Nucleation threshold and deactivation mechanisms of nanoscopic cavitation nuclei

The acoustic nucleation threshold for bubbles trapped in cavities has theoretically been predicted within the crevice theory by Atchley and Prosperetti [“The crevice model of bubble nucleation,” J. Acoust. Soc. Am. 86, 1065 (1989)]. Here, we determine this threshold experimentally, by applying\ud a single pressure pulse to bubbles trapped in cylindrical nanoscopic pits (“artificial crevices”) with radii down to 50 nm. By decreasing the minimum pressure stepwise, we observe the threshold for which the bubbles start to nucleate. The experimental results are quantitatively in good agreement with the theoretical predictions of Atchley and Prosperetti. In addition, we provide the mechanism which explains the deactivation of cavitation nuclei: gas diffusion together with an aspherical bubble collapse. Finally, we present superhydrophobic nuclei which cannot be deactivated, unless with a high-speed liquid jet directed into the pit

Role of Air in Granular Jet Formation

A steel ball impacting on a bed of very loose, fine sand results in a surprisingly vigorous jet which shoots up from the surface of the sand [D. Lohse et al., Phys. Rev. Lett. 93, 198003 (2004)]. When the ambient pressure p is reduced, the jet is found to be less vigorous [R. Royer et al., Nature Phys. 1, 164 (2005)]. In this Letter we show that p also affects the rate of penetration of the ball: Higher pressure increases the rate of penetration, which makes the cavity created by the ball close deeper into the sand bed, where the hydrostatic pressure is stronger, thereby producing a more energetic collapse and jetting. The origin of the deeper penetration under normal ambient pressure is found to lie in the extra sand fluidization caused by the air flow induced by the falling ball

A bouncing oil droplet in a stratified liquid and its sudden death

Droplets can self-propel when immersed in another liquid in which a concentration gradient is present. Here we report the experimental and numerical study of a self-propelling oil droplet in a vertically stratified ethanol/water mixture: At first, the droplet sinks slowly due to gravity, but then, before having reached its density matched position, jumps up suddenly. More remarkably, the droplet bounces repeatedly with an ever increasing jumping distance, until all of a sudden it stops after about 30 min. We identify the Marangoni stress at the droplet/liquid interface as responsible for the jumping: its strength grows exponentially because it pulls down ethanol-rich liquid, which in turn increases its strength even more. The jumping process can repeat because gravity restores the system. Finally, the sudden death of the jumping droplet is also explained. Our findings have demonstrated a type of prominent droplet bouncing inside a continuous medium with no wall or sharp interface.Comment: 6 pages, 4 figure

Drop impact on superheated surfaces

At impact of a liquid droplet on a smooth surface heated above the liquid's boiling point, the droplet either immediately boils when it contacts the surfaces (``contact boiling''), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back (``gentle film boiling''), or both forms the Leidenfrost layer and ejects tiny droplets upward (``spraying film boiling''). We experimentally determine conditions under which impact behaviors in each regime can be realized. We show that the dimensionless maximum spreading $\gamma$ of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes shows a universal scaling with the Weber number \We (\gamma\sim\We^{2/5}) -- regardless of surface temperature and of liquid properties -- which is much steeper than for the impact on non-heated (hydrophilic or hydrophobic) surfaces (\gamma\sim\We^{1/4}). We also intereferometrically measure the vapor thickness under the droplet

Validation of linguistic rating variables for the Mini-Mental State Examination sentence writing task - written language as indicator of cognitive impairments

