Dave F. Farson

Mechanism of keyhole formation and stability in stationary laser welding

The formation and stability of stationary laser weld keyholes are investigated using a numerical simulation. The effect of multiple reflections in the keyhole is estimated using the ray tracing method, and the free surface profile, flow velocity and temperature distribution are calculated numerically. In the simulation, the keyhole is formed by the displacement of the melt induced by evaporation recoil pressure, while surface tension and hydrostatic pressure oppose cavity formation. A transition mode having the geometry of the conduction mode with keyhole formation occurs between the conduction and keyhole modes. At laser powers of 500 W and greater, the protrusion occurs on the keyhole wall, which results in keyhole collapse and void formation at the bottom. Initiation of the protrusion is caused mainly by collision of upward and downward flows due to the pressure components, and Marangoni flow has minor effects on the flow patterns and keyhole stability.

Micropatterning and characterization of electrospun poly(ε-caprolactone)/gelatin nanofiber tissue scaffolds by femtosecond laser ablation for tissue engineering applications.

Experimental investigations aimed at assessing the effectiveness of femtosecond (FS) laser ablation for creating microscale features on electrospun poly(ε‐caprolactone) (PCL)/gelatin nanofiber tissue scaffold capable of controlling cell distribution are described. Statistical comparisons of the fiber diameter and surface porosity on laser‐machined and as‐spun surface were made and results showed that laser ablation did not change the fiber surface morphology. The minimum feature size that could be created on electrospun nanofiber surfaces by direct‐write ablation was measured over a range of laser pulse energies. The minimum feature size that could be created was limited only by the pore size of the scaffold surface. The chemical states of PCL/gelatin nanofiber surfaces were measured before and after FS laser machining by attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy and X‐ray photoelectron spectroscopy (XPS) and showed that laser machining produced no changes in the chemistry of the surface. In vitro, mouse embryonic stem cells (mES cells) were cultured on as‐spun surfaces and in laser‐machined microwells. Cell densities were found to be statistically indistinguishable after 1 and 2 days of growth. Additionally, confocal microscope imaging confirmed that spreading of mES cells cultured within laser‐machined microwells was constrained by the cavity walls, the expected and desired function of these cavities. The geometric constraint caused statistically significant smaller density of cells in microwells after 3 days of growth. It was concluded that FS laser ablation is an effective process for microscale structuring of these electrospun nanofiber tissue scaffold surfaces. Biotechnol. Bioeng. 2011; 108:116–126.

Structuring electrospun polycaprolactone nanofiber tissue scaffolds by femtosecond laser ablation

Meshes of electrospun (ES) polycaprolactone (PCL) and polyethylene terephthalate nanofiber meshes were structured by ablation of linear grooves with a scanned femtosecond laser. Focus spot size, pulse energy, and scanning speed were varied to determine their affects on groove size and the characteristics of the electrospun fiber at the edges of these grooves. The femtosecond laser was seen to be an effective means for flexibly structuring the surface of ES PCL scaffolds. Femtosecond ablation resulted in much more uniformly ablated patterns compared to Q-switched nanosecond pulse laser ablation. Also, the width of the ablated grooves was well controlled by laser energy and focus spot size, although the grooves were significantly larger than the spot size. Also, some melting of fibers was observed at the edges of grooves. These affects were attributed to optical radiation from laser-induced plasma at higher pulse energies and melting of fibers at laser fluences lower than the ablation threshold. The ablatio...

Rationalization of Microstructure Heterogeneity in INCONEL 718 Builds Made by the Direct Laser Additive Manufacturing Process

Simulative builds, typical of the tip-repair procedure, with matching compositions were deposited on an INCONEL 718 substrate using the laser additive manufacturing process. In the as-processed condition, these builds exhibit spatial heterogeneity in microstructure. Electron backscattering diffraction analyses showed highly misoriented grains in the top region of the builds compared to those of the lower region. Hardness maps indicated a 30 pct hardness increase in build regions close to the substrate over those of the top regions. Detailed multiscale characterizations, through scanning electron microscopy, electron backscattered diffraction imaging, high-resolution transmission electron microscopy, and ChemiSTEM, also showed microstructure heterogeneities within the builds in different length scales including interdendritic and interprecipitate regions. These multiscale heterogeneities were correlated to primary solidification, remelting, and solid-state precipitation kinetics of γ″ induced by solute segregation, as well as multiple heating and cooling cycles induced by the laser additive manufacturing process.

Femtosecond laser micromachining of dielectric materials for biomedical applications

Techniques for microfluidic channel fabrication in soda-lime glass and fused quartz using femtosecond laser ablation and ablation in conjunction with polymer coating for surface roughness improvement were tested. Systematic experiments were done to characterize how process variables (laser fluence, scanning speed and focus spot overlap, and material properties) affect the machining feature size and quality. Laser fluence and focus spot overlap showed the strongest influence on channel depth and roughness. At high fluence, the surface roughness was measured to be between 395 nm and 731 nm RMS. At low fluence, roughness decreased to 100 nm–350 nm RMS and showed a greater dependence on overlap. The surface roughness of laser ablation was also dependent on the material properties. For the same laser ablation parameters, soda-lime glass surfaces were smoother than fused quartz. For some applications, especially those using quartz, smoother channels are desired. A hydroxyethyl methacrylate (HEMA) polymer coating was applied and the roughness of the coated channels was improved to 10–50 nm RMS.

