a One-Component Material System.

The popularity of two-photon direct laser writing in biological research is remarkable as this technique is capable of 3D fabrication of microstructures with unprecedented control, flexibility and precision. Nevertheless, potential impurities such as residual monomers and photoinitiators remaining unnoticed from the photopolymerization in the structures pose strong challenges for biological applications. Here, the first use of high-precision 3D microstructures fabricated from a one-component material system (without monomers and photoinitiators) as a 3D cell culture platform is demonstrated. The material system consists of prepolymers with built- in crosslinker motieties, requiring only aliphatic C, H units as reaction partners following two-photon excitation. The material is written by direct laser writing using two-photon processes in a solvent-free state, which enables the generation of structures at a rapid scan speed of up to 500mms-1 with feature sizes scaling down to few micrometers. The generated structures possess stiffnesses close to those of common tissue and demonstrate excellent biocompatibility and cellular adhesion without any additional modification. The demonstrated approach holds great promise for fabricating high-precision complex 3D cell culture scaffolds that are safe in biological environments.

Additive manufacturing of 3D surface microstructures through repetitively nucleated plasmonic bubbles

Photothermally triggered bubbles have been widely used to fabricate various surface microstructures. So far, studies have mostly focused on the fabrication of planar microstructures. In this study, we present an approach to achieve three-dimensional (3D) fabrication of surface microstructures via plasmonic bubbles in an ethanol suspension of polystyrene (PS) microparticles. We will show that the dispersed PS microparticles will be guided by a thermal Marangoni convection flow around a plasmonic bubble and deposited at the three-phase contact line. Interestingly, we find that PS microparticles tend to deposit along the vertical direction driven by the repetitively nucleated plasmonic bubbles, resulting in the formation of a 3D hollow microcylinder. The detailed process and the mechanism of the microstructure formation were systematically investigated. Finally, microstructures of different sizes were readily fabricated, exhibiting good controllability. Our method paves the way for 3D microstructure manufacturing using photothermal microbubbles.

The Application of Electron-Beam Welding in Pellet Mold Preparation

This paper provides insight into the application of electron-beam welding in pellet mold preparation, highlighting the importance of the combination of electron-beam welding and pellet mold preparation in the fields of microstructure joining and micro- and nanostructure preparation. Precise material joining and microstructure fabrication can be achieved by the precise control of electron-beam welding and the shape adjustment of pellet molds. These applications hold significant potential in the modern industrial field, providing robust support for the development of new materials and the growth of the petrochemical industry. This paper asserts that in the future, the ongoing development of electron-beam welding and pelletizing template technology will unlock new possibilities in the field of petrochemicals, fostering progress in science and technology.

Fabrication of Plasmonic Microcubes by Laser Ablation of Au‐Nanoparticles‐Loaded Acrylate

The fabrication of plasmonic 3D microstructures is still a challenge. In this study, the fabrication of gold‐nanoparticle (Au‐NP)‐filled acrylate‐based micron‐scale cuboids forming plasmonic 3D particles is investigated. UV curable acrylate is mixed with HAuCl4 and spin‐coated onto a fused silica wafer. UV light and heat treatment of the layer result in polymerization of the acrylate and formation of Au‐NPs with a plasmonic resonance ranging from 535 to 550 nm. The plasmonic resonance wavelength decreases slightly with increasing UV dose. The resonance wavelength can be shifted by a subsequent annealing process. The decreasing resonance wavelength with increasing annealing temperature is discussed as a result of the increasing Au‐NPs. The size and the distribution of the resulting gold particles in the acrylate matrix is investigated by transmission electron microscopy. The Au‐NPs‐filled acrylate layer is laser structured using a UV‐femtosecond laser, which allows the fabrication of adjustable micro‐cuboids with edge lengths down to 15 μm. The laser‐cut micro‐cuboids are transferred into isopropanol and the plasmonic properties of the structures in the liquid are successfully demonstrated. The presented concept allows easy and large‐scale fabrication of plasmonic microstructures with independently adjustable plasmonic properties and microstructural size and shape.

Femtosecond laser assisted selective etching of microchannels in lithium niobate.

Here, we report on the fabrication of cm-long microchannels in LiNbO3 by selective etching of femtosecond laser inscribed tracks using hydrofluoric acid. We achieved a 1 cm long microchannel after 300 h of etching a single track inscribed into the volume along the optical axis of LiNbO3. Furthermore, we investigated the dependence of the etching behavior on various writing parameters. Highly selective etching with a selectivity up to 104 was achieved and a functional relationship between the etched depth and time was found. Thus, our results set the first milestone for future fabrication of 3D-hollow microstructures in the volume of LiNbO3 combining its outstanding physical properties such as the strong nonlinearity as well as the acousto- and electrooptic properties with both microfluidic and photonic structures in a monolithic setup.

