Multiple Tips Research Articles

Fingering in a channel and tripolar Loewner evolutions

A class of Laplacian growth models in the channel geometry is studied using the formalism of tripolar Loewner evolutions, in which three points, namely, the channel corners and the point at infinity, are kept fixed. Initially, the problem of fingered growth, where growth takes place only at the tips of slitlike fingers, is revisited and a class of exact solutions of the corresponding Loewner equation is presented for the case of stationary driving functions. A model for interface growth is then formulated in terms of a generalized tripolar Loewner equation and several examples are presented. It is shown that the growing interface evolves into a steadily moving finger and that tip competition arises for nonsymmetric initial configurations with multiple tips.

Removing Material Using Atomic Force Microscopy with Single‐ and Multiple‐Tip Sources

Atomic force microscopy (AFM) has been an effective material removing tool for fabricating various nanostructures because of its sub-nanometer precision and simplicity in operation. AFM material removing techniques have evolved from a solely mechanical process to one in which the tip can be loaded by additional energy sources, such as thermal, electric, or chemical, to enhance its fabrication abilities. In this paper, these material removing techniques are reviewed with an emphasis on their capabilities and recent progress. The recent hardware and software developments are first presented to provide a general view on the current status of the technology to be assessed. Following an overview of the feasibility and effectiveness of using mechanical scratching for removing various types of soft and hard materials, the processes of a wide range of approaches using multiple tip sources are then assessed with a focus on their principles, versatilities, and potentials for future applications.

Study of the twinned dendrite tip shape II: Experimental assessment

The favorable growth kinetics of twinned dendrites can be explained by their complex morphology, multiple side branching mechanisms, growth undercooling and tip morphology. Three models were proposed for the twinned dendrite tip shape: (i) a grooved tip [1] satisfying the Smith condition at the triple line; (ii) a doublon [2], i.e. a double-tip dendrite that grows with a narrow and deep liquid channel in its center; and (iii) a pointed (or edgy) tip [3], with consideration of the solid–liquid interfacial energy anisotropy. In the first part of this work, phase field simulations of half a twinned dendrite with an appropriate boundary condition to reproduce the Smith condition supported the doublon conjecture, with a narrow liquid channel ending its solidification with the formation of small liquid droplets. In this part, experimental observations of twinned dendrite tips reveal the presence of a small, but well-defined, groove, thus definitely eliminating the edged tip hypothesis. Focused ion beam nanotomography and energy-dispersive spectroscopy chemical analysis in a transmission electron microscope reveal the existence of a positive solute gradient in a region localized within 2 μm around the twin plane. In Al–Zn specimens, small particles aligned within the twin plane further support the doublon conjecture and the predicted formation of small liquid droplets below the doublon root.

Diamond tips for automated electrical probing inside a scanning electron microscopy system

Pyramidal tips made from boron doped diamond have become the ultimate choice for electrically measuring semiconductor device structures in electrical atomic force microscopy (AFM). An advanced measurement setup with diamond probing units directly integrated inside a scanning electron microscopy (SEM) system is highly wanted as this allows for accurate tip positioning compared to the optical microscope of a standard AFM and enables also multiple tip measurements. Therefore, we have developed highly conductive in-plane diamond tips with a triangular shape and attached them to Ni cantilevers. We have established a LabVIEW-based setup enabling automated electrical measurements inside a SEM system using stick-and-slip motion nanomanipulators and a parameter analyzer system. To our best knowledge, this paper presents first 1- and 2-tip electrical measurements of microfabricated diamond probes inside a SEM system. Measurements of Si staircase and Ge structures are shown and compared to scanning spreading resistance (SSRM) results. Our work demonstrates that doped diamond tips clearly outperform common tungsten probe needles enabling nanoprobing experiments which were impossible so far. Based on our results, we predict that doped diamond is going to be the standard tip material not only for electrical AFM but also for nanoprobing of semiconductor materials.

