Troy P. Anderson

Formation of multiscale surface structures on nickel via above surface growth and below surface growth mechanisms using femtosecond laser pulses

The formation of self-organized micro- and nano-structured surfaces on nickel via both above surface growth (ASG) and below surface growth (BSG) mechanisms using femtosecond laser pulse illumination is reported. Detailed stepped growth experiments demonstrate that conical mound-shaped surface structure development is characterized by a balance of growth mechanisms including scattering from surface structures and geometric effects causing preferential ablation of the valleys, flow of the surface melt, and redeposition of ablated material; all of which are influenced by the laser fluence and the number of laser shots on the sample. BSG-mound formation is dominated by scattering, while ASG-mound formation is dominated by material flow and redeposition. This is the first demonstration to our knowledge of the use of femtosecond laser pulses to fabricate metallic surface structures that rise above the original surface. These results are useful in understanding the details of multi-pulse femtosecond laser interaction with metals.

Extraordinary shifts of the Leidenfrost temperature from multiscale micro/nanostructured surfaces.

In the present work, the effects of surface chemistry and micro/nanostructuring on the Leidenfrost temperature are experimentally investigated. The functional surfaces were fabricated on a 304 stainless steel surface via femtosecond laser surface processing (FLSP). The droplet lifetime experimental method was employed to determine the Leidenfrost temperature for both machine-polished and textured surfaces. A precision dropper was used to control the droplet size to 4.2 μL and surface temperatures were measured by means of an embedded thermocouple. Extraordinary shifts in the Leidenfrost temperatures, as high as 175 °C relative to the polished surface, were observed with the laser-processed surfaces. These extraordinary shifts were attributed to nanoporosity, reduction in contact angle, intermittent liquid/solid contacts, and capillary wicking actions resulting from the presence of self-assembled nanoparticles formed on the surfaces. In addition to the shift in the Leidenfrost temperature, significant enhancement of the heat transfer in the film boiling regime was also observed for the laser-processed surfaces; water droplet evaporation times were reduced by up to 33% for a surface temperature of 500 °C.

Enhanced pool-boiling heat transfer and critical heat flux on femtosecond laser processed stainless steel surfaces

In this paper, we present an experimental investigation of pool boiling heat transfer on multiscale (micro/nano) functionalized metallic surfaces. Heat transfer enhancement in metallic surfaces is very important for large scale high heat flux applications like in the nuclear power industry. The multiscale structures were fabricated via a femtosecond laser surface process (FLSP) technique, which forms self-organized mound-like microstructures covered by layers of nanoparticles. Using a pool boiling experimental setup with deionized water as the working fluid, both the heat transfer coefficients and critical heat flux were investigated. A polished reference sample was found to have a critical heat flux of 91 W/cm2 at 40 °C of superheat and a maximum heat transfer coefficient of 23,000 W/m2 K. The processed samples were found to have a maximum critical heat flux of 142 W/cm2 at 29 °C and a maximum heat transfer coefficient of 67,400 W/m2 K. It was found that the enhancement of the critical heat flux was directly related to the wetting and wicking ability of the surface which acts to replenish the evaporating liquid and delay critical heat flux. The heat transfer coefficients were also found to increase when the surface area ratio was increased as well as the microstructure peak-to-valley height. Enhanced nucleate boiling is the main heat transfer mechanism, and is attributed to an increase in surface area and nucleation site density.

Comparison of the structural and chemical composition of two unique micro/nanostructures produced by femtosecond laser interactions on nickel

The structural and chemical composition of two unique microstructures formed on nickel, with nanoscale features, produced using femtosecond laser surface processing (FLSP) techniques is reported in this paper. These two surface morphologies, termed mounds and nanoparticle-covered pyramids, are part of a larger class of self-organized micro/nanostructured surfaces formed using FLSP. Cross-sections of the structures produced using focused ion beam milling techniques were analyzed with a transmission electron microscope. Both morphologies have a solid core with a layer of nanoparticles on the surface. Energy dispersive X-ray spectroscopy by scanning transmission electron microscopy studies reveal that the nanoparticles are a nickel oxide, while the core material is pure nickel.

Self-propelled droplets on heated surfaces with angled self-assembled micro/nanostructures

Directional and ratchet-like functionalized surfaces can induce liquid transport without the use of an external force. In this paper, we investigate the motion of liquid droplets near the Leidenfrost temperature on functionalized self-assembled asymmetric microstructured surfaces. The surfaces, which have angled microstructures, display unidirectional properties. The surfaces are fabricated on stainless steel through the use of a femtosecond laser-assisted process. Through this process, mound-like microstructures are formed through a combination of material ablation, fluid flow, and material redeposition. In order to achieve the asymmetry of the microstructures, the femtosecond laser is directed at an angle with respect to the sample surface. Two surfaces with microstructures angled at 45° and 10° with respect to the surface normal were fabricated. Droplet experiments were carried out with deionized water and a leveled hot plate to characterize the directional and self-propelling properties of the surfaces. It was found that the droplet motion direction is opposite of that for a surface with conventional ratchet microstructures reported in the literature. The new finding could not be explained by the widely accepted mechanism of asymmetric vapor flow. A new mechanism for a self-propelled droplet on asymmetric three-dimensional self-assembled microstructured surfaces is proposed.

