104 research outputs found
Air flow in a collapsing cavity
No full textWe experimentally study the airflow in a collapsing cavity created by the impact of a circular disc on a water surface. We measure the air velocity in the collapsing neck in two ways: Directly, by means of employing particle image velocimetry of smoke injected into the cavity and indirectly, by determining the time rate of change of the volume of the cavity at pinch-off and deducing the air flow in the neck under the assumption that the air is incompressible. We compare our experiments to boundary integral simulations and show that close to the moment of pinch-off, compressibility of the air starts to play a crucial role in the behavior of the cavity. Finally, we measure how the air flow rate at pinch-off depends on the Froude number and explain the observed dependence using a theoretical model of the cavity collapse
Zhu et al. Reply
No full textInternational audienceA Reply to the Comment by S. Gekle and A. Arnold. Original Article: Stephan Gekle and Axel Arnold, Comment on "Anomalous Dielectric Behavior of Nanoconfined Electrolytic Solutions", Phys. Rev. Lett. 111, 089801 (2013)
Generation and breakup of Worthington jets after cavity collapse. Part 1. Jet formation
No full textAt the beginning of the last century Worthington and Cole discovered that the high-speed jets ejected after the impact of an axisymmetric solid on a liquid surface are intimately related to the formation and collapse of an air cavity created in the wake of the impactor. In this paper, we combine detailed boundary-integral simulations with analytical modelling to describe the formation of such Worthington jets after the impact of a circular disk on water. We extend our earlier model in Gekle et al. (Phys. Rev. Lett., vol. 102, 2009a, 034502), valid for describing only the jet base dynamics, to describe the whole jet. We find that the flow structure inside the jet may be divided into three different regions: the axial acceleration region, where the radial momentum of the incoming liquid is converted to axial momentum; the ballistic region, where fluid particles experience no further acceleration and move constantly with the velocity obtained at the end of the acceleration region; and the jet tip region, where the jet eventually breaks into droplets. From our modelling of the ballistic region we conclude that, contrary to the case of other physical situations where high-speed jets are also ejected, the types of Worthington jets studied here cannot be described using the theory of hyperbolic jets of Longuet-Higgins (J. Fluid Mech., vol. 127, 1983, p. 103). Most importantly, we find that the velocity and the shape of the ejected jets can be well predicted at any instant in time with the only knowledge of quantities obtained before pinch-off occurs. This fact allows us to provide closed expressions for the jet velocity and the sizes of the ejected droplets as a function of the velocity and the size of the impactor. We show that our results are also applicable to Worthington jets emerging after the collapse of a bubble growing from an underwater nozzle, although this system creates thicker jets than the disk impact
Supersonic air flow due to solid-liquid impact
No full textA solid object impacting on liquid creates a liquid jet due to the collapse of the impact cavity. Using visualization experiments with smoke particles and multiscale simulations, we show that in addition, a high-speed air jet is pushed out of the cavity. Despite an impact velocity of only 1??m/s, this air jet attains supersonic speeds already when the cavity is slightly larger than 1 mm in diameter. The structure of the air flow closely resembles that of compressible flow through a nozzle—with the key difference that here the “nozzle” is a liquid cavity shrinking rapidly in time
Hydrodynamic mobility of a solid particle near a spherical elastic membrane. II. Asymmetric motion
No full textIn this paper, we derive analytical expressions for the leading-order hydrodynamic mobility of a small solid particle undergoing motion tangential to a nearby large spherical capsule whose membrane possesses resistance toward shearing and bending. Together with the results obtained in the first part [Daddi-Moussa-Ider and Gekle, Phys. Rev. E 95, 013108 (2017)], where the axisymmetric motion perpendicular to the capsule membrane is considered, the solution of the general mobility problem is thus determined. We find that shearing resistance induces a low-frequency peak in the particle self-mobility, resulting from the membrane normal displacement in the same way, although less pronounced, to what has been observed for the axisymmetric motion. In the zero-frequency limit, the self-mobility correction near a hard sphere is recovered only if the membrane has a nonvanishing resistance toward shearing. We further compute the in-plane mean-square displacement of a nearby diffusing particle, finding that the membrane induces a long-lasting subdiffusive regime. Considering capsule motion, we find that the correction to the pair-mobility function is solely determined by membrane shearing properties. Our analytical calculations are compared and validated with fully resolved boundary integral simulations where a very good agreement is obtained
Collapse and pinch-off of a non-axisymmetric impact-created air cavity in water
