Publications (37)126.15 Total impact
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ABSTRACT: Synthetic nanoparticles and other stiff objects injected into a blood vessel filled with red blood cells are known to marginate toward the vessel walls. By means of hydrodynamic latticeBoltzmann simulations, we show that active particles can strongly accelerate their margination by moving against the flow direction: particles located initially in the channel center migrate much faster to their final position near the wall than in the nonactive case. We explain our findings by an enhanced rate of collisions between the stiff particles and the deformable red blood cells. Our results imply that a significantly faster margination can be achieved either technically by the application of an external magnetic field (if the particles are magnetic) or biologically by selfpropulsion (if the particles are, e.g., swimming bacteria).  [Show abstract] [Hide abstract]
ABSTRACT: The special macroscopic properties of liquid water stem from its structure as a complex network of molecules connected by hydrogen bonds. While the dynamics of single molecules within this network has been extensively investigated, only little attention has been paid to the closed loops (meshes) of hydrogenbonded molecules which determine the network topology. Using molecular dynamics simulations we analyze the size, shape, geometrical arrangement, and dynamical stability of loops containing up to 10 hydrogen bonds. We find that sixmembered loops in liquid water even at room temperature retain a striking similarity with the wellknown structure of ice. Analyzing the network dynamics we find that rings of more than five hydrogen bonds are stabilized compared to a random collection containing the same number of single bonds. We finally show that in the vicinity of hydrophobic and hydrophilic interfaces loops arrange in a preferred orientation.  [Show abstract] [Hide abstract]
ABSTRACT: We calculate the local dielectric function ɛ(r) inside the hydration layer around a spherical solute (i) from molecular dynamics simulations including explicit solutes and (ii) theoretically using the nonlocal dielectric function of bulk water which includes the radial electric field, but not the explicit solute. The observed agreement between the two concepts shows that while ɛ(r) is strongly different from bulk, this difference is not due to restructuring of the hydrogen bond network but is mostly a consequence of the field geometry. The dielectric response differs for anions and cations, yielding a natural explanation for the wellknown charge asymmetry of ionic solvation in agreement with experimental data.  [Show abstract] [Hide abstract]
ABSTRACT: Using Molecular Dynamics simulations we elucidate in detail the dynamics of the ππ 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 approx. 1 kT in toluene, but not in the nonaromatic solvent hexane. A similar effect is observed tor two noncovalently linked monomers. The origin of this barrier is traced back to ππ 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 welldefined 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.  [Show abstract] [Hide abstract]
ABSTRACT: Using extensive equilibrium molecular dynamics simulations we determine the dielectric spectra of aqueous solutions of NaF, NaCl, NaBr, and NaI. The ionspecific and concentrationdependent shifts of the static dielectric constants and the dielectric relaxation times match experimental results very well, which serves as a validation of the classical and nonpolarizable ionic force fields used. The purely ionic contribution to the dielectric response is negligible, but determines the conductivity of the salt solutions. The ionwater cross correlation contribution is negative and reduces the total dielectric response by about 5%10% for 1 M solutions. The dominating water dielectric response is decomposed into different water solvation shells and ionpair configurations, by this the spectral blue shift and the dielectric decrement of salt solutions with increasing salt concentration is demonstrated to be primarily caused by firstsolvation shell water. With rising salt concentration the simulated spectra show more pronounced deviations from a singleDebye form and can be well described by a ColeCole fit, in quantitative agreement with experiments. Our spectral decomposition into ionic and different water solvation shell contributions does not render the individual contributions more Debyelike, this suggests the nonDebyelike character of the dielectric spectra of salt solutions not to be due to the superposition of different elementary relaxation processes with different relaxation times. Rather, the nonDebyelike character is likely to be an inherent spectral signature of solvation water around ions.  [Show abstract] [Hide abstract]
