Quantifying Human Engagement into Playful Activities.

Engaging in playful activities, such as playing a musical instrument, learning a language, or performing sports, is a fundamental aspect of human life. We present a quantitative empirical analysis of the engagement dynamics into playful activities. We do so by analyzing the behavior of millions of players of casual video games and discover a scaling law governing the engagement dynamics. This power-law behavior is indicative of a multiplicative (i.e., "happy- get-happier") mechanism of engagement characterized by a set of critical exponents. We also find, depending on the critical exponents, that there is a phase transition between the standard case where all individuals eventually quit the activity and another phase where a finite fraction of individuals never abandon the activity. The behavior that we have uncovered in this work might not be restricted only to human interaction with videogames. Instead, we believe it reflects a more general and profound behavior of how humans become engaged in challenging activities with intrinsic rewards.

Optimization of crystal nucleation close to a metastable fluid-fluid phase transition.

The presence of a metastable fluid-fluid critical point is thought to dramatically influence the crystallization pathway, increasing the nucleation rate by many orders of magnitude over the predictions of classical nucleation theory. We use molecular dynamics simulations to study the kinetics of crystallization in the vicinity of this metastable critical point and throughout the metastable fluid-fluid phase diagram. To quantitatively understand how the fluid-fluid phase separation affects the crystal nucleation, we evaluate accurately the kinetics and reconstruct the thermodynamic free-energy landscape of crystal formation. Contrary to expectations, we find no special advantage of the proximity of the metastable critical point on the crystallization rates. However, we find that the ultrafast formation of a dense liquid phase causes the crystallization to accelerate both near the metastable critical point and almost everywhere below the fluid-fluid spinodal line. These results unveil three different scenarios for crystallization that could guide the optimization of the process in experiments.

Persistence of iatrogenic atrial septal defect after interventional mitral valve repair with the MitraClip system: a note of caution.

OBJECTIVES: The purpose of this study was to investigate the persistence rates of iatrogenic atrial septal defect (iASD) after interventional edge-to-edge repair with serial transesophageal echocardiography examinations and close clinical follow-up (FU). BACKGROUND: Transcatheter mitral valve repair (TMVR) with the MitraClip system (Abbott Vascular, Abbott Park, Illinois) is a therapeutic alternative to surgery in selected high-risk patients. Clip placement requires interatrial transseptal puncture and meticulous manipulation of the steerable sheath. The persistence of iASD after MitraClip procedures and its clinical relevance is unknown. METHODS: A total of 66 patients (76.7% male, mean age 77.1 ± 7.9 years) with symptomatic mitral regurgitation (MR) at prohibitive surgical risk (EuroSCORE II 10.1 ± 6.1%) underwent MitraClip procedures and completed 6 months of FU. RESULTS: Transesophageal echocardiography after FU showed persistent iASD in 50% of cases. Patients with iASD did not significantly differ from patients without ASD concerning baseline characteristics, New York Heart Association functional class, severity of MR, and acute procedural success rates (p > 0.05). When comparing procedural details and hemodynamic measures between groups, MitraClip procedures took longer in patients without iASD (82.4 ± 39.7 min vs. 68.9 ± 45.5 min; p = 0.05), and echocardiography after FU showed less decrease of systolic pulmonary artery pressures in the iASD group (-1.6 ± 14.1 mm Hg vs. 9.3 ± 17.4 mm Hg; p = 0.02). Clinically, patients with iASD presented more often with New York Heart Association functional classes >II after FU (57% vs. 30%; p = 0.04), showed higher levels of N-terminal pro-brain natriuretic peptide (6,667.3 ± 7,363.9 ng/dl vs. 4,835.9 ± 6,681.7 ng/dl; p = 0.05), and had less improvement in 6-min walking distances (20.8 ± 107.4 m vs. 114.6 ± 116.4 m; p = 0.001). Patients with iASD showed higher death rates during 6 months (16.6% vs. 3.3%; p = 0.05). Cox regression analysis found that only persistence of iASD (p = 0.04) was associated with 6-month survival. CONCLUSIONS: The persistence rate of 50% iASD after MitraClip procedures is considerably high. Persistent interatrial shunting was associated with worse clinical outcomes and increased mortality. Further studies are warranted to investigate if persistent interatrial shunting is the mediator or marker of advanced disease in these patients.

