Bio-plastics are starting to graduate from the 'emerging technology' stage to market acceptance as everyday materials. In the present study, nanocomposite coatings embedding copper nanoparticles (CuNPs) were developed as new active packaging for fresh dairy products. In order to combine the bioactivity of CuNPs with a biodegradable polymer matrix, copper nanoparticles were satisfactorily incorporated into polylactic acid (PLA). Two different routes were carried out to prepare active films by picosecond-pulsed laser ablation. The nano-materials were characterized by UV-Vis spectroscopy and X-ray Photoelectron spectroscopy. Copper release was also measured through atomic absorption analyses. To assess the antimicrobial effects of nanocomposite systems, both in vitro and in vivo tests were carried out. The active polylactic acid films showed good antibacterial activity. In fiordilatte samples stored at 4 C during 9 days, proliferation of main spoilage microorganisms was delayed with a consequent preservation of sensory attributes. These results represent a step forward in the possible application of copper in the food packaging industry. Industrial relevance Bio-plastics with active properties represent the most emerging technology in food packaging field. Results from the current paper demonstrate that antimicrobial films of PLA embedding copper nanoparticles could be developed and applied to fresh dairy products as fiordilatte. In fact, the in vivo test confirmed the antimicrobial effects on fiordilatte spoilage, without compromising sensory attributes. Results could gain great importance from the industrial dairy sector.

A new type of nanomaterial has been developed as antibacterial additive for food packaging applications.This nanocomposite is composed of copper nanoparticles embedded in polylactic acid, combining the antibacterial properties of copper nanoparticles with the biodegradability of the polymer matrix. Metal nanoparticles have been synthesised by means of laser ablation, a rising and easy route to prepare nanostructures without any capping agent in a liquid environment. As prepared, nanoparticle suspensions have been easily mixed to a polymer solution. The resulting hybrid solutions have been deposited by drop casting, thus obtaining self-standing antibacterial packages. All samples have been characterized by UV-Vis spectroscopy, X-ray photoelectron spectroscopy and electro-thermal atomic absorption spectroscopy. Ion release data have been matched with bioactivity tests performed by Japanese Industrial Standard (JIS) method (JIS Z 2801:2000) against Pseudomonas spp., a very common Gram-negative microbial group able to proliferate in processed food.

Recent developments in laser joining show the applicability of spectral analysis of the plasma plume emission to monitor and control the quality of weld. The analysis of the complete spectra makes it possible to measure specific emission lines which reveal information about the welding process. The subsequent estimation of the electron temperature can be correlated with the quality of the corresponding weld seam. A typical quality parameter, for laser welds of stainless steel, is the achieved penetration depth of the weld. Furthermore adequate gas shielding of the welds has to be provided to avoid seam oxidation . In this paper monitoring and real-time control of the penetration depth during laser welding is demonstrated. Optical emissions in the range of 400nm and 560nm are collected by a fast spectrometer. The sensor data are used to determine the weld quality of overlap welds in AISI 304 stainless steel sheets performed both with CW Nd:YAG and CO2 lasers. A PI-controller adjusts the laser power aiming at a constant penetration. Optical inspection of the weld surface and microscopic analysis of weld cross sections were used to verify the results obtained with the proposed closed-loop system of spectroscopic sensor and controller.

In this paper we describe a novel spectroscopic closed loop control system capable of stabilizing the penetration depth during laser welding processes by controlling the laser power. Our novel approach is to analyze the optical emission from the laser generated plasma plume above the keyhole, to calculate its electron temperature as a process-monitoring signal. Laser power has been controlled by using a quantitative relationship between the penetration depth and the plasma electron temperature. The sensor is able to correlate in real time the difference between the measured electron temperature and its reference value for the requested penetration depth. Accordingly the closed loop system adjusts the power, thus maintaining the penetration depth.

