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Paper   IPM / Physic / 8765
School of Physics
  Title:   The Dynamics for the Soret Motion of a Charged Spherical Colloid
1.  S.N. Rasuli
2.  R. Golestanian
  Status:   In Proceedings
  Proceeding: Proceedings of The 8th International Meeting on Thermophoresis, 9-13 June 2008, Gustav-Stresemann-Institut, Bonn, Germany
  Year:  2008
  Supported by:  IPM
The Soret effect for a single charged colloidal particle, has been studied by different experimental groups in recent years [1, 2] and still seems a challenging topic. We know two distinct theoretical approaches to this phenomenon. The First, motivated by Ruckenstein in 1981 [3], is based on solving hydrodynamics equations for the charged fluid around colloid. This approach is restricted to weakly charged colloids with thin double-layer around them [3?5], and was verified with Piazza and Guarino in 2002 [1]. The second approach however, uses Gibbs enthalpy [2, 6] to predict the density profile of a colloid in a temperature field. It is seemed that this approach has been tested successfully for Polystyrene beads by Duhr and Braun [2]. Recently, Astumian [7] suggested that, we can interpret the Ruckenstein?s approach as the deterministic motion of a charged colloid in a temperature field, while attribute the second approach to colloid stochastic Langevin motion in the temperature field [7]. Accepting his suggestion, two mentioned approaches, may come together in a unified theory which addresses both kinds of motion simultaneously. Here, we extend the Ruckenstein?s hydrodynamics approach to a colloid with arbitrary doublelayer around it. We consider the dielectrophoretic force in our formalism, and since the Boltzmann weight is hardly reliable in the presence of a temperature gradient, we solve the diffusion equation to find ions densities. In the diffusion equation, we consider both the convective motion and ions Soret motion [8]. For a weakly charged colloid, our equations are explicitly solved. Our result has the Ruckenstein?s formula, as its limiting case. For a colloid with high surface potential also, we solve the equations numerically. We confront our results with existing experimental data [1, 2] and possible agreements and/or disagreements are discussed [8, 9].

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