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Numerical simulations and experimental validations of gas-focused liquid sheet dynamics Suitable for XFELs and synchrotrons
Abstract
The research presented in this thesis primarily focuses on the dynamics of fluids of microfluidic liquid flat jet and the numerical methodology associated understanding of this complex thin sheet formation therein. The gas focussing is based on the gas dynamic virtual nozzle (GDVN) principle. This liquid sheet has eminated with a huge scope in the upcoming utilization in the X-ray free electron laser (XFELs) and in the synchrotrons and laser spectroscopy setups which require continuous sample replenishment. This thin flat liquid sheet has developed huge interests for its utilization in the X-ray microscopy in comparison to that produced by the traditional inpinging liquid jet system. It is due to the ability to tune its thickness and increase the velocity of these complex liquid jets. Flatjet flipping mechanism involved in the generation of ultra thin liquid sheet produced due to the interdependent inertial and surface tension forces. This results to form the liquid jet to form a flat sheet between bulky liquid rims which initially move apart and then approach each other again to collide, in form of a nodal point, consequently forming a new rimmed flat sheet perpendicular to the preceding one.The numerical analysis has been performed using computational fluid dynamics (CFD) methodology. This approach therefore enables a greater understanding of the fluid behaviour which is difficult to monitor experimentally in the microscopic fluid regime. The flat jet flipping mechanism is clearly understood through the results obtained via numerical analysis. The outcome of this versatile work enriches multiple arenas such as XFELs, soft X-ray spectroscopy and industrial applications such as in the flat fiber fabrication.
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The physics and chemistry of liquid solutions play a central role in science, and our understanding of life on Earth. Unfortunately, key tools for interrogating aqueous systems, such as infrared and soft X-ray spectroscopy, cannot readily be applied because of strong absorption in water. Here we use gas-dynamic forces to generate free-flowing, sub-micron, liquid sheets which are two orders of magnitude thinner than anything previously reported. Optical, infrared, and X-ray spectroscopies are used to characterize the sheets, which are found to be tunable in thickness from over 1 μm down to less than 20 nm, which corresponds to fewer than 100 water molecules thick. At this thickness, aqueous sheets can readily transmit photons across the spectrum, leading to potentially transformative applications in infrared, X-ray, electron spectroscopies and beyond. The ultrathin sheets are stable for days in vacuum, and we demonstrate their use at free-electron laser and synchrotron light sources. X-ray spectroscopy is a tool used for the investigation of aqueous solutions but the strong absorption of water means that very thin liquid sheets are needed for accurate analysis. Here the authors produce free-flowing liquid sheets 2 orders of magnitude thinner than sheets obtained with existing techniques.
Serial femtosecond crystallography requires reliable and efficient delivery of fresh crystals across the beam of an X-ray free-electron laser over the course of an experiment. We introduce a double-flow focusing nozzle to meet this challenge, with significantly reduced sample consumption, while improving jet stability over previous generations of nozzles. We demonstrate its use to determine the first room-temperature structure of RNA polymerase II at high resolution, revealing new structural details. Moreover, the double flow-focusing nozzles were successfully tested with three other protein samples and the first room temperature structure of an extradiol ring-cleaving dioxygenase was solved by utilizing the improved operation and characteristics of these devices.
As the needs for low-cost rapidly-produced microfluidics are growing with the trend of Lab-on-a-Chip and distributed healthcare, the fully inkjet-printing of microfluidics can be a solution to it with numerous potential electrical and sensing applications. Inkjet-printing is an additive manufacturing technique featuring no material waste and a low equipment cost. Moreover, similar to other additive manufacturing techniques, inkjet-printing is easy to learn and has a high fabrication speed, while it offers generally a great planar resolution down to below 20 µm and enables flexible designs due to its inherent thin film deposition capabilities. Due to the thin film feature, the printed objects also usually obtain a high vertical resolution (such as 4.6 µm). This paper introduces a low-cost rapid three-dimensional fabrication process of microfluidics, that relies entirely on an inkjet-printer based single platform and can be implemented directly on top of virtually any substrates.