No full textThe axisymmetric collapse of a cylindrical air cavity in water follows a universal power law with logarithmic corrections. Nonetheless, it has been suggested that the introduction of a small azimuthal disturbance induces a long-term memory effect, reflecting in oscillations which are no longer universal but remember the initial condition. In this work, we create non-axisymmetric air cavities by driving a metal disc through an initially quiescent water surface and observe their subsequent gravity-induced collapse. The cavities are characterized by azimuthal harmonic disturbances with a single mode number and amplitude . For small initial distortion amplitude (1 or 2 % of the mean disc radius), the cavity walls oscillate linearly during collapse, with nearly constant amplitude and increasing frequency. As the amplitude is increased, higher harmonics are triggered in the oscillations and we observe more complex pinch-off modes. For small-amplitude disturbances we compare our experimental results with the model for the amplitude of the oscillations by Schmidt et al. (Nature Phys., vol. 5, 2009, pp. 343–346) and the model for the collapse of an axisymmetric impact-created cavity previously proposed by Bergmann et al. (J. Fluid Mech., vol. 633, 2009b, pp. 381–409). By combining these two models we can reconstruct the three-dimensional shape of the cavity at any time before pinch-off
Dispersion of solute released from a sphere flowing in a microchannel
No full textA solute is released from the surface of a sphere flowing freely in a cylindrical channel mimicking a modern drug delivery agent in a blood vessel. The solute then disperses by the combined action of advection and diffusion. We consider reflecting boundary conditions on the sphere and absorbing boundary conditions on the channel surface mimicking a biochemical reaction between the drug and endothelial cells on the vessel surface. The drug is released either instantaneously or continuously in time. The two key observables are the mean residence time in the flow before the drug is absorbed and the width over which it is spread on the vessel surface upon reaction. We numerically solve the Fokker–Planck equation for the time-dependent substance concentration combined with an analytical solution of the flow field. As expected, we find that the presence of the sphere leads to a substantial reduction in mean residence time and reaction width. Surprisingly, however, even in the limit of very large Péclet numbers (high velocities) the sphere-free case is not generally recovered. This observation can be attributed mainly to the small, but non-negligible radial flow component induced by the moving sphere. We further identify a strong influence of the release position which sharply separates two qualitatively different regimes. If the release position is between \unicode[STIX]{x1D703}_{0}=0 (front) and a critical \unicode[STIX]{x1D703}_{c} the substance is quickly advected away from the sphere and its overall behaviour is similar to free diffusion in an empty channel. For release between \unicode[STIX]{x1D703}_{c} and \unicode[STIX]{x1D703}_{0}=\unicode[STIX]{x03C0} (tail), on the other hand, the substance is pushed towards the sphere leading to behaviour reminiscent of confined diffusion between two infinitely long cylinders. The critical position \unicode[STIX]{x1D703}_{c} is generally smaller than \unicode[STIX]{x03C0}/2 which would correspond to an equatorial release position.</jats:p
Impact on Liquids: Void Collapse and Jet Formation
No full textA spectacular example of free surface flow is the impact of a solid object on a liquid: At impact a “crown” splash is created and a surface cavity (void) emerges which immediately starts to collapse due to the hydrostatic pressure of the surrounding liquid. Eventually the cavity closes in a single point about halfway down its length and shoots out a fast and extremely slender water jet. In the present thesis we study the impact of thin circular discs a few centimeters in radius with impact velocities of a few meters per second. Combining high-speed imaging with sophisticated boundaryintegral computer simulations we elucidate various aspects of this fascinating process. Next to their undisputable interest for fundamental science such impacts can also be of practical relevance in other disciplines. For oceanographic research it is important that raindrops falling on the ocean entrain small air bubbles after the pinch-off of the impact cavity. This behavior constitutes the main mechanism for carbon dioxide exchange between the sea and the atmosphere and is furthermore a major source of underwater noise. As a medical application, the thin liquid jets which are generated during the collapse of a liquid cavity such as the cavities produced during liquid impact represent a promising possibility for very localized drug delivery into cells or through a patient’s skin
Dynamic Stacking Pathway of Perylene Dimers in Aromatic and Nonaromatic Solvents
No full textUsing molecular dynamics simulations,
we elucidate in detail the
dynamics of the π–π stacking process of a perylene
bisimide (PBI) dimer solvated in toluene. Our calculations show that
the transition from the open (unstacked) to the stacked configuration
is hindered by a small free energy barrier of approximately 1<i>k</i><sub>B</sub><i>T</i> in toluene but not in the
nonaromatic solvent hexane. A similar effect is observed tor two non-covalently
linked monomers. The origin of this barrier is traced back to π–π
interactions between perylene and the aromatic solvent which are very
similar in nature to those between two PBI monomers. The stacking
process proceeds in three phases via two well-defined transition states:
(i) in the first phase, the two PBI molecules share part of their
respective solvation shells forming the first transition state. Further
approach needs to squeeze out the shared solvent layer, thus creating
the energy barrier. (ii) After removal of the separating solvent,
the two PBIs form a second transition state with one monomer located
at a random position in the other’s solvation shell. (iii)
Finally, the two PBIs slide on top of each other into their final
stacked position
Spatially resolved dielectric constant of confined water and its connection to the non-local nature of bulk water
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