Multiscale Surface Structures Research Articles

Wettability Behaviour of Metal Surfaces after Sequential Nanosecond and Picosecond Laser Texturing.

This study examines the wettability behaviour of 304 stainless steel (304SS) and Ti-6Al-4V (Ti64) surfaces after sequential nanosecond (ns) and picosecond (ps) laser texturing; in particular, how the multi-scale surface structures created influence the lifecycle of surface hydrophobicity. The effect of different post-process treatments is also examined. Surfaces were analysed using Scanning Electron Microscopy (SEM), a white light interferometer optical profiler, and Energy Dispersive X-ray (EDX) spectroscopy. Wettability was assessed through sessile drop contact angle (CA) measurements, conducted at regular intervals over periods of up to 12 months, while EDX scans monitored elemental chemical changes. The results show that sequential (ns + ps) laser processing produced multi-scale surface texture with laser-induced periodic surface structures (LIPSS). Compared to the ns laser case, the (ns + ps) laser processed surfaces transitioned more rapidly to a hydrophobic state and maintained this property for much longer, especially when the single post-process treatment was ultrasonic cleaning. Some interesting features in CA development over these extended timescales are revealed. For 304SS, hydrophobicity was reached in 1-2 days, with the CA then remaining in the range of 120 to 140° for up to 180 days; whereas the ns laser-processed surfaces took longer to reach hydrophobicity and only maintained the condition for up to 30 days. Similar results were found for the case of Ti64. The findings show that such multi-scale structured metal surfaces can offer relatively stable hydrophobic properties, the lifetime of which can be extended significantly through the appropriate selection of laser process parameters and post-process treatment. The addition of LIPSS appears to help extend the longevity of the hydrophobic property. In seeking to identify other factors influencing wettability, from our EDX results, we observed a significant and steady rate of increase in the carbon content at the surface over the study period.

Direct Fabrication of Te-Doped Black Si with an Enhanced Photoelectric Performance by Femtosecond Laser Irradiation under Water.

Tellurium (Te)-doped black silicon (Si) with enhanced absorption and photoelectric performance over a broad wavelength range of 0.2-2.5 μm was obtained using femtosecond (fs) laser irradiation in liquid water. Prior to laser irradiation, the Si sample was covered with a Te thin film (thickness 200 nm) over an adhesion layer of Cr (thickness 5 nm). Surface analyses by scanning electron microscopy and three-dimensional confocal microscopy evidence the presence of hierarchical surface structures combining quasi-periodic stripes with a spatial period of about 5 μm and subwavelength laser-induced periodic surface structures directed in directions parallel and perpendicular to the direction of the laser polarization, respectively. Moreover, the incorporation of Te generates intermediate levels within the Si bandgap. The Te-doped black Si shows a significant enhancement of the absorption, which reaches values of about 48% in the UV and visible (0.2-1.1 μm) and 70% in the near-infrared (1.1-2.5 μm) spectral ranges, respectively, due to the synergistic effects of multiscale surface structures and Te incorporation. Moreover, the surface reflectance is reduced to almost zero across the entire spectrum. The Te-doped black Si sample is used to realize a photodetector which displays an impressive photoelectric capability, being characterized by a responsivity of 328 mA/W, and an external quantum efficiency of 49.27% at a voltage bias of -10 V for 1064 nm light illumination, with rising and falling times of 55 and 67 ms, respectively. These figures remarkably outperform the response of unprocessed Si under the same experimental conditions.

