Control of Orientation, Formation of Ordered Structures, and Self-Sorting of Surface-Functionalized Microcubes at the Air–Water Interface

Abstract

The dependence of the orientation of microscale PS cubes, which are surface functionalized on only five faces, at the water/air interface and the ordered aggregates formed by capillary force assembly are reported. Depending on the wettability of the faces, the cubes were shown to adopt a preferred orientation that changes with decreasing wettability from face up to edge up and further to vertex up. Concomitantly, stable aggregates with different structures were formed by capillary force self-assembly. The unmodified bottom face of the cubes was localized by fluorescence labeling. Finally, self-sorting of differently surface functionalized microcubes was realized for the first time, due to the stronger capillary interactions of quadrupole–quadrupole and hexapole–hexapole interactions compared to quadrupole–hexapole interaction.

Chains of cubic colloids at fluid–fluid interfaces

“…The scenario experimentally found in Refs. [23,24] represents, at the same time, a confirmation and an extension of the theoretical predictions previously presented in Refs. [25,26].…”

“…While hydrophilic cubes with a small θ ∼ 0 give rise to square close-packed aggregates, by increasing θ first linear aggregates and finally hexagonal aggregates are observed. Cases of coexistence between linear and hexagonal aggregates have also been reported [23,24].…”

“…Among the several shapes studied so far, cubic particles have been proven to form 2D structures with striking features. In recent experiments [23,24] for example, polystyrene cubes with 30 µm edge length have been reported to self-assemble at a water-air interface into three distinct structures, namely square close-packed, linear, and hexagonal structures. Furthermore, the relative stability of these three phases has been shown to strongly depend on their wettability, which is specified by the so-called Young contact angle θ [13].…”

“…a "face up" configuration in the square closepacked structure, an "edge up" configuration in the linear structure, and a "vertex up" configuration in the hexagonal structure, the authors of Refs. [23,24] have argued that the formation of each 2D structure could be caused by a different kind of deformation field induced by the cubes at the interface. In particular these authors have proposed that cubes adsorbed in the "face up", "edge up", and "vertex up" configurations would induce monopolar, quadrupolar and hexapolar deformation fields, respectively.…”

A Journey Through the Landscapes of Small Particles in Binary Colloidal Assemblies: Unveiling Structural Transitions from Isolated Particles to Clusters upon Variation in Composition

“…Binary colloidal suspensions were obtained starting from polystyrene (PS) particles purchased from Thermo Scientific (Waltham, MA, USA) (5000 Series Polymer particles packaged as aqueous suspensions at 10 wt % solids) of nominal diameters equal to dS=220 nm and dL=500 nm and a coefficient of variation (CV) of less than 3%. For the assembly of binary colloids, we used interfacial self-assembly, which has emerged as an effective technique to assemble diverse types of objects, from particles of different materials and shapes to semiconductor nanocrystals and exfoliated graphene films [1,4,24,42,43,44,45,46,47,48]. In particular, colloidal assemblies were prepared at air/water interface by the technique of surface confinement and water discharge described in [4,24].…”