Single-shot femtosecond laser ablation of nano/polycrystalline titanium investigated using molecular dynamics and experiments

Laser ablation, a crucial technique in various scientific and industrial fields, plays a pivotal role in precision manufacturing. Industries such as aerospace rely on laser technology for tasks like drilling microscale holes in jet turbine components to enhance air-cooling efficiency. Moreover, laser-based material processing is indispensable in addressing healthcare challenges with facilitating the postprocessing of 3D-printed bespoke components like patient-specific implants as an example. Ultrashort pulsed laser ablation enables precise micro and nanofabrication, enhancing material properties like wettability, adhesion and biocompatibility. This is particularly important in medical applications like implant development, as it can help reduce the possibility of post-surgery infections. Scientifically, understanding the intricacies of ultrashort pulsed laser ablation contributes to ongoing research and development efforts in ablation technology, fostering the enhancement of new material properties related to surface modifications. Additionally, laser ablation plays a crucial role in additive manufacturing technology like 3D printing of metals by facilitating the post-processing stage. This thesis investigates the ultrashort pulsed laser ablation of titanium, utilising a combination of molecular dynamics simulations and experiments. Molecular dynamics simulations are used for their capability to model systems at the atomistic scale and ultrashort timescale (femtoseconds in this work), in contrast to the finite element method, and for their computational efficiency compared to methods employing more detailed calculations like density functional theory. The primary focus of this work is on exploring the size effect by examining variations in beam spot diameter and grain size with profound implications for ultraprecision manufacturing of titanium surfaces in sub-micron length scale, produced by casting and additive manufacturing techniques. It contributes a nuanced understanding of ultrashort pulsed laser ablation by bridging the gap between molecular dynamics simulations and experiments. It extends the boundaries by simulating the largest feasible atomistic models and measuring features at the smallest scale permitted by the available metrology devices in experiments. The key observations showed the critical importance of the beam spot diameter in determining the laser fluence necessary to achieve average plasma temperatures of around 9,000 K, as well as a direct correlation between the grain size and the response of the material to laser irradiation. Notably, the simulations indicated that the 10 nm laser beam spot diameter compared to the 25 nm requires 59% more absorbed laser energy for ablation. Furthermore, the investigation revealed that by increasing the grain size in alpha-phase titanium, when the number of grains in the volume of 500,000 nm³ were reduce from 500 grains to 10, 36% more absorbed laser fluence was necessary to achieve average plasma temperatures of approximately 9,000 K, despite the material exhibiting higher heat conductivity. Additionally, a comparative analysis of ultrashort pulsed laser ablation between atomistic models of pure titanium with single crystal and polycrystalline structures were carried out using molecular dynamics simulations. The results revealed that the nanocrystalline sample modelled in this work, which exhibited lower heat conduction, produced a relatively deeper crater compared to its single crystal counterpart. The single crystal sample had a greater resistance to ablation, leading to the formation of a recast layer with rougher edges in contrast to the nanocrystalline sample. In materials science and engineering "size effect" is attributed to a phenomenon where the mechanical, thermal, optical or electrical properties of a crystalline material changes as a function of its physical size where at least one dimension is in submicron length scale. Experimental examination of the size effect was carried out on commercially pure titanium (consisting of 99.6% titanium and the remaining 0.4% containing carbon, nitrogen, hydrogen, iron and oxygen atoms) and Ti-6Al-4V alloy where craters were formed on both materials using single-shots with identical fluence while varying the diameter of the laser beam. It was observed that reducing the beam spot diameter resulted in relatively shallower craters, suggesting an increased threshold for ablation. Experiments comparing single-shot laser ablation outcomes between casted and 3D-printed Ti-6Al-4V alloy revealed that the 3D-printed surface (\u1d445\u1d44e = 32 \u1d45b\u1d45a) produced a slightly cleaner crater and smoother recast layer compared to the casted material (\u1d445\u1d44e = 45 \u1d45b\u1d45a). This observation was made after subjecting both substrates to ultrashort pulsed laser irradiation with identical laser parameters

Personality Characteristics of Dramatics Majors at Selected Institutions of Higher Education in Tennessee

