Many critical policy decisions, from strategic investments to the allocation of humanitarian aid, rely on data about the geographic distribution of wealth and poverty. Yet many poverty maps are out of date or exist only at very coarse levels of granularity. Here we develop microestimates of the relative wealth and poverty of the populated surface of all 135 low- and middle-income countries (LMICs) at 2.4 km resolution. The estimates are built by applying machine-learning algorithms to vast and heterogeneous data from satellites, mobile phone networks, and topographic maps, as well as aggregated and deidentified connectivity data from Facebook. We train and calibrate the estimates using nationally representative household survey data from 56 LMICs and then validate their accuracy using four independent sources of household survey data from 18 countries. We also provide confidence intervals for each microestimate to facilitate responsible downstream use. These estimates are provided free for public use in the hope that they enable targeted policy response to the COVID-19 pandemic, provide the foundation for insights into the causes and consequences of economic development and growth, and promote responsible policymaking in support of sustainable development.
A microscale shear cell is used to study the formation of parabolic focal conic defects in the thermotropic smectic-A liquid crystal 8CB (4-octyl-4'-cyanobiphenyl). Defects are produced by four distinct methods: by the application of dilatational strain alone, by shear flow alone, by dilatational strain and subsequent shear flow, and by the simultaneous application of dilatational strain and shear flow. We confirm that defects originate within the bulk, consistent with the previously suggested undulation instability mechanism. In the presence of a shear flow, we observe that defect formation requires micrometer-level dilatations, whose magnitude depends on the sample thickness. The size and ordering of both disordered and ordered defect arrays is quantified using a pair distribution function. Deviations from the predictions of linear stability theory are observed that have not been reported previously. For example, defects form a square array with greater ordering in the principal flow direction. Ordering due to shear flow does not change the average defect size. It has been shown previously that the principal defect sizes of ordered defects scale differently with sample thickness than the wavelength of the small amplitude undulations. We find that disordered defects show a similar deviation from this predicted wavelength.
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/mame.201700256.
Azo PolymerThe accumulation of photoinduced deformation in azobenzene functionalized polymers has received a significant amount of attention in recent years. Critically, the induced photomechanical deformation in these systems experiences varying degrees of relaxation. Control over the persistence of photomechanical strains is vital to the broader utility of these materials in shape programmable systems including soft robotics and engineered origami. Furthermore, investigations of relaxation in light responsive polymer systems triggered by UV light are more prominent than those triggered by blue-green light. In this study, the impact of chain mobility and initially induced photostrain on the relaxation dynamics of azobenzene-functionalized polyimides after irradiation with blue light is examined. A modeling effort coupling chromophore population dynamics to material strain is carried out to further explore the relationship between material structure, relaxation dynamics, and macroscopic deformation. The implications for controlling strain persistence are highlighted by simulating one example of a photoprimed bistable actuator. 3 of 11) www.advancedsciencenews.com www.mme-journal.de
We examine the buckling of a thin elastic film floating on a viscous liquid layer which is itself supported on a prestretched rubber sheet. Releasing the prestretch in the rubber induces a viscous stress in the liquid, which in turn induces a compressive stress in the elastic film, leading to buckling. Unlike many previous studies on wrinkling of floating films, the buckling process in the present study is dominated by viscous effects whereas gravitational effects are negligible. An approximate shear lag model predicts the evolution of the stress profile in the unbuckled film that depends on three parameters: the rate at which the prestretch is released, the thickness of the liquid layer, and the length of the elastic film. A linear perturbation analysis is developed to predict the wavelength of wrinkles. Numerical simulations are conducted to predict nonlinear evolution of the wrinkle wavelength and amplitude. Experiments using elastic polymer films and viscous polymer liquids show trends that are qualitatively consistent with the predictions although quantitatively, the experimentally-observed wrinkle wavelengths are longer than predicted. Although this article is focused only on small-strain wrinkling behavior, we show that application of large nominal strains (on the order of 100%) leads to sharply localized folds. Thus this approach may be useful for developing buckled features with high aspect ratio on surfaces.
Directly transducing light into work is attractive for remotely powering soft mechanisms fabricated from photoresponsive polymers, but presents challenges for achieving reliable actuation within a control framework. Here, we utilize azobenzene-functionalized polyimides to fabricate actuators characterized by mechanically discrete states. Irradiation initiates the photochemically induced, quasistatic deformation to advance the actuator to the edge of instability. Following this latency, ultrafast snap through actuation (~10ms-scale) ensues. Restricting the role of control to the attainment of the edge of instability, strategies for achieving repetitive actuation via multiplexed irradiation are demonstrated. Approaches are examined for modulating the latency of the actuator using an all-optical strategy as well as mechanical design of the actuator. Prototypical assemblies of these actuators in arrays are used to fabricate morphable surfaces and structures, which is aided by the realization that the ultrafast actuation is characterized by a high power-density on the order of ~kW/m 3 .
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