Development of controlled nanosphere lithography technology

This work is devoted to the development of nanosphere lithography (NSL) technology, which is a low-cost and efficient method to form nanostructures for nanoelectronics, as well as optoelectronic, plasmonic and photovoltaic applications. Creating a nanosphere mask by spin-coating is a promising, but not sufficiently studied method, requiring a large experimental base for different sizes of nanospheres. So, in this work, we investigated the influence of the technological parameters of NSL by spin-coating on the substrate coverage area by a monolayer of nanospheres with a diameter of 300 nm. It was found that the coverage area increases with decreasing spin speed and time, isopropyl and propylene glycol content, and with increasing the content of nanospheres in solution. Moreover, the process of controllably reducing the size of nanospheres in inductively coupled oxygen plasma was studied in detail. It was determined that increasing the oxygen flow rate from 9 to 15 sccm does not change the polystyrene etching rate, whereas changing the high-frequency power from 250 to 500 W increases the etching rate and allows us to control the decreasing diameter with high accuracy. Based on the experimental data, the optimal technological parameters of NSL were selected and the nanosphere mask on Si substrate was created with coverage area of 97.8% and process reproducibility of 98.6%. Subsequently reducing the nanosphere diameter lets us obtain nanoneedles of various sizes, which can be used in field emission cathodes. In this work, the reduction of nanosphere size, silicon etching, and removal of polystyrene residues occurred in unified continuous process of plasma etching without sample unloading to atmosphere.

Damage-free LED lithography for atomically thin 2D material devices

Desired electrode patterning on two-dimensional (2D) materials is a foremost step for realizing the full potentials of 2D materials in electronic devices. Here, we introduce an approach for damage-free, on-demand manufacturing of 2D material devices using light-emitting diode (LED) lithography. The advantage of this method lies in mild photolithography by simply combining an ordinary optical microscope with a commercially available LED projector; the low-energy red component is utilized for optical characterization and alignment of devices, whereas the high-energy blue component is utilized for photoresist exposure and development of personal computer designed electrode patterns. This method offers maskless, damage-free photolithography, which is particularly suitable for 2D materials that are sensitive to conventional lithography. We applied this LED lithography to device fabrication of selected nanosheets (MoS₂, graphene oxides and RuO₂), and achieved damage-free lithography of various patterned electrodes with feature sizes as small as 1-2 μm. The LED lithography offers a useful approach for cost-effective mild lithography without any costly instruments, high vacuum, or complex operation.

Self-Assembled Asperities for Pressure-Tunable Adhesion

Control of adhesion is important in a host of applications including soft robotics, pick-and-place manufacturing, wearable devices, and transfer printing. While there are adhesive systems with discrete switchability between states of high and low adhesion, achieving continuously variable adhesion strength remains a challenge. In this work, a pressure-tunable adhesive (PTA) that is based on the self-assembly of stiff microscale asperities on an elastomeric substrate is presented. It is demonstrated that the adhesion strength of the PTA increases with the applied compressive preload due to the unique contact formation mechanism caused by the asperities. Additionally, a contact mechanics model is developed to explain the resulting trends. For a specific PTA design, the critical pull-off force can be increased from 0.4 to 30 mN by increasing the applied preload from 1 to 30 mN. Finally, the applicability of precision control of adhesion strength is demonstrated by utilizing the PTA for pick-and-place material handling. The approach in pressure-tunable adhesive design based on self-assembly of asperities presents a scalable and versatile approach that is applicable to a variety of material systems having different mechanical or surface properties.

A new class of transformable kirigami metamaterials for reconfigurable electromagnetic systems