3D Printed Bioinspired Hierarchical Surface Structure with Tunable Wettability

Abstract Nature has examples of impressive surfaces and interfaces with diverse wettability stemming from superhydrophilicity to superhydrophobicity. The multiscale surface structures found in biological systems generally have high geometric complexity, which makes it challenging to replicate their characteristics, especially using traditional fabrication techniques. It is even more challenging to fabricate such complex microstructures with tunable wettability. In this paper, we propose a method to tune the wettability of a micro-scale surface by changing the geometrical parameters of embedded micro-structures in the surface. By taking inspiration from an insect (springtails), we designed micropillar arrays with different roughness by adjusting geometric parameters such as reentrant angle, pitch distance, and the number of spikes and pillars. This study shows that, by changing geometrical parameters in microscale, the apparent contact angle, and hence the surface wettability can be calibrated. The microscale pillars were fabricated using a precise micro direct light processing (uDLP) three-dimensional (3D) printer. Different printing parameters were studied to optimize the geometric parameters to fabricate 3D hierarchical structures with high accuracy and resolution. By controlling the size of different features of the pillar, pillar number, and layout of the mushroom-shaped micropillars, the wettability of the surface is possible to be tuned from a highly nonwetting liquid/material combination to highly wetting material. Such wettability tuning capability expands the design space for many biomedical and thermofluidic applications.

Multiscale enhancement of refrigerant falling film boiling by combining commercially enhanced tubes with nanostructures

Multiscale surface structures offer the opportunity to combine the heat transfer enhancement provided by microscale structures with the dryout benefits provided by some nanostructures, which is particularly attractive for falling film evaporators, who have to prevent dryout to ensure high heat transfer coefficients can be maintained.In this study commercially produced plain, low-finned and 3D enhanced commercially produced tubes were tested uncoated as well as coated with fibrous nanostructures that induce wicking on the surface to investigate the opportunity that multiscale enhancements might offer falling film boiling evaporators. Single tubes were heated internally by water and tested in the horizontal position with refrigerant R-134a drizzled over the outside of the tube surface at saturation temperatures of 5 and 25 °C, heat fluxes from 20 to 100 kW/m2 and film Reynolds numbers of up to 2000. The tubes tested were a roughened plain tube, a low-finned Gewa-KS tube and two different 3D enhanced tubes, an Gewa-B5 and an Turbo EHPII, that have networks of re-entrant cavities.The uncoated 3D enhanced tubes achieved the highest heat transfer coefficients, outperforming a roughened plain tube by up to 500 %. The uncoated low-finned tube had heat transfer coefficients that were up to 140 % higher than the roughened plain tube, despite having only an 80 % larger surface area, thought to be due to bubbles sliding down the channels between the fins, increasing the surface area of the bubbles in contact with the surface and thus allowing for increased microlayer evaporation.The application of the copper oxide coating benefitted the low-finned tube, with heat transfer enhancement of up to 60 %, possibly due to liquid wicked underneath the sliding bubbles and onto the fin tips. However, the nanocoating largely decreased the heat transfer of the roughened plain tube and the 3D enhanced tubes, thought to be due flooding of nucleation sites on the roughened tube and interference with the hydraulics of the re-entrant cavity network on the surface of the 3D enhanced surfaces.The low-finned tube also benefitted the most from the falling film boiling heat transfer mode compared to previous pool boiling results, with heat transfer increased by up to 80 and 460 % for the uncoated and coated low-finned tube respectively, again likely due to the bubbles sliding in the fin channels under falling film conditions.The Gewa-B5 tubes had the best dryout performance, followed by the Turbo EHPII tubes and then the low-finned Gewa-KS tube. The open pore structure of the Gewa-B5 likely aided it in dryout prevention, while the fins of the Gewa-KS are thought to have prevented lateral fluid distribution and thus worsen dryout.The dryout performance of the tubes we not markedly improved by the nanocoating, highlighting the difficulty in improving wettability on high wetting low surface tensions fluids such as refrigerants. Microscale enhancements were thus shown to have significant influence on dryout performance, while nanoscale enhancements had little effect in this study.

Flexoelectricity at fractal rough surfaces

This study quantifies the role of surface roughness on interfacial flexoelectricity under normal compression and oscillation by examining a series of 3D-printed surfaces with diverse roughness features. For incipient contact, the measured flexoelectric charge is found to follow a power-law dependence on normal compression load, with the exponent positively correlated with the fractal dimension. The value of this power-law exponent is similar to that for contact stiffness. The underlying mechanism for this coincidence is elucidated by contact analyses based on geometric truncations of surface structures. Contact micromechanics show that the interfacial flexoelectric charge will increasingly concentrate on large microcontacts as the compression continues. A rougher surface with higher fractal dimension tends to demonstrate less heterogeneity for flexoelectric polarizations over microcontacts. This study provides systematic experimental measurements and comprehensive explanations for interfacial flexoelectricity and establishes quantitative links to multi-scale surface structures, shedding light on novel approaches for contact evaluation and flexoelectricity enhancement.

