JETSPIN: a specific-purpose open-source software for simulations of nanofiber electrospinning

We present the open-source computer program JETSPIN, specifically designed to simulate the electrospinning process of nanofibers. Its capabilities are shown with proper reference to the underlying model, as well as a description of the relevant input variables and associated test-case simulations. The various interactions included in the electrospinning model implemented in JETSPIN are discussed in detail. The code is designed to exploit different computational architectures, from single to parallel processor workstations. This paper provides an overview of JETSPIN, focusing primarily on its structure, parallel implementations, functionality, performance, and availability.Comment: 22 pages, 11 figures. arXiv admin note: substantial text overlap with arXiv:1507.0701

Design and Implementation of a Near-field Scanning Optical Module for Inverted Microscopes

This dissertation describes the design and implementation of a home-built near-field scanning optical microscope (NSOM) module for inverted microscopes. In this design, the NSOM module is built atop an inverted fluorescence microscope. This is particularly well suited for applications in which the normal imaging modes of the inverted microscope are still required to locate and study the sample before performing NSOM experiments. This module is used as a near-field instrument for nanostructure investigation of various samples including plane-ruled reflection gratings, AFM calibration standards, nanospheres and gold nanoparticles. In addition, we will demonstrate the ability to achieve nanometer-scale surface modification of specific polymer films using this technique

Analysis and design of rapid prototyped mechanisms using hybrid flexural pivots

The ability of fabricating flexure based mechanism is of great importance in modern technology fields such as nanotechnology and precision engineering. For an instance, a great number of nanopositioning systems are made out of flexures. Examples of these systems are those used in scanning probe microscopy and many other types of metrology tools. Not having friction is a requirement to achieve nanometer scale motion and thus flexural systems are preferred as they lack of sliding surfaces. Moreover, flexure hinges are able to produce accurate and repeatable motion when properly designed. Conventionally, flexure-type systems are manufactured from high performance metals such as stainless and alloyed steel or aluminum alloys for high material performance and durability. Functional requirements such as high bandwidth, accuracy performance and geometric complexity require them to be manufactured as monolithic structures using conventional precision machining and electro discharge machining (EDM). However, such an approach is expensive and not practical for mass production. They can only be used for custom and high-value added applications. Conventional and emerging additive manufacturing technologies such as Direct Metal Laser Sintering (DMLS) offer an opportunity to fabricate cost effective flexure-based mechanisms with complicated spatial structures. However, the reported limitations of this approach are: dimensional accuracy, low quality surface finish, anisotropic properties, thermal instability, low holding force capabilities and severely reduced durability of the flexural elements as most rapid prototyping materials are unsuitable in fatigue loading conditions. This thesis work envisions an approach to manufacture hybrid mechanisms that uses i) economic methods like casting and molding (for high volume production) or 3-D printing (for custom, one-off systems) for manufacturing the mechanism structures/skeletons and ii) inserts of simple geometry with specialized materials (e.g. spring steel, etc.) to get the right material properties where need it. The objective of this research is to develop and exemplify a methodology that integrates a host material (rapid prototyping) with a flexure material and combines them to create a much more easy to produce mechanism. For this purpose, we focus on the design of the interfaces between the two materials and, particularly, the penetration depth of the insert into the host. Using Finite Element simplified model and tracking mechanical variables such as stress, pressure and elastic energy we arrived to the functions relating the optimum penetration depth (insertion iii distance where the elastic work done by the host material is minimum relative to that one done by the flexure) with the thickness of the flexure and the elastic properties of the two materials. For example, in the case of an aluminum host and steel inserts; the optimum penetration distance is six times the thickness of the insert whereas in the case of an ABS structure and steel inserts, the optimum penetration distance is ten times greater than the insert thickness. Further results include the study of extra compliance introduced to the system in design scenarios considering materials and manufacturing consideration for the fabrication, alignment and assembly of the mechanism. Finally, we demonstrate a piezoelectric-actuated four-bar mechanism, and an XYZ force sensor for suture training as general applications of these devices to the precision motion field and the medical industry. The methodology implemented in this work poses a simple and affordable way to fabricate, assemble and customize low-cost devices for precision motion application and it applies to both, systems fabricated by polymer and metal rapid prototyping technologies