ABSTRACT: Recent femtosecondresolved spectroscopy experiments demonstrate the singlewater orientational dynamics in the first solvation shell around monatomic ions to be slowed down. In contrast, dielectric spectroscopy experiments exhibit a blue shift of the water dielectric relaxation time with rising salt concentration, indicative of faster water dynamics. Using molecular dynamics simulations employing nonpolarizable and thermodynamically optimized ion force fields, we reproduce both experimental trends and resolve these conflicting experimental findings by the simultaneous analysis of singlewater and collectivewater dynamics in the ion solvation shells. While the singlemolecule reorientational dynamics of first solvation shell water around ions indeed slows down, the collective dynamics, which furnishes the dominant contribution to the dielectric response, accelerates. This collective acceleration is rationalized by a dramatically decreasing water cooperativity around ions when compared to bulk water, quantified by the Kirkwood dielectric enhancement factor. The static dielectric decrement of salt solutions is thus reinterpreted as a dielectric structure breaking rather than a water alignment effect. Both the dielectric blue shift and the dielectric decrement become stronger with increasing anion size, meaning larger halide ions such as iodide are more efficient dielectric structure breakers than small halide ions such as fluoride.  [Show abstract] [Hide abstract]
ABSTRACT: We compare the results from timeresolved fluorescence anisotropy experiments and molecular modelling on perylene bisimide acrylate dimers which allows us to connect the observed spectral signatures unambiguously with the nonstacked and two (parallel and antiparallel) stacked conformations. For the parallel stacked conformation the experimental data can be reproduced quantitatively using a model that assumes structural relaxation in the electronically excited state of the stacked aggregate. For the nonstacked conformation we find quantitative agreement between experiment and modelling only if a fast hopping of the electronic excitation between the perylene bisimide subunits is taken into account.  [Show abstract] [Hide abstract]
ABSTRACT: Radiofrequency (RF) electromagnetic fields are readily absorbed in biological matter and lead to dielectric heating. To understand how RF radiation interacts with macromolecular structures and possibly influences biological function, a quantitative description of dielectric absorption and heating at nanometer resolution beyond the usual effective medium approach is crucial. We report an exemplary multiscale theoretical study for biomembranes that combines (i) atomistic simulations for the spatially resolved absorption spectrum at a single planar DPPC lipid bilayer immersed in water, (ii) calculation of the electric field distribution in planar and spherical cell models, and (iii) prediction of the nanometer resolved temperature profiles under steady RF radiation. Our atomistic simulations show that the only 2 nm thick lipid hydration layer strongly absorbs in a wide RF range between 10 MHz and 100 GHz. The absorption strength, however, strongly depends on the direction of the incident wave. This requires modeling of the electric field distribution using tensorial dielectric spectral functions. For a spherical cell model, we find a strongly enhanced RF absorption on an equatorial ring, which gives rise to temperature gradients inside a single cell under radiation. Although absolute temperature elevation is small under conditions of typical telecommunication usage, our study points to hitherto neglected temperature gradient effects and allows thermal RF effects to be predicted on an atomistically resolved level. In addition to a refined physiological risk assessment of RF fields, technological applications for controlling temperature profiles in nanodevices are possible.  [Show abstract] [Hide abstract]
ABSTRACT: The diffusional water dynamics in the hydration layer of a dipalmitoylphosphatidylcholine bilayer is studied using molecular dynamics simulations. By mapping the perpendicular water motion on the ordinary diffusion equation, we disentangle free energetic and friction effects and show that perpendicular diffusion is strongly reduced. The lateral water motion exhibits anomalous diffusion up to several nanoseconds and is characterized by even further decreased diffusion coefficients, which by comparison with coarsegrained simulations are explained by the transient corrugated effective free energy landscape imposed by the lipids. This is in contrast to homogenous surfaces, where boundary hydrodynamic theory quantitatively predicts the anisotropy of water diffusion.  [Show abstract] [Hide abstract]
