Polarization in hot water

  Statistical Mechanics Biological Physics

Many temperature measuring devices are thermocouples based on the Seebeck effect: a temperature gradient applied to a circuit made from dissimilar metals induces an electric current. The reverse phenomenon – the Peltier effect, in which a current drives a thermal gradient – is the basis for compact thermoelectric refrigerators. Such thermoelectric effects are well known in metals and semiconductors, but what about other materials such as insulators in strong thermal gradients? A collaboration from Imperial College London, the Norwegian Academy of Sciences, and the Norwegian Institute of Science and Technology now finds that with a big enough thermal gradient, even bulk liquid water shows signs of electrical polarization.

The Seebeck effect results from charge carrier diffusion along the thermal gradient. For water, however, Bresme, Lervik, Bedeaux, and Kjelstrip find that thermal reorientation of water molecules can lead to polarization of the bulk liquid, resulting in a sizeable electrostatic field. To examine this effect, the researchers carried out nonequilibrium molecular dynamics simulations with up to 3240 water molecules confined to a rectangular box having heat sources on the edges. As a reality check, the authors obtained good agreement between their simulated equation of state (which relates values such as temperature, pressure, volume and internal energy) and experimental data.

To obtain the electrostatic field gradient, Bresme et al. calculated the spatial charge distribution. For thermal gradients in the neighborhood of 10¹⁰ K/m they observe fields of about 10⁸ V/m, but where do such extreme conditions exist? In fact, the authors note, these field gradients are characteristic of biomembranes and ionic thin films as well as of the conditions found in nanoparticle systems that experience heating from absorption of electromagnetic radiation. A better understanding of such effects may be relevant in proposals to destroy cancer cells with nanoparticles and radiation sources. - David Voss