Nucleation of ethanol, propanol, butanol, and pentanol: a systematic experimental study along the homologous series.

We present homogeneous vapor-liquid nucleation rates of the 1-alcohols (C(n)H(2n+1)OH, n = 2-4) measured in the well-established two-valve nucleation pulse chamber as well as in a novel one-piston nucleation pulse chamber at temperatures between 235 and 265 K. The nucleation rates and critical cluster sizes show a very systematic behavior with respect to the hydrocarbon chain length of the alcohol, just as their thermo-physical parameters such as surface tension, vapor pressure, and density would suggest. For all alcohols, except ethanol, predictions of classical nucleation theory lie several orders of magnitude below the experimental results and show a strong temperature-dependence typically found in nucleation experiments. The more recent Reguera-Reiss theory [J. Phys. Chem. B 108(51), 19831 (2004)] achieves reasonably good predictions for 1-propanol, 1-butanol, and 1-pentanol, and independent of the temperature. Ethanol, however, clearly shows the influence of strong association between molecules even in the vapor phase. We also scaled all experimental results with classic nucleation theory to compare our data with other data from the literature. We find the same overall temperature trend for all measurement series together but inverted and inconsistent temperature trends for individual 1-propanol and 1-butanol measurements in other devices. Overall, our data establishe a comprehensive and reliable data set that forms an ideal basis for comparison with nucleation theory.

Crossover from nucleation to spinodal decomposition in a condensing vapor.

The mechanism controlling the initial step of a phase transition has a tremendous influence on the emerging phase. We study the crossover from a purely nucleation-controlled transition toward spinodal decomposition in a condensing Lennard-Jones vapor using molecular dynamics simulations. We analyze both the kinetics and at the same time the thermodynamics by directly reconstructing the free energy of cluster formation. We estimate the location of the spinodal, which lies at much deeper supersaturations than expected. Moreover, the nucleation barriers we find differ only by a constant from the classical nucleation theory predictions and are in very good agreement with semiempirical scaling relations. In the regime from very small barriers to the spinodal, growth controls the rate of the transition but not its nature because the activation barrier has not yet vanished. Finally, we discuss in detail the influence of the chosen reaction coordinate on the interpretation of such simulation results.

Homogeneous nucleation of nitrogen.

We investigated the homogeneous nucleation of nitrogen in a cryogenic expansion chamber [A. Fladerer and R. Strey, J. Chem. Phys. 124, 164710 (2006)]. Gas mixtures of nitrogen and helium as carrier gas were adiabatically expanded and cooled down from an initial temperature of 83 K until nucleation occurred. This onset was detected by constant angle light scattering at nitrogen vapor pressures of 1.3-14.2 kPa and temperatures of 42-54 K. An analytical fit function well describes the experimental onset pressures with an error of +/-15%. We estimate the size of the critical nucleus with the Gibbs-Thomson equation yielding critical sizes of about 50 molecules at the lowest and 70 molecules at the highest temperature. In addition, we estimate the nucleation rate and compare it with nucleation theories. The predictions of classical nucleation theory (CNT) are 9 to 19 orders of magnitude below the experimental results and show a stronger temperature dependence. The Reguera-Reiss theory [Phys. Rev. Lett. 93, 165701 (2004)] predicts the correct temperature dependence at low temperatures and decreases the absolute deviation to 7-13 orders of magnitude. We present an empirical correction function to CNT describing our experimental results. These correction parameters are remarkably close to the ones of argon [Iland et al., J. Chem. Phys. 127, 154506 (2007)] and even those of water [J. Wolk and R. Strey, J. Phys. Chem. B 105, 11683 (2001)].