The rapid development of radio frequency (RF) components requires smart multifunctional materials that can adapt their physical shapes and properties according to the environment. While most current reconfigurable systems provide limited flexibility with high manufacturing cost, this research proposes to harness the transformable properties of kirigami-inspired multistable mechanical metasurfaces that can repeatedly deform and lock into different configurations to realize a novel class of low-cost reconfigurable electromagnetic structures with a broad design space. The metasurfaces are formed by designing kinematic-based unit cells with metallised coating that can provide adjustable resonant electromagnetic (EM) properties while rotating with respect to each other. Tailoring the cut length and geometry parameters of the patterns, we demonstrate programming of the topologies and shapes of different configurations. The influence of critical parameters on the structural multistability is illustrated by means of both a simplified energy model and finite element simulations. As examples of the reconfigurable electromagnetic devices that can be realized, we report the development of a tuneable half-wave dipole and two frequency selective surface (FSS) designs featuring isotropic and anisotropic responses. While the kirigami dipole can be tuned by mechanically stretching its arms, the FSSs exhibit distinct transmittance and reflectance spectra in each of the kirigami patterns stable states. The functionality of these kirigami devices is validated both by full-wave EM simulations and experiments. The proposed transformable structures can be mechanically actuated to tune the EM response in frequency or induce anisotropies for wave propagation.

3D printing of hollow geometries using blocking liquid substitution stereolithography

Micrometer scale arbitrary hollow geometries within a solid are needed for a variety of applications including microfluidics, thermal management and metamaterials. A major challenge to 3D printing hollow geometries using stereolithography is the ability to retain empty spaces in between the solidified regions. In order to prevent unwanted polymerization of the trapped resin in the hollow spaces-known as print-through-significant constraints are generally imposed on the primary process parameters such as resin formulation, exposure conditions and layer thickness. Here, we report on a stereolithography process which substitutes the trapped resin with a UV blocking liquid to mitigate print-through. We investigate the mechanism of the developed process and determine guidelines for the formulation of the blocking liquid. The reported method decouples the relationship between the primary process parameters and their effect on print-through. Without having to optimize the primary process parameters to reduce print-through, hollow heights that exceed the limits of conventional stereolithography can be realized. We demonstrate fabrication of a variety of complex hollow geometries with cross-sectional features ranging from tens of micrometer to hundreds of micrometers in size. With the framework presented, this method may be employed for 3D printing functional hollow geometries for a variety of applications, and with improved freedom over the printing process (e.g. material choices, speed and resulting properties of the printed parts).

Gap-directed chemical lift-off lithographic nanoarchitectonics for arbitrary sub-micrometer patterning

We introduce a unique soft lithographic operation that exploits stamp roof collapse-induced gaps to selectively remove an alkanethiol self-assembled monolayer (SAM) on Au to generate surface patterns that are orders of magnitude smaller than structures on the original elastomer stamp. The smallest achieved feature dimension is 5 nm using a micrometer-scale structured stamp in a chemical lift-off lithography (CLL) process. Molecular patterns retained in the gaps between stamp features and their circumscribed or inscribed circles follow mathematical predictions, and their sizes can be tuned by altering the stamp structure dimensions, including height, pitch, and shape. These generated surface molecular patterns can function as biorecognition arrays or be transferred to the underneath Au layer for metallic structure creation. By combining CLL process with this gap phenomenon, soft material properties that are previously thought as demerits can be used to achieve sub-10 nm features in a straightforward sketch.

Observation of enhanced nanoscale creep flow of crystalline metals enabled by controlling surface wettability

Understanding and controlling interface friction are central to many science and engineering applications. However, frictional sliding is closely related to adhesion, surface roughness, surface chemistry, mechanical deformation of contact solids, which poses the major challenge to experimental studying and theoretical modeling of friction. Here, by exploiting the recent developed thermomechanical nanomolding technique, we present a simple strategy to decouple the interplay between surface chemistry, plastic deformation, and interface friction by monitoring the nanoscale creep flow of metals in nanochannels. We show that superhydrophobic nanochannels outperforming hydrophilic nanochannels can be up to orders of magnitude in terms of creep flow rate. The comparative experimental study on pressure and temperature dependent nanomolding efficiency uncovers that the enhanced creep flow rate originates from diffusion-based deformation mechanism as well as the superhydrophobic surface induced boundary slip. Moreover, our results reveal that there exists a temperature-dependent critical pressure below which the traditional lubrication methods to reduce friction will break down. Our findings not only provide insights into the understanding of mechanical deformation and nanotribology, but also show a general and practical technique for studying the fundamental processes of frictional motion. Finally, we anticipate that the increased molding efficiency could facilitate the application of nanoimprinting/nanomolding.

