An (in)efficiency based measurement of economic resilience

The ecosystem and the economic subsystem are interlinked. In fact, it is the overconsumption of scarce resources or the overproduction of bad outputs at economic system level that causes a great part of the imbalances at the ecosystem level. Some imbalances do not originate at the economic system level, but are due to external factors. Given the possibility of external shocks, respecting static sustainability thresholds is not a guarantee for system sustainability. In a dynamic setting, the concept of resilience is therefore helpful. In this paper we show how this concept can complement the traditional efficiency approach to come to a sustainable value creating economic system.Resource /Energy Economics and Policy,

Initial spreading of low-viscosity drops on partially wetting surfaces

Liquid drops start spreading directly after brought into contact with a partial wetting substrate. Although this phenomenon involves a three-phase contact line, the spreading motion is very fast. We study the initial spreading dynamics of low-viscosity drops, using two complementary methods: Molecular Dynamics simulations and high-speed imaging. We access previously unexplored length- and time-scales, and provide a detailed picture on how the initial contact between the liquid drop and the solid is established. Both methods unambiguously point towards a spreading regime that is independent of wettability, with the contact radius growing as the square root of time

The Knysna Elephants: A Relict Population

Population estimate for the Knysna elephants in 1876 was 500-600. By 1910 there were 40 elephants, by 1920 18, in 1970 between 11 and 13, and in 1981 this number further declined to 3: a mature bull, a cow and a calf. Poaching and human encroachment into elephant habitat are believed to be the major contributing factors for this decimation

Bubble formation during the collision of a sessile drop with a meniscus

The impact of a sessile droplet with a moving meniscus, as encountered in processes such as dip-coating, generically leads to the entrapment of small air bubbles. Here we experimentally study this process of bubble formation by looking through the liquid using high-speed imaging. Our central finding is that the size of the entrapped bubble crucially depends on the location where coalescence between the drop and the moving meniscus is initiated: (i) at a finite height above the substrate, or (ii) exactly at the contact line. In the first case, we typically find bubble sizes of the order of a few microns, independent of the size and speed of the impacting drop. By contrast, the bubbles that are formed when coalescence starts at the contact line become increasingly large, as the size or the velocity of the impacting drop is increased. We show how these observations can be explained from a balance between the lubrication pressure in the air layer and the capillary pressure of the drop

On the smoothness of nonlinear system identification

We shed new light on the \textit{smoothness} of optimization problems arising in prediction error parameter estimation of linear and nonlinear systems. We show that for regions of the parameter space where the model is not contractive, the Lipschitz constant and $\beta$-smoothness of the objective function might blow up exponentially with the simulation length, making it hard to numerically find minima within those regions or, even, to escape from them. In addition to providing theoretical understanding of this problem, this paper also proposes the use of multiple shooting as a viable solution. The proposed method minimizes the error between a prediction model and the observed values. Rather than running the prediction model over the entire dataset, multiple shooting splits the data into smaller subsets and runs the prediction model over each subset, making the simulation length a design parameter and making it possible to solve problems that would be infeasible using a standard approach. The equivalence to the original problem is obtained by including constraints in the optimization. The new method is illustrated by estimating the parameters of nonlinear systems with chaotic or unstable behavior, as well as neural networks. We also present a comparative analysis of the proposed method with multi-step-ahead prediction error minimization

Two candidate brown dwarf companions around core helium-burning stars

Hot subdwarf stars of spectral type B (sdBs) are evolved, core helium-burning objects. The formation of those objects is puzzling, because the progenitor star has to lose almost its entire hydrogen envelope in the red-giant phase. Binary interactions have been invoked, but single sdBs exist as well. We report the discovery of two close hot subdwarf binaries with small radial velocity amplitudes. Follow-up photometry revealed reflection effects originating from cool irradiated companions, but no eclipses. The lower mass limits for the companions of CPD-64$^{\circ}$481 ($0.048\,M_{\rm \odot}$) and PHL\,457 ($0.027\,M_{\rm \odot}$) are significantly below the stellar mass limit. Hence they could be brown dwarfs unless the inclination is unfavourable. Two very similar systems have already been reported. The probability that none of them is a brown dwarf is very small, 0.02%. Hence we provide further evidence that substellar companions with masses that low are able to eject a common envelope and form an sdB star. Furthermore, we find that the properties of the observed sample of hot subdwarfs in reflection effect binaries is consistent with a scenario where single sdBs can still be formed via common envelope events, but their low-mass substellar companions do not survive.Comment: accepted to A&