The primary purpose of the study was to determine significant differences and likenesses in personality characteristics of dramatics majors. A secondary purpose was to ascertain whether personality studies of this type might be effectively used by supervisors, departmental heads, and/or instructors in guiding students interested in dramatics in their choices of majors, colleges, and ultimately careers. Eighty-seven female dramatics majors and seventy-two male dramatics majors enrolled in nine colleges and universities in Tennessee volunteered to participate by taking the Omnibus Personality Inventory, the measuring instrument used for data gathering. The one-way analysis of variance was used to ascertain any significant personality difference at the .05 level between state and denominational dramatics groups, state and other private groups, and denominational and other private groups. The following findings were ascertained: 1. State and denominational dramatics groups differed significantly at the .05 level on three of the fourteen scales— Autonomy, Religious Orientation, and Impulse Expression. The state group was significantly different on the Autonomy and Impulse-Expression scales. The denominational group was significantly different on the Religious Orientation scale. 2. State and other private dramatics groups failed to differ significantly at the .05 level on any of the fourteen scales. 3. Denominational and other private dramatics groups differed significantly at the .05 level on two scales— Religious Orientation and Masculinity-Femininity. The denominational group was significantly different on the Religlous-Orientatlon and Masculinity-Femininity scales from the other private group. 4. Although the three groups were different on five of the scales at the .05 level of significance when comparisons were made, homogeneity was reflected on the other nine scales. The state and other private dramatics groups had almost identical personality profiles. The following conclusions were made: 1. The state group was characterized as more Independent, liberal, tolerant, antl-authoritarlan, expressive, sensual, Imaginative, and sometimes impractical than the denominational group. The denominational group was characterized as more traditional and judgmental with stronger Judaic-Christian commitments than the state group. 2. Dramatics majors, preferring to be with other dramatics majors who valued autonomy and impulsiveness, might be better advised to attend state institutions of higher education. If, however, religious emphasis was a prime consideration, then dramatics majors might be better advised to attend denominational institutions of higher education. 3. Since no significance differences existed between state and other private institutions of higher education, dramatics majors with similar personality characteristics, might well be advised to attend either state or other private institutions. 4. Since the denominational dramatics group reflected greater significance on the Rellgious-Orientation and Masculinity-Femininity scales, it was characterized as more traditional and judgmental with stronger Judaic-Christian commitments and more feminine with more sociable, esthetic, sensitive, and emotional inclinations. If dramatics majors prized these personality characteristics highly, they might well be advised to attend denominational institutions of higher education. 5. Collectively, the dramatics group majors were moderately esthetic, complex, and anxious, reflecting varied interests in all the arts by appreciating poetry, paintings, dramatics, sculpture, and architecture; being tolerant of ambiguities, being appreciative of the unusual, new ideas, and uncertainties, and being nervous, worried, tense, and excitable. 6. Collectively, the dramatics group majors were reflected as highly feminine and impulsive, characterized as sociable, esthetic, sensitive, emotional, imaginative, aggressive, expressive, and sometimes impractical

Effects of disorder in location and size of fence barriers on molecular motion in cell membranes

The effect of disorder in the energetic heights and in the physical locations of fence barriers encountered by transmembrane molecules such as proteins and lipids in their motion in cell membranes is studied theoretically. The investigation takes as its starting point a recent analysis of a periodic system with constant distances between barriers and constant values of barrier heights, and employs effective medium theory to treat the disorder. The calculations make possible, in principle, the extraction of confinement parameters such as mean compartment sizes and mean intercompartmental transition rates from experimentally reported published observations. The analysis should be helpful both as an unusual application of effective medium theory and as an investigation of observed molecular movements in cell membranes.Comment: 9 pages, 5 figure

Multispectral imaging flow cytometry reveals distinct frequencies of γ-H2AX foci induction in DNA double strand break repair defective human cell lines

Copyright @ 2012 International Society for Advancement of Cytometry. The article can be accessed from the links below.This article has been made available through the Brunel Open Access Publishing Fund.The measurement of γ-H2AX foci induction in cells provides a sensitive and reliable method for the quantitation of DNA damage responses in a variety of cell types. Accurate and rapid methods to conduct such observations are desirable. In this study we have employed the novel technique of multispectral imaging flow cytometry to compare the induction and repair of γ-H2AX foci in three human cell types with different capacities for the repair of DNA double strand breaks (DSB). A repair normal fibroblast cell line MRC5-SV1, a DSB repair defective ataxia telangiectasia (AT5BIVA) cell line, and a DNA-PKcs deficient cell line XP14BRneo17 were exposed to 2 Gy gamma radiation from a 60Cobalt source. Thirty minutes following exposure we observed a dramatic induction of foci in the nuclei of these cells. After 24 hrs there was a predictable reduction on the number of foci in the MRC5-SV1 cells, consistent with the repair of DNA DSB. In the AT5BIVA cells, persistence of the foci over a 24 hour period was due to the failure in the repair of DNA DSB. However, in the DNA-PKcs defective cells (XP14BRneo17) we observed an intermediate retention of foci in the nuclei indicative of partial repair of DNA DSB. In summary, the application of imaging flow cytometry has permitted an evaluation of foci in a large number of cells (20,000) for each cell line at each time point. This provides a novel method to determine differences in repair kinetics between different cell types. We propose that imaging flow cytometry provides an alternative platform for accurate automated high through-put analysis of foci induction in a variety of cell types.This article is made available through the Brunel Open Access Publishing Fund

Transport Properties of Random Walks on Scale-Free/Regular-Lattice Hybrid Networks

We study numerically the mean access times for random walks on hybrid disordered structures formed by embedding scale-free networks into regular lattices, considering different transition rates for steps across lattice bonds ($F$) and across network shortcuts ($f$). For fast shortcuts ($f/F\gg 1$) and low shortcut densities, traversal time data collapse onto an universal curve, while a crossover behavior that can be related to the percolation threshold of the scale-free network component is identified at higher shortcut densities, in analogy to similar observations reported recently in Newman-Watts small-world networks. Furthermore, we observe that random walk traversal times are larger for networks with a higher degree of inhomogeneity in their shortcut distribution, and we discuss access time distributions as functions of the initial and final node degrees. These findings are relevant, in particular, when considering the optimization of existing information networks by the addition of a small number of fast shortcut connections.Comment: 8 pages, 6 figures; expanded discussions, added figures and references. To appear in J Stat Phy

