Stephan Gekle

University of Bayreuth, Bayreuth, Bavaria, Germany

Are you Stephan Gekle?

Claim your profile

Publications (37)126.15 Total impact

  • Stephan Gekle
    [Show abstract] [Hide abstract]
    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 lattice-Boltzmann 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 self-propulsion (if the particles are, e.g., swimming bacteria).
    No preview · Article · Jan 2016 · Biophysical Journal
  • Miriam Jahn · Stephan Gekle
    [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 hydrogen-bonded 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 six-membered loops in liquid water even at room temperature retain a striking similarity with the well-known 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.
    No preview · Article · Nov 2015 · Physical Review E
  • Christian Schaaf · Stephan Gekle
    [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 well-known charge asymmetry of ionic solvation in agreement with experimental data.
    No preview · Article · Sep 2015 · Physical Review E
  • Manuel Edgar Hollfelder · Stephan Gekle
    [Show abstract] [Hide abstract]
    ABSTRACT: Using 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 approx. 1 kT in toluene, but not in the non-aromatic 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.
    No preview · Article · Jul 2015 · The Journal of Physical Chemistry B
  • Klaus F Rinne · Stephan Gekle · Roland R Netz
    [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 ion-specific and concentration-dependent 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 non-polarizable ionic force fields used. The purely ionic contribution to the dielectric response is negligible, but determines the conductivity of the salt solutions. The ion-water 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 ion-pair 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 first-solvation shell water. With rising salt concentration the simulated spectra show more pronounced deviations from a single-Debye form and can be well described by a Cole-Cole fit, in quantitative agreement with experiments. Our spectral decomposition into ionic and different water solvation shell contributions does not render the individual contributions more Debye-like, this suggests the non-Debye-like 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 non-Debye-like character is likely to be an inherent spectral signature of solvation water around ions.
    No preview · Article · Dec 2014 · The Journal of Chemical Physics
  • Klaus F Rinne · Stephan Gekle · Roland R Netz
    [Show abstract] [Hide abstract]
    ABSTRACT: Recent femtosecond-resolved spectroscopy experiments demonstrate the single-water 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 single-water and collective-water dynamics in the ion solvation shells. While the single-molecule 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.
    No preview · Article · Dec 2014 · The Journal of Physical Chemistry A
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We compare the results from time-resolved fluorescence anisotropy experiments and molecular modelling on perylene bisimide acrylate dimers which allows us to connect the observed spectral signatures unambiguously with the non-stacked and two (parallel and anti-parallel) 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 non-stacked 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.
    Preview · Article · Oct 2014 · Physical Chemistry Chemical Physics
  • Stephan Gekle · Roland R Netz
    [Show abstract] [Hide abstract]
    ABSTRACT: Radio-frequency (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.
    No preview · Article · Apr 2014 · The Journal of Physical Chemistry B
  • Yann von Hansen · Stephan Gekle · Roland R Netz
    [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 coarse-grained 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.
    No preview · Article · Sep 2013 · Physical Review Letters
  • Stephan Gekle · Axel Arnold
    [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.
    No preview · Article · Aug 2013 · Physical Review Letters
  • Source
    [Show abstract] [Hide abstract]
    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 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.
    Full-text · Article · Feb 2013 · Physics of Fluids
  • Stephan Gekle · Roland R Netz
    [Show abstract] [Hide abstract]
    ABSTRACT: Proteins, molecules, and macromolecular assemblies in water are surrounded by a nanometer-sized 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 Maxwell-Garnett 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.
    No preview · Article · Sep 2012 · The Journal of Chemical Physics
  • Source
    [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 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 $m$ 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 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. (2009) and the model for the collapse of an axisymmetric impact-created cavity previously proposed by Bergmann et al. (2009b). By combining these two models we can reconstruct the three-dimensional shape of the cavity at any time before pinch-off.
    Full-text · Article · May 2012 · Journal of Fluid Mechanics
  • Douwe Jan Bonthuis · Stephan Gekle · Roland R Netz
    [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 space-dependent dielectric function in a mean-field 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, ion-specific adsorption effects are found to be more pronounced at hydrophobic surfaces than at hydrophilic surfaces, in agreement with experimental trends.
    No preview · Article · Mar 2012 · Langmuir
  • [Show abstract] [Hide abstract]
    ABSTRACT: Using molecular dynamics simulations we demonstrate pumping of water through a carbon nanotube by time-dependent 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 polarization-dragging mechanism accounts quantitatively for our numerical findings.
    No preview · Article · Mar 2012 · Nano Letters
  • Stephan Gekle · José Manuel Gordillo
    [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, needle-like 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 boundary-integral 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.
    No preview · Article · Dec 2011 · International Journal for Numerical Methods in Fluids
  • Douwe Jan Bonthuis · Stephan Gekle · Roland R Netz
    [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 Poisson-Boltzmann equation, the trend of experimental double-layer capacitances is well reproduced. We show that the apparent Stern layer can be understood in terms of the dielectric profile of pure water.
    No preview · Article · Oct 2011 · Physical Review Letters
  • Source
    [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 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 $m$ 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 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. (2009) and the model for the collapse of an axisymmetric impact-created cavity previously proposed by Bergmann et al. (2009b). By combining these two models we can reconstruct the three-dimensional shape of the cavity at any time before pinch-off.
    Full-text · Article · Sep 2011
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: There is no abstract available for this article.
    Full-text · Article · Sep 2011 · Physics of Fluids
  • Source
    [Show abstract] [Hide abstract]
    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 micron-sized jets with ultra high-speed 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 pinch-off between the lubrication approximation model and a boundary integral (BI) calculation, showing deviations at the final moment of the pinch-off. Inspite of this discrepancy, the results on pinch-off times and droplet and satellite droplet velocity obtained from the lubrication approximation agree with the high-speed imaging results.
    Full-text · Article · Nov 2010 · Physics of Fluids

Publication Stats

504 Citations
126.15 Total Impact Points

Institutions

  • 2013-2015
    • University of Bayreuth
      • Institute of Physics
      Bayreuth, Bavaria, Germany
  • 2011-2013
    • University of Technology Munich
      • Faculty of Physics
      München, Bavaria, Germany
  • 2007-2012
    • Universiteit Twente
      • Group of Physics of Fluids
      Enschede, Overijssel, Netherlands
  • 2006-2007
    • Universität Stuttgart
      Stuttgart, Baden-Württemberg, Germany