An experimental investigation is presented on the ultrashort pulse laser drilling of different metals with diverse thermal properties in the high repetition rate and high average power regime. An Ytterbium-doped fiber CPA system was used, providing pulse energies and repetition rates up to 70 ?J and 1 MHz, respectively. It has been found that at a few hundred kilohertz particle shielding causes a decrease of the ablation rate, depending on the pulse energy. At higher repetition rates, the heat accumulation effect overbalances particle shielding, but significant melt ejection affects the hole quality. The influence, in this regime, of pulse duration (800 fs to 19 ps) and wavelength (1030 nm and 515 nm) on the drilling efficiency and on the achievable precision have been further experimentally studied.

The impact of quantum cascade lasers (QCLs) intrinsically high sensitivity to external optical feedback intended for sensing applications such as in-line ablation rate measurements is experimentally demonstrated. We developed a QCL-based sensor to assess the voltage modulation at the laser terminals induced by fast displacement of the ablation front during the process. This work shows that the detection range of our diagnostic system is only limited by the emission wavelength of the QCL probe source and the capability to measure ablation rates as high as 160 nm/pulse was reported. This sensing technique can be employed with the whole class of quantum cascade lasers, whose emission spans from mid-IR to THz spectral region, thus enabling the extension of its applications to ultra-fast laser ablation processes.

We demonstrate that a non-invasive sensing technique based on optical feedback interferometry is capable to instantaneously measure the ablation front displacement and the removal rate during ultrafast laser percussion drilling of metallic plates. The sawtooth-like modulation of the interferometric signal out of the detecting sensor has been analyzed to reveal the time dependence of the removal depth with sub-micrometric resolution. Various dynamic factors related to the influence of laser pulse duration and peak energy have been assessed by in-situ spatial- and time-dependent characterization all through the ablation process. The importance of realtime measurement of the ablation rate is crucial to improve the basic understanding of ultrafast lasermaterial interactions. Moreover, the detection system results high-sensitive, compact, and easily integrable in most industrial workstations, enabling the development of on-line control to improve the ablation efficiency and the quality of laser micromachining processes.

The recent development of ultrafast laser ablation technology in precision micromachining has dramatically increased the demand for reliable and real-time detection systems to characterize the material removal process. In particular, the laser percussion drilling of metals is lacking of non-invasive techniques able to monitor into the depth the spatial- and time-dependent evolution all through the ablation process. To understand the physical interaction between bulk material and high-energy light beam, accurate in-situ measurements of process parameters such as the penetration depth and the removal rate are crucial. We report on direct real time measurements of the ablation front displacement and the removal rate during ultrafast laser percussion drilling of metals by implementing a contactless sensing technique based on optical feedback interferometry. High aspect ratio micro-holes were drilled onto steel plates with different thermal properties (AISI 1095 and AISI 301) and Aluminum samples using 120-ps/110-kHz pulses delivered by a microchip laser fiber amplifier. Percussion drilling experiments have been performed by coaxially aligning the diode laser probe beam with the ablating laser. The displacement of the penetration front was instantaneously measured during the process with a resolution of 0.41 ?m by analyzing the sawtooth-like induced modulation of the interferometric signal out of the detector system.

Microinjection moulding combined with the use of removable inserts is one of the most promising manufacturing processes for microfluidic devices, such as lab-on-chip, that have the potential to revolutionize the healthcare and diagnosis systems. In this work, we have designed, fabricated and tested a compact and disposable plastic optical stretcher. To produce the mould inserts, two micro manufacturing technologies have been used. Micro electro discharge machining (µEDM) was used to reproduce the inverse of the capillary tube connection characterized by elevated aspect ratio. The high accuracy of femtosecond laser micromachining (FLM) was exploited to manufacture the insert with perfectly aligned microfluidic channels and fibre slots, facilitating the final composition of the optical manipulation device. The optical stretcher operation was tested using microbeads and red blood cells solutions. The prototype presented in this work demonstrates the feasibility of this approach, which should guarantee real mass production of ready-to-use lab-on-chip devices.

