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   Advanced imaging of droplet drying Alerter l'administrateur Recommander à un ami Lien de l'article 

  SSY-based LCLCs are composed of organic, charged, plank-like molecules that organize in water into column-like mesogenic stacks. The internal structure of these rods depends on a combination of non-covalent electrostatic, excluded volume, hydrophobic, and π–π stacking interactions46,49. The mesogen assemblies, in turn, organize into nematic or columnar LC phases, depending on temperature and concentration. Under ambient equilibrium conditions, the isotropic—nematic transition occurs at about 28% by weight and the nematic—columnar transition occurs at about 36% by weight50. Thus SSY concentration affects two levels of organization: mesogen assembly and LC formation. SSY was purchased from Sigma-Aldrich with 90% purity and was further purified using a precipitation method51. SSY solutions of various weight concentrations were prepared with deionized water (ρ≥18 MΩ cm).

  The SSY droplets were pipetted from vials containing the various initial solution concentrations and were deposited onto clean glass slides or coverslips. Initial contact angles of the drops were observed to vary depending on the SSY concentration and the substrate surface. Generally, as seen in Supplementary Fig. 7, drops with comparatively high initial concentrations of SSY tended to have larger contact angles than lower concentration solutions. Evidently, SSY molecules and associated mesogens adsorbed to the air-water interface cause the surface tension to increase with respect to its bare value, with the largest SSY concentrations causing the largest surface tension increments. We confirmed the surface tension increases by the pendant drop method as reported in Supplementary Fig. 8. In addition, drops at 20% SSY by weight had larger contact angles on coverslips (∼51°) than on glass slides (∼20°).

  Droplet microscopy observations

  To connect the observations in the drying droplets to the SSY equilibrium phase diagram, experiments were conducted on an SSY sample at room temperature with a concentration gradient (see Supplementary Note 2). The order of appearance of the phases was the same for both the ‘equilibrium' and drying droplets experiments, and the appearance of the phases themselves was similar for both the ‘equilibrium’ and drying droplet experiment.

  Typical droplet volumes were 0.2–0.5 μl. Droplet evaporation was observed in both ambient and slow-drying conditions. The latter conditions were achieved by placing droplets in semi-permeable cross-linked polydimethylsiloxane (PDMS) chambers bonded to the substrate and sealed with a cover slip. Evaporation times correspondingly varied, that is, from minutes in ambient conditions to hours in the PDMS chambers. Videos of the evaporation process were captured by transmission optical microscopy with and without crossed polarizers. The use of POM readily permitted assignment of LC phase (for example, isotropic, nematic and columnar) and provided structural information (director configuration). Finally, the predominant orientation of the columns was readily determined by measuring polarized LED High Bay Light absorption near the absorption peak of isotropic SSY, λ=470 nm (±15 nm FWHM). SSY assemblies exhibit linear dichroism, and since their absorption is greatest for light polarized perpendicular to the liquid crystal director51, absorption anisotropy can be utilized to assign director orientation.

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