Active waveguide Bragg lasers via conformal contact PDMS stamps

Lasing is observed in Bragg lasers formed through conformal contact of a patterned PDMS stamp with a plain active film, spincoated on glass. The thresholds, output efficiencies and spectral characteristics are compared to standard substrate patterned gratings and is discussed in relation to the coupling coefficient [Formula: see text]. The reported thresholds are highly sensitive in distributed feedback (DFB) lasers to grating duty cycles, for both PDMS-air and substrate-film lasers. Overall, laser thresholds of PDMS-air (PA) DFB lasers are found to be significantly higher than substrate-film (SF) lasers, which is attributed to an approximate three-fold reduction of optical-confinement in the grating region. Slope output efficiencies are found to be comparatively higher in PA lasers relative to SF lasers for both DFB and DBR configurations and is attributed to several competing factors. The PDMS can be removed from the surface of the active film repeatedly and conformal contact is limited mainly by the particle build up on the PDMS surface. The proposed PA system is expected to be useful in rapid laser metrology of new gain materials and in practical applications of optically pumped lasers.

Magnetic patterning of Co/Ni layered systems by plasma oxidation

We studied the structural, chemical, and magnetic properties of Ti/Au/Co/Ni layered systems subjected to plasma oxidation. The process results in the formation of NiO at the expense of metallic Ni, as clearly evidenced by X-ray photoelectron spectroscopy, while not affecting the surface roughness and grain size of the Co/Ni bilayers. Since the decrease of the thickness of the Ni layer and the formation of NiO increase the perpendicular magnetic anisotropy, oxidation may be locally applied for magnetic patterning. Using this approach, we created 2D heterostructures characterized by different combinations of magnetic properties in areas modified by plasma oxidation and in the regions protected from oxidation. As plasma oxidation is an easy to use, low cost, and commonly utilized technique in industrial applications, it may constitute an improvement over other magnetic patterning methods.

Rail induced lateral migration of particles across intact co-flowing liquids

This paper presents a rail guided method to apply a Layer-by-Layer (LbL) coating on particles in a microfluidic device. The passive microfluidic approach allows handling suspensions of particles to be coated in the system. The trajectory of the particles is controlled using engraved rails, inducing lateral movement of particles while keeping the axially oriented liquid flow (and the interface of different liquids) undisturbed. The depth and angle of the rails together with the liquid velocity were studied to determine a workable geometry of the device. A discontinuous LbL coating procedure was converted into one continuous process, demonstrating that the chip can perform seven consecutive steps normally conducted in batch operation, further easily extendable to larger cycle numbers. Coating of the particles with two bilayers was confirmed by fluorescence microscopy.

Copper laser patterning on a flexible substrate using a cost-effective 3D printer

We studied the cost effective direct laser patterning of copper (Cu) on thin polyimide substrates (PI thickness: 12.5-50 µm) using a 405 nm laser module attached to an inexpensive 3D printer. The focal length of the laser was intentionally controlled to reduce defects on patterned Cu and surface damage of PI under predetermined process conditions. The appropriate focal length was examined at various focal distances. Focal distances of - 2.4 mm and 3 mm were found for the shorter focal length (SFL) and longer focal length (LFL), respectively, compared to the actual focal length. This resulted in clean Cu line patterns without line defects. Interestingly, the SFL case had a different Cu growth pattern to that of LFL, indicating that the small difference in the laser incident angle could affect Cu precursor sintering. Cu square patterns had a lower resistivity of 70 μΩ·cm for an LFL after three or four laser scans, while the SFL showed a resistivity below 48 μΩ·cm for a one-time laser scan. The residues of the Cu precursor on PI were easily removed with flowing water and normal surfactants. However, the resistivity of the patterns decreased after cleaning. Among the scan gaps, the Cu square pattern formed at a 70 μm scan gap had the lowest sheet resistance and the least change in resistance from around 4 to 4.4 Ω/ϒ after cleaning. This result implies that the adhesion of the patterned Cu could be improved if the coated Cu precursor was well sintered under the proper process conditions. For the application of this method to bioelectronics, including biosensors, LEDs were connected to the Cu patterns on PI attached to the arm skin and worked well, even when the substrate PI was bent during power connecting.

