Deposition and drying dynamics of liquid crystal droplets

Drop drying and deposition phenomena reveal a rich interplay of fundamental science and engineering, give rise to fascinating everyday effects (coffee rings), and influence technologies ranging from printing to genotyping. Here we investigate evaporation dynamics, morphology, and deposition patterns of drying lyotropic chromonic liquid crystal droplets. These drops differ from typical evaporating colloidal drops primarily due to their concentration-dependent isotropic, nematic, and columnar phases. Phase separation occurs during evaporation, and in the process creates surface tension gradients and significant density and viscosity variation within the droplet. As a result, the drying multiphase drops exhibit different convective currents, drop morphologies, and deposition patterns (coffee-rings).

Optical coherence microscopy (OCM) is a high-resolution version of optical coherence tomography [26]. In OCM, a broadband light source, e.g. a supercontinuum laser, is utilized to provide an axial resolution as high as 1 − 2 µm in the sample, and a high numerical aperture (NA) objective is equipped in the sample arm to provide high transverse resolution for imaging. OCM is often employed to observe cellular structure and subcellular features in biomedical samples [27][28][29]. In our study, OCM is well-suited for droplet imaging because it provides rapid depth-resolved crosssectional images of the drying drop. Moreover, the high-resolution in both axial and transverse dimensions enables movement tracking of particles at the same scale, i.e. 1 − 2 µm, within the droplet. Previous studies have demonstrated the ability of OCT (OCM) to track particle motion within evaporating drops [30][31][32].
Supplementary Figure 1 shows a schematic diagram of a custom ultra-high resolution optical coherence microscopy system (UHR-OCM) [27]. A supercontinuum laser (SC-400-4, Fianium Ltd.) is employed to provide a broadband light with a central wavelength of 800 nm and a spectral range of ∼ 220 nm, yielding an axial resolution of ∼ 1.5 µm in the tissue. Incident light is split 50:50 at the coupler and transmitted to the reference arm and the sample arm separately. A 2D galvanometer (GVS002, Thorlabs) performs transverse scanning of the droplet. A 10x objective is used to image the droplet, yielding a transverse resolution of ∼ 3.5 µm. A grating spectrometer with an f-theta lens and a 2048-pixel line scan camera (e2V) is configured to detect back-scattered interferograms at a speed of 20,000 axial scans/s. The sensitivity of this system was measured to be 95dB.
The sample arm of the UHR-OCM system was configured as an inverted microscope, where the illumination beam was shone on the droplet from below (see Supplementary Figure 1). As illustrated in Supplementary Figure 2, with a standard upright illumination configuration, the bottom surface of the droplet (i.e. flat cover glass) will be optically distorted, due to a combination of light refraction and increased optical path length within the droplet. With an inverted illumination configuration, OCM imaging beam was shone on the droplet through a flat cover glass, minimizing distortion from light refraction. The acquired OCM images were further scaled in the axial dimension by the index of refraction of the SSY solution, approximated as water (n = 1.33), in order to characterize the physical dimension (i.e. height) of the droplet.
We probed the earliest stages of the evaporation process with UHR-OCM by initiating image acquisition prior to a LCLC drop being pipetted into a coverslip and PDMS chamber. The pipette tip was positioned directly above the center of the scanning beam, in order to obtain cross-sectional OCM images from the drop center. A coverslip was placed on top of the PDMS chamber immediately after pipetting. 2D scans were performed repeatedly at the same position, with each 2D scan consisting of 600 axial scans. A total of 800 2D scans were obtained, yielding a 24-second imaging time (∼ 33 frames per second). This integration time was sufficient for drop pipetting and observation of initial behavior of the LCLC drop. For visualization, ten consecutive frames were averaged to suppress the speckle noise and improve OCT image quality. From Supplementary Movie 2, we can clearly see that the convective flows are initialized within the LCLC drop immediately after the drop is pipetted into the chamber. Since the frames are acquired every 0.3 s (0.03 s without averaging), we have shown that the convective flows begin at the earliest stage (or at least a very early stage) of the evaporation process.
A SSY solution drop with 0.2 µl volume was pipetted onto a cover glass (Fisher, 12-548-5E). A humidty chamber enclosure was optionally used to slow down the drying rate. Time-lapse OCM imaging was initiated within ∼30 s after pipetting. 2D cross-sectional scanning at the drop center was performed and each 2D scan consisted of 400 axial scans. Transverse image range was adjusted accordingly to cover the whole drop. 2D scans were repeated 30 times at the same cross-section (∼ 0.6 s) and averaged to reduce the speckle noise and improve OCM image quality. Time-lapse OCM data were acquired every 3-4 s, depending on the evaporation rate and the configuration of computer system. Under the typical humidity-controlled condition, the drop dried in approximately 15 minutes. Over 300 OCM slices were obtained to make the time-lapse video of the drop drying progression.

Supplementary Note 2
A concentration gradient of SSY was created in a 20 µm thick cell at room temperature by filling the cell with an SSY solution in the isotropic phase and allowing evaporation of the water to occur very slowly from one side of the sample. The glass surfaces of the cell had been rubbed with a fine abrasive foam to produce alignment of the direction in the nematic phase (see Supplementary  Figure 9). Using the SSY equilibrium phase diagram [33], the SSY concentration at the nematicisotropic transition is 27.8 wt%, and the SSY concentration at the columnar-nematic transition is 35.9 wt%. The appearance of the three phases in this "equilibrium" experiment is similar to the appearance of the three phases in the drying droplet (main text Fig. 1).
An image of the columnar-nematic transition region at higher magnification (see Supplementary Figure 10) reveals that the appearance of the columnar phase in this "equilibrium" experiment is similar to its appearance in the drying droplet (main text Fig. 4 and Supplementary Figure 4).