Evaluating nucleation rates in direct simulations.

We compare different methods for obtaining nucleation rates from molecular dynamics simulations of nucleation, using the condensation of Lennard-Jones argon as an example. All methods yield the same nucleation rate at the conditions where they can be applied correctly, with discrepancies smaller than a factor of 2. We critically examine the different approaches and highlight their respective strengths and possible limitations.

Unraveling the "pressure effect" in nucleation.

The influence of the pressure of a chemically inert carrier gas on the nucleation rate is one of the biggest puzzles in the research of gas-liquid nucleation. Experiments can show a positive effect, a negative effect, or no effect at all. The same experiment may show both trends for the same substance depending on temperature, or for different substances at the same temperature. We show how this ambiguous effect naturally arises from the competition of two contributions: nonisothermal effects and pressure-volume work. Our model clarifies seemingly contradictory experimental results and quantifies the variation of the nucleation ability of a substance in the presence of an ambient gas. Our findings are corroborated by molecular dynamics simulations and might have important implications since nucleation in experiments, technical applications, and nature practically always occurs in the presence of an ambient gas.

Kinetic reconstruction of the free-energy landscape.

We present a new general method to trace back a lot of valuable information from direct simulations and experiments of activated processes. In particular, it allows the reconstruction of the free-energy landscape for an arbitrary reaction coordinate directly from the out-of-equilibrium dynamics of the process. We demonstrate the power of this concept by its application to a molecular dynamics simulation of nucleation of a Lennard-Jones vapor. The same method can be also applied to Brownian dynamics and stochastic simulations.

Nucleation rate isotherms of argon from molecular dynamics simulations.

We report six nucleation rate isotherms of vapor-liquid nucleation of Lennard-Jones argon from molecular dynamics simulations. The isotherms span three orders of magnitude in nucleation rates, 10(23)

What is the best definition of a liquid cluster at the molecular scale?

We investigate the ability of different cluster definitions to serve as a good reaction coordinate in molecular simulations of nucleation. In particular, the most commonly used Stillinger criterion [J. Chem. Phys. 38, 1486 (1963)] is compared with the cluster definition introduced by ten Wolde and Frenkel [J. Chem. Phys. 109, 9901 (1998)]. The accuracy of these two different cluster definitions is tested by using molecular dynamics to study the vapor-liquid nucleation of Lennard-Jones argon as a model system. We are able to compare the size of the critical cluster identified by each cluster definition with a completely model-independent value provided by the nucleation theorem, aided by a recently introduced method that accurately extracts the location of the transition state directly from the kinetics. It is found that the Stillinger definition strongly overestimates the size of small molecular clusters by up to a factor of 2. A simple change of the Stillinger radius is unable to rectify this deficiency. On the contrary, the ten Wolde-Frenkel definition, while being only slightly more elaborate than a simple Stillinger criterion, is remarkably successful in identifying the correct molecular excess of the small clusters if the parameters are chosen adequately. The method described here can also be generalized to identify a proper reaction coordinate in other activated processes.

Influence of thermostats and carrier gas on simulations of nucleation.

We investigate the influence of carrier gas and thermostat on molecular dynamics (MD) simulations of nucleation. The task of keeping the temperature constant in MD simulations is not trivial and an inefficient thermalization may have a strong influence on the results. Different thermostating mechanisms have been proposed and used in the past. In particular, we analyze the efficiency of velocity rescaling, Nose-Hoover, and a carrier gas (mimicking the experimental situation) by extensive MD simulations. Since nucleation is highly sensitive to temperature, one would expect that small variations in temperature might lead to differences in nucleation rates of up to several orders of magnitude. Surprisingly, the results indicate that the choice of the thermostating method in a simulation does not have--at least in the case of Lennard-Jones argon--a very significant influence on the nucleation rate. These findings are interpreted in the context of the classical theory of Feder et al. [Adv. Phys. 15, 111 (1966)] by analyzing the temperature distribution of the nucleating clusters. We find that the distribution of cluster temperatures is non-Gaussian and that subcritically sized clusters are colder while postcritically sized clusters are warmer than the bath temperature. However, the average temperature of all clusters is found to be always higher than the bath temperature.