Einleitung: Eine der Aufgaben des Mini-Mental Status Tests (MMST) besteht darin, spontan einen Satz zu schreiben. Die Bewertung dieses Satzes ist bislang zu wenig differenziert, um kognitive Störungen anhand der geschriebenen Sprachproduktion zu identifizieren. Um eine differenziertere Erfassung zu ermöglichen, werden in der vorliegenden Arbeit Bewertungsvariablen aus den linguistischen Strukturebenen Morphologie (M), Syntax (Syn) und Semantik (Sem) explorativ untersucht. Es soll geprüft werden, ob diese einzeln und in Kombination signifikant zwischen Personen mit subjektiver kognitiver Verschlechterung (SCD), leichter kognitiver Störung (MCI) und einer Demenz vom Alzheimer-Typ (DAT) diskriminieren können und damit Rückschlüsse auf kognitive Störungen zulassen. Methodik: Für die Untersuchung wurden 19 Bewertungsvariablen aus den genannten linguistischen Strukturebenen ausgewählt. Es erfolgte eine querschnittliche retrospektive Analyse von 565 Sätzen aus dem MMST von einer deutschsprachigen Stichprobe. Diese bestand aus 162 Personen mit SCD, 185 Personen mit MCI und 218 Personen mit DAT. Zudem wurde deskriptiv eine längsschnittliche Fallanalyse vorgenommen. Der MMST wurde als Bestandteil einer neuropsychologischen Testbatterie eingesetzt. Ergebnisse: Für drei morphologische, zwei syntaktische und eine semantische Bewertungsvariable ergaben sich signifikante Ergebnisse. Der Quotient aus der Buchstabenanzahl des kürzesten und längsten Wortes (MQuotient_k_l_Wort, H(2)=6,20, p=0,045) sowie der Quotient aus der Gesamtbuchstabenanzahl und der Anzahl der Wörter (MQ, H(2)=13,07, p=0,001) konnten signifikant zwischen mindestens zwei der untersuchten Gruppen diskriminieren. Zudem bestanden signifikante Zusammenhänge zwischen der Gruppenzugehörigkeit und dem Vorhandensein von Rechtschreibfehlern (MR, χ2(2)=21,30, p<0,001), Satzzeichen (SynS, χ2(2)=27,87, p<0,001), syntaktischen Fehlern (SynF, χ2(2)=11,27, p=0,004) und semantischen Fehlern (SemFehler, χ2(2)=7,50, p=0,016). Mittels multinomialer logistischer Regression konnte gezeigt werden, dass die Zugehörigkeit zur Gruppe DAT am besten durch einen konstanten Faktor und vier der genannten Variablen (MR, SynS, SynF, MQ) prädiziert werden kann. Daher wurden diese vier Variablen zu einem Gesamtindex („M-Syn-Index“) zusammengefasst, der ebenfalls signifikante Unterschiede zwischen allen Gruppen erzielte (H(2)=52,47, p<0,001). Schlussfolgerung: Sechs der untersuchten linguistischen Bewertungsvariablen ermöglichen eine differenziertere Erfassung der geschriebenen Sprache in der Aufgabe „Satz schreiben“ im MMST. Sowohl einzeln als auch in Kombination sind die Variablen in der Lage, signifikant zwischen Personen mit SCD, MCI und DAT zu diskriminieren. Die Bewertung des geschriebenen Satzes mittels dieser linguistischen Variablen kann somit Hinweise auf eine kognitive Störung liefern. Vor einem klinischen Einsatz empfiehlt sich eine konfirmatorische Validierung der vorliegenden Ergebnisse.Introduction: One of the tasks of the Mini-Mental State Examination (MMSE) is to write a sentence spontaneously. So far, the assessment of this sentence is not differentiated enough to identify cognitive impairments on the basis of written speech production. To allow for a more differentiated assessment, the present study examines variables from the linguistic structural levels of morphology (M), syntax (Syn) and semantics (Sem) in an exploratory manner. The aim is to explore whether these variables, individually and in combination, can significantly discriminate between persons with subjective cognitive decline (SCD), mild cognitive impairment (MCI) and dementia of Alzheimer´s disease (DAT) and thus indicate cognitive impairments. Methods: 19 variables were selected from the linguistic structural levels mentioned above. A cross-sectional retrospective analysis of 565 sentences from the MMSE from a German-speaking sample was conducted. This consisted of 162 subjects with SCD, 185 patients with MCI and 218 patients with DAT. In addition, the study included a longitudinal case analysis. The MMSE was applied as part of a neuropsychological test battery. Results. Significant results were obtained for three morphological variables, two syntactic variables and one semantic variable. The quotient of the number of letters of the shortest and the longest word (H(2)=6,20, p=0,045) as well as the quotient of the total number of letters and words (MQ, H(2)=13,07, p=0,001) could discriminate significantly between at least two groups. In addition, there was a significant association between group membership and spelling errors (MR, χ2(2)=21,30, p<0,001), punctuation (SynS, χ2(2)=27,87, p<0,001), syntactic errors (SynF, χ2(2)=11,27, p=0,004) and semantic errors (SemFehler, χ2(2)=7,50, p=0,016). Using multinomial logistic regression, group membership for DAT was best predicted by a constant factor and four of the mentioned variables (MR, SynS, SynF, MQ). Therefore these variables were combined into an overall index that also yielded significant differences between all groups (H(2)=52,47, p<0,001). Conclusion. Six of the evaluated variables allow for a more differentiated assessment of written language in the writing task in the MMSE. Both individually and in combination, the variables are able to significantly discriminate between individuals with SCD, MCI and DAT. Using these variables to assess the written sentence can provide an indication of a cognitive impairment. Confirmatory validation of the findings is required before clinical use

