Although monodisperse amorphous silica nanoparticles have been widely investigated, their formation mechanism is still a topic of debate. Here, we demonstrate the formation of monodisperse nanoparticles from colloidally stabilized primary particles, which at a critical concentration undergo a concerted association process, concomitant with a morphological and structural collapse. The formed assemblies grow further by addition of primary particles onto their surface. The presented mechanism, consistent with previously reported observations, reconciles the different theories proposed to date.

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Silica nanoparticles are imaged in solution with scanning transmission electron microscopy (STEM) using a liquid cell with silicon nitride (SiN) membrane windows. The STEM images reveal that silica structures are deposited in well-defined patches on the upper SiN membranes upon electron beam irradiation. The thickness of the deposits is linear with the applied electron dose. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) demonstrate that the deposited patches are a result of the merging of the original 20 nm-diameter nanoparticles, and that the related surface roughness depends on the electron dose rate used. Using this approach, sub-micrometer scale structures are written on the SiN in liquid by controlling the electron exposure as function of the lateral position.

which changes their surface characteristics and compromises their performance.One way to increase the durability of functional coatings is to create a system which is able to relocate the low surface energy groups at the damaged surfaces and recover the low surface energy properties, i.e., to introduce a self-healing function. In the case of surface-structured coatings, the repair of the surface roughness is an additional challenge. [ 5,6 ] One of the possible solutions reported by Li et al., [ 7 ] is the creation of a porous polymer coating with micro-and nano-scale structures, for which the 'healing agent' consists of a low surface energy component previously impregnated in the porous layer. After damage, this component migrates to the new porous coating surface restoring its (super)hydrophobicity when exposed to a humid environment. Another possibility is to encapsulate a surface-active material in nano-containers [ 8 ] which will be released at the surface upon damage. The self-organization of colloidal particles at interfaces has also been used to design materials with surface-healing properties by embedding colloidal silica particles in a perfl uorinated wax. [ 9,10 ] Upon damage and an annealing step (at 60 °C), the wax melts and the particles migrate to the surface, recreating a new topography covered by the waxy layer. [ 9 ] Recently, a surface-healing approach was reported by our group [ 11 ] which uses a polymer network that is able to replenish damaged surfaces with low surface energy components, autonomously and at room temperature. Perfl uoro-alkyl-terminated dangling chains connected to a poly(ester urethane) crosslinked network are able to spontaneously re-orient to new air interfaces created by damage. A polymeric spacer was included in the dangling chains in order to control their mobility and ensure surface reorientation, but also to provide the proper miscibility with the bulk network and avoid complete surface segregation. [ 11 ] This self-replenishing principle can be applied to recover surface properties such as hydrophobicity. However, for structured surfaces topography needs to be equally repaired or recreated. Hence we applied the self-replenishing principle on polymer layers with dual-scale roughness, [ 12 ] created by introducing inorganic nanoparticles with two different sizes (≈70/700 nm). For this system a multi-scale surface roughness is re-created by the damage, while the polymer layer between the particles serves as the source of low surface energy To investigate self-replenishing on surface-structured composite coatings a dual simulation-experimental approach is employed to study the decisive role of polymer-air and polymer-particle interfaces. Experimentally, the composite system consists of a cross-linked polymer network with fl uorinated-dangling chains, embedding colloidal SiO 2 nanoparticles which are incorporated in the network via covalent bonding. These particles provide the desired surface structure at the air-interface before and after damage. Any damage replicates the...