ABSTRACT: A Comment on the Letter by H. Zhu et al., Phys. Rev. Lett. 109, 107801 (2012). The authors of the Letter offer a Reply. 
Article: Air flow in a collapsing cavity
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ABSTRACT: We experimentally study the airflow in a collapsing cavity created by the impact of a circular disk 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 pinchoff 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 pinchoff, 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 pinchoff depends on the Froude number and explain the observed dependence using a theoretical model of the cavity collapse. 
Article: Anisotropy in the dielectric spectrum of hydration water and its relation to water dynamics
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ABSTRACT: Proteins, molecules, and macromolecular assemblies in water are surrounded by a nanometersized hydration layer with properties very different from bulk water. Here, we use classical molecular dynamics simulations to study the dielectric response of hydration water next to hydrophobic and hydrophilic planar surfaces. We find the interfacial dielectric absorption of water to be strongly anisotropic: compared to bulk water, which shows a broad dielectric absorption maximum around 15 GHz in the imaginary part of the dielectric function, the absorption for electric fields parallel to the surface is of similar strength and shows a slight redshift, while for perpendicular electric fields it is strongly attenuated and blueshifted. This anisotropy is generic for hydrophobic and hydrophilic surfaces. From our spatially resolved dielectric functions and a modified MaxwellGarnett theory that accounts for anisotropic hydration layers around spherical particles, the dielectric absorption of solutions of organic molecules and micelles is derived to exhibit the experimentally known attenuation in combination with a redshift. These two features are traced back to the subtle interplay of interfacial depolarization effects and the dielectric anisotropy in the hydration layer. By a detailed analysis of the individual water molecule dynamics the perpendicular blueshift is shown not to be linked to accelerated water reorientation, but rather to dielectric boundary effects. Carefully conducted angularly resolved experiments at planar aqueous interfaces will be able to resolve this dielectric anisotropy and thus to confirm the subtle connection between spectral absorption features and the molecular water dynamics in hydration layers.  [Show abstract] [Hide abstract]
ABSTRACT: The 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 nonaxisymmetric air cavities by driving a metal disc through an initiallyquiescent water surface and observe their subsequent gravityinduced collapse. The cavities are characterized by azimuthal harmonic disturbances with a single mode number $m$ and amplitude $a_m$. 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 pinchoff modes. For small amplitude disturbances we compare our experimental results with the model for the amplitude of the oscillations by Schmidt et al. (2009) and the model for the collapse of an axisymmetric impactcreated cavity previously proposed by Bergmann et al. (2009b). By combining these two models we can reconstruct the threedimensional shape of the cavity at any time before pinchoff.  [Show abstract] [Hide abstract]
ABSTRACT: We derive the theoretical framework to calculate the dielectric response tensor and determine its components for water adjacent to hydrophilic and hydrophobic surfaces using molecular dynamics simulations. For the nonpolarizable water model used, linear response theory is found to be applicable up to an external perpendicular field strength of ∼2 V/nm, which is well beyond the experimental dielectric breakdown threshold. The dipole contribution dominates the dielectric response parallel to the interface, whereas for the perpendicular component it is essential to keep the quadrupole and octupole terms. Including the spacedependent dielectric function in a meanfield description of the ion distribution at a single charged interface, we reproduce experimental values of the interfacial capacitance. At the same time, the dielectric function decreases the electrostatic part of the disjoining pressure between two charged surfaces, unlike previously thought. The difference in interfacial polarizability between hydrophilic and hydrophobic surfaces can be quantized in terms of the dielectric dividing surface. Using the dielectric dividing surface and the Gibbs dividing surface positions to estimate the free energy of a single ion close to an interface, ionspecific adsorption effects are found to be more pronounced at hydrophobic surfaces than at hydrophilic surfaces, in agreement with experimental trends.  [Show abstract] [Hide abstract]
