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Nearly monodisperse nanoparticle (NP) spheres attached to a nonvolatile ionic liquid surface were tracked by in situ scanning electron microscopy to obtain the tracer diffusion coefficient D-tr as a function of the areal fraction phi. The in situ technique resolved both tracer (gold) and background (silica) particles for similar to 1-2 min, highlighting their mechanisms of diffusion, which were strongly dependent on phi. Structure and dynamics at low and moderate phi paralleled those reported for larger colloidal spheres, showing an increase in order and a decrease in D-tr by over 4 orders of magnitude. However, ligand interactions were more important near jamming, leading to different caging and jamming dynamics for smaller NPs. The normalized D-tr at ultrahigh phi depended on particle diameter and ligand molecular weight. Increasing the PEG molecular weight by a factor of 4 increased D-tr by 2 orders of magnitude at ultrahigh phi, indicating stronger ligand lubrication for smaller particles. 

Abstract An active droplet system, programmed to repeatedly move autonomously at a specific velocity in a well‐defined direction, is demonstrated. Coulombic energy is stored in oversaturated interfacial assemblies of charged nanoparticle‐surfactants by an applied DC electric field and can be released on demand. Spontaneous emulsification is suppressed by an increase in the stiffness of the oversaturated assemblies. Rapidly removing the field releases the stored energy in an explosive event that propels the droplet, where thousands of charged microdroplets are ballistically ejected from the surface of the parent droplet. The ejection is made directional by a symmetry breaking of the interfacial assembly, and the combined interaction force of the microdroplet plume on one side of the droplet propels the droplet distances tens of times its size, making the droplet active. The propulsion is autonomous, repeatable, and agnostic to the chemical composition of the nanoparticles. The symmetry‐breaking in the nanoparticle assembly controls the microdroplet velocity and direction of propulsion. This mechanism of droplet propulsion will advance soft micro‐robotics, establishes a new type of active matter, and introduces new vehicles for compartmentalized delivery. 

Abstract The in‐plane packing of gold (Au), polystyrene (PS), and silica (SiO₂) spherical nanoparticle (NP) mixtures at a water–oil interface is investigated in situ by UV–vis reflection spectroscopy. All NPs are functionalized with carboxylic acid such that they strongly interact with amine‐functionalized ligands dissolved in an immiscible oil phase at the fluid interface. This interaction markedly increases the binding energy of these nanoparticle surfactants (NPSs). The separation distance between the Au NPSs and Au surface coverage are measured by the maximum plasmonic wavelength (λ_max) and integrated intensities as the assemblies saturate for different concentrations of non‐plasmonic (PS/SiO₂) NPs. As the PS/SiO₂content increases, the time to reach intimate Au NP contact also increases, resulting from their hindered mobility. λ_maxchanges within the first few minutes of adsorption due to weak attractive inter‐NP forces. Additionally, a sharper peak in the reflection spectrum at NP saturation reveals tighter Au NP packing for assemblies with intermediate non‐plasmonic NP content. Grazing incidence small angle X‐ray scattering (GISAXS) and scanning electron microscopy (SEM) measurements confirm a decrease in Au NP domain size for mixtures with larger non‐plasmonic NP content. The results demonstrate a simple means to probe interfacial phase separation behavior using in situ spectroscopy as interfacial structures densify into jammed, phase‐separated NP films.