  In this article, we demonstrate a method for inducing reversible
crystal-to-crystal transitions in binary mixtures of soft colloidal particles.
Through a controlled decrease of salinity and increasingly dominating
electrostatic interactions, a single sample is shown to reversibly organize
into entropic crystals, electrostatic attraction-dominated crystals or
aggregated gels, which we quantify using microscopy and image analysis. We
furthermore analyze crystalline structures with bond order analysis to discern
between two crystal phases. We observe the different phases using a sample
holder geometry that allows both in situ salinity control and imaging through
Confocal Laser Scanning Microscopy, and apply a synthesis method producing
particles with high resolvability in microscopy with control over particle
size. The particle softness provides for an enhanced crystallization speed,
while altering the re-entrant melting behavior as compared to hard sphere
systems. This work thus provides several tools for use in the reproducible
manufacture and analysis of binary colloidal crystals.