In this paper we describe a setup for x-ray scattering experiments on complex fluids using a liquid jet. The setup supports Small and Wide Angle X-ray Scattering (SAXS/WAXS) geometries. The jet is formed by a gas-dynamic virtual nozzle (GDVN) allowing for diameters ranging between 1 μm and 20 μm at a jet length of several hundred μm. To control jet properties such as jet length, diameter, or flow rate, the instrument is equipped with several diagnostic tools. Three microscopes are installed to quantify jet dimensions and stability in situ. The setup has been used at several beamlines performing both SAXS and WAXS experiments. As a typical example we show an experiment on a colloidal dispersion in a liquid jet at the X-ray Correlation Spectroscopy instrument at the Linac Coherent Light Source free-electron laser.
In this work, we developed a shape-controllable nozzle inside a multilayer PDMS microchip. The nozzle was able to control the shape of the fluid channel in three dimensions. All the four walls of the fluid channel were comprised of pneumatic PDMS membrane valves, and their deformation was controlled by air pressure. As both the limitation of the fluid flux and the shape of the fluid channel were adjustable spatially in three dimensions, this valve-based nozzle generated droplets with less response time and in a more effectively controlled manner comparing to conventional droplet devices. It could also function as a microinjector to modulate the compositions of droplets precisely and continuously. In addition, the nozzle was able to form a specific shape to generate core–shell particles.
Serial femtosecond crystallography using ultrashort pulses from x-ray free electron lasers (XFELs) enables studies of the light-triggered dynamics of biomolecules. We used microcrystals of photoactive yellow protein (a bacterial blue light photoreceptor) as a model system and obtained high-resolution, time-resolved difference electron density maps of excellent quality with strong features; these allowed the determination of structures of reaction intermediates to a resolution of 1.6 angstroms. Our results open the way to the study of reversible and nonreversible biological reactions on time scales as short as femtoseconds under conditions that maximize the extent of reaction initiation throughout the crystal.
A new approach for collecting data from many hundreds of thousands of microcrystals using X-ray pulses from a free-electron laser has recently been developed. Referred to as serial crystallography, diffraction patterns are recorded at a constant rate as a suspension of protein crystals flows across the path of an X-ray beam. Events that by chance contain single-crystal diffraction patterns are retained, then indexed and merged to form a three-dimensional set of reflection intensities for structure determination. This approach relies upon several innovations: an intense X-ray beam; a fast detector system; a means to rapidly flow a suspension of crystals across the X-ray beam; and the computational infrastructure to process the large volume of data. Originally conceived for radiation-damage-free measurements with ultrafast X-ray pulses, the same methods can be employed with synchrotron radiation. As in powder diffraction, the averaging of thousands of observations per Bragg peak may improve the ratio of signal to noise of low-dose exposures. Here, it is shown that this paradigm can be implemented for room-temperature data collection using synchrotron radiation and exposure times of less than 3 ms. Using lysozyme microcrystals as a model system, over 40 000 single-crystal diffraction patterns were obtained and merged to produce a structural model that could be refined to 2.1 Å resolution. The resulting electron density is in excellent agreement with that obtained using standard X-ray data collection techniques. With further improvements the method is well suited for even shorter exposures at future and upgraded synchrotron radiation facilities that may deliver beams with 1000 times higher brightness than they currently produce.
This paper aims to deal with a four-quadrant gratings alignment method benefiting from phase demodulation for proximity lithography, which combines the advantages of interferometry with image processing. Both the mask alignment mark and the wafer alignment mark consist of four sets of gratings, which bring the convenience and simplification of realization for coarse alignment and fine alignment. Four sets of moiré fringes created by superposing the mask alignment mark and the wafer alignment mark are highly sensitive to the misalignment between them. And the misalignment can be easily determined through demodulating the phase of moiré fringe without any external reference. Especially, the period and phase distribution of moiré fringes are unaffected by the gap between the mask and the wafer, not excepting the wavelength of alignment illumination. Disturbance from the illumination can also be negligible, which enhances the technological adaptability. The experimental results bear out the feasibility and rationality of our designed approach.