Microfluidic optical stretchers are valuable optofluidic devices for studying single cell mechanical properties. These usually consist of a single microfluidic channel where cells, with dimensions ranging from 5 to 20 ?m are trapped and manipulated through optical forces induced by two counter-propagating laser beams. Recently, monolithic optical stretchers have been directly fabricated in fused silica by femtosecond laser micromachining (FLM). Such a technology allows writing in a single step in the substrate volume both the microfluidic channel and the optical waveguides with a high degree of precision and flexibility. However, this method is very slow and cannot be applied to cheaper materials like polymers. Therefore, novel technological platforms are needed to boost the production of such devices on a mass scale. In this work, we propose integration of FLM with micro-injection moulding (?IM) as a novel route towards the cost-effective and flexible manufacturing of polymeric Lab-on-a-Chip (LOC) devices. In particular, we have fabricated and assembled a polymethylmethacrylate (PMMA) microfluidic optical stretcher by exploiting firstly FLM to manufacture a metallic mould prototype with reconfigurable inserts. Afterwards, such mould was employed for the production, through ?IM, of the two PMMA thin plates composing the device. The microchannel with reservoirs and lodgings for the optical fibers delivering the laser radiation for cell trapping were reproduced on one plate, while the other included access holes to the channel. The device was assembled by direct fs-laser welding, ensuring sealing of the channel and avoiding thermal deformation and/or contamination.

Minimizing mechanical losses and friction invehicle engines would have a great impact on reducingfuel consumption and exhaust emissions, to the benefit ofenvironmental protection. With this scope, laser surfacetexturing (LST) with femtosecond pulses is an emergingtechnology, which consists of creating, by laser ablation,an array of high-density microdimples on the surface of amechanical device. The microtexture decreases the effectivecontact area and, in case of lubricated contact, actsas oil reservoir and trap for wear debris, leading to anoverall friction reduction. Depending on the lubricationregime and on the texture geometry, several mechanismsmay concur to modify friction such as the local reductionof the shear stress, the generation of a hydrodynamic liftbetween the surfaces or the formation of eddy-like flowsat the bottom of the dimple cavities. All these effectshave been investigated by fabricating and characterizingseveral LST surfaces by femtosecond laser ablation withdifferent features: partial/full texture, circular/ellipticaldimples, variable diameters, and depths but equivalentareal density. More than 85% of friction reduction hasbeen obtained from the circular dimple geometry, but theelliptical texture allows adjusting the friction coefficientby changing its orientation with respect to the slidingdirection.

Femtosecond-pulsed laser welding of transparent materials on a micrometer scale is a versatile tool for the fabrication and assembly of electronic, electromechanical, and especially biomedical micro-devices. In this paper, we report on microwelding of two transparent layers of polymethyl methacrylate (PMMA) with femtosecond laser pulses at 1030 nm in the MHz regime. We aim at exploiting localized heat accumulation to weld the two layers without any preprocessing of the sample and any intermediate absorbing media, by focusing fs-laser pulses at the interface. The modifications produced by the focused laser beam into the bulk material have been firstly investigated depending on the laser process parameters aiming to produce continuous melting. Results have been evaluated based on heat accumulation models. Finally, fs-laser welding of PMMA samples have been successfully demonstrated and tested by leakage tests for application in direct laser assembly of microfluidic devices.

In this work we present a micro manufacturing platform for the production of polymeric microfluidic devices on a mass scale, based on the integration of microinjection moulding and femtosecond laser (fs-laser) micromachining technologies. A mould prototype was designed for the fabrication of polymeric thin plates characterized by simplified microfeatures representative of typical Lab-on-a-Chip (LoC) devices. The injection moulding master tool includes replaceable metallic inserts, which were fabricated by exploiting the extreme flexibility and accuracy of fs-laser milling. Here, the laser process parameters have been studied and properly adjusted to meet the target geometry and surface quality of the mould inserts, which were subsequently characterized by confocal and SEM microscopy. The micro injection moulding (?IM) process parameters for the device production have been defined by complete three-dimensional filling and packing process simulations. Finally, the micro-injection mould with reconfigurable inserts was employed for the production of thin plates with simplified microfeatures using PMMA. The ability to reproduce these microfeatures via ?IM is an essential step to approach to the mass-production of a polymeric LoC and the use of replaceable micro-inserts fabricated by direct fs-laser ablation promises high flexibility in the design and manufacturing of such devices.