Leidenfrost temperature increase for impacting droplets on carbon-nanofiber surfaces

Droplets impacting on a superheated surface can either exhibit a contact boiling regime, in which they make direct contact with the surface and boil violently, or a film boiling regime, in which they remain separated from the surface by their own vapor. The transition from the contact to the film boiling regime depends not only on the temperature of the surface and kinetic energy of the droplet, but also on the size of the structures fabricated on the surface. Here we experimentally show that surfaces covered with carbon-nanofibers delay the transition to film boiling to much higher temperature compared to smooth surfaces. We present physical arguments showing that, because of the small scale of the carbon fibers, they are cooled by the vapor flow just before the liquid impact, thus permitting contact boiling up to much higher temperatures than on smooth surfaces. We also show that, as long as the impact is in the film boiling regime, the spreading factor of impacting droplets follows the same \We^{3/10} scaling (with \We the Weber number) found for smooth surfaces, which is caused by the vapor flow underneath the droplet.Comment: 10 pages, 6 figure

Validation of an approximate model for the thermal behavior in acoustically driven bubbles

The chemical production of radicals inside acoustically driven bubbles is determined by the local temperature inside the bubbles. Therefore, modeling of chemical reaction rates in bubbles requires an accurate evaluation of the temperature field and the heat exchange with the liquid. The aim of the present work is to compare a detailed partial differential equation model in which the temperature field is spatially resolved with an ordinary differential equation model in which the bubble contents are assumed to have a uniform average temperature and the heat exchanges are modeled by means of a boundary layer approximation. The two models show good agreement in the range of pressure amplitudes in which the bubble is spherically stable

Giant bubble pinch-off

Self-similarity has been the paradigmatic picture for the pinch-off of a drop. Here we will show through high-speed imaging and boundary integral simulations that the inverse problem, the pinch-off of an air bubble in water, is not self-similar in a strict sense: A disk is quickly pulled through a water surface, leading to a giant, cylindrical void which after collapse creates an upward and a downward jet. Only in the limiting case of large Froude number the neck radius $h$ scales as $h(-\log h)^{1/4} \propto \tau^{1/2}$, the purely inertial scaling. For any finite Froude number the collapse is slower, and a second length-scale, the curvature of the void, comes into play. Both length-scales are found to exhibit power-law scaling in time, but with different exponents depending on the Froude number, signaling the non-universality of the bubble pinch-off.Comment: 5 pages, 2 figures. Figure quality was reduced considerably and converted to greyscale to decrease file siz

Mehrwertsteuer-Erhebung durch Quellensteuer statt Reverse Charge

Seit der Einführung des Binnenmarktes im Jahre 1993 bis zur sog. Osterweiterung am 1. Mai 2004 haben die EU-Finanzminister im Durchschnitt einmal jährlich das gemeinsame Mehrwertsteuerrecht geändert. Im Europa der 25 haben sie wegen des Einstimmigkeitsprinzips keinen einzigen Änderungsvorschlag der Kommission angenommen. Auch die Mehrwertsteuerberatungen in der letzten Sitzung unter Österreichischer Ratspräsidentschaft sind ergebnislos verlaufen. Das ifo institut schlägt zur Überwindung der Widerstände im EU-Ministerrat ein Quellensteuerverfahren vor, das auf der Fortentwicklung bereits früher angedachter Überlegungen beruht.Umsatzsteuer, Quellensteuer, EU-Staaten