Conformal Dynamics of Precursors to Fracture

An exact integro-differential equation for the conformal map from the unit circle to the boundary of an evolving cavity in a stressed 2-dimensional solid is derived. This equation provides an accurate description of the dynamics of precursors to fracture when surface diffusion is important. The solution predicts the creation of sharp grooves that eventually lead to material failure via rapid fracture. Solutions of the new equation are demonstrated for the dynamics of an elliptical cavity and the stability of a circular cavity under biaxial stress, including the effects of surface stress.Comment: 4 pages, 3 figure

Nonlinear evolution of surface morphology in InAs/AlAs superlattices via surface diffusion

Continuum simulations of self-organized lateral compositional modulation growth in InAs/AlAs short-period superlattices on InP substrate are presented. Results of the simulations correspond quantitatively to the results of synchrotron x-ray diffraction experiments. The time evolution of the compositional modulation during epitaxial growth can be explained only including a nonlinear dependence of the elastic energy of the growing epitaxial layer on its thickness. From the fit of the experimental data to the growth simulations we have determined the parameters of this nonlinear dependence. It was found that the modulation amplitude don't depend on the values of the surface diffusion constants of particular elements.Comment: 4 pages, 3 figures, published in Phys. Rev. Lett. http://link.aps.org/abstract/PRL/v96/e13610

The role played by the molecular weight and acetylation degree in modualting the stiffness and elasticity of chitosan gels

A broad library of chitosans was produced varying the molecular weight and the fraction of acetylated units, FA. The produced chitosans were used for the formation of wall-to-wall cylindrical gels through a controlled external gelation using tripolyphosphate (TPP) as cross-linker. The resulting gels were analyzed by rheometry. Viscosity average degree of polymerization (DPv\uaf)\u202f>\u202f152 was shown to be required for the formation of stable gels. Both gel stiffness and gel rupture strength were proportional to the molecular weight regardless of the applied deformation. Increasing acetylation produced a marked reduction of the shear modulus, but, in parallel, switched the networks from rigid and brittle to weak and elastic. Intriguingly, gels made of chitosan with FA\u202f=\u202f0.37 displayed notable elasticity, i.e. up to 90% of applied strain falls into linear regime. These findings suggest that the frequency of glucosamine (D unit) and N-acetyl-glucosamine (A unit) contribute to a subtle structure-property relationship in chitosan-TPP gels

Connections, Symbols, and the Meaning of Intelligence

More recently, debates in AI have focused on the implications of Connectionism. Connectionism is the hypothesis that distributed computations are capable of instantiating intelligent functions without relying on the representational character of symbols, but rather on the computational states themselves which are cal1~ distributed representations (Haugeland, 1991). This distinction puts connectionism at odds with symbolic theory. The current debates tend to be over which theory will yield intelligent systems--symbolic or connectionist? But as we will soon see, this really amounts to a debate over which representational scheme is required for general intelligence

Phase Field Modeling of Fast Crack Propagation

We present a continuum theory which predicts the steady state propagation of cracks. The theory overcomes the usual problem of a finite time cusp singularity of the Grinfeld instability by the inclusion of elastodynamic effects which restore selection of the steady state tip radius and velocity. We developed a phase field model for elastically induced phase transitions; in the limit of small or vanishing elastic coefficients in the new phase, fracture can be studied. The simulations confirm analytical predictions for fast crack propagation.Comment: 5 pages, 11 figure

Robot Rights? Let's Talk about Human Welfare Instead

The 'robot rights' debate, and its related question of 'robot responsibility', invokes some of the most polarized positions in AI ethics. While some advocate for granting robots rights on a par with human beings, others, in a stark opposition argue that robots are not deserving of rights but are objects that should be our slaves. Grounded in post-Cartesian philosophical foundations, we argue not just to deny robots 'rights', but to deny that robots, as artifacts emerging out of and mediating human being, are the kinds of things that could be granted rights in the first place. Once we see robots as mediators of human being, we can understand how the `robots rights' debate is focused on first world problems, at the expense of urgent ethical concerns, such as machine bias, machine elicited human labour exploitation, and erosion of privacy all impacting society's least privileged individuals. We conclude that, if human being is our starting point and human welfare is the primary concern, the negative impacts emerging from machinic systems, as well as the lack of taking responsibility by people designing, selling and deploying such machines, remains the most pressing ethical discussion in AI.Comment: Accepted to the AIES 2020 conference in New York, February 2020. The final version of this paper will appear in Proceedings of the 2020 AAAI/ACM Conference on AI, Ethics, and Societ