Precise atom manipulation through deep reinforcement learning

Atomic-scale manipulation in scanning tunneling microscopy has enabled the creation of quantum states of matter based on artificial structures and extreme miniaturization of computational circuitry based on individual atoms. The ability to autonomously arrange atomic structures with precision will enable the scaling up of nanoscale fabrication and expand the range of artificial structures hosting exotic quantum states. However, the a priori unknown manipulation parameters, the possibility of spontaneous tip apex changes, and the difficulty of modeling tip-atom interactions make it challenging to select manipulation parameters that can achieve atomic precision throughout extended operations. Here we use deep reinforcement learning (DRL) to control the real-world atom manipulation process. Several state-of-the-art reinforcement learning (RL) techniques are used jointly to boost data efficiency. The DRL agent learns to manipulate Ag adatoms on Ag(111) surfaces with optimal precision and is integrated with path planning algorithms to complete an autonomous atomic assembly system. The results demonstrate that state-of-the-art DRL can offer effective solutions to real-world challenges in nanofabrication and powerful approaches to increasingly complex scientific experiments at the atomic scale.

Electron-beam patterned calibration structures for structured illumination microscopy

Super-resolution fluorescence microscopy can be achieved by image reconstruction after spatially patterned illumination or sequential photo-switching and read-out. Reconstruction algorithms and microscope performance are typically tested using simulated image data, due to a lack of strategies to pattern complex fluorescent patterns with nanoscale dimension control. Here, we report direct electron-beam patterning of fluorescence nanopatterns as calibration standards for super-resolution fluorescence. Patterned regions are identified with both electron microscopy and fluorescence labelling of choice, allowing precise correlation of predefined pattern dimensions, a posteriori obtained electron images, and reconstructed super-resolution images.

Superrepellent Porous Polymer Surfaces by Replication from Wrinkled Polydimethylsiloxane/Parylene F

Superrepellent surfaces, such as micro/nanostructured surfaces, are of key importance in both academia and industry for emerging applications in areas such as self-cleaning, drag reduction, and oil repellence. Engineering these surfaces is achieved through the combination of the required surface topography, such as porosity, with low-surface-energy materials. The surface topography is crucial for achieving high liquid repellence and low roll-off angles. In general, the combination of micro- and nanostructures is most promising in achieving high repellence. In this work, we report the enhancement of wetting properties of porous polymers by replication from wrinkled Parylene F (PF)-coated polydimethylsiloxane (PDMS). Fluorinated polymer foam “Fluoropor” serves as the low-surface-energy polymer. The wrinkled molds are achieved via the deposition of a thin PF layer onto the soft PDMS substrates. Through consecutive supercritical drying, superrepellent surfaces with a high surface porosity and a high water contact angle (CA) of >165° are achieved. The replicated surfaces show low roll-off angles (ROA) <10° for water and <21° for ethylene glycol. Moreover, the introduction of the micro-wrinkles to Fluoropor not only enhances its liquid repellence for water and ethylene glycol but also for liquids with low surface tension, such as n-hexadecane.

Patterned Aluminum/Polydimethylsiloxane-Laminated Film for a Solvent-Driven Soft Actuator with Programmable and Multistable Shape Morphing

Recently, soft actuators capable of deforming in predictable ways under external stimuli have attracted increasing attention by showing great potential in emerging industries. However, limited efforts are being spent on the untethered actuators with multistable deformations. Also, there is a lack of mechanically guiding design principles for multistable structures. Here, the patterned aluminum/polydimethylsiloxane (Al/PDMS)-laminated films with surface wrinkles are fabricated by magnetron sputtering the Al layer on the PDMS substrate. By tuning the geometric parameters and surface constraints of the patterned Al/PDMS-laminated films, a series of solvent-driven actuators with multiform stable configurations (such as monostable arc, multistable cylinder, and monostable/bistable spiral) are proposed. The deformation mechanism is revealed using a linear elastic theory. Combined with the finite element analysis method, the deformations of Al/PDMS-laminated films with different surface constraints and geometric configurations are visually predicted. Besides, we modulate the deformation of different parts of the Z-shaped actuators by tuning the surface constraints in different regions of the Z-shaped Al/PDMS bilayer films to achieve multiple stable deformations in a single actuator. The concept offers a huge design scope for reconfigurable soft robots. Finally, two bionic applications are proposed to demonstrate the practical applications of the soft solvent-driven actuator based on the patterned Al/PDMS films in artificial muscles and bionic robotics. This work provides a strategy for the design and fabrication of programmable and controllable soft actuators, laying the foundation for a wide range of applications in smart materials.

