A Dynamic Model for Concentric Tube Robots Till, John; Aloi, Vincent; Riojas, Katherine E. ...
IEEE transactions on robotics,
12/2020, Letnik:
36, Številka:
6
Journal Article
Recenzirano
Odprti dostop
Existing static and kinematic models of concentric tube robots are based on the ordinary differential equations of a static Cosserat rod. In this article, we provide the first dynamic model for ...concentric tube continuum robots by adapting the partial differential equations of a dynamic Cosserat rod to describe the coupled inertial dynamics of precurved concentric tubes. This generates an initial-boundary-value problem that can capture robot vibrations over time. We solve this model numerically at high time resolutions using implicit finite differences in time and arc length. This approach is capable of resolving the high-frequency torsional dynamics that occur during unstable "snapping" motions and provides a simulation tool that can track the true robot configuration through such transitions. Furthermore, it can track slower oscillations associated with bending and torsion as a robot interacts with tissue at real-time speeds. Experimental verification of the model shows that this wide range of effects is captured efficiently and accurately.
Computational interfacial rheology Jaensson, Nick O.; Anderson, Patrick D.; Vermant, Jan
Journal of non-Newtonian fluid mechanics,
April 2021, 2021-04-00, 20210401, Letnik:
290
Journal Article
Recenzirano
Odprti dostop
Fluid–fluid interfaces, laden with polymers, particles or other surface-active moieties, often show a rheologically complex response to deformations, in particular when strong lateral interactions ...are present between these moieties. The response of the interface can then no longer be described by an isotropic surface tension alone. These “structured” soft-matter interfaces are found in many industrial applications, ranging from foods, cosmetics and pharmaceuticals, to oil recovery. Also many biomedical applications involve such interfaces, including those involving lung surfactants and biofilms. In order to understand, design and optimize processes in which structured interfaces are present, flow predictions of how such multiphase systems deform are of the utmost importance, which is the goal of “computational interfacial rheology”, the main topic of this review. We start by rigorously establishing the stress boundary condition used in the computation of multi-phase flows, and show how this changes when the interface is rheologically complex. Then, constitutive models for the extra stress in interfaces, ranging from 2D generalized Newtonian to hyperelastic and viscoelastic, are reviewed extensively, including common pitfalls when applying these models. This is followed by an overview of different approaches to measure interfacial rheological properties, and a discussion of advanced numerical implementations for deforming interfaces. We conclude with an outlook for this relatively young and exciting field.
•Review of the advances in computational interfacial rheology.•Derivation of the stress boundary condition in the presence of structured interfaces.•Extensive treatment of constitutive models for structured interfaces.•Overview of numerical methods used in interfacial rheometry.•Overview of advanced numerical implementations for deforming structured interfaces.
•Forest dynamics between 1985 and 2012 were modeled using vegetation indices.•Normalized Difference Moisture Index was the best indicator of forest conditions.•Linear trend analysis used to identify ...areas of forest vulnerable to drought.•25% of area had significant negative trend, compared to only 10% with positive trend.•Plots with negative trends had lower live tree density and more standing dead trees.
Drought has long been recognized as a driving mechanism in the forests of western North America and drought-induced mortality has been documented across genera in recent years. Given the frequency of these events are expected to increase in the future, understanding patterns of mortality and plant response to severe drought is important to resource managers. Drought can affect the functional, physiological, structural, and demographic properties of forest ecosystems. Remote sensing studies have documented changes in forest properties due to direct and indirect effects of drought; however, few studies have addressed this at local scales needed to characterize highly heterogeneous ecosystems in the forest-shrubland ecotone. We analyzed a 22-year Landsat time series (1985–2012) to determine changes in forest in an area that experienced a relatively dry decade punctuated by two years of extreme drought. We assessed the relationship between several vegetation indices and field measured characteristics (e.g. plant area index and canopy gap fraction) and applied these indices to trend analysis to uncover the location, direction and timing of change. Finally, we assessed the interaction of climate and topography by forest functional type. The Normalized Difference Moisture Index (NDMI), a measure of canopy water content, had the strongest correlation with short-term field measures of plant area index (R2=0.64) and canopy gap fraction (R2=0.65). Over the entire time period, 25% of the forested area experienced a significant (p-value<0.05) negative trend in NDMI, compared to less than 10% in a positive trend. Coniferous forests were more likely to be associated with a negative NDMI trend than deciduous forest. Forests on southern aspects were least likely to exhibit a negative trend while north aspects were most prevalent. Field plots with a negative trend had a lower live density, and higher amounts of standing dead and down trees compared to plots with no trend. Our analysis identifies spatially explicit patterns of long-term trends anchored with ground based evidence to highlight areas of forest that are resistant, persistent or vulnerable to severe drought. The results provide a long-term perspective for the resource management of this area and can be applied to similar ecosystems throughout western North America.
WikiLeaks is among the most controversial institutions of the last decade, and this essay contributes to an understanding of WikiLeaks by revealing the philosophical paradigm at the foundation of ...Julian Assange’s worldview: cypherpunk ethics. The cypherpunk movement emerged in the early-1990s, advocating the widespread use of strong cryptography as the best means for defending individual privacy and resisting authoritarian governments in the digital age. For the cypherpunks, censorship and surveillance were the twin evils of the computer age, but they viewed encryption as a means to circumvent both. As a cypherpunk, Assange advocates for the use of cryptography in the fight for individual privacy as well as the fight for global justice. His cosmopolitan disposition is informed by his hacker background, antiwar principles, and Enlightenment outlook. This essay places Assange’s philosophical idea in historical context, exploring his views on censorship, surveillance, and the right to communicate. It also connects his cypherpunk principles to WikiLeaks, showing that the strategy of encouraging data leaks from powerful political and economic organizations is classic cypherpunk political praxis.