A new class of microscopic jet flows is here reported: for a certain range of physical parameters and
geometrical configurations, a perfectly steady microscopic liquid thread can be formed by a laminar
accelerating gas stream, eventually giving rise to a nearly monodisperse fine spray. Some interesting
characteristics for many applications of this robust and versatile flow and related atomization technique
are highlighted. Concentric multicomponent liquid threads can also be produced. A theoretical model
is presented that shows agreement with experiments.
Electrohydrodynamically (EHD) driven capillary jets are analysed in this work in the parametrical limit of negligible charge relaxation effects, i.e. when the electric
relaxation time of the liquid is small compared to the hydrodynamic times.
This regime can be found in the electrospraying of liquids when Taylor's
charged capillary jets are formed in a steady regime. A quasi-one-dimensional EHD model comprising temporal balance equations of mass, momentum, charge, the capillary balance across the surface, and the inner and outer electric fields equations is presented.
The steady forms of the temporal equations take into account surface charge convection
as well as Ohmic bulk conduction, inner and outer electric field equations, momentum
and pressure balances. Other existing models are also compared. The propagation
speed of surface disturbances is obtained using classical techniques. It is shown
here that, in contrast with previous models, surface charge convection provokes a
difference between the upstream and the downstream wave speed values, the upstream
wave speed, to some extent, being delayed. Subcritical, supercritical and convectively
unstable regions are then identified. The supercritical nature of the microjets emitted
from Taylor's cones is highlighted, and the point where the jet switches from a stable
to a convectively unstable regime (i.e. where the propagation speed of perturbations
become zero) is identified. The electric current carried by those jets is an eigenvalue
of the problem, almost independent of the boundary conditions downstream, in an
analogous way to the gas flow in convergent–divergent nozzles exiting into very low
pressure. The EHD model is applied to an experiment and the relevant physical
quantities of the phenomenon are obtained. The EHD hypotheses of the model
are then checked and confirmed within the limits of the one-dimensional assumptions.
Link: http://dx.doi.org/10.1017/S0022112096004466
The requirements for better control, linearity, and uniformity of critical dimension (CD) on photomasks in fabrication of 180 and 150 nm generation devices result in increasing demand for thinner, more etching durable, and more sensitive e-beam resists. Novolac based resists with chemical amplification have been a choice for their sensitivity and stability during etching . However, difficult CD control due to the acid catalyzer diffusion and quite narrow post exposure bake (PEB) process window are some of the major drawbacks of these resists. SU-8 is recently introduced to the market negative photoresist. High sensitivity, fairly good adhesion properties, and relatively simple processing of SU-8 make it a good substitution for novolac based chemically amplified negative e-beam resists in optical mask manufacturing. The replacement of traditional chemically amplified resists by SU-8 can increase the process latitude and reduce resist costs. Among the obvious drawbacks of SU-8 are the use of solvent-based developer and demand of oxygen plasma for resist removal. In this paper the use of SU-8 for optical mask manufacturing is reported. All steps of resist film preparation, exposure and development are paid a share of attention. Possibilities to use reactive ion etching (RIE) with oxygen in order to increase resist mask contrast are discussed. Special exposure strategy (pattern outlining) was employed to further improve the edge definition. The resist PEB temperature and time were studied to estimate their weight in overall CD control performance. Specially designed test patterns with 0.25 µm design rule could be firmly transferred into a chromium layer both by wet etching and ion milling. Influence of exposure dose variation on the pattern CD change was studied.