High-speed, scanned laser structuring of multi-layered eco/bioresorbable materials for advanced electronic systems

Physically transient forms of electronics enable unique classes of technologies, ranging from biomedical implants that disappear through processes of bioresorption after serving a clinical need to internet-of-things devices that harmlessly dissolve into the environment following a relevant period of use. Here, we develop a sustainable manufacturing pathway, based on ultrafast pulsed laser ablation, that can support high-volume, cost-effective manipulation of a diverse collection of organic and inorganic materials, each designed to degrade by hydrolysis or enzymatic activity, into patterned, multi-layered architectures with high resolution and accurate overlay registration. The technology can operate in patterning, thinning and/or cutting modes with (ultra)thin eco/bioresorbable materials of different types of semiconductors, dielectrics, and conductors on flexible substrates. Component-level demonstrations span passive and active devices, including diodes and field-effect transistors. Patterning these devices into interconnected layouts yields functional systems, as illustrated in examples that range from wireless implants as monitors of neural and cardiac activity, to thermal probes of microvascular flow, and multi-electrode arrays for biopotential sensing. These advances create important processing options for eco/bioresorbable materials and associated electronic systems, with immediate applicability across nearly all types of bioelectronic studies.

Prediction of textile pilling resistance using optical coherence tomography

This paper describes a new method of textile pilling prediction, based on multivariate analysis of the spatial layer above the surface. The original idea of the method is the acquisition of 3D fabric image using optical coherence tomography (OCT) with infrared light, which allows for the fabric fuzz visualization with high sensitivity. The pilling layer, reconstructed with the resolution of [Formula: see text], includes reliable textural information related to the amount of loose fibers and bunches appearing as a result of abrasion. Pilling intensity was assigned by supervised classification of the textural features using both linear (PLS-DA - partial least squares discriminant analysis, LDA - linear discriminant analysis) and non-linear (SVM - support vector machine) classifiers. The results demonstrated that the method is more suitable for fabrics after short-term abrasion, when the fuzz prevails over tangled fibers in the pilling layer. In that case, pilling grades were predicted with [Formula: see text] accuracy, sensitivity and specificity (for SVM model). The validation accuracy of the tested models after machine abrasion achieves lower values (up to [Formula: see text] for LDA model). With our method, we clearly showed that OCT can be used to quantitatively trace appearance changes of fabric samples due to test abrasion.

Micro-scale aerosol jet printing of superparamagnetic Fe3O4 nanoparticle patterns

The opportunity to create different patterns of magnetic nanoparticles on surfaces is highly desirable across many technological and biomedical applications. In this paper, this ability is demonstrated for the first time using a computer-controlled aerosol jet printing (AJP) technology. AJP is an emerging digitally driven, non-contact and mask-less printing process which has distinguishing advantages over other patterning technologies as it offers high-resolution and versatile direct-write deposition of a wide range of materials onto a variety of substrates. This research demonstrates the ability of AJP to reliably print large-area, fine-feature patterns of superparamagnetic iron oxide nanoparticles (SPIONs) onto both rigid material (glass) and soft and flexible materials (polydimethylsiloxane (PDMS) films and poly-L-lactic acid (PLLA) nanofilms). Investigation identified and controlled influential process variables which permitted feature sizes in the region of 20 μm to be realised. This method could be employed for a wide range of applications that require a flexible and responsive process that permits high yield and rapid patterning of magnetic material over large areas. As a first proof of concept, we present patterned magnetic nanofilms with enhanced manipulability under external magnetic field gradient control and which are capable of performing complex movements such as rotation and bending, with applicability to soft robotics and biomedical engineering applications.

