Abstract:We study the Soret coefficient of binary molecular mixtures with dispersion forces. Relying on standard transport theory for liquids, we derive explicit expressions for the thermophoretic mobility and the Soret coefficient. Their sign depends on composition, the size ratio of the two species, and the ratio of Hamaker constants. Our results account for several features observed in experiment, such as a linear variation with the composition; they confirm the general rule that small molecules migrate to the warm,… Show more
Recent thermophoretic experiments on colloidal suspensions revived an old debate, namely whether the Soret effect is properly described by thermostatics, or necessarily requires nonequilibrium thermodynamics. Based on colloidal transport theory and the entropy production of the related viscous flow, our analysis leads to the conclusion that the equilibrium approach may work for small ions, yet fails for colloidal particles and polymers. Regarding binary molecular mixtures, our results shed some doubt on the validity of thermostatic approaches that derive the Soret coefficient from equilibrium potentials.
Recent thermophoretic experiments on colloidal suspensions revived an old debate, namely whether the Soret effect is properly described by thermostatics, or necessarily requires nonequilibrium thermodynamics. Based on colloidal transport theory and the entropy production of the related viscous flow, our analysis leads to the conclusion that the equilibrium approach may work for small ions, yet fails for colloidal particles and polymers. Regarding binary molecular mixtures, our results shed some doubt on the validity of thermostatic approaches that derive the Soret coefficient from equilibrium potentials.
“…(4), which consider the gradient of μ 12 /T as the driving force for mass diffusion, predict that, even in the absence of coupling terms (i. e., L ϕq = L qϕ = 0), a temperature gradient, applied to an otherwise uniform regular binary mixture symmetric around composition ϕ = 0.5, induces a material flux. This effect, called thermal diffusion, is described by the Soret non-dimensional coefficient S T , defined as [12,[20][21][22]…”
Non-equilibrium thermodynamics provides a general framework for the description of mass and thermal diffusion, thereby including also cross-thermal and material diffusion effects, which are generally modeled through the Onsager coupling terms within the constitutive equations relating heat and mass flux to the gradients of temperature and chemical potential. These so-called Soret and Dufour coefficients are not uniquely defined, though, as they can be derived by adopting one of the several constitutive relations satisfying the principles of non-equilibrium thermodynamics. Therefore, mass diffusion induced by a temperature gradient and heat conduction induced by a composition gradient can be implicitly, and unexpectedly, predicted even in the absence of coupling terms. This study presents a critical analysis of different formulations of the constitutive relations, with special focus on regular binary mixtures. It is shown that, among the different formulations presented, the one which adopts the chemical potential gradient at constant temperature as the driving force for mass diffusion allows for the implicit thermo-diffusion effect to be strictly absent while the resulting Dufour effect is negligibly small. Such a formulation must be preferred to the other ones since cross-coupling effects are predicted only if explicitly introduced via Onsager coupling coefficients.
“…Flow field induced by a thermophilic NP of a colloid on its solvent. Reproduced with permission . Copyright 2011, AIP Publishing LLP.…”
Section: Multiphysics Effects Involvedmentioning
confidence: 99%
“…Furthermore, particles may tend to aggregate as a consequence of their enhanced interactions, leading to non‐Newtonian effects. Fortunately, the several phenomena described here are cumulative, and the net slip‐flux of the concentration of solids in the FF is the sum of the diffusive, thermophoretic, and magnetophoretic contributions . To give a quantitative dimension to our discussion, the velocity gradient of the continuum distribution of the mixture can be written as …”
Section: Multiphysics Effects Involvedmentioning
confidence: 99%
“…Fortunately, the several phenomena described here are cumulative, and the net slip-flux of the concentration of solids in the FF is the sum of the diffusive, thermophoretic, and magnetophoretic contributions. [31] To give a quantitative dimension to our discussion, the velocity gradient of the continuum distribution of the mixture can be written as [33]…”
Colloidal devices—in the liquid state—possibly protected from harsh planetary environments by a soft protective deformable skin, represent a totally new paradigm in the field of robotics. They include the autonomy features of conventional robotics and the shape‐changing advantages of soft robotics and offer new opportunities to science and technology. For these systems, energy management is considered to be the most essential function to guarantee autonomy and operation. Herein, smart fluids as autonomous systems with energy‐harvesting/storage capabilities are described and an initial assessment of existing possibilities that can be leveraged to pursue the emerging topic of colloidal autonomous systems is provided.
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