Influence of uniaxial stress on the lamellar spacing of eutectics

Directional solidification of lamellar eutectic structures submitted to uniaxial stress is investigated. In the spirit of an approximation first used by Jackson and Hunt, we calculate the stress tensor for a two-dimensional crystal with triangular surface, using a Fourier expansion of the Airy function. crystal with triangular surface in contact with its melt, given that a uniaxial external stress is applied. The effect of the resulting change in chemical potential is introduced into the standard model for directional solidification of a lamellar eutectic. This calculation is motivated by an observation, made recently [I. Cantat, K. Kassner, C. Misbah, and H. M\"uller-Krumbhaar, Phys. Rev. E, in press] that the thermal gradient produces similar effects as a strong gravitational field in the case of dilute-alloy solidification. Therefore, the coupling between the Grinfeld and the Mullins-Sekerka instabilities becomes strong, as the critical wavelength of the former instability gets reduced to a value close to that of the latter. Analogously, in the case of eutectics, the characteristic length scale of the Grinfeld instability should be reduced to a size not extremely far from typical lamellar spacings. In a Jackson-Hunt like approach we average the undercooling, including the stress term, over a pair of lamellae. Following Jackson and Hunt, we assume the selected wavelength to be determined by the minimum undercooling criterion and compute its shift due to the external stress. we realize the shifting of the wavelength by the application of external stress. In addition, we find that in general the volume fraction of the two solid phases is changed by uniaxial stress. Implications for experiments on eutectics are discussed.Comment: 8 pages RevTex, 6 included ps-figures, accepted for Phys. Rev.

Guar gum/borax hydrogel: Rheological, low field NMR and release characterizations

Guar gum (GG) and Guar gum/borax (GGb) hydrogels are studied by means of rheology, Low Field Nuclear Magnetic Resonance (LF NMR) and model drug release tests. These three approaches are used to estimate the mesh size (ζ) of the polymeric network. A comparison with similar Scleroglucan systems is carried out. In the case of GGb, the rheological and Low Field NMR estimations of ζ lead to comparable results, while the drug release approach seems to underestimate ζ. Such discrepancy is attributed to the viscous effect of some polymeric chains that, although bound to the network to one end, can freely fluctuate among meshes. The viscous drag exerted by these chains slows down drug diffusion through the polymeric network. A proof for this hypothesis is given by the case of Scleroglucan gel, where the viscous contribution is not so significant and a good agreement between the rheological and release test approaches was found

Cell Activities on Viscoelastic Substrates Show an Elastic Energy Threshold and Correlate with the Linear Elastic Energy Loss in the Strain-Softening Region

Energy-sensing in viscoelastic substrates has recently been shown to be an important regulator of cellular activities, modulating mechanical transmission and transduction processes. Here, this study fine-tunes the elastic energy of viscoelastic hydrogels with different physical and chemical compositions and shows that this has an impact on cell response in 2D cell cultures. This study shows that there is a threshold value for elastic energy (≈0.15 J m−3) above which cell adhesion is impaired. When hydrogels leave the linear stress–strain range, they show softening (plastic) behavior typical of soft tissues. This study identifies a correlation between the theoretical linear elastic energy loss in the strain-softening region and the number of cells adhering to the substrate. This also has implications for the formation of vinculin-rich anchorage points and the ability of cells to remodel the substrate through traction forces. Overall, the results reported in this study support that the relationship between cell activities and energy-sensing in viscoelastic substrates is an important aspect to consider in the development of reliable ex vivo models of human tissues that mimic both normal and pathological conditions

Early Neuromuscular Blockade in Moderate-to-Severe Pediatric Acute Respiratory Distress Syndrome

Objectives: The use of neuromuscular blocking agents (NMBAs) in pediatric acute respiratory distress syndrome (PARDS) is common but unsupported by efficacy data. We sought to compare the outcomes between patients with moderate-to-severe PARDS receiving continuous NMBA during the first 48 hours of endotracheal intubation (early NMBA) and those without. Design: Secondary analysis of data from the Randomized Evaluation of Sedation Titration for Respiratory Failure (RESTORE) clinical trial, a pediatric multicenter cluster randomized trial of sedation. Setting: Thirty-one PICUs in the United States. Patients: Children 2 weeks to 17 years receiving invasive mechanical ventilation (MV) for moderate-to-severe PARDS (i.e., oxygenation index >= 8 and bilateral infiltrates on chest radiograph on days 0-1 of endotracheal intubation). Interventions: NMBA for the entire duration of days 1 and 2 after intubation. Measurements and Main Results: Among 1,182 RESTORE patients with moderate-to-severe PARDS, 196 (17%) received early NMBA for a median of 50.0% ventilator days (interquartile range, 33.3-60.7%). The propensity score model predicting the probability of receiving early NMBA included high-frequency oscillatory ventilation on days 0-2 (odds ratio [OR], 7.61; 95% CI, 4.75-12.21) and severe PARDS on days 0-1 (OR, 2.16; 95% CI, 1.50-3.12). After adjusting for risk category, early use of NMBA was associated with a longer duration of MV (hazard ratio, 0.57; 95% CI, 0.48-0.68; p < 0.0001), but not with mortality (OR, 1.62; 95% CI, 0.92-2.85; p = 0.096) compared with no early use of NMBA. Other outcomes including cognitive, functional, and physical impairment at 6 months post-PICU discharge were similar. Outcomes did not differ when comparing high versus low NMBA usage sites or when patients were stratified by baseline PaO2/FIO2 less than 150. Conclusions: Early NMBA use was associated with a longer duration of MV. This propensity score analysis underscores the need for a randomized controlled trial in pediatrics