Versatile Surface Patterning with Low Molecular Weight Photoswitches

Surface patterning of functional materials is a key technology in various fields such as microelectronics, optics, and photonics. In micro- and nanofabrication, polymers are frequently employed either as photoreactive or thermoresponsive resists that enable further fabrication steps, or as functional adlayers in electronic and optical devices. In this article, a method is presented for imprint lithography using low molecular weight arylazoisoxazoles photoswitches instead of polymer resists. These photoswitches exhibit a rapid and reversible solid-to-liquid phase transition upon photo-isomerization at room temperature, making them highly suitable for reversible surface functionalization at ambient conditions. Beyond photo-induced imprint lithography with multiple write-and-erase cycles, prospective applications as patterned matrix for nanoparticles and etch resist on gold surfaces are demonstrated.

RGD Nanoarrays with Nanospacing Gradient Selectively Induce Orientation and Directed Migration of Endothelial and Smooth Muscle Cells

Directed migration of cells through cell-surface interactions is a paramount prerequisite in biomaterial-induced tissue regeneration. However, whether and how the nanoscale spatial gradient of adhesion molecules on a material surface can induce directed migration of cells is not sufficiently known. Herein, we employed block copolymer micelle nanolithography to prepare gold nanoarrays with a nanospacing gradient, which were prepared by continuously changing the dipping velocity. Then, a self-assembly monolayer technique was applied to graft arginine-glycine-aspartate (RGD) peptides on the nanodots and poly(ethylene glycol) (PEG) on the glass background. Since RGD can trigger specific cell adhesion via conjugating with integrin (its receptor in the cell membrane) and PEG can resist protein adsorption and nonspecific cell adhesion, a nanopattern with cell-adhesion contrast and a gradient of RGD nanospacing was eventually prepared. In vitro cell behaviors were examined using endothelial cells (ECs) and smooth muscle cells (SMCs) as a demonstration. We found that SMCs exhibited significant orientation and directed migration along the nanospacing gradient, while ECs exhibited only a weak spontaneously anisotropic migration. The gradient response was also dependent upon the RGD nanospacing ranges, namely, the start and end nanospacings under a given distance and gradient. The different responses of these two cell types to the RGD nanospacing gradient provide new insights for designing cell-selective nanomaterials potentially used in cell screening, wound healing, etc.

UV- and Visible-Light Photopatterning of Molecular Gradients Using the Thiol-yne Click Reaction

The rational design of chemical coatings is used to control surface interactions with small molecules, biomolecules, nanoparticles, and liquids as well as optical and other properties. Specifically, micropatterned surface coatings have been used in a wide variety of applications, including biosensing, cell growth assays, multiplexed biomolecule interaction arrays, and responsive surfaces. Here, a maskless photopatterning process is studied, using the photocatalyzed thiol-yne "click" reaction to create both binary and gradient patterns on thiolated surfaces. Nearly defect-free patterns are produced by first coating glass surfaces with mercaptopropylsilatrane, a silanizing agent that forms smoother self-assembled monolayers than the commonly used 3-mercaptopropyltrimethoxysilane. Photopatterning is then performed using UV (365 nm) or visible (405 nm) light to graft molecules onto the surface in tunable concentrations based on the local exposure. The technique is demonstrated for multiple types of molecular grafts, including fluorescent dyes, poly(ethylene glycol), and biotin, the latter allowing subsequent deposition of biomolecules via biotin-avidin binding. Patterning is demonstrated in water and dimethylformamide, and the process is repeated to combine molecules soluble in different phases. The combination of arbitrary gradient formation, broad applicability, a low defect rate, and fast prototyping thanks to the maskless nature of the process creates a particularly powerful technique for molecular surface patterning that could be used for a wide variety of micropatterned applications.