Stress corrosion in titanium alloys and other metallic materials

Multiple physical and chemical techniques including mass spectroscopy, atomic absorption spectroscopy, gas chromatography, electron microscopy, optical microscopy, electronic spectroscopy for chemical analysis (ESCA), infrared spectroscopy, nuclear magnetic resonance (NMR), X-ray analysis, conductivity, and isotopic labeling were used in investigating the atomic interactions between organic environments and titanium and titanium oxide surfaces. Key anhydrous environments studied included alcohols, which contain hydrogen; carbon tetrachloride, which does not contain hydrogen; and mixtures of alcohols and halocarbons. Effects of dissolved salts in alcohols were also studied. This program emphasized experiments designed to delineate the conditions necessary rather than sufficient for initiation processes and for propagation processes in Ti SCC

Investigating TriHaloMethanes with respect to humidity

The disinfection of potable water has dramatically reduced the instances of Cholera and similar ailments within a population drawing upon that water source. There is however the possibility that Natural Organic Matter (NOM) can interact with the disinfection compounds to form Disinfection By-Products (DBP). One group of DBPs are Trihalomethanes (THMs) with several compounds of the group being suspected carcinogens. Within the UK the total concentration of all THMs within drinking water must not exceed 100µg/l. At present water authorities take samples of the water supply and return them to a central laboratory for analysis. This provides an accurate test but one which can involve a long lead time in discovering a potential hazard to public health. A Field Asymmetric Ion Mobility Spectrometer (FAIMS) sensor may be ideally placed to perform in situ continuous monitoring at particular sites. As part of a PhD co-sponsored by The Open University and Owlstone Nanotech Plc an investigation is ongoing to discover how sensitive a FAIMS device is with respect to THMs and humidity when sampling. Initial results and the method of data processing, which involves peak fitting to evolving spectra are presented

The effect of high-frequency rTMS over left DLPFC and fluid abilities on goal neglect

Goal neglect refers to when an aspect of task instructions is not utilised due to increased competition between goal representations, an attentional limit theoretically linked to working memory. In an attempt to alleviate goal neglect and to investigate the association between dorsolateral prefrontal cortex (DLPFC)-supported working memory and goal neglect, we used high-frequency repetitive transcranial magnetic stimulation to the left DLPFC whilst participants completed the letter-monitoring task, a measure of goal neglect, and an N3-back task, a working memory task known to be affected by rTMS of the left DLPFC, following 20 min of active and sham stimulation (run on separate days). We found increased accuracy on the N3-back task in addition to decreased goal neglect in the active compared to sham condition when controlling for age and fluid abilities (as assessed by matrix reasoning performance). Furthermore, analysis showed that active stimulation improvements on both the N3-back and letter-monitoring tasks were greater for those with higher fluid abilities. These findings provide support for the link between the DLPFC-support working memory and goal neglect. Increased performance on the N3-back task also supports the literature reporting a link between left DLPFC and verbal working memory. Results are evaluated in the context of potential use to alleviate symptoms of disorders related to goal neglect

Quantification of ethyl acetate using FAIMS

The enjoyment of wine is based not only on its flavour but also its aroma. The presence of compounds, such as ethyl acetate, can add a desirable depth of body, richness and sweetness to a wine. However, if the levels exceed the human perception threshold (100 - 200 mg/l) an aroma similar to acetone becomes prevalent and the wine is regarded as spoilt.A study was undertaken to determine the concentration of ethyl acetate within wine using a Gas Chromatograph coupled with Field Asymmetric Ion Mobility Spectrometry (FAIMS). The use of FAIMS (also known as Differential Mobility Spectrometry) as a selective detector has increasingly become prevalent within analytical science. There is now a vibrant community of researchers working with the technique and different commercial systems are readily available. Two such systems are the Tourist and Lonestar instruments available from Owlstone Ltd. The Owlstone systems are characterised by the sensor being a solid state device, which imposes constraints on operational parameters but also offers opportunities over comparable devices. FAIMS technology requires the interaction of compounds both within the ionisation and separation region. To influence the interactions in these two regions studies were completed at elevated (compared to ambient) pressures and humidity. The study of ethyl acetate within wine provided an opportunity to investigate these effects and also provided a route for optimisation of the instrument for the study. It was found, in contrast to a previously reported investigation, that increasing the pressure resulted in a greater resolution of compounds. A more complicated relationship was observed with respect to humidity, believed attributable to water being an important constituent of reactive and product ion formation. Additional effects due to the presence of co-solvents were observed and subsequently managed so as to provide increased sensitivity. The quantification of ethyl acetate within wine was accomplished considerably below the human perception threshold providing the opportunity to better manage this multifarious compound throughout manufacture and the product lifecycle