We investigate the dynamics of a vesicle in a shear flow on the basis of the newly proposed advected field (AF) method [T. Biben and C. Misbah, Eur. Phys. J. E 67, 031908 (2003)]. We also solve the same problem with the boundary integral formulation for the sake of comparison. We find that the AF results presented previously overestimated the tumbling threshold due to the finite size of the membrane, inherent to the AF model. A comparison between the two methods shows that only in the sharp interface limit (extrapolating the results to a vanishing width) the AF method leads to accurate quantitative results. We extensively investigate the tank-treading to tumbling transition, and compare our numerical results to the theory of Keller and Skalak which assumes a fixed ellipsoidal shape for the vesicle. We show that this theory describes correctly the two regimes, at least in two dimensions, even for the quite elongated non-convex shapes corresponding to red blood cells (and therefore far from ellipsoidal), This theory is, however, not fully quantitative. Finally we investigate the effect of a confinement on the tank-treading to tumbling transition, and show that the tumbling regime becomes unfavorable in a capillary vessel, which should have strong effects on blood rheology in confined geometries.

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Bubbles at a free surface usually burst in ejecting myriads of droplets. Focusing on the bubble bursting jet, prelude for these aerosols, we propose a simple scaling for the jet velocity and we unravel experimentally the intricate roles of bubble shape, capillary waves, gravity, and liquid properties. We demonstrate that droplets ejection unexpectedly changes with liquid properties. In particular, using damping action of viscosity, self-similar collapse can be sheltered from capillary ripples and continue closer to the singular limit, therefore producing faster and smaller droplets. These results pave the road to the control of the bursting bubble aerosols. C 2014 AIP Publishing LLC. [http://dx.Savoring a glass of champagne would not be as enjoyable without this fizzy sensation coming from bursting bubbles at the surface. More than just triggering a simple tingling sensation, the tiny droplets ejected during bursting are crucial for champagne tasting as their evaporation highly contribute to the diffusion of wine aroma in air. 1 Airborne droplets resulting from sea surface bubble bursting are also known since the late forties 2-4 to play a major role in the interaction between ocean and atmosphere. 5,6 Two distinct types of droplets are involved, lying on two different mechanisms appearing during bubble bursting. 7 When the thin liquid film-the bubble cap-separating the bubble from the atmosphere disintegrates, film drops are produced 8,9 with radius mainly less than 1 µm. Then the resulting opened cavity collapses and a jet emerges producing jet drops by breaking up 10-12 (see Fig. 1 and corresponding Multimedia view). For example, this latter mechanism accounts for the majority of sea-spray aerosol particles in the atmosphere with radius between 1 and 25 µm. 13 The last sixty years have witnessed a number of laboratory studies documenting jet drops properties, such as the ejection speed, the maximum height, or the size distribution as a function of bubble volume, 7,14-17 but a comprehensive picture of the mechanisms at play in bubble bursting is still lacking. In particular, the sequence of violent events preluding jet formation 11,18 and the roles of liquid properties remain elusive.In this article, we unravel the tangled roles of liquid properties, gravity, and capillary waves in the cavity collapse and show that these waves invariably adopt a self-similar behavior. We evidence the critical role of viscosity, that shelters self-similar collapse from remnant ripples, and therefore promotes the emergence of thinner and faster jets. Optimal conditions for singular jets as well as general scaling laws for the jet dynamics are assessed from detailed bubble bursting experiments. The consequences for aerosol generation are finally outlined, in particular in the context of champagne fizz, where liquid properties are tunable.Our experiment consists in releasing a gas bubble from a submerged needle in a liquid and recording the upward jet after the bubble bursts at the free surface. Bubbles are quasi-steadily formed us...