Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session KM: Materials Processing |
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Chair: Malcom Andrews, Texas A&M University Room: Hilton Chicago PDR 1 |
Monday, November 21, 2005 4:10PM - 4:23PM |
KM.00001: Elastohydrodynamics of Step and Flash Imprint Lithography Shravanthi Reddy, Roger Bonnecaze We study theoretically and numerically the elastohydrodynamics of template filling and deformation in Step and Flash Imprint Lithography (SFIL). This is a photolithography process in which the photoresist is compressed in its liquid monomer form between a silicon wafer and a quartz template with desired features. The monomer is cured into the template pattern by flashing UV light through the quartz template, instead of using traditional optical systems. Features as small as 20 nm can be produced with SFIL. Surprisingly, the lubrication pressures in the filling process can be large enough to cause distortions in the template that are comparable to the feature size and hence reduce the fidelity of the imprinting process. An elastohydrodynamic simulation is developed combining lubrication theory with capillary forces for the fluid flow and thin plate theory for the template deformation to understand the dynamics of the process and how to mitigate the undesirable deformation. Imprint time, template deformation and possible contact of the template with the wafer are presented as a function of number of drops, their placement and imprinting speed. [Preview Abstract] |
Monday, November 21, 2005 4:23PM - 4:36PM |
KM.00002: Multiscale modeling of interfacial flow in particle-solidification front dynamics Justin Garvin, Yi Yang, H.S. Udaykumar Particle-solidification front interactions are important in many applications, such as metal-matrix composite manufacture, frost heaving in soils and cryopreservation. The typical length scale of the particles and the solidification fronts are of the order of microns. However, the force of interaction between the particle and the front typically arises when the gap between them is of the order of tens of nanometers. Thus, a multiscale approach is necessary to analyze particle-front interactions. Solving the Navier-Stokes equations to simulate the dynamics by including the nano-scale gap between the particle and the front would be impossible. Therefore, the microscale dynamics is solved using a level-set based Eulerian technique, while an embedded model is developed for solution in the nano-scale (but continuum) gap region. The embedded model takes the form of a lubrication equation with disjoining pressure acting as a body force and is coupled to the outer solution. A particle is pushed by the front when the disjoining pressure is balanced by the viscous drag. The results obtained show that this balance can only occur when the thermal conductivity ratio of the particle to the melt is less than 1.0. The velocity of the front at which the particle pushing/engulfment transition occurs is predicted. In addition, this novel method allows for an in-depth analysis of the flow physics that cause particle pushing/engulfment. [Preview Abstract] |
Monday, November 21, 2005 4:36PM - 4:49PM |
KM.00003: A `Coating' Operating-Window for a Metal-Casting Flow Cormac J. Byrne, Paul H. Steen, Steven J. Weinstein Planar-flow single roll melt spinning is a promising technology for the next generation of continuous casting machines. In the process, a planar nozzle is held close to a rotating metal wheel and liquid metal is forced through the nozzle into the gap region between the nozzle and wheel where a puddle, constrained by surface tension, is formed. A solidification front grows along the wheel as it translates, forming a solid ribbon ($\sim $100 $\mu $m thick) which is continually ejected from the puddle ($\sim $ 10 m/s). An operating window can be predicted based on the possible curvatures of the upstream meniscus, similar to the approach taken for some liquid film coating flows. While the derivation of the window mirrors that of the coating-flow literature, the flow regimes are different. Apart from the presence of solidification, the pressure losses in casting flows are predominantly inertia dominated, while viscous effects are usually dominant in coating flows. A range of accessible product thicknesses can be predicted when casting open to atmosphere. This predicted thickness range is compared with well-established empirical windows. The predicted window also indicates considerable benefits (extended operability) from applying a pressure difference between the upstream and downstream menisci, as will be discussed. [Preview Abstract] |
Monday, November 21, 2005 4:49PM - 5:02PM |