In this paper, linear hydrodynamic stability analysis is used to study the response of a capillary jet and a coflowing fluid to both axisymmetric and nonaxisymmetric perturbations. The temporal analysis revealed that nonaxisymmetric perturbations were damped (or overdamped) within the region of parameter space explored, which involved equal velocities for the jet and focusing fluid. It is explained how an extension to a spatiotemporal analysis implies that those perturbations can yield no transition from convective (jetting) to absolute (whipping) instability for that parameter region. This result provides a theoretical explanation for the absence of that kind of transition in most experimental results in the literature.
X-ray crystallography provides the vast majority of macromolecular structures, but the success of the method relies on growing crystals of sufficient size. In conventional measurements, the necessary increase in X-ray dose to record data from crystals that are too small leads to extensive damage before a diffraction signal can be recorded. It is particularly challenging to obtain large, well-diffracting crystals of membrane proteins, for which fewer than 300 unique structures have been determined despite their importance in all living cells. Here we present a method for structure determination where single-crystal X-ray diffraction 'snapshots' are collected from a fully hydrated stream of nanocrystals using femtosecond pulses from a hard-X-ray free-electron laser, the Linac Coherent Light Source. We prove this concept with nanocrystals of photosystem I, one of the largest membrane protein complexes. More than 3,000,000 diffraction patterns were collected in this study, and a three-dimensional data set was assembled from individual photosystem I nanocrystals (∼200 nm to 2 μm in size). We mitigate the problem of radiation damage in crystallography by using pulses briefer than the timescale of most damage processes. This offers a new approach to structure determination of macromolecules that do not yield crystals of sufficient size for studies using conventional radiation sources or are particularly sensitive to radiation damage.
Large eddy simulations were performed on a modular pump-turbine to study oxygen dissolution inside the draft tube. Air injection was applied over the runner cone surface during turbine operation. Data regarding bubble size, void fraction and interfacial area concentration were presented to understand their influence on oxygen dissolution. Transient single phase and multiphase flow simulations were carried out to investigate the influence of air injection and dissolution within the flow field and turbine performance. Multiphase simulations were conducted by using the mixture multiphase model. The mathematical modeling of oxygen dissolution employed was validated by comparing predicted oxygen dissolution against experimental measurements performed by Zhou et al. (2013). The averaged dissolved oxygen concentration in the range of 1.2–1.4 mg/l was obtained; which is sufficient for an active aerobic microorganism activity for wastewater treatment processes. Dissolution efficiency and the amount of averaged dissolved oxygen inside the draft tube were sensitive to the inlet bubble size. The efficiency of the dissolution increases strongly as the inlet bubble size was reduced. The obtained results revealed that vortex suppression was achieved through air admission within multiphase flow simulation. Moreover, the power generation of the turbine was hardly influenced by the aeration through the runner cone.
The great Italian scientist Leonardo da Vinci was the first person to use the word "turbulence" (or turbolenza) to describe the complex motion of water or air. By carefully examining the turbulent wakes created behind obstacles placed in the path of a fluid, he found that there are three key stages to turbulent flow. Turbulence is first generated near an obstacle. Long-lived "eddies" – beautiful whirls of fluid – are then formed. Finally, the turbulence rapidly decays away once it has spread far beyond the obstacle.
Polypyridyl transition-metal complexes are an intriguing class of compounds due to the relatively facile chemical designs and variations in ligand-field strengths that allow for spin-state changes and hence electronic configurations in response to external perturbations such as pressure and light. Light-activated spin-conversion complexes have possible applications in a variety of molecular-based devices, and ultrafast excited-state evolution in these complexes is of fundamental interest for understanding of the origins of spin-state conversion in metal complexes. Knowledge of the interplay of structure and valence charge distributions is important to understand which degrees of freedom drive spin-conversion and which respond in a favorable (or unfavorable) manner.