New method to analyze simulations of activated processes.

We present a new method to analyze molecular and Brownian dynamics simulations of activated processes based on the concept of mean first-passage times. The new method provides a simple and efficient strategy to evaluate reaction rates and it facilitates the localization of the transition state directly from the kinetics of the system without the need of thermodynamical considerations. It also provides a more rigorous value of the steady-state transition rate and gives valuable information about many important characteristics of the process. We illustrate the power of this new technique by its application to the study of nucleation in rare gases.

Finite-size effects in simulations of nucleation.

We investigate the importance of finite-size effects in simulations of nucleation processes. Most molecular dynamics simulations of first order phase transitions, such as vapor-liquid nucleation, are performed in the canonical NVT ensemble where, owing to the fixed total number of molecules N, the growth of the new phase causes the depletion of the metastable phase. This effect may lead to significant errors in the simulation and even to the impossibility of observing nucleation in a small finite system. We present a theory to estimate the system size beyond which these finite-size effects are expected to be negligible. This optimization saves valuable calculation time and can extend the range of supersaturations and rates attainable by simulations by several orders of magnitude. Our results are applicable to diverse situations, such as crystallization, capillary condensation, or the melting of nanoclusters.

Persistence of iatrogenic atrial septal defect after pulmonary vein isolation--an underestimated risk?

BACKGROUND: According to present knowledge, pulmonary vein isolation (PVI) bears a low interventional risk and has a high feasibility. For completion of PVI, left atrial access is achieved via single or double transseptal puncture. We sought to determine the incidence and echocardiographic characteristics of persistent iatrogenic atrial septal defect (iASD) after PVI. Further objectives were to define clinical and periprocedural risk factors for the development of iASD. METHODS: Every patient admitted for PVI at our hospital was screened for eligibility for study participation. Exclusion criteria were inability for undergoing transesophageal echocardiography, preexisting atrial septal defect, open-heart surgery or another transseptal procedure during the follow-up period. Transesophageal echocardiography was performed before PVI and after 9 months. Interatrial shunt was characterized by echocardiographic parameters; right-to-left-shunting (RLS) was quantified by contrast echocardiography. RESULTS: Forty-two patients were included, 27 patients underwent PVI with single transseptal puncture and additional advancement of a second electrophysiologic catheter (group A) 15 patients underwent PVI with double transseptal puncture (group B). In 8 patients of group A, iASD persisted after the follow-up period, including 6 patients with distinct RLS. We saw no iASD in group B (P = .011, CI -0.79 to -0.11). Preprocedural pulmonary artery pressure was significantly higher in patients with iASD and accompanying RLS, compared with patients with iASD and no evidence of RLS (23.75 +/- 0.50 vs 17.59 +/- 5.82, P = .048, CI 0.048-12.27). CONCLUSION: This is the first study that demonstrates a high incidence of long-term persistent iatrogenic atrial septal defect with RLS after PVI. All interatrial shunts occurred after single transseptal puncture with passage of 2 electrophysiologic catheters into the left atrium. Increased preprocedural pulmonary artery pressure seems to promote the occurrence of RLS across iASD.

Homogeneous nucleation rates of 1-pentanol.

We have measured isothermal homogeneous nucleation rates J for 1-pentanol vapor in two different carrier-gases, argon, and helium, using a two-valve nucleation pulse chamber. The nucleation rates cover a range of 10(5)