Abstract
Colloidal supraparticles are micron-scale spherical assemblies of uniform primary particles, which exhibit emergent properties of a colloidal crystal, yet exist as a dispersible powder. A prerequisite to utilize these emergent functionalities is that the supraparticles maintain their mechanical integrity upon the mechanical impacts that are likely to occur during processing. Understanding how the internal structure relates to the resultant mechanical properties of a supraparticle is therefore of general interest. Here, we take the example of supraparticles templated from water/fluorinated oil emulsions in droplet-based microfluidics and explore the effect of surfactants on their mechanical properties. Stable emulsions can be generated by nonionic block copolymers consisting of a hydrophilic and fluorophilic block and anionic fluorosurfactants widely available under the brand name Krytox. The supraparticles formed in the presence of both types of surfactants appear structurally similar, but differ greatly in their mechanical properties. While the nonionic surfactant induces superior mechanical stability and ductile fracture behavior, the anionic Krytox surfactant leads to weak supraparticles with brittle fracture. We complement this macroscopic picture with Brillouin light spectroscopy that is very sensitive to the interparticle contacts for subnanometer-thick adsorbed layers atop of the nanoparticle. While the anionic Krytox does not significantly affect the interparticle bonds, the amphiphilic nonionic surfactant drastically strengthens these bonds to the point that individual particle vibrations are not resolved in the experimental spectrum. Our results demonstrate that seemingly subtle changes in the physicochemical properties of supraparticles can drastically impact the resultant mechanical properties.
Highlights
From bulk studies of colloidal assemblies, it has been established that surprisingly attractive mechanical properties can result from a strengthening of the interparticle bonds, for example, by the addition of binders,[48] sintering,[62] or chemically interconnecting organic ligands on the surface of the materials.[63−66] Recently, we focused on the mechanical properties of supraparticles in the absence of such binders
Colloidal supraparticles with a well-defined internal structure, spherical shape, and low size polydispersity are fabricated using emulsion droplets prepared by droplet-based microfluidics as confinements.[9,21]
In the course of the self-assembled process, water is removed from the droplet by evaporation and diffusion into the oil phase until the colloidal primary particles eventually consolidate into compact supraparticles
Summary
Supraparticles are finite assemblies of primary colloidal particles with micron-scale dimensions fabricated by confined self-assembly processes, for example, within emulsion droplets.[1−4] If the primary particles are sufficiently monodispersed, such supraparticles exhibit a well-defined structural order determined by the interplay between the curvature imposed by the templating droplet,[5−7] the system size,[8,9] and the shape of the primary particle,[10−12] as well as the drying kinetics.[13,14] As inherently hierarchical materials, supraparticles bridge the microscopic and macroscopic scales and combine emergent properties arising from the collective behavior of the primary building blocks with characteristics of a macroscopic powder and finite properties that can be different from selfassembled materials in the bulk. Incorporating functional primary particles encodes additional properties into such supraparticles, such as magnetic actuation,[31] efficient separation,[32] object identification,[33]
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