ABSTRACT: Using molecular dynamics simulations we demonstrate pumping of water through a carbon nanotube by timedependent electric fields. The fields are generated by electrodes with oscillating charges in a broad gigahertz frequency range that are attached laterally to the tube. The key ingredient is a phase shift between the electrodes to break the spatiotemporal symmetry. A microscopic theory based on a polarizationdragging mechanism accounts quantitatively for our numerical findings.  [Show abstract] [Hide abstract]
ABSTRACT: We present a multiscale approach to simulate the impact of a solid object on a liquid surface: upon impact a thin liquid sheet is thrown upwards all around the rim of the impactor while in its wake a large surface cavity forms. Under the influence of hydrostatic pressure the cavity immediately starts to collapse and eventually closes in a single point from which a thin, needlelike jet is ejected. Existing numerical treatments of liquid impact either consider the surrounding air as an incompressible fluid or neglect air effects altogether. In contrast, our approach couples a boundaryintegral method for the liquid with a Roe scheme for the gas domain and is thus able to handle the fully \emph{compressible} gas stream that is pushed out of the collapsing impact cavity. Taking into account air compressibility is crucial, since, as we show in this work, the impact crater collapses so violently that the air flow through the cavity neck attains supersonic velocities already at cavity diameters larger than 1 mm. Our computational results are validated through corresponding experimental data.  [Show abstract] [Hide abstract]
ABSTRACT: The framework for deriving tensorial interfacial dielectric profiles from bound charge distributions is established and applied to molecular dynamics simulations of water at hydrophobic and hydrophilic surfaces. In conjunction with a modified PoissonBoltzmann equation, the trend of experimental doublelayer capacitances is well reproduced. We show that the apparent Stern layer can be understood in terms of the dielectric profile of pure water.  [Show abstract] [Hide abstract]
ABSTRACT: The 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 nonaxisymmetric air cavities by driving a metal disc through an initiallyquiescent water surface and observe their subsequent gravityinduced collapse. The cavities are characterized by azimuthal harmonic disturbances with a single mode number $m$ and amplitude $a_m$. 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 pinchoff modes. For small amplitude disturbances we compare our experimental results with the model for the amplitude of the oscillations by Schmidt et al. (2009) and the model for the collapse of an axisymmetric impactcreated cavity previously proposed by Bergmann et al. (2009b). By combining these two models we can reconstruct the threedimensional shape of the cavity at any time before pinchoff.  [Show abstract] [Hide abstract]
ABSTRACT: There is no abstract available for this article. 
Article: Breakup of Diminutive Rayleigh Jets
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ABSTRACT: Discharging a liquid from a nozzle at sufficient large velocity leads to a continuous jet that due to capillary forces breaks up into droplets. Here we investigate the formation of microdroplets from the breakup of micronsized jets with ultra highspeed imaging. The diminutive size of the jet implies a fast breakup time scale $\tau_\mathrm{c} = \sqrt{\rho r^3 / \gamma}$ of the order of 100\,ns{}, and requires imaging at 14 million frames per second. We directly compare these experiments with a numerical lubrication approximation model that incorporates inertia, surface tension, and viscosity [Eggers and Dupont, J. Fluid Mech. 262, 205 (1994); Shi, Brenner, and Nagel, Science 265, 219 (1994)]. The lubrication model allows to efficiently explore the parameter space to investigate the effect of jet velocity and liquid viscosity on the formation of satellite droplets. In the phase diagram we identify regions where the formation of satellite droplets is suppressed. We compare the shape of the droplet at pinchoff between the lubrication approximation model and a boundary integral (BI) calculation, showing deviations at the final moment of the pinchoff. Inspite of this discrepancy, the results on pinchoff times and droplet and satellite droplet velocity obtained from the lubrication approximation agree with the highspeed imaging results.
Publication Stats
504  Citations  
126.15  Total Impact Points  
Top Journals
Institutions

20132015

University of Bayreuth
 Institute of Physics
Bayreuth, Bavaria, Germany


20112013

University of Technology Munich
 Faculty of Physics
München, Bavaria, Germany


20072012

Universiteit Twente
 Group of Physics of Fluids
Enschede, Overijssel, Netherlands


20062007

Universität Stuttgart
Stuttgart, BadenWürttemberg, Germany
