Phase behavior of a de-ionized binary mixture of charged spheresin the presence of gravity

摘要

We report on the phase behavior of an aqueous binary charged sphere suspension under exhaustivelyde-ionized conditions as a function of number fraction of small particles p and total number densityn. The mixture of size ratio F =0.557 displays a complex phase diagram. Formation of bcc crystalswith no compositional order dominates. We observe a region of drastically decreased crystalstability at 0.55 < 0.95 with the minimum located at p= 0.8 ± 0.05 at densities above n=26 μm-3. A peaked region of enhanced stability is observed at p =0.4. Further light scatteringexperiments were conducted to characterize the crystallization time scales, the density profiles, andthe composition of formed phases. For 0.82 >p > 0.95 crystal formation is partially assisted bygravity, i.e., gravitational separation of the two species precedes crystal formation for samples in thecoexistence range. In the composition range corresponding to the decreased crystal stability onlylower bounds of the freezing and melting line are obtained, but the general shape of the phasediagram is retained. At p =0.93 and n =43 μm-3 two different crystalline phases coexist in the bulk,while at p =0.4 additional Bragg peaks appear in the static light scattering experiments. Thisstrongly suggests that we observe an eutectic in the region of decreased stability, while the enhancedstability at p =0.4 seems to correlate with compound formation.