Micromachining on Stainless Steel 304 for Improved Water Condensation Properties

Microstructure fabrication and chemical surface functionalization with low-surface-energy materials are the key steps to achieve hydrophobic surfaces with high water droplet contact angles (CA). In this work we employed wire Electric Discharge Machining (EDM) as a way to induce microstructure topography on stainless steel 304 coupons. The resulting topography was rendered hydrophobic using trichloro-1H,1H,2H,2H-perfluorooctyl silane (PFOTS) via gas phase deposition. The channels created by machining and PFOTS functionalization facilitate water condensation by increasing nucleation sites and enhancing droplet coalescence. The resulting surface is hydrophobic (CA~140o) in contrast to the bare stainless steel 304, which is hydrophilic (CA~76o).

Femtosecond laser direct writing of pure three-dimensional fluorescent protein and its application to physiological pH sensing

Femtosecond laser direct write (fs-LDW), a three-dimensional (3D) printing technology, is a promising method for creating microstructures made of proteins that retain their original function, enabling the development of complex biomimetic 3D microenvironments and versatile enhancements of medical microdevices. Fabrication using pure proteins via photoactivator-free femtosecond laser multiphoton crosslinking has recently been demonstrated This approach avoids the undesirable effects caused by the leaching of photoactivator molecules, and is thus regarded as suitable for biological applications. Here, we present the 3D fabrication of microstructures made of pure fluorescent protein variants, namely mScarlet, enhanced green fluorescent protein (EGFP), and enhanced blue fluorescent protein (EBFP2). Multicolor fluorescent microstructures are created using a sequential procedure with various precursors. We evaluate the dependence of fluorescence retention on fabrication parameters such as the total accumulated fluence. We demonstrate that microstructures made of EGFP can be applied to the detection of physiological pH changes. The results show that fs-LDW fabrication can broaden the application scope of fluorescent protein variants.

Microchannels Formed Using Metal Microdroplets.

The metal microdroplet deposition manufacturing technique has gained extensive attention due to its potential applications in microstructure fabrication. In order to fabricate components such as microchannel heat sinks and microchannel reactors, this paper investigates the interactions and influences between microdroplets and substrates, as well as between microdroplets themselves. The transient phenomena during the fusion of metal microdroplets in contact with the substrate and the formation of inclined columns, as well as the solid-liquid coupling and morphology formation processes during the collision between microdroplets, are analyzed. The influence of microdroplet spacing on the morphology of microchannels during their formation is specifically studied. A three-dimensional finite element numerical model for the deposition of metal microdroplets forming inclined pillars is established based on the volume of fluid (VOF) method. The model treats the protective gas around the microdroplet as an empty zone and the microdroplet as a single-phase fluid. Simulation analysis is conducted to investigate the forming patterns of unsupported microdroplets at different spacing and their impact on the fusion morphology of microchannel components. Building upon this, a series of validation experiments are conducted using a piezoelectric microdroplet generator to produce uniform aluminum alloy microdroplets with a diameter of approximately 600 μm. A method for fabricating metal microchannel structures is obtained, which is expected to be applied in fields such as scattering structures for high-power electronic devices and microreactors in microchemical fields.

Unleashing multi-scale mechanical enhancement in NiTi shape memory alloy via annular intra-laser deposition with homogenized Ti2Ni nanoprecipitates

Additive manufacturing (AM) of composition-sensitive NiTi shape memory alloys (SMAs) is hindered by uncontrollable non-equilibrium microstructures, leading to functional performance degradation and limitations in high-throughput net-shaping applications. To address these challenges, we propose a novel approach combining annular intra-laser directed energy deposition (AIL-DED) with a tailored gradient post-treatment process. This enables the fabrication of nanocomposite microstructures in NiTi SMAs with precise control over the homogenization of Ti2Ni nanoprecipitates. In-situ monitoring and characterization of thermal history, microstrain, and constitutional phases, were utilized to uncover the relationship between multi-scale structural features and mechanical responses. The interplay between laser reflectivity, energy input, and lattice thermal vibrations affects forming quality in AIL-DED. Dynamic temperature variations were studied before reaching thermal equilibrium, revealing insights into thermal and mechanical interactions during the process. These findings contribute to understanding challenges in fabricating high-quality NiTi SMAs. Examining phase transformation behavior and its correlation with processing parameters deepens understanding of microstructural evolution and provides insights for tailoring material properties. The stress relaxation and dislocation evacuation at elevated temperatures caused the expansion of lattice and interplanar spacing in the age-treated SMA's B2 matrix phase, exhibiting a stable two-step phase transformation behavior (B19′↔R↔B2). AIL-DED combined with two-stage heat treatment strategy demonstrated enhanced homogeneity of non-equilibrium microstructures, resulting in the formation of high-density, well-dispersed Ti2Ni nanoprecipitates, with a semi-coherent interface to B2 matrix. The combination of load-bearing strengthening through well-dispersed Ti2Ni nanoprecipitates and Orowan strengthening at the semi-coherent interface yielded simultaneous enhancements in nanohardness, ultimate compressive strength, compressive fracture strain, and deformation recovery, following the AIL-DED combined with two-stage heat treatment process. The fracture mechanism transitioned from a mixture of brittle-ductile behavior to predominantly ductile characteristics. This study provides valuable insights that advance additive manufacturing and enhance the functional performance of SMAs in critical applications.