Kinetics of copolymer localization at a selective liquid-liquid interface

The localization kinetics of a regular block-copolymer of total length $N$ and block size $M$ at a selective liquid-liquid interface is studied in the limit of strong segregation between hydrophobic and polar segments in the chain. We propose a simple analytic theory based on scaling arguments which describes the relaxation of the initial coil into a flat-shaped layer for the cases of both Rouse and Zimm dynamics. For Rouse dynamics the characteristic times for attaining equilibrium values of the gyration radius components perpendicular and parallel to the interface are predicted to scale with block length $M$ and chain length $N$ as $\tau_{\perp} \propto M^{1+2\nu}$ (here $\nu\approx 0.6$ is the Flory exponent) and as $\tau_{\parallel} \propto N^2$, although initially the characteristic coil flattening time is predicted to scale with block size as $\propto M$. Since typically $N\gg M$ for multiblock copolymers, our results suggest that the flattening dynamics proceeds faster perpendicular rather than parallel to the interface, in contrast to the case of Zimm dynamics where the two components relax with comparable rate, and proceed considerably slower than in the Rouse case. We also demonstrate that, in the case of Rouse dynamics, these scaling predictions agree well with the results of Monte Carlo simulations of the localization dynamics. A comparison to the localization dynamics of {\em random} copolymers is also carried out.Comment: 11 pages, 15 figure

Displacive model of deformation twinning in hexagonal close-packed metals. Case of the (90 deg, a) and (86 deg, a) extension twins in magnesium

A crystallographic displacive model is proposed for the extension twins in magnesium. It is based on a hard-sphere assumption previously used for martensitic transformations. The atomic displacements are established, and the homogeneous lattice distortion is analytically expressed as a continuous angular-distortive matrix that takes the usual form of shear when the distortion is complete. The calculations prove that a volume change of 3 percents occurs for the intermediate states and that the twinning plane, even if untilted and restored when the distortion is complete, is not fully invariant during the transient states. The crystallographic calculations also show that the (90 deg, a) twins observed in magnesium nano-pillars and the (86 deg, a) twins observed in bulk samples come from the same mechanism, the only difference being the existence of a slight obliquity angle (+/- 3.4 deg) required to reduce the strains in the latter case. Continuous features in the pole figures between the low-misoriented (86 deg, a) twin variants are expected; they are confirmed by EBSD maps acquired on a deformed magnesium single crystal. As the continuous mechanism of extension twinning is not a simple shear, a "virtual work" criterion using the value of the intermediate distortion matrix at the maximum volume change is proposed in place of the usual Schmid's law. It allows predicting the formation of extension twins for crystal orientations associated with negative Schmid factors.Comment: 41 pages, 12 figures, 1 Appendix with 3 figures, 6 Suppl. Material

Large, long range tensile forces drive convergence during

Indirect evidence suggests that blastopore closure during gastrulation of anamniotes, including amphibians such as Xenopus laevis, depends on circumblastoporal convergence forces generated by the marginal zone (MZ), but direct evidence is lacking. We show that explanted MZs generate tensile convergence forces up to 1.5 mN during gastrulation and over 4 mN thereafter. These forces are generated by convergent thickening (CT) until the midgastrula and increasingly by convergent extension (CE) thereafter. Explants from ventralized embryos, which lack tissues expressing CE but close their blastopores, produce up to 2 mN of tensile force, showing that CT alone generates forces sufficient to close the blastopore. Uniaxial tensile stress relaxation assays show stiffening of mesodermal and ectodermal tissues around the onset of neurulation, potentially enhancing long-range transmission of convergence forces. These results illuminate the mechanobiology of early vertebrate morphogenic mechanisms, aid interpretation of phenotypes, and give insight into the evolution of blastopore closure mechanisms. © Shook et al

Numerical Evaluation of Alford Forces Acting on an Axial Expander for Supercritical CO2 Application

