Conical Microstructures Research Articles

Study of the Impact of Droplets on Superhydrophobic Surfaces

The potential applications of superhydrophobicity for self-cleaning, microfluidic systems, and so on, inspired by the “lotus leaf effect”, have recently attracted great attention from academia and industry. To date, however, neither experimental nor theoretical studies have reached the scalable application of superhydrophobic surfaces, and no simple methods have been developed to produce durable superhydrophobic surfaces. In this project, conical, cylindrical, and cube microstructures with different shapes and dimensions were prepared by 3D printing. Among these structures, the conical microstructures without further modification exhibited the best hydrophobic performance with a contact angle of 120°. However, the hydrophobic performance of the cube microstructures was significantly improved with the addition of an FAS coating to the surface, reaching 131°. This indicates that, with the help of the coating, the 3D printed materials with special features can be transformed into hydrophobic surfaces. The outcome of this project will be useful in promoting the efficient and low-cost preparation of superhydrophobic functional surfaces and their engineering applications.

Fabrication of Conical Microstructure Array for Stable Droplet Generation Over Wide Flow Rate Range.

Droplet generators with the ability to resist flow fluctuations are of importance for microfluidic chip analysis systems. However, obtaining stably desired-size droplets is still a bugbear since even slight fluctuations can cause polydisperse droplets. In this study, a high-performance droplet generator is achieved with a functional conical array housed in the junction of the channels. The conical microstructures are fabricated through the selective etching of the scratched silicon nitride/silicon (Si3N4/Si) substrate in potassium hydroxide (KOH) etchant, where the combination of lateral and normal material removal contributes to the structure formation. It is found that the key role of the conical microstructures is to regulate the flow rate of the continuous phase, which allows droplet generation to turn to the necking phase and enables droplets to shed more easily. It is also noted that the droplet generator with such a conical array can produce monodisperse droplets in wide-range flow, providing new insights for high-quality device design.

Adhesive Wearable Sensors for Electroencephalography from Hairy Scalp.

Electroencephalography has garnered interest for applications in mobile healthcare, human-machine interfaces, and Internet of Things. Conventional electroencephalography relies on wet and dry electrodes. Despite favorable interface impedance of wet electrodes and skin, the application of a large amount of gel at their interface with skin limits the electroencephalography spatial resolution, increases the risk of shorting between electrodes, and makes them unsuited for long-term mobile recording. In contrast, dry electrodes are better suited for long-term recordings but susceptible to motion artifacts. In addition, both wet and dry electrodes are non-adhesive to the hairy scalp and mechanical support, or chemical adhesives are used to hold them in place. Herein, a conical microstructure array (CMSA) based sensor made of carbon nanotube-polydimethylsiloxane composite is reported. The CMSA sensor is fabricated using the innovative, cost-effective, and scalable method of viscosity-controlled dip-pull process. The sensor adheres to the hairy scalp by generating negative pressure in its conical microstructures when it is pressed against scalp. Aided by the application of a trace amount of gel, CMSA sensor establishes good electrical contact with the skin, enabling its applications in mobile electroencephalography over extended periods. Notably, the signal quality of CMSA sensors is comparable to that of medical-grade wet gel electrodes.

The Criterion of the Cassie–Baxter and Wenzel Wetting Modes and the Effect of Elastic Substrates on It (Adv. Mater. Interfaces 12/2023)

Wettability of Bio-Mimetic Microstructures In article number 2202439, Il Woong Park, Jonas M. Ribe, Maria Fernandino, and Carlos A. Dorao replicate bio-mimetic conical microstructures from a hard substrate onto an elastic substrate to explore the effect of microstructures on wettability. The cover art illustrates an elastic surface with replicated microstructures that can mimic the superhydrophobic behavior of the original hard surface. [NTNU Nano is acknowledged for support through the NTNU Nano Impact Fund.]

