Droplet drying was also visualized with a custom ultrahigh resolution spectral domain optical coherence microscopy system (UHR-OCM)40. Differences in flow patterns are easily visualized with UHR-OCM by adding 1 μm polystyrene particles to the suspension (Supplementary Note 1 and Supplementary Figs 1 and 2). The patterns are shown in Fig. 2 and in the Supplementary Movies 2–4. Pure circular convective flows are seen from the earliest observed times (at <0.3 s) and persist until the emergence of a nematic phase. The difference in flow circulation direction compared to previous observations with surfactants arises because higher concentrations of SSY at the interface cause the surface tension to increase rather than decrease from its bare value. This behaviour is also observed among many salts and described phenomenologically by the Hofmeister series, which orders anions by their effect on surface tension41,42. Since SSY has two sulfate groups, which are high in the Hofmeister series, we surmise that the SSY molecules tend to induce a large increase in surface tension. The microscopic causes of these effects in LCLC drops may be related to the unusual amphiphilic structure of SSY, which leads to assemblies of molecules that do not align like conventional surfactant amphiphiles at an interface43.

 

  The drying progression imaged by UHR-OCM. The drop is placed inside a humidity trapping enclosure that slows its drying rate. White spots in the image are micron-diameter polystyrene particles that strongly reflect light and act as tracers of convective fluid flows and LC phase boundaries. Image capture begins within 30 s of placing the drop. (a) In the initial drying stage, convective flows move towards the pinned contact line along the drop-air interface and move inward to the drop center along the substrate (see arrows in the frame taken at 0+s). (b) At later times, a phase boundary, identified by the arrows, shows that particles in the isotropic phase (I) are prevented from entering the viscous and comparatively dense nematic (N) region that is nucleating from the droplet edge; the particle concentration tends to be large at these phase boundaries. (c) Eventually, particles are swept towards the droplet center where they form a shell around a remaining isotropic fluid bubble (arrow, 538 s), and as the region of isotropic phase shrinks to zero volume, the particles irreversibly cluster. The columnar phase first appears in the OCM images as white lines near the droplet edge. These bright lines are not caused by particles; rather, they are cuts through boundaries between domains of varying columnar orientation and thus strongly scatter LED High Bay Light . (d) The white lines at the edges of the drop in the last frame (695 s, dashed arrow) show the boundaries of columnar phase (C) regions.