In Situ Monitoring of Structural Changes during Colloidal Self-Assembly

Abstract

Reflectance spectroscopy is utilized to monitor structural changes during the self-assembly of a monodisperse colloidal system at the meniscus of a sessile drop on an inert substrate. Treating the ordered colloidal structure as a photonic crystal is equivalent to monitoring the changes in the photonic band gap (PBG) as the colloidal system self-assembles heterogeneously into a crystal through solvent evaporation in ambient conditions. Using a modified Bragg's law model of the photonic crystal, we can trace the structural evolution of the self-assembling colloidal system. After a certain induction period, a face-centered cubic (FCC) structure emerges, albeit with a lattice parameter larger than that of a true close-packed structure. This FCC structure is maintained while the lattice parameter shrinks continuously with further increase in the colloidal concentration due to drying. When the structure reaches a lattice parameter 1.09 times the size of that of a true close-packed structure, it undergoes an abrupt decrease in lattice spacing, apparently similar to those reported for lattice-distortive martensitic transformations. This abrupt final lattice shrinkage agrees well with the estimated Debye screening length of the electric double layer of charged colloids and could be the fundamental reason behind the cracking commonly seen in colloidal crystals.

Formation of silica colloidal crystals on soft hydrophobic vs rigid hydrophilic surfaces

“…In the vertical deposition process, NPs align into an expanded hexagonal structure near the triple-phase line with large spacing as illustrated in Fig. 4 (condition A) [27]. When the interstitial solvent evaporates, NPs move closer to each other, and eventually form the hexagonal close packing structure (condition B).…”

Synthesis of Porous Bi2WO6 Thin Films as Efficient Visible‐Light‐Active Photocatalysts

“…The layer was composed of an hexagonal array of carbon spheres with diameters of 340 nm. Cracks were observed in the array; Koh and Wong [14] have pointed out that the occurrence these is due to the disappearance of the Debye screening layers. The disappearance of Debye screening layers resulted in high stresses on the array, and the response for this COMMUNICATION www.advmat.de Scheme 1.…”

Wetting dynamics of colloidal dispersions on agar gel surfaces

“…As mentioned in the introduction, a solid-like ordering of the particles was observed in the confined three-phase contact region at the edge of the wetting fluid [13][14][15]. This condensation can induce thickening and inhibit wetting at the three-phase contact line.…”

“…Furthermore, a solid-like ordering of suspended spheres occurs in the confined three-phase contact region at the edge of the wetting fluid, as described by Wassan et al who used video microscopy to demonstrate the two-dimensional crystal-like ordering of charged nanometer-sized polystyrene spheres in water [13,14]. Koh and Wong utilized reflectance spectroscopy to monitor structural changes during the self-assembly of a monodisperse colloidal system at the three-phase contact line of a sessile drop on an inert substrate [15]. Solids dispersed in a drying drop will migrate to the edge of the drop and form a solid ring called a "coffee ring.…”