Study of surface wettability on fused silica by ultrafast laser-induced micro/nano-surface structures

Glass materials integrated with advanced functions, such as anti-fogging and self-cleaning functions, are urgently demanded in applications such as vehicle windshields, eyeglasses, goggles, electronic device screens, and windows. Inspired by nature, researchers have found multiscale surface structures to be a crucial factor in determining a material's surface wettability. The ultrafast pulsed laser is a promising tool to induce such surface structures and subsequently control the surface functionalities on different materials. The behavior of surface wettability with different micro/nano-scale surface structures created by femtosecond laser irradiation on fused silica is investigated in this research. The effects of laser fluence, overlapping ratio, and repetition rate on the structure morphology are studied. Seven different sorts of structures can be created by manipulating these parameters: (S1) micro-dots array, (S2) micro-dots array covered with laser-induced periodic surface structures (LIPSSs), (S3) LIPSSs, (S4) microgrooves covered with LIPSSs, (S5) microgrooves covered with irregular nanostructures, (S6) micro-channels and (S7) micro-channels covered with LIPSSs. Investigations into the static contact angle show that hydrophilic and superhydrophilic surfaces can be generated through the introduction of these laser-induced hierarchical structures. The relationship between laser parameters, surface structures, surface roughness, and surface wettability is established. Different distinctive mean static contact angle values are identified for different surface structures.

One step machining of hierarchical optical structures for autostereoscopic images

This work presents a new optical design and the corresponding novel machining strategy to achieve the one step generation of hierarchical optical structures on a flat surface for autostereoscopic effects. The hierarchical structures combine V-groove arrays as a parallax barrier for stereopsis and integrated diffraction gratings with variable grating spacing for motion parallax. The multi-scale surface structures are machined simultaneously using the tool geometry to form V-grooves while utilizing elliptical tool vibration to generate blazed gratings concurrently. An autostereoscopic image with strong depth perception is fabricated with a pixel density exceeding 1,660 pixels per inch.

Surface engineering of biomaterials in orthopedic and dental implants: Strategies to improve osteointegration, bacteriostatic and bactericidal activities

The success of biomedical implants in orthopedic and dental applications is usually limited due to insufficient bone-implant integration, and implant-related infections. Biointerfaces are critical in regulating their interactions and the desirable performance of biomaterials in biological environment. Surface engineering has been widely studied to realize better control of the interface interaction to further enhance the desired behavior of biomaterials. This review aims to investigate surface coating strategies in hard tissue applications to address insufficient osteointegration and implant-related infection problems. We first focused on surface coatings to enhance the osteointegration and biocompatibility of implants by emphasizing calcium phosphate-related, nanoscale TiO2 -related, bioactive tantalum-based and biomolecules incorporated coatings. Different coating strategies such as plasma spraying, biomimetic deposition, electrochemical anodization and LENS are discussed. We then discussed techniques to construct anti-adhesive and bactericidal surface while emphasizing multifunctional surface coating techniques that combine potential osteointegration and antibacterial activities. The effects of nanotopography via TiO2 coatings on antibacterial performance are interesting and included. A smart bacteria-responsive titanium dioxide nanotubes coating is also attractive and elaborated. Developing multifunctional surface coatings combining osteogenesis and antimicrobial activity is the current trend. Surface engineering methods are usually combined to obtain hierarchical multiscale surface structures with better biofunctionalization outcomes.

Magnetic Field-Assisted Stereolithography for Productions of Multimaterial Hierarchical Surface Structures.

