Colloidal self-assembly is a process in which dispersed matter spontaneously form higher-order structures without external intervention. During self-assembly, packed particles are subject to solvent-evaporation induced dynamic structuring phases, which leads to microscale defects called the grain boundaries. While it is imperative to precisely control detailed grain boundaries to fabricate well-defined self-assembled crystals, the understanding of the colloidal physics that govern grain boundaries remains a challenge due to limited resolutions of current visualization approaches. In this work, we experimentally report in situ interparticle interactions during evaporative colloidal assembly by studying a novel microscale laser induced fluorescence technique. The fluorescence microscopy measures the saturation levels with high fidelity to identify distinct colloidal structuring regimes during self-assembly as well as cracking mechanics. The techniques discussed in this work not only enables unprecedented levels of colloidal self-assembly analysis but also have potential to be used for various sensing applications with microscopic resolutions.
In situ monitoring of evaporative colloidal self-assembly through the use of microscale laser induced fluorescence (μLIF) techniques. (a) Particles are guided towards a growth front by evaporation-induced convective flows. An ordered structure of colloidal spheres forms as more spheres accumulate. to and t1 represent different time frames. (b) As nanospheres start to assemble into crystalline opals, a relatively constant thickness film develops. (c-e) The number of dye particles loaded onto colloidal particles and mean distances between doped particles dictates the saturation level of the film and thereby the fluorescence intensity. The figures illustrate how the local dye concentration changes during the three distinctive regimes called the (c) saturated regime, the (d) wet regime, and the (e) dry regime. The change in fluorescence intensity is illustrated as orange luminance. (f) During the μLIF measurement, a light source (green) excites the dye particles, causing it to fluoresce light at a higher wavelength. The light passes through an optical filter to a charge-coupled device (CCD) camera and is postprocessed in PC with software.
Main Findings
We employ a novel μLIF technique that overcomes extant resolution barriers to experimentally characterizes real-time interparticle interactions in colloidal assemblies that decide grain boundary formation mechanisms with up to 300 nm spatial resolution. Our method identifies three distinctive saturation level-dictated structuring phases called the saturated, the wet, and the dry regime. The fluorescence intensity increases by 44% and 153% during saturated to wet regime and wet to dry regime transition, respectively. The saturation level jumps indicate discrete saturation level reductions during the self-assembly process. The quick transition (~15 s) from saturated to wet regime suggest a rapid structuring of nanospheres. In addition, grain boundaries form in the wet regime where the local saturation level measurements shows that the near-crack domain has 25% lower normalized dryness than the opal domain, suggesting the existence of microscale evaporation rate misbalances between the two domains. The evaporation rate misbalances in turn can cause auxiliary tensile forces that especially promote longitudinal cracks. Furthermore, in situ saturation level monitoring shows that transverse cracks function as hydraulic barriers that trigger local grain dry-out, which eventuate in non-simultaneous film dry-out. The findings in this study provides foundational grounds for engineering novel micro/nanostructures and promises innovational advances in large scale fabrication of self-assembled structures.
Distinctive saturation regimes during evaporative self-assembly. (a) The image shows the wet regime expanding through the saturated regime. (b) As the wet regime expands, cracks initiate and propagate in the drying direction (i.e., longitudinal cracks), which are indicated by arrows. (c) At a later stage, cracks perpendicular to the drying direction (i.e., transverse cracks) form and create an isolated domain (i.e., grain). The opal film dries in discrete steps as local grain dry-out occurs. Individual grains are identified with numbers for the dry-out analysis in Figure 4. (d) The fluorescence intensity abruptly increases after the complete dry-out. The scale bars represent 100 μm. (e) The average fluorescence intensity of the dashed box in (a) shows stepwise increases with time. Each step represents a different saturation regime (e.g., saturated regime, wet regime, and dry regime). The inset shows the average intensity of each saturation regime. Crack formation and propagation phenomena associated with the local evaporation. (a-c) High magnified fluorescence images show a longitudinal crack propagating through the film. (a) The drying direction and cracking direction are shown. (b) The opal domain and the near-crack domain is indicated in the figure. (c) Dry-out causes the fluorescence intensity to drastically increase. (d) The fluorescence intensity profile of the dashed line drawn in (a) exhibits high intensities at the near-crack domain (red block). The abrupt dip in fluorescence intensity is caused by the absence of solution at the fissure caused by the crack. (e) Time-dependent plots of the opal and the near-crack domains show fluorescence intensity differences between the two domains decreases as the film dries. (f) The normalized dryness shows that the near-crack domain has a ~25% higher fluorescence intensity than the opal domain during the wet regime, which indicates a lower saturation level. The crack formation and propagation associated with local evaporation phenomena are illustrated from the (g-i) top and (j-l) front view. The blue and grey color represent dry and wet nanospheres, respectively. Grain dry-out mechanism. (a) As longitudinal cracks form, the separated structures are continuously supplied with liquid via capillary wicking. Longitudinal cracks form due to tensile forces (indicated in black arrows) in the direction perpendicular to the drying direction. On the other hand, (b) transverse cracks form due to tensile forces parallel to the drying direction (indicated in black arrows). Transverse cracks prevents further liquid supply to the isolated structure (i.e., grain), which leads to local grain dry-out. (c) The average intensity of individual grains in Figure 2c displays a stepwise increase right after transverse cracks form. The arrows indicate when longitudinal and transverse cracks start to form.