We present measurements of friction coefficient of lubricated laser surface textured (LST) microstructures with two different geometries. The former is made of a square lattice of microholes; the latter is constituted by a series of microgrooves. We analyze sliding velocities spanning more than two orders of magnitude to cover the entire range from the boundary to the hydrodynamic regime. In all cases, the interfacial pressure is limited to values (relevant to particular manufacturing processes) which allow to neglect macroscopic elastic deformations, piezo-viscosity and oil compressibility effects. The measured Stribeck curves data are compared with those obtained for the flat control surface and show that the regular array of microholes allows to reduce friction over the entire range of lubrication regimes with a decrease of about 50 % in the hydrodynamic regime. On the contrary, the parallel microgrooves lead to an increase of friction compared to the flat control surface with a maximum increase of about 80-100 % in the mixed lubrication regime. These remarkably opposite friction results are then explained with the aid of numerical simulations. Our findings confirm that LST may have cutting edge applications in engineering, not only in classical applications (e.g.; to reduce piston-ring friction losses in internal combustion engines) but also, in particular, in technological processes, such as hydroforming, superplastic forming, where the mapping of the frictional properties of the mold has a crucial role in determining the final properties of the mechanical component. © 2012 Springer Science+Business Media New York.

Despite its advantages with respect to precision, ultrashort pulse micromachining often suffers from a low processing speed. We will discuss the opportunities for high repetition rate and high average power ultrafast fiber lasers to overcome these problems. © OSA / FILAS 2011.

We present a cost-effective and highly-portable plastic prototype that can be interfaced with a cell phone to implement an optofluidic imaging cytometry platform. It is based on a PMMA microfluidic chip that fits inside an opto-mechanical platform fabricated by a 3D printer. The fluorescence excitation and imaging is performed using the LED and the CMOS from the cell phone increasing the compactness of the system. A custom developed application is used to analyze the images and provide a value of particle concentration.

The concept of replaceable micro cavities can be applied in the design of moulds for different applications and the efficiency of the product development stage is greatly improved. The inserts allow easy testing of the design prototypes especially in highly interdisciplinary fields where manufacturing challenges such as replication, structuring of moulds, polymers, sealing, integration of functional elements, and the production of (customised) disposables at competitive prices all still need to be addressed. In this direction laser micromachining technology meets the need for fabricating micro-injection mould inserts with complex shapes and a high level of accuracy.

We report on an experimental study of the incubation effect during laser ablation of stainless steel with fs- and ps-pulses at high repetition rates. Ablation thresholds for multiple pulses N have been estimated. As expected, the ablation threshold decreases with N due to damage accumulation. The related incubation coefficient has been determined at different repetition rates, from 50-kHz to 1-MHz and two pulse durations: 650-fs and 10-ps. Results show that the incubation effect is lower for fs-pulses below 600 kHz. At higher repetition rates incubation is more pronounced regardless of the pulse duration, probably due to heat accumulation. © 2013 The Authors.

We studied the laser ablation dynamics of steel in the thermal regime both experimentally and theoretically. The real-time monitoring of the process shows that the ablation rate depends on laser energy density and ambient pressure during the exposure time. We demonstrated that the ablation efficiency can be enhanced when the pressure is reduced with respect to the atmospheric pressure for a given laser fluence, reaching an upper limit despite of high-vacuum conditions. An analytical model based on the Hertz-Knudsen law reproduces all the experimental results.