KM.00004: Fluid Mechanical Properties of Silkworm Fibroin Solutions Akira Matsumoto, Amil Lindsay, David Kaplan, Behrouz Abedian The aqueous solution behavior of silk fibroin is of interest due to the assembly and processing of this protein related to the spinning of protein fibers that exhibit remarkable mechanical properties. To gain insight into the origins of this functional feature, it is desired to determine how the protein behaves under a range of solution conditions. Pure fibroin at different concentrations in water was studied for surface tension, as a measure of surfactancy. In addition, shear induced changes on these solutions in terms of structure and morphology was also determined. Fibroin solutions exhibited shear rate-sensitive viscosity changes and precipitated at a critical shear rate where a dramatic increase of 75-150\% of the initial value was observed along with a decrease in viscosity. In surface tension measurements, critical micelle concentrations were in the range of 3-4\% w/v. The influence of additional factors, such as sericin protein, divalent and monovalent cations, and pH on the solution behavior in relation to structural and morphological features will also be described. [Preview Abstract] |
Monday, November 21, 2005 5:02PM - 5:15PM |
KM.00005: Numerical Modeling of Hollow Fiber Drawing Jing Yang, Yogesh Jaluria Hollow optical fibers, which are widely used in medicine and in diagnostics, power delivery and communications, are typically manufactured by drawing a specially fabricated hollow preform down to a fiber in a conventional fiber-drawing tower. In this work, a numerical model based on the mass, momentum and energy equations is developed to investigate the drawing process. The axisymmetric flow of air in the central cavity, as well as the flow of glass and aiding purge gas, are considered. The complex computational domains are converted to cylindrical regions by using coordinate transformations. The two neck-down profiles, which are the inner and outer surfaces of the hollow fiber, are generated by using an iterative scheme. The optical thick approximation, as well as the zone model, are applied to calculate the radiative transport within the glass and the Boussinesq approximations are used for the buoyancy effects. The results obtained show that it is possible to predict the geometry of the final hollow fiber and to provide feasible combinations of parameters for successful hollow fiber drawing. The validation of the model is carried out by comparing the predictions with the results for solid-core fiber drawing and with available experimental and numerical results for hollow fibers. It is shown that the results from the model are consistent with the physical trends and agree well with the results in the literature. [Preview Abstract] |
Monday, November 21, 2005 5:15PM - 5:28PM |
KM.00006: Effect of Curtet Number Variation on Dispersion of SWNTs in Epoxy Composites Prepared by a Continuous Impingement Mixing Process Gopinath Subramanian, Malcolm Andrews Dispersion of nanoparticles is a key issue in preparing nanocomposites. Preparation of polymer nanocomposites is normally done by batch processing, with nanoparticles synthesized in-situ by a chemical reaction, which leads to a good dispersion. However, the in-situ synthesis technique is not readily applicable to the dispersion of single wall carbon nanotubes (SWNTs). A novel approach is presented here to improve the dispersion of SWNTs in polymers to enhance the structural, electrical and thermal properties that uses a continuous, high output impingement mixing process. In particular, we report on the dispersion and properties of composites of SWNTs in a Shell EPON-862/W system. The primary mechanism of dispersion is a high-speed jet immersed in a secondary stream confined by a constant area duct. The degree of dispersion is governed by the Curtet number (C$_{t}$) calculated using the diameter ratio and the velocity ratio. It was found that dispersion was affected by a critical value for C$_{t}$ of 0.75. Poor performance above the critical Curtet number is attributed to a reduction in residence time of fluid within the mixer. A series of composites with various SWNT loading were prepared at various C$_{t}$. The effect of C$_{t}$ on the degree of dispersion was evaluated by scanning electron microscopy and electrical conductivity measurements. Percolation curves were also obtained. [Preview Abstract] |
Monday, November 21, 2005 5:28PM - 5:41PM |
KM.00007: Solvent-Free Thermal Spraying of Polymer Particles Milan Ivosevic, Richard A. Cairncross, Richard Knight During thermal spray deposition, jets of high temperature and high velocity gases are used to melt and accelerate particles towards the surface to be coated. Upon impact at the surface, multiple droplets deform, cool and consolidate to form a coating. A 3D model of particle impact and deformation on flat and rough surfaces has been developed for thermally sprayed polymer and metal particles. Fluid flow and particle deformation were predicted by the Volume of Fluid Method using Flow-3D software. A comparison between polymer and metal splatting demonstrates how the large physical property differences between these materials affect their flow behavior under similar thermal spray conditions. The higher viscosity of molten polymers leads to lower Reynolds numbers and less deformation, and lower thermal conductivity of polymers leads to higher Biot numbers and large temperature gradients in the polymer particles. Temperature gradients in a particle lead to a ``fried egg'' shaped splat characteristic of experimental observations of thermally sprayed polymer particles. The effect of roughness on the mechanics of splatting and final splat shapes was explored through the use of several prototypical rough surfaces, e.g. steps and grooves. [Preview Abstract] |