SunB, a novel Sad1 and UNC‐84 domain‐containing protein required for development of Dictyostelium discoideum

A gene, sunB, encoding a novel class of Sad1 and UNC-84 (SUN) domain, was isolated from a cDNA screen for suppressors of a mutation in Dd-STATa - a Dictyostelium homologue of metazoan STAT (signal transducers and activators of transcription). The SunB protein localized in the area around the nucleus in growing cells, but in the multicellular stages it was predominantly found in prespore vacuoles (PSVs). A disruptant of sunB was multinucleated in the vegetative phase; during development it formed mounds with multiple tips and failed to culminate. The mutation was cell autonomous, and showed reduced expression of the prespore marker gene pspA and elevated expression of marker genes for prestalk AB cells. Interestingly, the level of SunB was abnormally high in the prestalk cells of Dd-STATa mutants, which are defective in culmination. We conclude that SunB is essential for accurate prestalk/prespore differentiation during Dictyostelium development and that its cell-type dependent localization is regulated by a Dd-STATa-mediated signaling pathway.

Chromosome Tips Damaged in Anaphase Inhibit Cytokinesis

Genome maintenance is ensured by a variety of biochemical sensors and pathways that repair accumulated damage. During mitosis, the mechanisms that sense and resolve DNA damage remain elusive. Studies have demonstrated that damage accumulated on lagging chromosomes can activate the spindle assembly checkpoint. However, there is little known regarding damage to DNA after anaphase onset. In this study, we demonstrate that laser-induced damage to chromosome tips (presumptive telomeres) in anaphase of Potorous tridactylis cells (PtK2) inhibits cytokinesis. In contrast, equivalent irradiation of non-telomeric chromosome regions or control irradiations in either the adjacent cytoplasm or adjacent to chromosome tips near the spindle midzone during anaphase caused no change in the eventual completion of cytokinesis. Damage to only one chromosome tip caused either complete absence of furrow formation, a prolonged delay in furrow formation, or furrow regression. When multiple chromosome tips were irradiated in the same cell, the cytokinesis defects increased, suggesting a potential dose-dependent mechanism. These results suggest a mechanism in which dysfunctional telomeres inhibit mitotic exit.

Enhancement of the Stress Corrosion Sensitivity of AA5083 by Heat Treatment

In this study, the stress corrosion cracking (SCC) resistance of AA5083 is intentionally degraded by a series of progressively longer annealing treatments at 448 K (175 °C) that create a two-phase microstructure. Precipitation of strongly anodic Mg2Al3, known as β-phase, occurs heterogeneously with substantial precipitation along the grain boundaries, as observed by differential interference microscopy. Ultimate tensile strength, yield strength, and strain to failure of AA5083 alloy were found to be independent of the amount of β-phase precipitates, making AA5083 an ideal system to study the relative contributions of anodic dissolution and hydrogen embrittlement. Open circuit dropwise exposure SCC tests with precracked double cantilever beam (DCB) specimens made from the AA5083 alloy with different heat treatment conditions were conducted using 3.5 pct NaCl solution at an initial stress intensity factor (K I ) of $$ 1 5\,{\text{ksi}}\sqrt {\text{in}} .\;\left( { 1 6. 5\,{\text{MPa}}\sqrt {\text{m}} } \right). $$ Two SCC characteristics, initial crack growth rate and incubation time, were found to be strongly dependent on the amount of β-phase precipitates. Initial crack growth rate increased sigmoidally as a function of heat treatment time with an inflection point between 120 and 240 hours of sensitization time, while the incubation time decreases monotonically with sensitization time. Additionally, fracture surfaces investigated by scanning electron microscopy demonstrated characteristics of intergranular cracking with multiple crack tips. Discussion centers on the evidence supporting anodic dissolution of β-phase grain boundary precipitates as a primary mechanism of SCC in severely sensitized AA5083 alloy and the potential contribution of hydrogen embrittlement in the failure of grain boundary ligaments between β-phase grain boundary precipitates in less severely sensitized conditions.

Fatigue growth prediction of center slant crack in rectangular plate

ABSTRACTThis paper presents a numerical prediction model of mixed‐mode crack fatigue growth in a plane elastic plate. It involves a formulations of fatigue growth of multiple crack tips under mixed‐mode loading and a displacement discontinuity method with crack‐tip elements (a boundary element method) proposed recently by Yan is extended to analyse the fatigue growth process of multiple crack tips. Due to an intrinsic feature of the boundary element method, a general growth problem of multiple cracks can be solved in a single‐region formulation. In the numerical simulation, for each increment of crack extension, remeshing of existing boundaries is not necessary. Crack extension is conveniently modelled by adding new boundary elements on the incremental crack extension to the previous crack boundaries. At the same time, the element characters of some related elements are adjusted according to the manner in which the boundary element method is implemented. As an example, the present numerical approach is used to analyse the fatigue growth of a centre slant crack in a rectangular plate. The numerical results illustrate the validation of the numerical prediction model and can reveal the effect of the geometry of the cracked plate on the fatigue growth.