Surface hardening with discrete laser spot treatment is an interesting solution since the adoption of a single pulse allows the treatment of different surface geometries avoiding the effect of back tempering. The aim of this work is to find a suitable process window in which operate to get best results in terms of hardness, diameter and depth of the treated region. A single pulse out of a fiber laser source impinging on a bearing hypereutectoid steel was used using different power values, pulse energy and defocussing distances, in order to get the optimal process parameters. The dimensions of the hardened zone and its hardness were then acquired and related to the laser process parameters, to the prior microstructure of the steel (spheroidized and tempered after oil quenching) and to the roughness on the specimen before the laser treatment. Experimental results highlighted that both the surface condition (in terms of roughness) and the initial steel microstructure have a great influence on the achieved hardness values and on the dimension of the laser hardened layer. The pulse energy and power strongly affected the dimension of the hardened layer, too.

Direct real-time measurements of the penetration depth during laser micromachining has been demonstrated by developing a novel ablation sensor based on laser diode feedback interferometry. Percussion drilling experiments have been performed by focusing a 120-ps pulsed fiber laser onto metallic targets with different thermal conductivity. In-situ monitoring of the material removal rate was achieved by coaxially aligning the beam probe with the ablating laser. The displacement of the ablation front was revealed with sub-micrometric resolution by analyzing the sawtooth-like induced modulation of the interferometric signal out of the detector system.

Surface micro-texturing has been widely theoretically and experimentally demonstrated to be beneficial to friction reduction in sliding contacts under lubricated regimes. Several microscopic mechanisms have been assessed to concur to this macroscopic effect. In particular, the micro-textures act as lubricant reservoirs, as well as traps for debris. Furthermore, they may produce a local reduction of the shear stress coupled with a stable hydrodynamic pressure between the lubricated sliding surfaces. All these mechanisms are strongly dependent both on the micro-texturing geometry and on the operating conditions. Among the various micro-machining techniques, laser ablation with ultrashort pulses is an emerging technology to fabricate surface textures, thanks to the intrinsic property of laser light to be tightly focused and the high flexibility and precision achievable. In addition, when using sub-ps pulses, the thermal damage on the workpiece is negligible and the laser surface textures (LST) are not affected by burrs, cracks or resolidified melted droplets, detrimental to the frictional properties. In this work several LST geometries have been fabricated by fs-laser ablation of steel surfaces, varying the diameter, depth and spacing of micro-dimples squared patterns. We compared their frictional performance with a reference nontextured sample, on a range of sliding velocities from the mixed lubrication to the hydrodynamic regime. The measured Stribeck curves data show that the depth and diameter of the microholes have a huge influence in determining the amount of friction reduction at the interface. Different theoretical interpretations to explain the experimental findings are also provided. © 2014 SPIE.

We have investigated the friction properties of lubricated laser micro-textured surfaces. The micro-texture consists of a square lattice of micro-holes whose diameter, depth and spacing are controlled during the laser texturing process. All surfaces have the same texture area density, but different diameters and depths of the micro-holes. We measure the coefficient of friction on a range of sliding velocities from the mixed lubrication regime to the hydrodynamic regime. We find that the depth and the diameter of the micro-holes have a huge influence in determining the amount of friction reduction at the interface. Interestingly experiments also show that optimal micro-hole depth values, minimizing the friction in the hydrodynamic regime, are remarkably effective also in the mixed lubrication regime. (C) 2014 Elsevier Ltd. All rights reserved.

The instantaneous measurement of both ablation front displacement and removal rate during ultrafast laser microdrilling is demonstrated by on line sensing technique based on optical feedback interferometry in both unipolar and bipolar semiconductor laser. The dependence of laser ablation dynamics on pulse duration, energy density and working pressure has been investigated, thus allowing a significant advancement of the basic understanding of the ultrafast laser-material interactions. Moreover, the detection system results high-sensitive, compact, and easily integrable in most industrial workstations, enabling the development of real-time control to improve ablation efficiency and quality of laser micro-machining processes. © 2013 The Authors.