We present microfluidic chip based devices that produce liquid jets with micrometer diameters while operating at very low flow rates. The chip production is based on established soft-lithographical techniques employing a three-layer design protocol. This allows the exact, controlled and reproducible design of critical parts such as nozzles and the production of nozzle arrays. The microfluidic chips reproducibly generate liquid jets exiting at perfect right angles with diameters between 20 μm and 2 μm, and under special circumstances, even down to 0.9 μm. Jet diameter, jet length, and the domain of the jetting/dripping instability can be predicted and controlled based on the theory for liquid jets in the plate-orifice configuration described by Gañán-Calvo et al. Additionally, conditions under which the device produces highly reproducible monodisperse droplets at exact and predictable rates can be achieved. The devices operate under atmospheric and under vacuum conditions making them highly relevant for a wide range of applications, for example, for free-electron lasers. Further, the straightforward integration of additional features such as a jet-in-jet is demonstrated. This device design has the potential to integrate more features based on established microfluidic components and may become a standard device for small liquid jet production.
Computational Fluid Dynamics (CFD) has emerged as a powerful and economical alternative to empiricism in the prediction of aerosol deposition inside the extrathoracic airways (ETA). In RANS-type treatments, a main difficulty is the specification of turbulent fluid fluctuations experienced by the particles, and hence recent research has concentrated on Large Eddy Simulations (LES) in conjunction with Lagrangian Particle Tracking (LPT). While providing close agreement with data, LES/LPT approaches are extremely time consuming, thus the motivation to investigate whether better Lagrangian stochastic models can help RANS-based treatments achieve comparable accuracy. In this study, the RANS-RSM model is used to obtain the mean carrier flow field, whereas turbulent fluid velocities are defined through a stochastic Continuous Random Walk (CRW) model based on the normalized Langevin equation. With extensive validation against flow field and particle deposition data, we demonstrate that RANS, combined with the Langevin CRW provides accuracy which compares very favorably with the more computationally intensive LES approaches.
This study presents reports on the fabrication and testing of micro-nozzle/diffuser with curved profile boundary. In this paper, different polymers are used for the fabrication of micro-nozzle/diffuser, SU-8 negative bone photo-resist is introduced for the molding structure, and polydimethylsiloxane (PDMS) is used for the structure of nozzle/diffuser. The structure is then bonded with glass by using low-temperature bonding technique.Through measurement of the pressure and flow, the analysis and investigation of fluid dynamics characteristics are then achieved. The experimental data shows that for a given Reynolds number, the pressure loss for the diffuser is lower than that of the nozzle due to a difference in the momentum change. Furthermore, the results which indicate the pressure loss coefficients for both nozzle and diffuser decrease with the Reynolds number show a good agreement with the predicted results. In sum, the theoretical analysis and design basis from this study can then be formulated as the reference and application need in the fabrication of micro-nozzle/diffuser.
A fully developed turbulent flow over a matrix of cubes has been studied using the large Eddy simulation (LES) and Reynolds-averaged Navier–Stokes (RANS) [more specifically, the standard k–ε model] approaches. The numerical method used in LES of an incompressible fluid flow was a second-order accurate, fully conservative discretization scheme. This scheme was used in conjunction with a dynamic semi-coarsening multigrid method applied on a staggered grid as proposed originally by Ham et al. (Proceedings of the Seventh Annual Conference of the Computational Fluid Dynamics Society of Canada, Halifax, Nova Scotia, Canada, 1999; J. Comput. Phys. 177 (2002) 117). The effects of the unresolved subgrid scales in LES are modeled using three different subgrid-scale models: namely, the standard Smagorinsky model; the dynamic model with time-averaging procedure (DMT); and, the localized dynamic model (LDM). To reduce the computational time, LES calculations were conducted on a Linux-based PC cluster using the message passing interface library. RANS calculations were performed using the STREAM code of Lien and Leschziner (Comp. Meth. Appl. Mech. Eng. 114 (1994) 123). The Reynolds number for the present flow simulations, based on the mean bulk velocity and the cube height, was 3800 which is in accordance with the experimental data of Meinders (Ph.D. Thesis, Faculty of Applied Sciences, Delft University of Technology, Delft, Netherlands, 1998). A comparison of predicted model results for mean flow and turbulence with the corresponding experimental data showed that both the LES and RANS approaches were able to predict the main characteristics of the mean flow in the array of cubes reasonably well. LES, particularly when used with LDM, was found to perform much better than RANS in terms of its predictions of the spanwise mean velocity and Reynolds stresses. Flow structures in the proximity of a cube, such as separation at the sharp leading top and side edges of the cube, recirculation in front of the cube, and the arch-type vortex in the wake are captured by both the LES and RANS approaches. However, LES was found to give a better overall quantitative agreement with the experimental data than RANS.