Nitrogen-Mediated Wet-Chemical Formation of Carbon Nitride/ZnO Heterojunctions for Enhanced Field Emission

Heterojunction structures of nanocrystalline materials are of great importance in scientific and industrial research for their potential applications in nanoscale electronics and photonics. Here, we report a simple wet-chemical method of nitrogen-mediated growth of ZnO nanocrystals on carbon nitride (CNx) nanotubes. SEM and TEM analyses show self-organized ZnO nanoflowers on CNx stems. PL spectra exhibiting a blue emission at 449 nm confirms the junction formation between CNx and ZnO. The field emission (FE) properties of CNx-ZnO film are greatly improved over those of pristine CNx. The turn-on and threshold fields for bare CNx film are 1.70 and 2.95 V/v, whereas those for CNx-ZnO hybrid are found to be 0.75 and 1.3 V/microm, respectively. This significant downshift in the turn-on and threshold fields is believed to occur via lowering of the Schottky barrier at the metal-semiconductor interface. Three-dimensionally blossomed ZnO nanopetals with multiple sharp tips effectively enhance the FE performance. Moreover, this heterojunction reinforces the electron emission lifetime and protects the CNx tubes against thermal degradation.

Single-molecule force spectroscopy of DNA-based reversible polymer bridges: Surface robustness and homogeneity

Single-molecule force spectroscopy, as implemented in an atomic force microscope, provides a rarely used method by which to monitor dynamic processes that occur near surfaces. Here, a methodology is presented and characterized that facilitates the study of polymer bridging across nanometer-sized gaps. The model system employed is that of DNA-based reversible polymers, and an automated procedure is introduced that allows the AFM tip–surface contact point to be automatically determined, and the distance d between opposing surfaces to be actively controlled. Using this methodology, the importance of several experimental parameters was systematically studied, e.g. the frequency of repeated tip/surface contacts, the area of the substrate surface sampled by the AFM, and the use of multiple AFM tips and substrates. Experiments revealed the surfaces to be robust throughout pulling experiments, so that multiple touches and pulls could be carried out on a single spot with no measurable affect on the results. Differences in observed bridging probabilities were observed, both on different spots on the same surface and, more dramatically, from one day to another. Data normalization via a reference measurement allows data from multiple days to be directly compared.

Test Taking Tips to Give You an Edge

Test Taking Tips to Give You an Edge

Comment on “Atomic structure model of the reconstructed Si(557) surface with a triple step structure: Adatom-parallel dimer model”

Recently, [Oh et al.Phys. Rev. B 77, 155430 (2008)] reported on an atomic model for the so-called Si(557)-reconstructed surface with regular triple steps. The atomic structure model proposed was developed on the basis of high-resolution scanning tunneling microscopy (STM) images and first-principles calculations performed to match such STM images. Here we argue that the STM image presented is affected by an artifact, most likely related to a multiple tip image effect. Among other issues with the image presented, the corner holes for $(7\ifmmode\times\else\texttimes\fi{}7)$ reconstruction on Si(111) terraces are not visible, since a doubled image superimposed smears out such characteristic depression. The $(\ifmmode\times\else\texttimes\fi{}7)$ periodicity deduced for the edge facets, in contrast with previously reported STM images and electron-diffraction studies which unambiguously indicate a $(\ifmmode\times\else\texttimes\fi{}2)$ periodicity, arises from the same artifact. Finally, we remark that the (557) surface orientation and (112) orientation of triple steps assumed for first-principles calculations disregard the latest experimental data available, which suggests this surface has a (7 7 10) orientation and the steps in triple step do not form a well-defined plane [Surf. Sci. 600, 4878 (2006)].

Type B phosphatidylinositol-4-phosphate 5-kinases mediate Arabidopsis and Nicotiana tabacum pollen tube growth by regulating apical pectin secretion.

Phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P(2)] occurs in the apical plasma membrane of growing pollen tubes. Because enzymes responsible for PtdIns(4,5)P(2) production at that location are uncharacterized, functions of PtdIns(4,5)P(2) in pollen tube tip growth are unresolved. Two candidate genes encoding pollen-expressed Arabidopsis thaliana phosphatidylinositol-4-phosphate 5-kinases (PI4P 5-kinases) of Arabidopsis subfamily B were identified (PIP5K4 and PIP5K5), and their recombinant proteins were characterized as being PI4P 5-kinases. Pollen of T-DNA insertion lines deficient in both PIP5K4 and PIP5K5 exhibited reduced pollen germination and defects in pollen tube elongation. Fluorescence-tagged PIP5K4 and PIP5K5 localized to an apical plasma membrane microdomain in Arabidopsis and tobacco (Nicotiana tabacum) pollen tubes, and overexpression of either PIP5K4 or PIP5K5 triggered multiple tip branching events. Further studies using the tobacco system revealed that overexpression caused massive apical pectin deposition accompanied by plasma membrane invaginations. By contrast, callose deposition and cytoskeletal structures were unaltered in the overexpressors. Morphological effects depended on PtdIns(4,5)P(2) production, as an inactive enzyme variant did not produce any effects. The data indicate that excessive PtdIns(4,5)P(2) production by type B PI4P 5-kinases disturbs the balance of membrane trafficking and apical pectin deposition. Polar tip growth of pollen tubes may thus be modulated by PtdIns(4,5)P(2) via regulatory effects on membrane trafficking and/or apical pectin deposition.

New drop weight analysis for surface tension determination of liquids

The validity of the existing drop weight analyses for determining surface tension could be influenced by the liquid properties. This study aimed to verify the validity of the Harkins–Brown correction factors and the Bond number correlation, as well as to develop a new drop weight analysis that is insensitive to liquid properties. The liquid samples of known surface tension and viscosity were chosen and classified into six different groups. Validation was done by comparing the data compiled in this study with the existing correlations as well as by dimensionless analysis. Results show that the existing drop weight analyses are valid for most liquid groups provided that the surface tension number ( N γ ) exceeds 10 −1 or the Ohnersorge number ( Oh) is below 10 2. This, however, confirms the influence of liquid properties on the validity of the analyses. The LCP coefficient method was developed to eliminate this problem by using the drop weight results generated from multiple tips of different sizes. The surface tension could then be calculated by using a semi-empirical linear correlation. Error analysis shows that the LCP coefficient method gives the best reliability and accuracy among various drop weight analyses.

Magnetic field enhanced nano-tip fabrication for four-probe STM studies

We present an improved method for fabricating tungsten STM tips for measuring nanoscaleobjects using a multiprobe STM. The tips were prepared by a ‘drop-off’ techniquecombined with a magnetic field. For magnetic field strengths above 150 Oe the electrolytewas seen to rotate about the tungsten anode. This rotation reduced the adhesion of bubblesand led to a reduction in the cone angle of the tips produced. This property isadvantageous for multiprobe measurements as it allows one to bring multiple tips into closeproximity.

The effect of aging on crack-growth resistance and toughening mechanisms in human dentin

Crack-growth experiments in human dentin have been performed in situ in an environmental scanning electron microscope to measure, for the first time, the crack-growth resistance curve ( R-curve) for clinically relevant (<250 μm) crack extensions and to simultaneously identify the salient toughening mechanisms. “Young” dentin from donors 19–30 years in age and “aged” dentin from donors 40–70 years in age were evaluated. The “young” group had 0–4% of its tubules filled with apatite; the “aged” group was subdivided into “opaque” with 12–32% filled tubules and “transparent” with 65–100% filled tubules. Although crack-initiation toughnesses were similar, the crack-growth resistance of “young” dentin was higher by about 40% compared to “aged” dentin. Mechanistically, this behavior is interpreted in terms of three phenomena: (i) gross crack deflection of the growing crack, (ii) microcracks which initiated at unfilled tubules in the high stress region in the vicinity of a propagating crack (no microcracks formed at filled tubules), and (iii) crack propagation which followed a local trajectory through unfilled tubules and deflected around filled tubules. The higher toughness of the “young” dentin was related to enhanced microcracking (at unfilled tubules) ahead of the growing crack, which (i) shields the crack by activating multiple crack tips and by reducing the local stress intensity through crack deflection and (ii) leads to the formation of crack bridges from “uncracked ligaments” due to the incomplete coalescence of these microcracks with the main crack tip. With age, the role of these toughening mechanisms was diminished primarily to the lower fraction of unfilled, and hence microcracked, tubules.