The plasma optical radiation emitted during CO2 laser welding of stainless steel samples has been detected with a Si-PIN photodiode and analyzed under different process conditions. The discrete wavelet transform (DWT) has been used to decompose the optical signal into various discrete series of sequences over different frequency bands. The results show that changes of the process settings may yield different signal features in the range of frequencies between 200 Hz and 30 kHz. Potential applications of this method to monitor in real time the laser welding processes are also discussed.

The in-process monitoring and real-time control of the penetration depth during laser welding is evaluated. An optical collimator collects the optical emission for measurement with a fast spectrometer. The sensor data are used to calculate the electron temperature and subsequently to determine the weld quality of overlap welds in AISI 304 stainless steel sheets performed both with CW Nd:YAG and CO2 lasers. A PI-controller adjusts the laser power aiming at a constant penetration depth and has been tested for Nd:YAG laser welding. Optical inspection of the weld verifies the results obtained with the proposed closed-loop system of spectroscopic sensor and controller.

We demonstrated a sensing technique for in-line ablation rate detection using a quantum cascade laser (QCL) under external optical feedback. The design of the QCL-based diagnostic system allowed to monitor the voltage modulation at the laser terminals induced by fast dynamics in the ablation process. Real-time detection of the ablation front velocity as well as in-situ investigations of the surface temperature were provided. Experimental results on fast ablation rates per pulse correlate well with the theoretical prediction. The detection range was demonstrated to be limited only by the QCL-probe emission wavelength, which is scalable up to the THz spectral region.

We have introduced a new hybrid fabrication method for lab-on-a-chip devices through the combination of femtosecond laser micromachining and removable insert micro-injection molding. This method is particularly suited for the fast prototyping of new devices, while maintaining a competitive low cost. To demonstrate the effectiveness of our approach, we designed, fabricated, and tested a completely integrated flow cytometer coupled to a portable media device. The system operation was tested with fluorescent plastic micro-bead solutions ranging from 100 beads/?L to 500 beads/?L. We demonstrated that this hybrid lab-on-a-chip fabrication technology is suitable for producing low-cost and portable biological microsystems and for effectively bridging the gap between new device concepts and their mass production.

We report on the instantaneous detection of the ablation rate as a function of depth during ultrafast microdrilling of metal targets. The displacement of the ablation front has been measured with a sub-wavelength resolution using an all-optical sensor based on the laser diode self-mixing interferometry. The time dependence of the laser ablation process within the depth of aluminum and stainless steel targets has been investigated to study the evolution of the material removal rate in high aspect-ratio micromachined holes.

High-energy ultra-short pulse laser ablation is a fast-growing technology in precision laser micromachining of transparent as well as opaque materials. Accurate in-situ measurements of physical parameters such as the penetration depth and the removal rate are crucial to fully characterize the ultrafast laser-material interactions [1-5]. Nonetheless, the laser drilling is still lacking of a real-time technique able to monitor and control the spatial- and time-dependent evolution of the hole-depth in metallic plates.

We study the incubation effect during laser ablation of stainless steel with ultrashort pulses to boost the material removal efficiency at high repetition rates. The multi-shot ablation threshold fluence has been estimated for two pulse durations, 650-fs and 10-ps, in a range of repetition rates from 50kHz to 1 MHz. Our results show that the threshold fluence decreases with the number of laser pulses N due to damage accumulation mechanisms, as expected. Moreover, approaching the MHz regime, the onset of heat accumulation enhances the incubation effect, which is in turn lower for shorter pulses at repetition rates below 600 kHz. A saturation of the threshold fluence value is shown to occur for a significantly high number of pulses, and well fitted by a modified incubation model. (C) 2014 Optical Society of America