Geometrically Induced Selectivity and Unidirectional Electroosmosis in Uncharged Nanopores

Selectivity toward positive and negative ions in nanopores is often associated with electroosmotic flow, the control of which is pivotal in several micro-nanofluidic technologies. Selectivity is traditionally understood to be a consequence of surface charges that alter the ion distribution in the pore lumen. Here we present a purely geometrical mechanism to induce ionic selectivity and electroosmotic flow in uncharged nanopores, and we tested it via molecular dynamics simulations. Our approach exploits the accumulation of charges, driven by an external electric field, in a coaxial cavity that decorates the membrane close to the pore entrance. The selectivity was shown to depend on the applied voltage and becomes completely inverted when reversing the voltage. The simultaneous inversion of ionic selectivity and electric field direction causes a unidirectional electroosmotic flow. We developed a quantitatively accurate theoretical model for designing pore geometry to achieve the desired electroosmotic velocity. Finally, we show that unidirectional electroosmosis also occurs in much more complex scenarios, such as a biological pore whose structure presents a coaxial cavity surrounding the pore constriction as well as a complex surface charge pattern. The capability to induce ion selectivity without altering the pore lumen shape or the surface charge may be useful for a more flexible design of selective membranes.

Wettability and Surface Roughness Analysis of Laser Surface Texturing of AISI 430 Stainless Steel

Due to its wide applicability in industry, devising microstructures on the surface of materials can be easily implemented and automated in technological processes. Laser Surface Texturing (LST) is applied to modify the chemical composition, morphology, and roughness of surfaces (wettability), cleaning (remove contaminants), reducing internal stresses of metals (hardening, tempering), surface energy (polymers, metals), increasing the adhesion (hybrid joining, bioengineering) and decreasing the growth of pathogenic bacteria (bioengineering). This paper is a continuation and extension of our previous studies in laser-assisted texturing of surfaces. Three different patterns (crater array-type C, two ellipses at 90° overlapping with its mirror-type B and 3 concentric octagons-type A) were applied with a nanosecond pulsed laser (active medium Nd: Fiber Diode-pumped) on the surface of a ferritic stainless steel (AISI 430). Micro texturing the surface of a material can modify its wettability behavior. A hydrophobic surface (contact angle greater than 90°) was obtained with different variations depending on the parameters. The analysis performed in this research (surface roughness, wettability) is critical for assessing the surface functionality, characteristics and properties of the stainless steel surface after the LST process. The values of the surface roughness and the contact angle are directly proportional to the number of repetitions and inversely proportional to the speed. Recommendations for the use of different texturing pattern designs are also made.

Swelling-Induced Information Camouflage and Optical Decryption on a Transparent Recoverable Hydrogel Surface

Information camouflage and decryption on hydrogels rely on chemical stimuli such as pH, ultraviolet light, and chemical reactions, in which the cyclability is limited. This work develops a simpler yet effective physical method that can achieve the information camouflage on hydrogels by water swelling and decrypt it under white light. The information camouflage and decryption can proceed with unlimited cycles. To successfully reach the information camouflage, the hydrogel is synthesized with the water swelling ratio in weight as high as 250, which is enabled by the strong electrostatic repulsion of cationic moieties inside the network. At such a high water-swollen state, the hydrogel is still robust and elastic, which provides a mechanical basis to maintain the stability of the camouflaged information. We write information on the hydrogel surface by laser cutting. Upon immersing the hydrogel in water, the high swelling results in huge expansion of the hydrogel, thus inducing the information camouflage. With exposure to white light, the information can be decrypted and becomes visible again. Our protocol utilizes a simple physical process to enable the camouflage and decryption of complex information, which might open an alternative pathway for the development of hydrogel materials in the application of informatics.

Effect of Nanopatterning on Concentration Polarization during Nanofiltration

Membranes used for desalination still face challenges during operation. One of these challenges is the buildup of salt ions at the membrane surface. This is known as concentration polarization, and it has a negative effect on membrane water permeance and salt rejection. In an attempt to decrease concentration polarization, a line-and-groove nanopattern was applied to a nanofiltration (NF) membrane. Aqueous sodium sulfate (Na2SO4) solutions were used to test the rejection and permeance of both pristine and patterned membranes. It was found that the nanopatterns did not reduce but increased the concentration polarization at the membrane surface. Based on these studies, different pattern shapes and sizes should be investigated to gain a fundamental understanding of the influence of pattern size and shape on concentration polarization.