Extraction of polymer from mechanically blended poly(dimethylsiloxane)-silica mixtures was conducted at room temperature using chloroform. The amount of silicon still bound to silica after removal of the polymer solution was studied as a function of the average chain molecular weight. A multiple-link structure of binding sites of PDMS chains onto the plain surface of silica is proposed from the analysis of results.
Numerical simulations reveal the formation of singular structures in the polymer stress field of a viscoelastic fluid modeled by the Oldroyd-B equations driven by a simple body force. These singularities emerge exponentially in time at hyperbolic stagnation points in the flow and their algebraic structure depends critically on the Weissenberg number. Beyond a first critical Weissenberg number the stress field approaches a cusp singularity, and beyond a second critical Weissenberg number the stress becomes unbounded exponentially in time. A local approximation to the solution at the hyperbolic point is derived from a simple ansatz, and there is excellent agreement between the local solution and the simulations. Although the stress field becomes unbounded for a sufficiently large Weissenberg number, the resultant forces of stress grow subexponentially. Enforcing finite polymer chain lengths via a FENE-P penalization appears to keep the stress bounded, but a cusp singularity is still approached exponentially in time.
The flow orientation of anisotropic particles through narrow channels is of importance in many fields, ranging from the spinning and molding of fibers to the flow of cells and proteins through thin capillaries. It is commonly assumed that anisotropic particles align parallel to the flow direction. When flowing through narrowed channel sections, one expects the increased flow rate to improve the parallel alignment. Here, we show by microfocus synchrotron X-ray scattering and polarized optical microscopy that anisotropic colloidal particles align perpendicular to the flow direction after passing a narrow channel section. We find this to be a general behavior of anisotropic colloids, which is also observed for disk-like particles. This perpendicular particle alignment is stable, extending downstream throughout the remaining part of the channel. We show by microparticle image velocimetry that the particle reorientation in the expansion zone after a narrow channel section occurs in a region with considerable extensional flow. This extensional flow is promoted by shear thinning, a typical property of complex fluids. Our discovery has important consequences when considering the flow orientation of polymers, micelles, fibers, proteins, or cells through narrow channels, pipes, or capillary sections. An immediate consequence for the production of fibers is the necessity for realignment by extension in the flow direction. For fibrous proteins, reorientation and stable plug flow are likely mechanisms for protein coagulation.
For patterning thick photoresist films, x-ray lithography is superior to optical lithography because of the use of a shorter wavelength and a very large depth of focus. SU-8 negative resist is well suited to pattern tall, high-aspect ratio microstructures in UV optical and x-ray lithography with rapid prototyping capability due to its high sensitivity. The negative tone of the SU-8 resist offers advantages in fabricating multi-level and non-planar microstructures using x-ray lithography or a combination of x-ray and UV optical lithography. In this paper, we present a fabrication process for multi-level metallic mold insert by a combination of multi-layer SU-8 patterning, poly-dimethylsiloxane (PDMS) molding, and nickel electroplating to make final nickel mold inserts that are suitable for injection molding and hot embossing of plastics and ceramics.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
The concept of large-eddy simulation is applied to simulate turbulent
flow over a square rib mounted on the bottom of a plate channel. The
effects of different spatial resolution on mean flow and on second order
correlations is demonstrated. With sufficient spatial resolution,
excellent agreement with experimental data can be achieved.