Fabrication of magnetic force microscopy probes via localized electrochemical deposition of cobalt

Magnetic force microscopy probes were obtained via the solution phase electrochemical deposition of cobalt nanostructures at the probe apexes. Single tips were fabricated in an atomic force microscope fluid cell. Multiple tips were produced in a single batch with an alternating potential in an electrochemical cell. The probes achieve 50nm spatial resolution.

Cadherin Conformations Associated with Dimerization and Adhesion

To investigate conformations of C-cadherin associated with functional activity and physiological regulation, we generated monoclonal antibodies (mAbs) that bind differentially to monomeric or dimeric forms. These mAbs recognize conformational epitopes at multiple sites along the C-cadherin ectodomain aside from the well known Trp-2-mediated dimer interface in the N-terminal EC1 domain. Group 1 mAbs, which bind monomer better than dimer and the Trp-2-mutated protein (W2A) better than wild type, recognize epitopes in EC4 or EC5. Dimerization of the W2A mutant protein via a C-terminal immunoglobulin Fc domain restored the dimeric mAb-binding properties to EC4-5 and partial homophilic binding activity but did not restore full cell adhesion activity. Group 2 and Group 3 mAbs, which bind dimer better than monomer and wild type better than W2A, recognize epitopes in EC1 and the interface between EC1 and EC2, respectively. None of the mAbs could distinguish between different physiological states of C-cadherin at the cell surface of either Xenopus embryonic cells or Colo 205 cultured cells, demonstrating that changes in dimerization do not underlie regulation of adhesion activity. On the cell surface the EC3-EC5 domains are much less accessible to mAb binding than EC1-EC2, suggesting that they are masked by the state of cadherin organization or by other molecules. Thus, the EC2-EC5 domains either reflect, or are involved in, cadherin dimerization and organization at the cell surface.

Computational Analysis of Wake Vortices Generated by Notched Wings

The motivation behind the present investigations is the alleviation of vortex wake hazard. The present paper contains a 3-D computational analysis of the trailing wake vortices generated by notched (rectangular) wings, at a chord-based Reynolds number of 5.34 x 10 4 . Various wing configurations were tested to determine the quantitative and qualitative effects of wing geometries on vortex dynamics. Two sets of computations were conducted up to 80 chord lengths downstream from the tip region. The first set used the conventional and notched-wing geometries for correlation of experimental and computed results. The second set of simulations concentrated on the development and testing of new wing designs. Wing geometries combined multiple tip (triangular/ogee) forms with the original notch design. The nature of vortices was classified in terms of their location, strength, and trajectories. Flow data were analyzed within these categories, at different crossflow planes behind the trailing edge

The Diagnostic Effect of Various Needle Tip Positions in Selective Lumbar Nerve Blocks: An Analysis of 1202 Injections

Retrospective chart review. To determine the effect of various needle tip positions on immediate postinjection pain in selective lumbar nerve blocks. To our knowledge, no large study has examined the effect of various needle tip positions within or adjacent to the intervertebral foramen on immediate pain outcome. A total of 1774 cases of intervertebral lumbar nerve blocks performed by our radiology staff between April 7, 1997 and May 31, 2002, were reviewed. Of the 1774 cases, 1202 met the study criteria (i.e., a single-level injection in an adult). The position of the needle tip and degree of immediate postinjection pain relief were examined. The blocks resulted in an average pain reduction of 4.14 U, as graded on a 0-10 ordinate scale (95% confidence interval 3.98-4.30). The degree of pain reduction was not associated with the needle tip position. Our results suggest that precise needle tip position within or adjacent to the intervertebral foramen made no difference on immediate pain reduction. These findings allow the practitioner more freedom in performing an injection. This study does not evaluate the long-term effects of various needle tip positions in selective lumbar nerve blocks.