Natural organisms provide inspirations for various functional structures and surfaces with significant applications in multidisciplinary fields. These biological systems are generally composed of multiscale surface structures with high geometric complexity and a variety of materials, making it challenging to replicate their characteristics in engineering. This study presents a novel multiscale multimaterial 3D printing method, magnetic field-assisted stereolithography (M-SL), for fabricating hierarchical particle-polymer structures with surface features ranging from a few nanometers to millimeters or even centimeters. Taking inspiration from nature, this study describes the design and fabrication of a bioinspired multiscale hierarchical surface structure, which is characterized of microscale cones, nanoscale pores, and surface wrinkles at a few nanometers. To understand the fundamental physics underlying the hierarchical surface structure fabrication in the proposed M-SL process, the complexities among the M-SL process parameters, material parameters, and printed geometries are discussed. The accuracy of the developed printing method is investigated by comparing the printed geometries and digital designs. Effects of the printed hierarchical surface structure on hydrophobicity and cell viability were characterized and discussed. It was found that the highly hierarchical surface structure changed the polymer composite surface from hydrophilic (contact angle: ∼38°) to hydrophobic (∼146°). In addition, the hierarchical surface structure also created a better environment for cell attachment and growth, with 900% more living cells at 72 h after cell seeding, compared with cells on the nonstructured smooth surface. Local and selective cell seeding can also be enabled by the surface structure design. Experimental results validated the effectiveness of the M-SL 3D printing method on fabricating multimaterial functional objects with hierarchically structured surfaces for a wide spectrum of applications.

Oil-water separation device based on super-hydrophobic material with multi-scale surface structure

A buoyancy-adaptive oil-water separation device is developed based on the prepared oil-water separation materials with a micro-nano hierarchical structure. This device can achieve the efficient recovery of diesel oil pads in the process of salt cavern construction without an external power source. The surface structure of superhydrophobic materials and the wettability of droplets on their surfaces are characterized. Then the separation mechanism is theoretically analysed. In addition, the brine breakthrough pressure and the separation efficiency of the device are studied considering the different surface structures of the separation materials. The mesh number and the layer number of separation materials are explored to determine their effects on the breakthrough pressure. It is found that this pressure increases with an increasing mesh number, but the influence of the screen layers is weak. Furthermore, the mesh number and the number of cycle separations that affect the separation efficiency are investigated. The results show that the sieve hole is smaller when the mesh number is larger, and the volume flow rate of diesel oil is thus reduced. Therefore, the screen mesh number should be reasonably selected based on the actual needs. The results of the cyclic separation test indicate that the separation material has good reusability. After 30 cycles of recycling, the separation efficiency of the device can still reach 90.1 ± 1.7%.

Multiscale Structures Aggregated by Imprinted Nanofibers for Functional Surfaces.

Multiscale surface structures have attracted increasing interest owing to several potential applications in surface devices. However, an existing challenge in the field is the fabrication of hybrid micro-nano structures using a facile, cost-effective, and high-throughput method. To overcome these challenges, this paper proposes a protocol to fabricate multiscale structures using only an imprint process with an anodic aluminum oxide (AAO) filter and an evaporative self-aggregation process of nanofibers. Unlike previous attempts that have aimed to straighten nanofibers, we demonstrate a unique fabrication method for multiscale aggregated nanofibers with high aspect ratios. Furthermore, the surface morphology and wettability of these structures on various liquids were investigated to facilitate their use in multifunctional surfaces.

Fabrication and Wettability Analysis of Hydrophobic Stainless Steel Surfaces With Microscale Structures From Nanosecond Laser Machining

In this work, nanosecond laser machining is used to fabricate hydrophobic 17-4 PH stainless steel surfaces with microscale and submicron structures. Four surface structures were designed, with microscale channels and pillars (100 μm pitch size) of uniform heights (100 μm) or alternating heights (between 100 μm and 50 μm). During fabrication, the high-power laser beams also created submicron features on top of the microscale ones, leading to hierarchical, multiscale surface structures. Detailed wettability analysis was conducted on the fabricated samples. Measured static contact angles of water on these surfaces are over 130 deg without any coating, compared to ∼70 deg on the original steel surface before laser machining. Slightly lower contact angle hysteresis was also observed on the laser machined surfaces. Overall, these results agree with a simple Cassie–Baxter model for wetting that assumes only fractional surface area contact between the droplet and the surface. This work demonstrates that steel surfaces machined with relatively inexpensive nanosecond laser can achieve excellent hydrophobicity even with simple microstructural designs.

