On the Convective Self-Assembly of Colloidal Particles in Nanofluid Based on in Situ Measurements of Interaction Forces

Abstract

While the currently available techniques for the self-assembly of colloidal particles show great promise owing to their simplicity and high efficiency, they are plagued by the fact that they result in colloidal crystals with defects. Here, in order to overcome this problem, we propose a strategy that uses a suspension of nanoparticles (i.e., a nanofluid) as the "solvent" for the colloidal particles. We fabricated colloidal films of microspheres using such a nanofluid suspension and performed in situ measurements of the interaction forces between the microspheres in the nanofluid. This was done in order to systematically elucidate the effects of the nanoparticle size and the thickness of the electric double layer (Debye length) on the self-assembly process. The obtained results confirm that the use of the nanofluid results in a monolayer with a higher degree of order than that in the case of films formed using pure water. Further, the optimal size of the nanoparticles is determined based on the balance between their physical size and the Debye length. We also show that the lodging of the nanoparticles between the microspheres decreases both the lubrication force and the friction force between them. Thus, in this study, we show, for the first time, that a nanofluid can be used in the self-assembly process for improving the regularity of the fabricated colloidal particle arrays, as it inhibits the aggregation of the particles and limits the lubrication and friction forces between them.

A Novel Technique for Large-Scale Fabrication of 3D Colloidal Crystals: Suspending Self-Assembly in Water Medium (SSAM)

“…26,27 It should be noteworthy that the threshold is far smaller than the diameter of PS microspheres so that longrange ordered assembly occurs, instead of coagulation. 28 Most importantly, this transformation process is so short that the microspheres finally assembled into colloidal crystals without breaking the ordered arrangement. An additional experiment is conducted to show the difference of SSAM from the conventional gas−liquid interface self-assembly approach.…”

“…The colorful suspension is subjected to further evaporation, during which PS colloidal crystals are forced to approach each other without breaking the ordered arrangement. The interactive force between microspheres changes with the variation of their spacing, which is the result of the competition between electrostatic repulsion and van der Waals attractive forces based on DLVO theory. , Meanwhile, a threshold exists during the evaporation, and once the distance is smaller than the threshold, the force will change from repulsion to attractive forces; that is, the van der Waals force becomes dominant immediately allowing the PS microspheres to contact each other. , It should be noteworthy that the threshold is far smaller than the diameter of PS microspheres so that long-range ordered assembly occurs, instead of coagulation . Most importantly, this transformation process is so short that the microspheres finally assembled into colloidal crystals without breaking the ordered arrangement.…”

A Review on Self-Assembled Colloidal Nanoparticle Clusters, Patterns and Films: Emerging Synthesis Techniques and Applications

“…Another effective strategy is to create a thin film of the colloidal solution by capillary force so that the evaporation from the thin film region will lead to the formation of a closed packed nanoparticle film while the nanoparticles from the bulk will continuously replenish the evaporating region, Figure 9 (f). [114] This is similar to evaporative self-assembly on a vertical substrate as in this case also the solvent evaporate form the meniscus where the layer of the solvent is thin leaving behind the nanoparticles. The evaporation can also be controlled by microstructures on the substrate.…”

“…(f) Horizontal convective selfassembly induced by capillary force. Reprinted with permission [114] © 2019, American Chemical Society. (g) Controlled evaporation of nanoparticle colloid over an array of larger nanostructures lead to the formation of satellite-like binary assemblies.…”

Size-Selective Sub-micrometer-Particle Confinement Utilizing Ionic Entropy-Directed Trapping in Inscribed Nanovoid Patterns

“…Hence, separation and sorting sub-micrometer-sized particles and biological objects with high throughput are still very challenging. Another widely exploited strategy is to utilize the more scalable self-assembly of particles. − While the self-assembly processes relying on “particle–particle interaction” have nicely worked for setups such as particle-trapping floating electrodes in microfluidic DEP cells and their interaction force measurement systems, assembly into high-density arrays and with desired patterns is still tough.…”