Experimental explanation of the formation mechanism of surface mound-structures by femtosecond laser on polycrystalline Ni60Nb40

Femtosecond laser surface processing (FLSP) is an emerging technique for creating functionalized surfaces with specialized properties, such as broadband optical absorption or superhydrophobicity/superhydrophilicity. It has been demonstrated in the past that FLSP can be used to form two distinct classes of mound-like, self-organized micro/nanostructures on the surfaces of various metals. Here, the formation mechanisms of below surface growth (BSG) and above surface growth (ASG) mounds on polycrystalline Ni60Nb40 are studied. Cross-sectional imaging of these mounds by focused ion beam milling and subsequent scanning electron microscopy revealed evidence of the unique formation processes for each class of microstructure. BSG-mound formation during FLSP did not alter the microstructure of the base material, indicating preferential valley ablation as the primary formation mechanism. For ASG-mounds, the microstructure at the peaks of the mounds was clearly different from the base material. Transmission electron microscopy revealed that hydrodynamic melting of the surface occurred during FLSP under ASG-mound forming conditions. Thus, there is a clear difference in the formation mechanisms of ASG- and BSG-mounds during FLSP.

A Fundamental Understanding of the Dependence of the Laser-Induced Breakdown Spectroscopy (LIBS) Signal Strength on the Complex Focusing Dynamics of Femtosecond Laser Pulses on Either Side of the Focus:

We correlate the focusing dynamics of 50 femtosecond (fs) laser radiation as it interacts with a silicon sample to laser-induced breakdown spectroscopy (LIBS) signal strength. Presented are concentric ring-shaped variations in the electric field in the prefocus region due to lens aberrations and nonsymmetry between the prefocus and post-focus beam profile as a result of continuum generation, occurring around the focus. Experimental results show different signal trends for both atmospheric and vacuum conditions, attributed to the existence of a continuum for the former. Lens aberrations effects on the LIBS signal strength are investigated using a plano-convex spherical lens and an aspherized achromatic lens. High-resolution scanning electron micrographs of the silicon surface after ablation, along with theoretical simulations, reveal the electric field patterns near the focus. The research results contribute to fundamental understanding of the basic physics of ultrashort, femtosecond laser radiation interacting with materials.

Micro/nanostructures formation by femtosecond laser surface processing on amorphous and polycrystalline Ni60Nb40

Femtosecond laser surface processing is a technology that can be used to functionalize many surfaces, imparting specialized properties such as increased broadband optical absorption or superhydrophilicity/superhydrophobicity. In this study, two unique classes of surface structures, below surface growth (BSG) and above surface growth (ASG) mounds, were formed by femtosecond laser surface processing on amorphous and polycrystalline Ni60Nb40 with two different grain sizes. Cross sectional imaging of these mounds revealed thermal evidence of the unique formation processes for each class of surface structure. BSG mounds formed on all three substrates using the same laser parameters had similar surface morphology. The microstructures in the mounds were unaltered compared with the substrate before laser processing, suggesting their formation was dominated by preferential valley ablation. ASG mounds had similar morphology when formed on the polycrystalline Ni60Nb40 substrates with 100 nm and 2 [H9262]m grain size. However, the ASG mounds had significantly wider diameter and higher peak-to-valley heights when the substrate was amorphous Ni60Nb40. Hydrodynamic melting was primarily responsible for ASG mound formation. On amorphous Ni60Nb40 substrates, the ASG mounds are most likely larger due to lower thermal diffusivity. There was clear difference in growth mechanism of femtosecond laser processed BSG and ASG mounds, and grain size does not appear to be a factor.

Superhydrophobic metallic surfaces functionalized via femtosecond laser surface processing for long term air film retention when submerged in liquid

Femtosecond laser surface processing (FLSP) is a powerful technique used to create self-organized microstructures with nanoscale features on metallic surfaces. By combining FLSP surface texturing with surface chemistry changes, either induced by the femtosecond laser during processing or introduced through post processing techniques, the wetting properties of metals can be altered. In this work, FLSP is demonstrated as a technique to create superhydrophobic surfaces on grade 2 titanium and 304 stainless steel that can retain an air film (plastron) between the surface and a surrounding liquid when completely submerged. It is shown that the plastron lifetime when submerged in distilled water or synthetic stomach acid is critically dependent on the specific degree of surface micro- and nano-roughness, which can be tuned by controlling various FLSP parameters. The longest plastron lifetime was on a 304 stainless steel sample that was submerged in distilled water and maintained a plastron for 41 days, the length of time of the study, with no signs of degradation. Also demonstrated for the first time is the precise control of pulse fluence and pulse count to produce three unique classes of surface micron/nano-structuring on titanium.

Understanding the formation of self-organized micro/nanostructures on metal surfaces from femtosecond laser ablation using stop-motion SEM imaging

There are a growing number of unique self-organized micro/nanostructures created using femtosecond laser surface processing that have been demonstrated. Although researchers have provided insight into the formation processes for distinctive morphologies on specific materials, there is a need for a broader understanding of the physics behind the formation of a wide range of morphologies and what parameters affect their formation. In this work, the formation processes for mound structures on 316 stainless steel (SS) with growth above the original sample surface are studied. The formation process for the structures on 316 SS is compared to similar structures formed on nickel using the same technique. The structures are formed using 800 nm, 50 fs laser pulses, and are self-organized, meaning the structure dimensions are much smaller than the spot size of the pulses used to create them. The formation dynamics were studied using a stop-motion scanning electron microscope (SEM) technique, where the same location of an irradiated sample was imaged in the SEM at various pulse counts. The result is a series of images showing the developmental progress with increasing pulse counts. The structures form through a combination of fluid flow of the surface melt that results after irradiation, preferential ablation of the center of the pits between structures, and material/nanoparticle redeposition.