Distributions of the inclination angle of the projection of the
vorticity vectors in selected locations of the flow field are presented,
and from two-point spatial velocity correlation coefficients, the
corresponding integral scales have been evaluated.
Recent years have seen significant progress in the field of soft- and
hard-X-ray microscopy, both technically, through developments in source,
optics and imaging methodologies, and also scientifically, through a
wide range of applications. While an ever-growing community is pursuing
the extensive applications of today's available X-ray tools, other
groups are investigating improvements in techniques, including new
optics, higher spatial resolutions, brighter compact sources and
shorter-duration X-ray pulses. This Review covers recent work in the
development of direct image-forming X-ray microscopy techniques and the
relevant applications, including three-dimensional biological
tomography, dynamical processes in magnetic nanostructures, chemical
speciation studies, industrial applications related to solar cells and
batteries, and studies of archaeological materials and historical works
of art.
The thermal and mechanical properties of a new negative photoresist, SU8, were characterized. The influence of curing conditions, such as baking temperature, baking time and UV dosage, on the thermal and mechanical properties of the resultant coatings was studied in detail. It was found that the glass-transition temperature (Tg) of the coatings was coincident with the baking temperature over the temperature range of 25 °C–220 °C for coatings being baked for just 20 min. However, the Tg reached a limiting value (about 240 °C) once the cross-linking reaction was complete, and would not increase further with the baking temperature. The peak temperature of the dimension versus temperature plots, where heat shrinkage occurred, was about a factor of 1.16 times higher than the baking temperature for the temperature range studied. Both the Tg and the shrinkage temperature were affected by the baking time. The thermal expansion coefficients (TEC), including the volumetric TEC (αv), the in-plane TEC (α1) and the out-of-plane TEC (α2), were measured by a pressure–volume–temperature (PVT) apparatus and thermal–mechanical analyzer (TMA). Great residual stress could be generated during the process, and the change in residual stress with the environmental humidity was investigated using vibrational holographic interferometry.
Detailed investigations of the limits of a new negative-tone near-UV resist (IBM SU-8) have been performed. SU-8 is an epoxy-based resist designed specifically for ultrathick, high-aspect-ratio MEMS-type applications. We have demonstrated that with single-layer coatings, thicknesses of more than 500 μm can be achieved reproducibly. Thicker resist layers can be made by applying multiple coatings, and we have achieved exposures in 1200 μm thick, double-coated SU-8 resist layers. We have found that the aspect ratio for near-UV (400 nm) exposed and developed structures can be greater than 18 and remains constant in the thickness range between 80 and 1200 μm. Vertical sidewall profiles result in good dimensional control over the entire resist thickness. To our knowledge, this is the highest aspect ratio reported for near-UV exposures and the given range of resist thicknesses. These results will open up new possibilities for low-cost LIGA-type processes for MEMS applications. The application potential of SU-8 is demonstrated by several examples of devices and structures fabricated by electroplating and photoplastic techniques. The latter is especially interesting as SU-8 has attractive mechanical properties.
Rapid prototyping of polydimethylsiloxane (PDMS) is frequently used to build microfluidic devices. PDMS is inherently hydrophobic; however, the surface can be temporarily rendered hydrophilic by exposing the surface to oxygen plasma. Hydrophilic microchannels are sometimes advantageous over hydrophobic microchannels due to increased cell adhesion or increase in electro osmotic flow (EOF) leading to ease of liquid filling in microchannels. However, the hydrophilic surface is unstable and that low molecular weight (LMW) chains diffuse from the bulk of the PDMS and cover up the thermodynamically unstable surface. This is one reason for the hydrophilic unstability of PDMS. Present study shows that not only chemistry of the creation of silanol groups on the surface, but also morphology of the film surface nanostructuring of PDMS plays an important role in hydrophilization of PDMS. Present paper tries to understand the mechanism of hydrophobic recovery taking into consideration physical and chemical parameters using SEM characterization.
