Stephan Gekle

University of Bayreuth, Bayreuth, Bavaria, Germany

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Publications (6)39.59 Total impact

  • Stephan Gekle, Roland R Netz
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    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.
    The Journal of Physical Chemistry B 04/2014; · 3.61 Impact Factor
  • Yann von Hansen, Stephan Gekle, Roland R Netz
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    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.
    Physical Review Letters 09/2013; 111(11):118103. · 7.73 Impact Factor
  • Stephan Gekle, Roland R Netz
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    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.
    The Journal of Chemical Physics 09/2012; 137(10):104704. · 3.12 Impact Factor
  • [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.
    Langmuir 03/2012; 28(20):7679-94. · 4.38 Impact Factor
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    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.
    Nano Letters 03/2012; 12(4):1780-3. · 13.03 Impact Factor
  • [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.
    Physical Review Letters 10/2011; 107(16):166102. · 7.73 Impact Factor

Publication Stats

38 Citations
39.59 Total Impact Points

Institutions

  • 2014
    • University of Bayreuth
      Bayreuth, Bavaria, Germany
  • 2012–2013
    • Freie Universität Berlin
      • Department of Physics
      Berlin, Land Berlin, Germany
  • 2011–2013
    • University of Technology Munich
      • Faculty of Physics
      München, Bavaria, Germany