By performing measurements on a large class of macromolecular and colloidal systems, we show that thermophoresis (particle drift induced by thermal gradients) in aqueous solvents displays a distinctive universal dependence on temperature. For systems of particles interacting via temperature-independent forces, this behavior is strictly related to the solvent thermal expansivity, while an additional, T-independent term is needed to account for the behavior of "thermophilic" (migrating to the warmth) particles. The former relation between thermophoresis and thermal expansion may be exploited to envisage other fruitful studies of colloidal diffusion in inhomogeneous fluids.
Thermophoresis, unlike thermal diffusion in simple mixtures, consists in particle drift induced by a temperature gradient ∇T . We show that thermophoresis in lysozyme solutions has a very distinctive behavior: particle motion can indeed be tuned from "thermophobic" (towards the cold) to "thermophilic" (along ∇T ) by decreasing T . The observed temperature behaviour weakly depends on electrostatic effects, and rather suggests a primary role of hydrophobic interactions, further supported by comparison with the temperature dependence of lysozyme equilibrium solubility. Most of the observed features can be qualitatively understood by envisaging thermophoresis as a "microscopic Marangoni effect", due to thermally induced gradients of the interfacial free energy.
Highly cross-linked polystyrene microgel colloids dispersed in an index and density matching solvent provide a system with hard-sphere-like interactions, where gravity effects are effectively minimized. They are a suitable target for time-resolved observations of solidification in purely repulsive systems. We have investigated the crystallization kinetics at increasing undercooling using time resolved light scattering. Crystallization starts always with the formation of compressed, structurally heterogeneous precursor domains. In the coexistence region the precursors, after being converted into true crystallites, start growing fast by assimilating particles from the melt. The resulting polycrystalline material consists of high quality crystals and seems not to undergo long time-scale rearrangements. As the particle concentration grows, the higher undercooling and reduced particle mobility increasingly compromise the conversion-growth process. The growth of crystallites relies then on much slower ripeninglike processes, while refining of the crystal structure is detected up to the longest observed times.
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