Anti- and De-Icing Behaviors of Superhydrophobic Fabrics

This paper reports the application of superhydrophobic coatings on cotton fabrics and their functionalities for anti- and de-icing efficacy. Superhydrophobic cotton fabrics with different water-repellent properties have been achieved by decorating the surface of pristine cotton fibers with ZnO structures of varying sizes and shapes through an in situ solution growth process, followed by the treatment of the surface with low-surface-energy coating such as Teflon. The surface morphology of the treated cotton fabrics was characterized using scanning electron microscopy (SEM). The surface wettability of the treated fabrics was evaluated through the measurement of static contact angle (SCA), contact angle hysteresis (CAH), and sliding angle (SA) of a water droplet. The anti- and de-icing behaviors of the treated fabrics were evaluated through both static (sessile droplet) and dynamic (spraying) tests. The results show that the superhydrophobic fabric with a higher SCA and the lower CAH/SA has superior anti- and de-icing behaviors in both the static and dynamic conditions. Compared to hard substrates, the soft, flexible, and porous (air-permeable) superhydrophobic fabrics can lead to broader applicability of textile-based materials for the design and fabrication of anti- and de-icing materials. Furthermore, the multi-scale surface structures of fabrics (fibers, yarns, and weaving constructions) combining with the hierarchical micro-nanostructures of the ZnO coating provides an ideal platform for anti-icing studies.

Multiple Liquid Manipulations on Patterned Surfaces with Tunable Adhesive Property

AbstractMicro/nanostructured surfaces with designable wettability show great promise in many fields. This paper reports a new strategy for fabricating superhydrophobic surfaces with tunable water adhesion force by combining multiscale surface structures and mussel‐inspired surface chemistry, which can be used for multiple liquid manipulations. Highly ordered porous films prepared by the breath figure method enable convenient construction of surfaces with honeycomb‐patterned or nanosized pincushion‐like structures. Multiple liquid manipulations including no‐loss transportation of water droplets, water collection, programmable movement of water droplets, and chemical reaction monitoring are demonstrated on the micro/nanostructured surfaces. The proposed method is facile, energy‐efficient, and can be conducted for the preparation of superhydrophobic surfaces with high contrast adhesion patterns.

Contact stiffness of multiscale surfaces by truncation analysis

In this paper, we study the contact stiffness of a fractal rough surface compressed by a rigid flat plane. A numerical model based on the analysis of flat punch indentation is proposed for simulated hierarchical surfaces, which are generated using statistical and fractal descriptors collected by surface profilometry. The contact stiffness of surfaces under increasing normal load is determined on the basis of the total truncated area at varying heights. The results are compared with experimental data from nanoindentation on four types of treated rough surfaces, showing good agreement with experimental observations below a certain truncation depth. Furthermore, the limits of the model's validity are discussed by focusing on surface geometries and deformation of contacting asperities. With this proposed truncation method, we present a parametric analysis to establish a correlation between contact stiffness and surface roughness descriptors. The contact stiffness shows a unified power-law scaling with respect to the applied load over a wide range for simulated surfaces with distinct sets of roughness descriptors. The exponent of the power-law relationship is found to correlate positively to the fractal dimension while its amplitude is inversely correlated to the surface roughness amplitude. This study provides an easily implemented and computationally efficient method to connect mechanical behaviour with multi-scale surface structure, which can be utilized in design and optimization of engineering applications involving rough contacts.

Deposition of Ultrathin Nano-Hydroxyapatite Films on Laser Micro-Textured Titanium Surfaces to Prepare a Multiscale Surface Topography for Improved Surface Wettability/Energy.

