| Authors: Arghya Majee(a,b) and Alois W�rger(c) a)Max-Planck Institut f�r Intelligent Systeme, Heisenbergstr. 3, 70569 Stuttgart, Germany b)4. Institut f�r Theoretische Physik, Univesit�t Stuttgart, Germany c)LOMA, Universit� de Bordeaux & CNRS, 33405 Talence, France Abstract: Selective transport and controlled pattern formation are of fundamental interest in microfluidics and biotechnology. Common methods are based on electrophoresis, or motion driven by thermal and chemical gradients. As a versatile tool for addressing colloidal solutes, temperature gradients have been used for designing particle focusing devices and macromolecular trap [1, 2]. We propose a novel actuation mechanism for hot colloidal particles which relies on the electrolyte Seebeck effect [3]. Because of their temperature dependent solvation energy, positive and negative ions of the solution migrate along an applied thermal gradient grad(T), resulting in a macroscopic thermoelectric field E=S grad(T), with the Seebeck coefficient S. Over past few years it has become clear that this Seebeck effect is essential for thermophoresis of charged colloids; because of the ion-specific Seebeck coefficient, transport velocity is very sensitive to the electrolyte composition [4-7]. Here we discuss the Seebeck effect in the vicinity of a hot colloidal particle with radius a and excess temperature dT (In recent experiments dT values of hundreds of Kelvin have been achieved by absorption of a focused laser beam [8]). Temperature gradient grad(T)=-dT.a/r^2 results in an unscreened thermoelectric field E=-S.dT.a/r^2 (1-(r+k)/(a+k) e^((a-r)/k)), which beyond one Debye length k, varies with the inverse square of the distance r. This field is related to a net charge Q=-4.pi.epsilon.a.S.dT accumulated within one Debye length from the particle surface. With the permittivity (epsilon) of water and the Seebeck coefficient S(=-0.22 mV/K) of NaOH solution, one finds that a (non-ionic) micron-size particle with dT=30K carries a thermocharge of several hundred elementary charges. The nearby field attains values of tens of kV/m [3]. This charge and the field are intimately related to the non-equilibrium nature of the Seebeck effect; in thermal equilibrium, charges in an electrolyte solution are screened and their electric field decays exponentially. As possible applications, we discuss how the size-dependence of the thermocharge can be used for sieving colloidal particles by size, or carbon nanotubes by their electronic and optical absorption properties. [1] H.-R. Jiang, H. Wada, N. Yoshinaga, M. Sano, Phys. Rev. Lett. 102, 208301 (2009) [2] C. J. Wienken, Ph. Baaske, U. Rothbauer, D. Braun, S. Duhr, Nature Comm. 1, 100 (2010) [3] A. Majee, A. W�rger, Phys. Rev. Lett. 108, 118301 (2012) [4] S. A. Putnam, D. G. Cahill, Langmuir 21, 5317 (2005) [5] D. Vigolo, S. Buzzaccaro, R. Piazza, Langmuir 26, 7792 (2010) [6] A. W�rger, Rep. Prog. Phys. 73, 126601 (2010) [7] A. Majee, A. W�rger, Phys. Rev. E 83, 061403 (2011) [8] D. Rings, R. Schachoff, M. Selmke, F. Cichos, K. Kroy, Phys. Rev. Lett. 105, 090604 (2010) |
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