The Criterion of the Cassie–Baxter and Wenzel Wetting Modes and the Effect of Elastic Substrates on It

AbstractControlling the wettability using microstructures has been studied because of many applications. In particular, bio‐mimetic microstructures modeled after the self‐cleaning properties of the lotus leaf have been extensively studied. Despite many studies successfully achieving the fabrication of superhydrophobic to superhydrophilic surfaces through the manipulation of microstructures, the effect of rough surfaces on contact angle remains an area of inquiry. In this study, conical microstructures with well‐defined geometric parameters are fabricated over a silicon wafer. They are replicated into soft matter that has transparent and flexible characteristics. From the measurement of the contact angle for fabricated surfaces, the prediction of the criteria for transitioning from the Cassie‐Baxter state to the Wenzel state can be suggested. Furthermore, the fabrication of an inexpensive, transparent, elastic, and superhydrophobic surface based on truncated micro‐conical structures in PDMS can be suggested.

Oxygen evolution reaction enhancement of copper electrodes in alkaline medium using ultrafast femtosecond laser structuring

In the pursuit of enhancing the oxygen evolution reaction (OER) activity, we employed femtosecond (FS) laser-induced surface nano-structuring to extend the surface area of copper (Cu) electrodes. Compelling surface hierarchical microstructures have been generated on the electrode surface using this approach. By optimizing the laser processing parameters, regular arrays of conical microstructures are fully covered by a thick and porous layer of Cu2O. Electrochemical measurements reveal that FS laser structuring developed large and structure-tunable surface areas with relative roughness factor between 51 and 61. The enhanced surface area and FS laser-induced Cu2O layers significantly enhanced the OER activity leading to a reduced overpotential (η10 = 345 mV) that is 70% less than that of pristine Cu at a current density of 10 mA cm−2. The results demonstrate the effectiveness of femtosecond laser-based surface structuring in enhancing the electrocatalytic OER activity of Cu electrodes in alkaline media.

A Fully Integrated Flexible Electronic System With Highly Sensitive MWCNTs Piezoresistive Array Sensors for Pressure Monitoring

Pressure ulcers are a common and costly health problem in clinical medicine, caused primarily by continuous vertical pressure on local tissues of the body. Recently, flexible devices have been used to alert pressure ulcers by continuously monitoring the pressure applied to the human skin. Although many pressure sensors provide excellent sensing performance, they still rely on uncomfortable, unstretchable systems with rigid electronics. These devices are mounted on the human body, which hinders continuous pressure monitoring. Herein, we propose a fully integrated flexible electronic system (FIFES) with highly sensitive multi-walled carbon nanotubes (MWNTs) piezoresistive array sensors for pressure monitoring, which accurately achieves external pressure signals acquisition and independently performs processing and wireless transmission. We develop a heat seal connecting technology for integrating the piezoresistive array sensors and its flexible processing circuit, which overcomes the compatibility defect of a rigid circuit board and the flexible sensors. Furthermore, the Cu–Ge metal sputtering and transferring technologies are employed to achieve an extensible circuit. The as-fabricated fully integrated electronic system has excellent flexibility, a rapid response time (the response/recovery time is 0.11/0.08 s, respectively), a wide dynamic range (6–50 kPa), and an excellent sensitivity (1.61 kPa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> for the range under the pressure of 18 kPa, 0.47 kPa <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> for the range of 18–50 kPa) attributes to the optimized conical microstructures number. Owing to its excellent performance, the FIFES has extensive applications for pressure monitoring in different human epidermises, such as elbow bending and knee-bending. Also, it has immense potential applications in sleep posture monitoring and pressure ulcer warning.

Effect of surface micromorphology and hydrophobicity on condensation efficiency of droplets using the lattice Boltzmann method

In the present study, the effects of the surface morphology and surface hydrophobicity on droplet dynamics and condensation efficiency are investigated using the lattice Boltzmann method. Different surface morphologies may have different condensation heat transfer efficiencies, resulting in diverse condensation rates under the same conditions. The obtained results show that among the studied morphologies, the highest condensation rate can be achieved for conical micro-structures followed by the triangle micro-structure, and the columnar micro-structure has the lowest condensation rate. Moreover, it is found that when the surface micro-structure spacing is smaller and the surface micro-structure is denser, the condensation heat transfer between the surface structure and water vapor facilitates, thereby increasing the condensation efficiency of droplets. Furthermore, the condensation process of droplets is associated with the surface hydrophobicity. The more hydrophobic the surface, the more difficult the condensation heat transfer and the longer the required time for droplet nucleation. Meanwhile, a more hydrophobic surface means that it is harder for droplets to gather and merge, and the corresponding droplet condensation rate is also lower.

