Dried supraball powders and films were directly imaged using an Olympus BX 51 microscope under the dark-field mode. Here, we used white Teflon tape as a white standard. We measured reflectance spectra of single supraballs and films using a CRAIC AX10 ultraviolet-visible-near-infrared microspectrophotometer (CRAIC Technologies Inc.). We used a 50× objective and a 75-W Xenon short arc lamp (Ushio UXL-75XE) for the light source. We reported the averaged spectra from six measurements using pavo package in R programming software (51).

We combined SEM and TEM to investigate the distribution of SPs and SMPs in the photonic assemblies. We imaged the surface of both supraballs and films using an SEM (JEOL-7401, JEOL Ltd) without any sputter-coating to maintain the inherent contrast from SMPs and SPs. To quantify the SEM images, we used a MATLAB code [imfindcircles (52)] for images containing different sizes of particles and a python code [Trackpy (53)] for images containing particles with different brightness. Using these centers and radii, we plotted the measured circles over the original image to ensure that this analysis captured the real features of particles (fig. S13). The output of running these algorithms was a list of particle sizes and positions within each image, which we used to calculate the surface volume fraction and g(r) for the surface layer. To prepare thin cross sections of supraballs for TEM imaging, we embedded dried supraball powders with EMbed 812 resin and cut the samples to 80-nm-thick sections using a diamond knife on a Leica UC7 ultramicrotome. TEM images and EDS elemental map were acquired using a Hitachi HD-2300 Dual EDS Cryo STEM equipped with dual Thermo Fisher Scientific (Waltham, MA) EDS detectors (total x-ray collection angle, ~0.8 steradian). Both TEM images and EDS data were obtained using Thermo Fisher Scientific NSS software under EDX operation mode and TE mode (phase contrast mode).

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