In the present work hydrophilic stability of Sylgard 184 poly(dimethyl siloxane) (PDMS) was studied with the objective to create more stable hydrophilic surfaces. The surface modification of PDMS was carried out by conventional (oxygen plasma) and unconventional plasma modification (2-step modification using oxygen and C2F6) processes and also by chemical grafting using oxygen plasma polymerization of 2-hydroxyethyl methacrylate (HEMA). The hydrophilic stability of the modified surfaces was monitored as a function of time elapsed after treatment and quantified. The surfaces were characterized using static contact angle measurements and Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR). (c) 2006 Elsevier B.V. All rights reserved.
This paper describes a mask alignment technique for a projection printer in which the mask-to-wafer offset is determined interferometrically by a phase-locked method. The alignment patterns used in this method are cross grids which are illuminated with a He-Ne laser beam. By diffraction and interference a harmonic intensity signal is obtained whose phase is representative for the relative displacement of mask and wafer. Phase modulation and fine alignment are done with a tiltable glass plate driven by a galvanometric scanner. The total alignment operation including registration and fine alignment is controlled by a PC. With a registration precision in the nanometer region and cycle times below 0.3 s, this method is particularly suited for each field alignment in projection steppers. Results of theoretical and experimental investigations are discussed.
A capillary steady microjet produced at the discharge region of a gas jet expanding into vacuum is imaged using scanning electron microscopy. The co-flowing gas focuses by inertia and pulls the liquid fed through a capillary tube concentrically positioned at the discharge orifice. The jet breaks up into well-collimated droplets. Numerical simulation explains the persistence of the liquid phase after discharge into vacuum.
A method is described for quick alignment of photolithographic masks over silicon substrates. Moiré reflection patterns are observed from grids etched on the silicon wafer and grids located on the photolithographic masks. A restricted class of moiré patterns provides information on both the direction and degree of misalignment during all phases of alignment. Alignment accuracy of 0.2 microm has been achieved even when there were separations of 30 microm between the mask and wafer. The moiré patterns can be observed visually through standard mask-alignment facilities. The etched grids are found to retain their diffraction properties during the operations of reoxidation and epitaxial growth.
Since accurate alignment is essential for projection lithography, an extended dual-grating based alignment scheme is proposed. This method is an extension of the basic dual-grating alignment model, the mechanism of which is explained to make it clear how the extended scheme performs in projection lithography. The framework of the extended alignment scheme for projection lithography is constructed, and the process of key parameter determination is then detailed. In both cases, a tiny shift of the wafer during the alignment process can be resolved by a conspicuous displacement or phase variation of corresponding fringes. Analytical results indicate that alignment is independent of the gap between wafer and mask, disturbance from the fluctuation in illumination can be neglected, and alignment resolution in subnanometers can be realized with this scheme.
If your new or interested in the field of CFD this book with introduction to cfd by john d anderson should be in your personal library.
As shown by Ganan-Calvo and co-workers, a free liquid jet can be compressed in iameter through gas-dynamic forces exerted by a co-flowing gas, obviating the need for a solid nozzle to form a microscopic liquid jet and thereby alleviating the clogging problems that plague conventional droplet sources of small diameter. We describe in this paper a novel form of droplet beam source based on this principle. The source is miniature, robust, dependable, easily fabricated, and eminently suitable for delivery of microscopic liquid droplets, including hydrated biological samples, into vacuum for analysis using vacuum instrumentation. Monodisperse, single file droplet streams are generated by triggering the device with a piezoelectric actuator. The device is essentially immune to clogging.
- Herrada
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A liquid flatjet system for solution phase soft-x-ray spectroscopy
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Simple and low cost fabrication of embedded micro-channels by using a new thick-film photoplastic
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Whole cell tomography/molecular biology/ structural biology: Affordable x-ray microscopy with nanoscale resolution
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