Microstructured layered targets for improved laser-induced x-ray backlighters.

We present the usage of two-layer targets with laser-illuminated front-side microstructures for x-ray backlighter applications. The targets consisted of a silicon front layer and copper back side layer. The structured layer was irradiated by the 500-fs PHELIX laser with an intensity above 10^{20}Wcm^{-2}. The total emission and one-dimensional extent of the copper Kα x-ray emission as well as a wide spectral range between 7.9 and 9.0 keV were recorded with an array of crystal spectrometers. The measurements show that the front-side modifications of the silicon in the form of conical microstructures maintain the same peak brightness of the Kα emission as flat copper foils while suppressing the thermal emission background significantly. The observed Kα source sizes can be influenced by tilting the conical microstructures with respect to the laser axis. Overall, the recorded copper Kα photon yields were in the range of 10^{11}sr^{-1}, demonstrating the suitability of these targets for probing applications without subjecting the probed material to additional heating from thermal line emission.

Improving superamphiphobicity by mimicking tree-branch topography

when a droplet impacts on a superhydrophobic structured surface below a certain impact velocity, the droplet can bounce off completely from the surface. However, above such velocity a fraction of the droplet will pin on the surface. Surfaces capable of repelling water droplets are ubiquitous in nature or have been artificially fabricated. However, as the surface tension of the liquid is reduced, the capability of the surface to remain non-wetting gets hindered. Despite progress in previous research, the understanding and development of superamphiphobic surface to impacting low surface tension droplets remains elusive. It is proposed that multi-layer re-entrant like roughness can further enhance the anti-wetting properties also for low surface tension fluids.In this work, we produce patterned conical micro-structures with lateral nano-sized roughness. Furthermore, the droplet impact experiments are conducted on various surfaces with variable surface tensions (27 mN/m - 72 mN/m) by using droplets with different Weber numbers (2–170).We show that conical microstructures with lateral roughness mimicking tree-branches provides a surface topology capable of absorbing the force exerted by the droplet during the impact which prevents the droplet from pinning on the surface at higher impact velocity even for low surface tension droplets. Our study has significance for understanding the liquid interaction mechanism with the surface during the impact process and for the associated surface design considerations.

Sub-bandgap absorption and photo-response of molybdenum heavily doped black silicon fabricated by a femtosecond laser.

Molybdenum (Mo)-doped black silicon (Si) is obtained by using femtosecond laser irradiation. The concentration of Mo atoms at the depth from 10 to 200 nm has exceeded 1019cm-3. In contrast, the carrier concentration in the Mo-doped layer is lower than 1015cm-3. The surface morphologies with ripple and conical spike microstructures are formed by changing the pulsed laser fluences. The Mo-doped Si samples exhibit a sub-bandgap (1100∼2500nm) absorptance of more than 60% at a wavelength of 1310 nm. A Mo-doped Si photodetector is made, and the responsivity of the device for 1310 nm is up to 76 mA/W at a -10V bias.

FIB-SEM Investigation of Laser-Induced Periodic Surface Structures and Conical Surface Microstructures on D16T (AA2024-T4) Alloy

The use of aluminum alloy AA2024-T4 (Russian designation D16T) in applications requiring a high strength-to-weight ratio and fatigue resistance such as aircraft fuselage often demands the control and modification of surface properties. A promising route to surface conditioning of Al alloys is laser treatment. In the present work, the formation of ripples and conical microstructures under scanning with femtosecond (fs) laser pulses was investigated. Laser treatment was performed using 250 fs pulses of a 1033 nm Yb:YAG laser. The fluence of the pulses varied from 5 to 33 J/cm2. The scanning was repeated from 1 to 5 times for different areas of the sample. Treated areas were evaluated using focused ion beam (FIB)- scanning electron microscopy (SEM) imaging and sectioning, energy-dispersive X-ray (EDX) spectroscopy, atomic force microscopy (AFM), and confocal laser profilometry. The period of laser-induced periodic surface structures (LIPSS) and the average spacing of conical microstructures were deduced from SEM images by FFT. Unevenness of the treated areas was observed that is likely to have been caused by ablation debris. The structural and elemental changes of the material inside the conical microstructures was revealed by FIB-SEM and EDX. The underlying formation mechanisms of observed structures are discussed in this paper.

