The article by Stefan Duhr and Dieter Braun discusses the phenomenon of thermophoresis, or thermodiffusion, where molecules drift along temperature gradients in liquids. The authors present experimental results using an all-optical microfluidic fluorescence method to study DNA and polystyrene beads over a wide range of particle sizes, salt concentrations, and temperatures. They support a unifying theory based on solvation entropy, where the Soret coefficient (the ratio of thermodiffusion to ordinary diffusion) is given by the negative solvation entropy divided by \( kT \). This theory predicts the thermodiffusion of polystyrene beads and DNA without any free parameters, assuming local thermodynamic equilibrium of solvent molecules around the particles. The study reveals that thermophoretic motion changes sign at lower temperatures, attributed to increasing positive entropy of hydration and negative entropy of ionic shielding. The understanding of thermodiffusion has implications for detailed probing of solvation properties of colloids and biomolecules, such as determining the effective charge of DNA and beads over a size range not accessible with electrophoresis. The experimental techniques used, including fluorescence microfluidic imaging, allow for the measurement of thermodiffusion over a wide molecule size range without artifacts induced by thermal convection. The theoretical approach is based on local equilibrium and the relationship between small concentration changes and small Gibbs-free energy differences, leading to a quantitative description of thermodiffusion. The study also explores the size dependence of thermodiffusion, showing that the Soret coefficient scales with particle surface area and thermodiffusion coefficient scales linearly with particle diameter. Additionally, the effective charge of particles can be inferred from thermodiffusion measurements, providing valuable information for colloid science, biology, and biotechnology.The article by Stefan Duhr and Dieter Braun discusses the phenomenon of thermophoresis, or thermodiffusion, where molecules drift along temperature gradients in liquids. The authors present experimental results using an all-optical microfluidic fluorescence method to study DNA and polystyrene beads over a wide range of particle sizes, salt concentrations, and temperatures. They support a unifying theory based on solvation entropy, where the Soret coefficient (the ratio of thermodiffusion to ordinary diffusion) is given by the negative solvation entropy divided by \( kT \). This theory predicts the thermodiffusion of polystyrene beads and DNA without any free parameters, assuming local thermodynamic equilibrium of solvent molecules around the particles. The study reveals that thermophoretic motion changes sign at lower temperatures, attributed to increasing positive entropy of hydration and negative entropy of ionic shielding. The understanding of thermodiffusion has implications for detailed probing of solvation properties of colloids and biomolecules, such as determining the effective charge of DNA and beads over a size range not accessible with electrophoresis. The experimental techniques used, including fluorescence microfluidic imaging, allow for the measurement of thermodiffusion over a wide molecule size range without artifacts induced by thermal convection. The theoretical approach is based on local equilibrium and the relationship between small concentration changes and small Gibbs-free energy differences, leading to a quantitative description of thermodiffusion. The study also explores the size dependence of thermodiffusion, showing that the Soret coefficient scales with particle surface area and thermodiffusion coefficient scales linearly with particle diameter. Additionally, the effective charge of particles can be inferred from thermodiffusion measurements, providing valuable information for colloid science, biology, and biotechnology.