The primary aim of this study was to analyse the correlation between topographical features and chemical composition with the changes in wettability and the surface free energy of microstructured titanium (Ti) surfaces. Periodic microscale structures on the surface of Ti substrates were fabricated via direct laser interference patterning (DLIP). Radio-frequency magnetron sputter deposition of ultrathin nanostructured hydroxyapatite (HA) films was used to form an additional nanoscale grain morphology on the microscale-structured Ti surfaces to generate multiscale surface structures. The surface characteristics were evaluated using atomic force microscopy and contact angle and surface free energy measurements. The structure and phase composition of the HA films were investigated using X-ray diffraction. The HA-coated periodic microscale structured Ti substrates exhibited a significantly lower water contact angle and a larger surface free energy compared with the uncoated Ti substrates. Control over the wettability and surface free energy was achieved using Ti substrates structured via the DLIP technique followed by the deposition of a nanostructured HA coating, which resulted in the changes in surface chemistry and the formation of multiscale surface topography on the nano- and microscale.

Bio-inspired hierarchical patterning of silicon by laser interference lithography.

This paper presents a facile approach for the rapid and maskless fabrication of hierarchical structures by multibeam laser interference. In the work, three- and four-beam laser interference lithographies were proposed to fabricate ordered multiscale surface structures instead of six or more beam interference with a complicated system setup. The pitch and shape of hierarchical structures can be controlled by adjusting the parameters of incident light. The experiment results have shown that the hierarchical anisotropy and isotropy surface structures can be fabricated by this method with the control of the parameters of each incident beam, which is in accordance with the theoretical analysis and computer simulations.

Static friction at fractal interfaces

Tribological phenomena are governed by combined effects of material properties, topology and surface-chemistry. We study the interplay of multiscale surface structures with molecular-scale interactions towards interpreting static frictional interactions at fractal interfaces. By spline-assisted-discretization we analyse asperity interactions in pairs of contacting fractal surface-profiles. For elastically deforming asperities, force analysis reveals greater friction at surfaces exhibiting higher fractality, with increasing molecular-scale friction amplifying this trend. Increasing adhesive strength yields higher overall friction at surfaces of lower fractality owing to greater true-contact-area. In systems where adhesive-type interactions play an important role, such as those where cold-welded junctions form, friction is minimised at an intermediate value of surface profile fractality found to be around 1.3–1.5. Results have implications for systems exhibiting evolving surface structures.

Mussel-Inspired Modification of Honeycomb Structured Films for Superhydrophobic Surfaces with Tunable Water Adhesion

In this paper, we report novel superhydrophobic surfaces with tunable water adhesion by combining mussel-inspired surface chemistry and bioinspired multiscale surface structures. Highly ordered honeycomb porous films were prepared from a diblock copolymer polystyrene-block-poly(N,N-dimethylaminoethyl methacrylate) by the breath figure method. Removing the top surface layer of the honeycomb films leads to pincushion-like surfaces. The films were coated with polydopamine after completely prewetting by ethanol, followed by the reaction with 1H,1H,2H,2H-perfluorodecanethiol for fluorination. As a result of the surface modification, both the honeycomb and the pincushion-like surfaces are superhydrophobic with a water contact angle higher than 150°. Interestingly, the former is highly adhesive, on which water droplets are pinned at any tilted angles, whereas the latter is relatively low adhesive. Calculations indicate that the honeycomb films are in the Cassie state, whereas the pincushion-like surfaces are in ...

Facile fabrication of functional PDMS surfaces with tunable wettablity and high adhesive force via femtosecond laser textured templating

Femtosecond laser processing is emerged as a promising tool to functionalize surfaces of various materials, including metals, semiconductors, and polymers. However, the productivity of this technique is limited by the low efficiency of laser raster scanning. Here we report a facile approach for efficiently producing large-area functional polymer surfaces, by which metal is firstly textured by a femtosecond laser, and the as-prepared hierarchical structures are subsequently transferred onto polydimethylsiloxane (PDMS) surfaces. Aluminum pieces covered by laser induced micro/nano-structures act as template masters and their performance of displaying diverse colors are investigated. Polymer replicas are endowed with tunable wetting properties, which are mainly attributed to the multi-scale surface structures. Furthermore, the surfaces are found to have extremely high adhesive force for water drops because of the high water penetration depth and the resultant high contact angle hysteresis. This characteristic facilitates many potential applications like loss-free tiny water droplets transportation. The reusability of metal master and easiness of soft lithography make it to be a very simple, fast and cost-efficient way for mass production of functional polymeric surfaces.