Conical micro-structures as a route for achieving super-repellency in surfaces with intrinsic hydrophobic properties

Hydrophobic surfaces like Lotus leaves show amazing self-cleaning properties with the apparent water contact angle above 150° and contact angle hysteresis below 10°. Thus, at low inclination angles, millimeter drops can roll-off easily. This effect can be a consequence of the air trapped below the drop, which allows the droplet to reach a superhydrophobic Cassie-Baxter state. However, the superhydrophobic state can be accompanied by very different adhesive properties due to the pinning of the droplet to the microstructures, implying that even in a hydrophobic or superhydrophobic state, the droplet might not roll-off easily. A superhydrophobic state with minimum adhesion to the surface has been the pursuit in many applications where a super-repellent state is highly desired. Many microstructures have been shown to be able to reach a superhydrophobic state, but only a few have been shown to be capable of achieving a super-repellent state without the help of more complex hierarchical structures. Here, we show that conical structures provide a template for designing super-repellent surfaces where the wetting characteristics look to be invariant in the microscale range. The conical structures can maintain a super-repellent state for all intrinsic contact angles larger than 90°, and the transition from the Cassie-Baxter to the Wenzel state is controlled by the apex angle of the conical structures. This finding advances the understanding of why conical structures can show a superhydrophobic state, which will be beneficial for the design of super-repellent surfaces with a wider intrinsic contact angle range.

Stability and water wetting behavior of superhydrophobic polyurethane films created by hot embossing and plasma etching and coating

AbstractThe surface characteristics, the stability against erosion, and the water wetting behavior of superhydrophobic polyurethane (PU) films are described. Hot embossing is used to imprint conical microstructures in PU films. Afterwards, about 200‐nm thick hydrophobic plasma polymers are deposited by PECVD, using different fluorocarbons (CHF3, C3F6, or C4F8) or hexamethyldisiloxane as precursors. Ar or O2 plasma etching is used to increase the surface nanoroughness. The plasma coatings show stability against sand abrasion and in a long‐term outdoor test but are completely degraded by an industrial UV/water weathering test. Superhydrophobicity is achieved on the coated microstructures with base diameters between 35 µm and 50 µm, top diameters between 14 µm and 20 µm, and distances between 50 µm and 70 µm.

Effects of laser processing parameters on glass light guide plate scattering dot performance

With the development of ultra-thin liquid-crystal displays, glass has been adopted to replace polymethyl methacrylate (PMMA) in light guide plates (LGPs). In this study, the backlight unit and conical microstructures of the edge-lit LGP were acquired from a software program, namely, TracePro, and the output light effect of the LGP was simulated. The uniformity could exceed 90% after the optimization of the dot parameters and decreased as the size, depth and cone angle of the dots increased substantially. Experiments were conducted to evaluate the accuracy of the simulation results. The features and cross-section of the dots were measured in detail using a digital microscope. Comparison results demonstrate that optimized laser micromachining technology can be used to process scattering dots on glass LGPs, and the features of the processed scattering dots are consistent with the simulation results.

Excimer Laser Three-Dimensional Micromachining Based on Image Projection and the Optical Diffraction Effect

An excimer laser three-dimensional (3D) micromachining system is proposed based on a mask image projection method and the optical diffraction effect. The effects of optical diffraction on the laser machining rate are evaluated using a hole-arrayed mask pattern with various feature sizes and hole-area opening ratios. The practical feasibility of the proposed method is demonstrated by machining conical, trihedral, and pyramidal 3D microstructures on polycarbonate substrates. The proposed method greatly simplifies the photo-mask design and preparation task in traditional excimer laser 3D micromachining systems and provides a powerful technique for achieving large-area 3D microstructures with complex patterns and atypical profiles.