Nowadays, supercritical carbon dioxide (S-CO2) cycles are of great interest in the scientific research especially considering the energy transition that is occurring. The S-CO2 high density and relatively low viscosity make it an interesting fluid for power generation. For large heat sources, large flowrates of fluid can be obtained. Therefore, the development of axial flow expanders can allow large power generations. In the presence of rotor eccentricities, the aerodynamic loading of free-standing blades is not constant tangentially and will promote the lateral vibration of the rotor. The dynamic phenomenon that arises is known as Thomas-Alford force. The Thomas-Alford force determines an increase of the vibration level of the machine and a higher risk of instabilities. In this paper, a preliminary investigation of a S-CO2 axial expander stage is performed. Different correlations proposed in the literature are adopted to estimate the magnitude of the Thomas-Alford force. A mono-dimensional code and a simplified computational fluid dynamics (CFD) model are adopted to obtain the parameters of the stage considered. In this preliminary investigation, only free-standing blades are considered. The results obtained show a good agreement between 1D and CFD inputs required by the different correlation used. Despite this, the cross coupled stiffness calculated are widely dependent on the correlation used; then, this study can be considered as the starting point for more detailed investigations validating the correlations behavior in this environment through an unsteady CFD and/or a proper test campaign

DEVELOPMENT OF A VERSATILE HIGH SPEED NANOMETER LEVEL SCANNING MULTI-PROBE MICROSCOPE

The motivation for development of a multi-probe scanning microscope, presented in this dissertation, is to provide a versatile measurement tool mainly targeted for biological studies, especially on the mechanical and structural properties of an intracellular system. This instrument provides a real-time, three-dimensional (3D) scanning capability. It is capable of operating on feedback from multiple probes, and has an interface for confocal photo-detection of fluorescence-based and single molecule imaging sensitivity. The instrument platform is called a Scanning Multi-Probe Microscope (SMPM) and enables 45 microm by 45 microm by 10 microm navigation of specimen with simultaneous optical and mechanical probing with each probe location being adjustable for collocation or for probing with known probe separations. The 3D positioning stage where the specimen locates was designed to have nanometer resolution and repeatability at 10 Hz scan speed with either open loop or closed loop operating modes. The fine motion of the stage is comprises three orthogonal flexures driven by piezoelectric actuators via a lever linkage. The flexures design is able to scan in larger range especially in z axis and serial connection of the stages helps to minimize the coupling between x, y and z axes. Closed-loop control was realized by the capacitance gauges attached to a rectangular block mounted to the underside of the fine stage upon which the specimen is mounted. The stage's performance was studied theoretically and verified by experimental test. In a step response test and using a simple proportional and integral (PI) controller, standard deviations of 1.9 nm 1.8 nm and 0.41 nm in the x, y and z axes were observed after settling times of 5 ms and 20 ms for the x and y axes. Scanning and imaging of biological specimen and artifact grating are presented to demonstrate the system operation. For faster, short range scanning, novel ultra-fast fiber scanning system was integrated into the xyz fine stage to achieve a super precision dual scanning system. The initial design enables nanometer positioning resolution and runs at 100 Hz scan speed. Both scanning systems are capable of characterization using dimensional metrology tools. Additionally, because the high-bandwidth, ultra-fast scanning system operates through a novel optical attenuating lever, it is physically separate from the longer range scanner and thereby does not introduce additional positioning noise. The dual scanner provides a fine scanning mechanism at relatively low speed and large imaging area using the xyz stage, and focus on a smaller area of interested in a high speed by the ultra-fast scanner easily. Such functionality is beneficial for researchers to study intracellular dynamic motion which requires high speed imaging. Finally, two high end displacement sensor systems, a knife edge sensor and fiber interferometer, were demonstrated as sensing solutions for potential feedback tools to boost the precision and resolution performance of the SMPM

Development and Performance of the Nanoworkbench: A Four Tip STM for Electrical Conductivity Measurements Down to Sub-micrometer Scales

A multiple-tip ultra-high vacuum (UHV) scanning tunneling microscope (MT-STM) with a scanning electron microscope (SEM) for imaging and molecular-beam epitaxy growth capabilities has been developed. This instrument (nanoworkbench) is used to perform four-point probe conductivity measurements at micrometer spatial dimension. The system is composed of four chambers, the multiple-tip STM/SEM chamber, a surface analysis and preparation chamber, a molecular-beam epitaxy chamber and a load-lock chamber for fast transfer of samples and probes. The four chambers are interconnected by a unique transfer system based on a sample box with integrated heating and temperature-measuring capabilities. We demonstrate the operation and the performance of the nanoworkbench with STM imaging on graphite and with four-point-probe conductivity measurements on a silicon-on-insulator (SOI) crystal. The creation of a local FET, whose dimension and localization are respectively determined by the spacing between the probes and their position on the SOI surface, is demonstrated.Comment: 39 pages, 15 figure