In-process monitoring and feedback control are fundamental actions for stable and good quality laser welding process. In particular, penetration depth is one of the most critical features to be monitored. In this research, overlap welding of stainless steel is investigated to stably reproduce a fixed penetration depth using both CO 2 and Nd:YAG lasers. Plasma electron temperatures of Fe(I) and Cr(I) are evaluated as in process monitoring using the measurement of intensities of emission lines with fast spectrometers. The sensor system is calibrated using a quantitative relationship between electron temperature and penetration depth in different welding conditions. Finally closed loop control of the weld penetration depth is implemented by acquiring the electron temperature value and by adjusting the laser power to maintain a pre-set penetration depth. A PI controller is successfully used to stabilize the electron temperature around the set point corresponding to the right penetration depth starting from a wrong value of any initial laser power different than the set point. Optical inspection of the weld surface and macroscopic analyses of cross sections verify the results obtained with the proposed closed-loop system based on a spectroscopic controller and confirms the reliability of our system.

The plasma electron temperature has been estimated starting from the spectroscopic analysis of the optical emission of the laser-generated plasma plume during quite diverse stainless steel welding procedures (c.w. CO 2 and pulsed Nd:YAG). Although the optical emissions present different spectral features, a discrete contribution of several iron lines can be highlighted in both types of welding. We have found that the electron temperature decreases as the laser power is enhanced, in static as well as dynamic conditions. Such a result could be useful to develop a closed loop control system of the weld penetration depth.

Ultrafast laser ablation in liquids is an easy, fast and versatile method to generate nanoparticles. Metal nanoparticles have been demonstrated to possess excellent antimicrobial properties thanks to their very high surface area to volume ratios which provide better contact with microorganisms [1]. For this reason, they are attracting growing interest as a base to develop novel nanocomposites preventing biocontamination in several application fields. In particular, copper nanoparticles (CuNPs) can be used, under controlled ionic release conditions [2], to inhibit bacteria proliferation in food packaging [3] as well as in other applications in medicine, agriculture or pharmaceuticals. In order to prevent human toxicity, CuNPs need to be carefully embedded into polymer matrices acting as immobilizing component and potentially bringing additional properties to the final nanocomposite [4]. Among the possible dispersing polymers, chitosan (CS) is a well-known antimicrobial material, widely exploited for its biodegradability and nontoxicity [5]. © 2013 IEEE.

We experimentally investigate and theoretically interpret the effect of varying the microstructure geometry introduced by laser surface texturing (LST), on the frictional properties of interacting components. The ability to control the coefficient of friction under lubricated conditions is demonstrated. Particularly, the LST optimization of a regular pattern of microholes on steel allows to reduce friction over the entire range of sliding velocities with respect to the untextured case. Moreover, we measure the Stribeck curves on a range of sliding velocity covering the entire lubrication range, i.e. from the boundary to the hydrodynamic regime under the so called iso-viscous rigid condition. Our measurements show a friction reduction up to 50% in the hydrodynamic regime. © 2013 The Authors.

Developing versatile joining techniques to weld transparentmaterials on a micrometer scale is of great importance in a growing numberof applications, especially for the fabrication and assembly of biomedicaldevices. In this paper, we report on fs-laser microwelding of two transparentlayers of polymethyl methacrylate (PMMA) based on nonlinear absorptionand localized heat accumulation at high repetition rates. A fiber CPA lasersystem was used delivering 650-fs pulses at 1030 nm with repetition rates inthe MHz regime. The laser-induced modifications produced by the focusedbeam into the bulk PMMA were firstly investigated, trying to find a suitableset of process parameters generating continuous and localized melting.Results have been evaluated based on existing heat accumulation models.Then, we have successfully laser welded two 1-mm-thick PMMA layers ina lap-joint configuration. Sealing of the sample was demonstrated throughstatic and dynamic leakage tests. This fs-laser micro-welding process doesnot need any pre-processing of the samples or any intermediate absorbinglayer. Furthermore, it offers several advantages compared to other joiningtechniques, because it prevents contamination and thermal distortion of thesamples, thus being extremely interesting for application in direct laserfabrication of microfluidic devices.