Weld pool flows during initial stages of keyhole formation in laser welding

Weld pool transport phenomena during the transition from conduction-mode laser spot welding to keyhole laser spot welding of titanium were studied by numerical simulation. A range of laser powers were simulated and temperature dependent evaporation recoil pressure and cooling were applied as boundary conditions on the weld pool surface. Simulation results predicted a complex time-varying flow pattern during weld pool development. The surface-normal flow at the weld pool centre oscillated between upwards and downwards during the simulation time due to interaction of competing effects of evaporation recoil and surface tension pressures and laser heating and evaporation cooling. The results show that the laser weld pool flow dynamics play a key role during the transition from conduction-mode laser welding to keyhole welding.

Nonhomogeneous surface premelting of Au nanoparticles

A reversible nonhomogeneous surface premelting of Au nanoparticles is demonstrated through molecular dynamics simulations. With increasing temperature, liquid-like atoms first appear at some vertices and edges of surface facets, then small liquid regions grow and, at temperatures close to the particle melting temperature, most of the remaining solid-like surface atoms reside on {111} planes which are the most stable against surface premelting. The appearance of a contiguous liquid layer (complete surface premelting) is size dependent and is not observed in very small nanoparticles.

GENERATION OF OPTICAL AND ACOUSTIC EMISSIONS IN LASER WELD PLUMES

The goal of this work is the formulation of an analytical model of optical and acoustic emission generation by laser welds. A simplified model of the visible portion of the laser weld plume is used as a basis for the prediction of optical and acoustic signals. The optical emission is calculated from the strongest helium and iron lines in the visible spectrum. The sound pressure is calculated as a function of vapor flow rate by considering the displacement of ambient air by vapor emanating from the keyhole. The model reveals that generation of visible light increases roughly in proportion to the flow rate of vapor from the plume. However, the acoustic signal is predicted to vary as the time derivative of the flow rate. Experimental measurements of visible light are found to be of the same magnitude as predicted by the model and the fluctuation of the optical and acoustic signals are found to be consistent with the hypothesis that they are both fundamentally due to vapor flow rate variations.

Design of a Microchannel-Nanochannel-Microchannel Array Based Nanoelectroporation System for Precise Gene Transfection

A micro/nano-fabrication process of a nanochannel electroporation (NEP) array and its application for precise delivery of plasmid for non-viral gene transfection is described. A dip-combing device is optimized to produce DNA nanowires across a microridge array patterned on the polydimethylsiloxane (PDMS) surface with a yield up to 95%. Molecular imprinting based on a low viscosity resin, 1,4-butanediol diacrylate (1,4-BDDA), adopted to convert the microridge-nanowire-microridge array into a microchannel-nanochannel-microchannel (MNM) array. Secondary machining by femtosecond laser ablation is applied to shorten one side of microchannels from 3000 to 50 μm to facilitate cell loading and unloading. The biochip is then sealed in a packaging case with reservoirs and microfluidic channels to enable cell and plasmid loading, and to protect the biochip from leakage and contamination. The package case can be opened for cell unloading after NEP to allow for the follow-up cell culture and analysis. These NEP cases can be placed in a spinning disc and up to ten discs can be piled together for spinning. The resulting centrifugal force can simultaneously manipulate hundreds or thousands of cells into microchannels of NEP arrays within 3 minutes. To demonstrate its application, a 13 kbp OSKM plasmid of induced pluripotent stem cell (iPSC) is injected into mouse embryonic fibroblasts cells (MEFCs). Fluorescence detection of transfected cells within the NEP biochips shows that the delivered dosage is high and much more uniform compared with similar gene transfection carried out by the conventional bulk electroporation (BEP) method.

Micronozzle Array Enhanced Sandwich Electroporation of Embryonic Stem Cells

Electroporation is one of the most popular nonviral gene transfer methods for embryonic stem cell transfection. Bulk electroporation techniques, however, require a high electrical field and provide a nonuniform electrical field distribution among randomly distributed cells, leading to limited transfection efficiency and cell viability, especially for a low number of cells. We present here a membrane sandwich electroporation system using a well-defined micronozzle array. This device is capable of transfecting hundred to millions of cells with good performance. The ability to treat a small number of cells (i.e., a hundred) offers great potential to work with hard-to-harvest patient cells for pharmaceutical kinetic studies. Numerical simulation of the initial transmembrane potential distribution and propidium iodide (PI) dye diffusion experiments demonstrated the advantage of highly focused and localized electric field strength provided by the micronozzle array over conventional bulk electroporation.