Interaction of ultrafast laser pulses with nanostructure surfaces

The interaction of ultrafast laser pulses with surfaces on the nanoscale paves the way for various innovative technologies in spectroscopy, photovoltaics, photocatalysis, or medicine, to mention only a few. The basic mechanisms, however, are still the subject of intense research. We take a closer look at this topic from different viewpoints. The first aspect is the enhancement of the efficiency of physical or chemical processes by producing local field maxima and resonances at top-down or bottom-up structured surfaces. A further aspect is the dynamic change of optical properties by inducing free carriers and plasmons. Last but not least, permanent nanostructures can be obtained as a result of nano-feedback and self-organization. In high-energy laser physics, all three aspects play a role at once. Therefore, particular attention will be paid to this emerging field

Nanosecond laser textured superhydrophobic metallic surfaces and their chemical sensing applications

tThis work demonstrates superhydrophobic behavior on nanosecond laser patterned copper and brasssurfaces. Compared with ultrafast laser systems previously used for such texturing, infrared nanosecondfiber lasers offer a lower cost and more robust system combined with potentially much higher processingrates. The wettability of the textured surfaces develops from hydrophilicity to superhydrophobicity overtime when exposed to ambient conditions. The change in the wetting property is attributed to the par-tial deoxidation of oxides on the surface induced during laser texturing. Textures exhibiting steady statecontact angles of up to ∼152◦with contact angle hysteresis of around 3–4◦have been achieved. Inter-estingly, the superhydrobobic surfaces have the self-cleaning ability and have potential for chemicalsensing applications. The principle of these novel chemical sensors is based on the change in contactangle with the concentration of methanol in a solution. To demonstrate the principle of operation ofsuch a sensor, it is found that the contact angle of methanol solution on the superhydrophobic surfacesexponentially decays with increasing concentration. A significant reduction, of 128◦, in contact angle onsuperhydrophobic brass is observed, which is one order of magnitude greater than that for the untreatedsurface (12◦), when percent composition of methanol reaches to 28%

Laser textured surface gradients

This work demonstrates a novel technique for fabricating surfaces with roughness and wettability gradients and their subsequent applications for chemical sensors. Surface roughness gradients on brass sheets are obtained directly by nanosecond laser texturing. When these structured surfaces are exposed to air, their wettability decreases with time (up to 20 days) achieving both spatial and temporal wettability gradients. The surfaces are responsive to organic solvents. Contact angles of a series of dilute isopropanol solutions decay exponentially with concentration. In particular, a fall of 132° in contact angle is observed on a surface gradient, one order of magnitude higher than the 14° observed for the unprocessed surface, when the isopropanol concentration increased from 0 to 15.6 wt%. As the wettability changes gradually over the surface, contact angle also changes correspondingly. This effect offers multi-sensitivity at different zones on the surface and is useful for accurate measurement of chemical concentration

Laser textured superhydrophobic surfaces and their applications for homogeneous spot deposition

This work reports the laser surface modification of 304S15 stainless steel to develop superhydrophobic properties and the subsequent application for homogeneous spot deposition. Superhydrophobic surfaces, with steady contact angle of ∼154° and contact angle hysteresis of ∼4°, are fabricated by direct laser texturing. In comparison with common pico-/femto-second lasers employed for this patterning, the nanosecond fiber laser used in this work is more cost-effective, compact and allows higher processing rates. The effect of laser power and scan line separation on surface wettability of textured surfaces are investigated and optimized fabrication parameters are given. Fluid flows and transportations of polystyrene (PS) nanoparticles suspension droplets on the processed surfaces and unprocessed wetting substrates are investigated. After evaporation is complete, the coffee-stain effect is observed on the untextured substrates but not on the superhydrophobic surfaces. Uniform deposition of PS particles on the laser textured surfaces is achieved and the deposited material is confined to smaller area

Flame-Assisted Spray Pyrolysis Using an Annular Flame Nozzle with Decoupled Velocity Control

Flame spray pyrolysis, widely used in chemical industries, is a technology to synthesize nanoparticles. While the flame spray pyrolysis uses fuels as a solution liquid, the flame-assisted spray pyrolysis method uses aqueous solutions. Since process parameters such as concentration of precursor, size of droplets, and ratio of the air–gas mixture affect the size of nanoparticles, developing a flexible system to control these parameters is required. This paper proposes a new type of nozzle system to produce nanoparticles using flame-assisted spray pyrolysis. The annular nozzle design allows flexible control of particle flow and temperature, and an ultrasonic nebulizer was used to produce droplets with different size. Experiments were conducted to analyze the relationship between nanoparticle size and process parameters, concentration of precursor, frequency of the atomizer, and flame temperature. A precursor solution consisting of silver nitrate (AgNO3) mixed in deionized water is used. The effects of the process parameters are discussed, and analysis of the nanoparticles shows that silver nanoparticles are deposited with an average size of 25~115 nm

One-step Fabrication of Superhydrophobic Hierarchical Structures by Femtosecond Laser Ablation

Hydrophobic surface properties are sought after in many areas of research, engineering, and consumer product development. Traditionally, hydrophobic surfaces are produced by using various types of coatings. However, introduction of foreign material onto the surface is often undesirable as it changes surface chemistry and cannot provide a long lasting solution (i.e. reapplication is needed). Therefore, surface modification by transforming the base material itself can be preferable in many applications. Femtosecond laser ablation is one of the methods that can be used to create structures on the surface that will exhibit hydrophobic behavior. The goal of the presented research was to create micro and nano-scale patterns that will exhibit hydrophobic properties with no additional post treatment. As a result, dual scale patterned structures were created on the surface of steel aluminum and tungsten carbide samples. Ablation was performed in the open air with no subsequent treatment. Resultant surfaces appeared to be strongly hydrophobic or even superhydrophobic with contact angle values of 140 degrees and higher. In conclusion, the nature of surface hydrophobicity proved to be highly dependent on surface morphology as the base materials used are intrinsically hydrophilic. It was also proven that the hydrophobicity inducing structures could be manufactured using femtosecond laser machining in a single step with no subsequent post treatment. (C) 2014 Elsevier B.V. All rights reserved.X113026sciescopu

Flame-Assisted Spray Pyrolysis Using an Annular Flame Nozzle with Decoupled Velocity Control

Flame spray pyrolysis, widely used in chemical industries, is a technology to synthesize nanoparticles. While the flame spray pyrolysis uses fuels as a solution liquid, the flame-assisted spray pyrolysis method uses aqueous solutions. Since process parameters such as concentration of precursor, size of droplets, and ratio of the air⁻gas mixture affect the size of nanoparticles, developing a flexible system to control these parameters is required. This paper proposes a new type of nozzle system to produce nanoparticles using flame-assisted spray pyrolysis. The annular nozzle design allows flexible control of particle flow and temperature, and an ultrasonic nebulizer was used to produce droplets with different size. Experiments were conducted to analyze the relationship between nanoparticle size and process parameters, concentration of precursor, frequency of the atomizer, and flame temperature. A precursor solution consisting of silver nitrate (AgNO3) mixed in deionized water is used. The effects of the process parameters are discussed, and analysis of the nanoparticles shows that silver nanoparticles are deposited with an average size of 25~115 nm