Colloidal Nanoparticles: Aggregation Patterns at Model Molecular Surfaces
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Apart from some specific applications related to microfluidics and targeted delivery where functionalized and freely moving nanocolloids are suited, nanoparticles mostly enter nanostructured materials as one-, two-, or three-dimensional networks, either grown (or embedded) at a solid–solid, solid–fluid, or fluid–fluid interface. As in many natural and technological processes and phenomena, this brings the basic question on the role of the underlying substrate properties on the spatial organization and stability of the nanoparticle network. The answer to this question at least partly depends on the methodology used to create the nanoparticle patterns. The following discussion mainly deals with some of the basic features related to nanoparticle assemblies that may result from the adsorption and drying of nanocolloidal suspensions onto a substrate. In addition to the elaboration of well-ordered, two-dimensional nanoparticles arrays, applications and phenomena related to nanocolloidal dispersions nearly concern all domains and materials, ranging from inorganic, to polymers and biological objects. For these nanocolloidal suspensions, the evaporation of the solvent may either result in random and featureless aggregates or lead to the formation of well-ordered complex fractal or dendrites structures of nanosize particles on the substrate. For some of these drying patterns, the mechanisms and interface phenomena that drive the aggregation and ultimately determine their fascinating topological features (order, fractality, symmetry) still remain insufficiently understood. Comprehension of the underlying mechanisms and critical parameters governing the emergence and topological features of these nanocolloid drying patterns is thus fundamental for both biocolloid applications and the controlled elaboration of fractal surface networks that are increasingly used by engineers to model flow and percolation in complex media.
Even on an ideal substrate, pattern formation in drying nanocolloids suspensions is a rather complex process, involving the whole range of interface phenomena (wetting dynamics, adsorption, adhesion). This complexity essentially arises from the strong time dependence of both the internal flow (particle diffusion) and competing interparticle and particle–substrate interactions through the solvent, which govern the particle aggregation, especially in low-to-semidilute suspensions. This, of course, is less the case for highly concentrated suspensions that are already close to their gelation point at drop deposition. For these gel-like suspensions, it is the extraction kinetic of the residual (interstitial) solvent and the very short-range repulsive interactions, rather than the interparticle and particle–substrate interactions, that dominates the final morphology (porosity) and mechanical properties of the particle network, as observed for solvent-based synthesis of ceramic and zeolite films. As opposed to these dense and relatively “thick” gel particle networks, the drying of low- to semiconcentrated nanocolloid dispersions rather form open aggregates of lower surface coverage Γs and fractal dimension D ≤ dspace. It is these structures that are strongly sensitive and critically determined by the drying kinetics and the above-mentioned competitive interparticle and particle–substrate interactions that constitute the main focus of the following discussion. One can produce nanoparticle assemblies (well-ordered arrays, open aggregates, or thick deposits) from colloidal suspensions, either by deep coating or by drying deposited drops or films. In both cases, the final morphology of the assembly is strongly determined by the surface properties of the underlying substrate, which control the extent of the surface coverage, the particle–substrate adhesion, and henceforth, the whole late-stage drying events. These involve the contact line pining vs. slip, the often-observed thick particle deposit at the drop edge, the hydrodynamic flow and related shear at the substrate–suspension interface, and the possible formation and rupture of a residual film. A complete understanding of these structure formations thus requires both the use of model surfaces and the simultaneous investigations and correlation between the aggregation patterns forming in deep-coating adsorption, and may also develop to some extent in evaporating drops, and serve as sublayer to the arising patterns. Therefore, we will successively discuss and relate these adsorption and drying nanocolloidal patterns, highlighting the critical role of the surface and the potential its functionalization offers toward the elaboration of nanocolloidal patterns of well-defined morphology. But first, we will start with the section “Materials,” which briefly describes the elaboration and main characteristics of both the model molecular surfaces and nanocolloidal gold particles used herein.