Quantitative experiments on the evaporation from sessile droplets of aqueous saline (NaCl) solutions show a strong dependence on salt concentration and droplet shape. The experiments were performed with seven decades of initial NaCl concentrations, with various droplet sizes and with different contact angles. The evaporation rate is significantly lower for high salt concentrations and small contact angles than what is expected from the well-accepted diffusion-controlled evaporation scenario for sessile droplets, even if the change of the vapor pressure due to the salt is taken into account. Particle tracking velocimetry reveals that this modification of the evaporation behavior is caused by marangoni flows that are induced by surface tension gradients originating from the local evaporative peripheral salt enrichment. In addition it is found that already very low salt concentrations lead to a pinning of the three phase contact line. Whereas droplets with concentration ≥10(-6) M NaCl are pinned as soon as evaporation starts, droplets with lower salt concentration do evaporate in a constant contact angle mode. Aside from new, fundamental insights the findings are also relevant for a better understanding of the widespread phenomenon of corrosion initiated by sessile droplets.
We compare the superficial segregations of the Cu-Ag system for a nanoparticle and for surfaces that are structurally equivalent to each of its facet. Based on a lattice-gas model and within a mean-field formalism, we derive segregation isotherms at various temperatures in the canonical ensemble, i.e., for a given overall solute concentration, and in the semigrand canonical ensemble, i.e., for a given bulk solute concentration. If both processes are very similar for high temperatures, they differ substantially at lower temperatures. Due to the finite-size effect and the indirect coupling between facets and edges, the relative position of the phase transitions of the facets and the corresponding surfaces is inversed when displayed as a function of the solute bulk concentration. Moreover, we show that working in the semigrand canonical ensemble is a much more efficient way to study this phenomenon, although nanoparticles are "canonical" objects in essence.
Using Monte Carlo simulations on a lattice-gas model within the pseudo-grand-canonical ensemble, we study the competition between superficial segregation, wetting and a core dynamical equilibrium for nanoparticles made of thousands of atoms in a system that tends to phase separate, e.g., Cu-Ag. Increasing the chemical potential difference ⌬ between Ag and Cu ͑or the nominal Ag concentration͒ at a temperature lower than the critical temperature for the phase separation in the infinite crystal, we show that the cluster goes through different stages: ͑i͒ Ag-superficial segregation that involves the vertices first, then the edges, and finally the ͑111͒ and ͑001͒ facets; ͑ii͒ prewetting that leads to Ag enrichment on the shells close to the cluster surface; ͑iii͒ a dynamical equilibrium that affects all the internal shells jointly, similar to the first-order phase transition due to the miscibility gap in an infinite crystal; and ͑iv͒ again standard segregation. Moreover, we show that a similar behavior occurs for the cluster facets if the temperature is lower than the critical temperatures of the first-order phase transition of the corresponding surfaces of semi-infinite crystals. A remarkable consequence of those dynamical equilibria is that very different concentrations of the facets on one hand and of the whole cluster on the other hand can be observed at a given ⌬.
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