Femtosecond laser fabrication of silicon surface microstructures for enhanced infrared radiation under electric drive

Silicon surface microstructures can significantly improve their reflection and absorption characteristics and are widely used in photovoltaics and optoelectronic detection. But there is little research on the infrared emission characteristics of microstructure silicon since silicon is an indirect bandgap material. Here, we fabricate surface microstructures on highly doped silicon by femtosecond laser scanning, enhancing its broadband infrared radiation significantly under electric driving. At 12 μm, it can be enhanced to 274% of the untreated silicon. Our results show that silicon with surface microstructures can be used as a wideband infrared emitter. This study provides a new method for realizing a broad-spectrum silicon-based electrical drive infrared light source.

Fabrication of large-depth and multiscale gradient microstructure in dual-phase high entropy alloy via shock loading

Large-depth and multiscale gradient microstructure in dual-phase Al0.6CoCrFeNi high entropy alloy (HEA) were fabricated by explosion hardening (EH) technique. The gradient microstructure evolution was investigated in detail. The microhardness distribution and tensile properties near EH affected surface were also determined. The formation and strengthening mechanisms of gradient microstructure after EH treatment were subsequently analyzed. Experimental results showed that the multiscale gradient microstructure in terms of grain size, phase morphology, dislocation structure, texture was induced in dual-phase (BCC + FCC) HEA by EH technique. The closer to EH affected surface, the finer grain size, higher dislocation density and stronger texture intensity. The deformation and break of spinodal decomposition and formation of new grain boundaries by dislocation structure contributed to grain refinement and transition of equiaxed grains to dendritic grains from lower area to EH affected surface. After EH treatment, the microhardness generally increased with a maximum increase of ∼67% within 5 mm from EH affected surface, the tensile yield stress increased from 320 MPa to maximum of 700 MPa. The strengthening mechanism is different at various depth positions: near EH affected surface, the main strengthening mechanism is high-density dislocations, stacking faults and grain refinement; in areas relatively far from EH affected surface, only dislocation structure contributed to the strengthening of mechanical properties.

ZnO-Bi2O3 Heterostructured Composite for the Photocatalytic Degradation of Orange 16 Reactive Dye: Synergistic Effect of UV Irradiation and Hydrogen Peroxide

The development of semiconductor photocatalysts has recently witnessed notable momentum in the photocatalytic degradation of organic pollutants. ZnO is one of the most widely used photocatalysts; however, its activity is limited by the inefficient absorption of visible light and the fast electron–hole recombination. The incorporation of another metal or semiconductor with ZnO boosts its performance. In this present study, a heterostructured ZnO-Bi2O3 composite was synthesized via a simple co-precipitation method and was investigated for the UV-driven photocatalytic degradation of the Reactive Orange 16 (RO16), a model textile dye. The successful fabrication of ZnO-Bi2O3 microstructures with crystalline nature was characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX). The discoloration of the dye solution was quantified using UV–Vis spectroscopy to determine the photocatalytic efficiency. The photocatalytic activity results demonstrated that the photodegradation at ZnO-Bi2O3 heterojunction was more efficient and 300 and 33% faster than individual Bi2O3 and ZnO catalysts, respectively, an effect that is indicative of a synergistic effect. In the presence of ZnO-Bi2O3 particles, the UV light-driven activity for RO16 degradation was twice as high as in its absence. The influence of adding the oxidant H2O2 on the UV-induced photocatalytic degradation was investigated and the results revealed a two-time increase in the photocatalytic activity of ZnO-Bi2O3 compared to UV irradiation alone, which could be ascribed to a summative degradative effect between UV and H2O2. Hence, this approach holds the potential for environmentally friendly wastewater treatment.