Femtosecond laser pulse energy accumulation optimization effect on surface morphology of black silicon

Arrays of sharp conical spike microstructures are created by repeatedly irradiating silicon surfaces with focused femtosecond laser pulses in SF6. The absorbance of light is increased to approximately 90% in a wavelength range from the near ultraviolet (0.25 m) to the near infrared (2.5 m) by the microstructured silicon surface. The microstructured surface presents pitch-black because of enhanced absorption with a broad wavelength range, which is called black silicon. The unique microstructure morphology of black silicon surface formed by femtosecond laser can also bring a lot of other surface functions, for example, self-cleaning and field emission. These functions make black silicon highly desirable in solar energy, detectors and other fields. Therefore, the forming mechanism and conditions of fabrication optimization for black silicon microstructure have always been the focus of research. In our work, the sample is moved by motor-controlled stage while the laser beam is fixed. In the case of laser beam scanning, arrays of sharp conical spikes on the silicon are manufactured in 70 kPa SF6. The aim of the experiment is to find how to optimize the distribution of the laser energy in a number of laser accumulation pulses (the combination of single pulse energy and pulse number) to control the surface morphology of the black silicon. Experimental results show that there appears a bottleneck effect of morphology size growth with the increase of laser irradiation (improving the single pulse energy or increasing pulse accumulation number). Excessive energy accumulation brings no extra effect on optimizing and controlling of microstructure morphology on the surface. Based on theoretical results obtained from a physical model we proposed, we find that the reason for this phenomenon is that the microstructure morphology induced by former sequence pulse modulates the laser energy absorption of current laser pulse, and changes the laser ablation efficiency of the current pulse. According to this physical mechanism, we propose a new way of optimizing surface morphology, with fixing the total laser irradiation energy. And the size and distribution of surface morphology can be achieved by optimizing the distribution of the laser energy in a number of laser accumulation pulses. This approach can not only improve the efficiency of silicon surface preparation of microstructures but also reduce the surface defects and damage. Furthermore, the proposed method can reduce the energy consumption in the process of femtosecond machining. It is of great significance for the engineering application of black silicon.

A large-scale water-harvesting device with β-Al(OH)3 microcone arrays by simple hydrothermal synthesis

Here we report a novel approach to fabricate conical microstructures on aluminum substrates using a cost-effective and scalable hydrothermal synthesis method.

Localized Plasmon Resonances in Gold and Silver Microcones Fabricated Using Single-Pulse Femtosecond Nanoprocessing

In this paper we report on the one-step fabrication of silver and gold conical microstructures using femtosecond single-pulse nanoablation. The lateral and vertical size of the microcones was found to be fully determined by the pulse energy. Using dark-field confocal microspectroscopy technique we observe and measure the resonant light scattering from the individual nanostructures in visible spectral range with the resonant peak position being dependent on the microcone size. This resonant light scattering was attributed to the excitation of the localized surface plasmon modes in the upper part of the microcones, where the main part of the molten materials accumulates and resolidifies during the formation process. The plasmon-mediated EM field enhancement near the microcones was experimentally probed and revealed by means of photoluminescent (PL) microscopy. The averaged 6-fold enhancement of the PL signal from the layer of alkylated europium complex embedded into the organic matrix was experimentally demonstrated.

Properties of conical microstructures formed on silicon surfaces via nanosecond laser ablation under vacuum

We fabricated conical microstructures onto a silicon surface via nanosecond laser ablation at 355 nm in a vacuum. The silicon surface coved with these conical microstructures exhibited strong optical absorbance for UV–VIS–NIR wavelengths (from 0.25 to 2.5 μm). The transmission properties at infrared wavelengths (2.5–24 μm) for this ns laser ablated silicon was also discussed. In addition, the microstructured silicon surface exhibited excellent super-hydrophobicity with the highest water contact angle near 160°.