Monday, November 21, 2005 5:41PM - 5:54PM |
KM.00008: Controlling Release Kinetics of PLG Microspheres Using a Manufacturing Technique Nader Berchane, Malcolm Andrews Controlled drug delivery offers numerous advantages compared with conventional free dosage forms, in particular: improved efficacy and patient compliance. Emulsification is a widely used technique to entrap drugs in biodegradable microspheres for controlled drug delivery. The size of the formed microspheres has a significant influence on drug release kinetics. Despite the advantages of controlled drug delivery, previous attempts to achieve predetermined release rates have seen limited success. This study develops a tool to tailor desired release kinetics by combining microsphere batches of specified mean diameter and size distribution. A fluid mechanics based correlation that predicts the average size of Poly(Lactide-co-Glycolide) [PLG] microspheres from the manufacturing technique, is constructed and validated by comparison with experimental results. The microspheres produced are accurately represented by the Rosin-Rammler mathematical distribution function. A mathematical model is formulated that incorporates the microsphere distribution function to predict the release kinetics from mono-dispersed and poly-dispersed populations. Through this mathematical model, different release kinetics can be achieved by combining different sized populations in different ratios. The resulting design tool should prove useful for the pharmaceutical industry to achieve designer release kinetics. [Preview Abstract] |
Monday, November 21, 2005 5:54PM - 6:07PM |
KM.00009: Boltzmann Monte-Carlo simulations of a suspension of non-spherical particles in a parallel-wall channel Mauricio Zurita-Gotor, Jerzy Blawzdziewicz, Eligiusz Wajnryb The evolution of a dilute suspension of axisymmetric particles confined between two parallel planar walls is investigated under creeping-flow conditions. The suspension undergoes a shear flow that results from the relative motion of the walls. The hydrodynamic interactions are accurately evaluated using our Cartesian-representation algorithm. In the absence of interparticle interactions the suspended particles undergo periodic motions, similar to Jefferey orbits in free space. However, the periods in the confined system are not identical. Due to the associated phase shifts a stationary state is reached at long times. Finite-concentration effects are included via a Boltzmann Monte-Carlo method. The state of the system is described by an ensemble of periodic particle trajectories, which are characterized by the vertical position and orientation of the particles crossing the horizontal plane. The ensemble is updated by computing a large number of binary collisions. Implications of our simulations for particle separation in microchannels are examined. [Preview Abstract] |
Monday, November 21, 2005 6:07PM - 6:20PM |
KM.00010: Flow-induced instability of mushy layer permeability Jerome Neufeld, John S. Wettlaufer The coupling of an external shear flow with the permeability of a solidifying mushy layer is investigated experimentally and theoretically. We grow a mushy layer from a trans-eutectic aqueous ammonium chloride solution from the base of a laboratory flume. The growth rate is constant and a laminar shear flow is applied. We find a threshold speed above which a spatiotemporal variation of the permeability of the layer appears with a planform wherein the long axis is transverse to the flow direction. Upon removal of the flow, the material returns to a uniform state. The growth of the pattern compares favorably with an analytical and numerical stability analysis which incorporates dissolution of the solid matrix. [Preview Abstract] |
Monday, November 21, 2005 6:20PM - 6:33PM |
KM.00011: Three dimensional Solutocapillary Convection in Spherical Shells Pravin Subramanian, Abdelfattah Zebib Nonlinear, time-dependent, three-dimensional, variable viscosity, infinite Schmidt number solutocapillary convection in spherical shells is computed by a finite-volume method. The shell contains a solute and a solvent, and the inner boundary is impermeable and stress free. The solvent evaporates at the outer surface into a water-solvent environment with a prescribed mass transfer coefficient. Convection is driven by surface tension dependence on the solvent concentration $C$. A time-dependent diffusive state characterized by concentration $C_{d}(r,t)$ and a receding outer surface $r_{2d}(t)$ is possible and is a function of the mass transfer Biot number, a partition coefficient, and ambient solvent concentration $C_{\infty}$. It loses stability at critical values of the Marangoni number and degree of surface harmonic. In the limit of small Capillary number $Ca\to0$ the outer radius deviation from sphericity $\delta(\theta,\phi,t)$ is $O(Ca)$ and $r_{2}(\theta,\phi,t)$ is given by $r_{2d}(t)$ in the $O(1)$ convection. We compute supercritical motions and companion $\delta(\theta,\phi,t)$ in this moving boundary problem subject to random initial conditions and compare nonlinear results with those from linear theory, axisymmetric calculations and available experiments. [Preview Abstract] |
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