For three decades, the cypherpunk movement has fought for the right of individuals and publics to use digital cryptography—or crypto—to defend their individual privacy and promote institutional ...transparency and accountability. The cypherpunks have also fought for institutional transparency through various strategies of sousveillance. Yet the movement’s contribution to theories of surveillance and the praxis of resistance have been largely overlooked by scholars. This essay bridges the gap between the movement and the academy by outlining the normative and epistemological aspects of cypherpunk philosophy. Cypherpunk ethics is captured by the normative cypherpunk slogan “privacy for the weak, transparency for the powerful.” Cypherpunk epistemology is a form of data activism that calls for a hands-on response to the datafication of surveillance and relies upon both pro-active (transparency) and re-active (privacy) strategies. While the cypherpunks are famously concerned about privacy, because cypherpunk philosophy also calls for “transparency for the powerful,” cypherpunks have practiced a distinctive form of cypherpunk sousveillance. By understanding that cypherpunk theory and practice each consist of two complementary dynamics—privacy/transparency, pro-active/re-active—it becomes possible to understand that the cypherpunk movement provides the basis for activists and citizens to resist large surveillance institutions like the National Security Agency and Google by altering information fluxes at the systemic level. The cypherpunk movement provides an intelligible, viable, and effective model of data activism and strategic agency, and this essay contributes to the pluralistic, multidisciplinary understanding of resistance to surveillance and practice of sousveillance by outlining the basic normative, epistemic, and pragmatic aspects of cypherpunk theory and practice.
For suspensions with electromagnetic particles exposed to an external field, we examine the effect of the solidification of the suspending medium on the formation of particle structures, ...representative of the curing of a photo-reactive resin during stereolithography. To that end, Brownian Dynamics (BD) simulations are examined in which the solidification of the suspending medium can be incorporated by increasing its viscosity in the course of time. For illustrative purposes, it is assumed that the viscosity function is known apriori in explicit and parametrized form, however, experimental data can be used as well. It is demonstrated that one can study the effects of the viscosity increase by a transformation of time, akin to the time–temperature superposition principle, but here also in the presence of thermal noise on the suspended particles. Therefore, instead of performing BD simulations with a continuously increasing viscosity (computationally inefficient), we advocate performing simulations at a constant (low) viscosity and subsequently transform time (nonlinearly) for re-interpretation of the simulation results. So doing, one can predict the formation of particle structures during on-going solidification of the suspending medium. In practice, the viscosity increase is so drastic that further evolution of the particle structure can be considered as arrested after the characteristic transition-time of the viscosity is reached. Semi-quantitative rules of thumb are formulated for the 3D-printing practitioner.
Graphical Abstract
Government whistleblowers are those who disclose classified government documents in violation of the law but do so to bring to light serious government wrongdoing. Scholarly debates have identified ...various procedural requirements for whistleblowing, and this paper expands upon these insights by providing an account of Edward Snowden's moderate theory and Julian Assange's radical theory of government whistleblowing ethics. Through the practice of ethical listening, this essay places Snowden and Assange into conversation with academic theories of government whistleblowing. By including the previously neglected voices of real-world government whistleblowers and their publishers, this paper provides a more dynamic understanding of whistleblowing's procedural requirements, interrogates the ethical paradigms of whistleblowers, and shows that different theories of government whistleblowing depend upon a wide range of assumptions about audience, professionalism, and ultimate aims.
To predict double-emulsion formation in a capillary microfluidic device, a ternary diffuse-interface model is presented. The formation of double emulsions involves complex interfacial phenomena of a ...three-phase fluid system, where each component can have different physical properties. We use the Navier-Stokes/Cahn-Hilliard model for a general ternary system, where the hydrodynamics is coupled with the thermodynamics of the phase field variables. Our model predicts important features of the double-emulsion formation which was observed experimentally by Utada et al. Utada et al., Science, 2005, 308, 537. In particular, our model predicts both the dripping and jetting regimes as well as the transition between those two regimes by changing the flow rate conditions. We also demonstrate that a double emulsion having multiple inner drops can be formed when the outer interface is more stable than the inner interface.
Activating transcription factor 3 (ATF3) belongs to the ATF/cyclic AMP responsive element binding family of transcription factors and is often described as an adaptive response gene whose activity is ...usually regulated by stressful stimuli. Although expressed in a number of splice variants and generally recognized as a transcriptional repressor, ATF3 has the ability to interact with a number of other transcription factors including c-Jun to form complexes which not only repress, but can also activate various genes. ATF3 expression is modulated mainly at the transcriptional level and has markedly different effects in different types of cell. The levels of ATF3 mRNA and protein are normally very low in neurons and glia but their expression is rapidly upregulated in response to injury. ATF3 expression in neurons is closely linked to their survival and the regeneration of their axons following axotomy, and that in peripheral nerves correlates with the generation of a Schwann cell phenotype that is conducive to axonal regeneration. ATF3 is also induced by Toll-like receptor (TLR) ligands but acts as a negative regulator of TLR signaling, suppressing the innate immune response which is involved in immuno-surveillance and can enhance or reduce the survival of injured neurons and promote the regeneration of their axons.
PDF
