Particle Size Determines the Shape of Supraparticles in Self-Lubricating Ternary Droplets

Abstract

Supraparticles are large clusters of much smaller colloidal particles. Controlling the shape and anisotropy of supraparticles can enhance their functionality, enabling applications in fields such as optics, magnetics, and medicine. The evaporation of self-lubricating colloidal ouzo droplets is an easy and efficient strategy to create supraparticles, overcoming the problem of the “coffee-stain effect” during drop evaporation. Yet, the parameters that control the shape of the supraparticles formed in such evaporating droplets are not fully understood. Here, we show that the size of the colloidal particles determines the shape of the supraparticle. We compared the shape of the supraparticles made of seven different sizes of spherical silica particles, namely from 20 to 1000 nm, and of the mixtures of small and large colloidal particles at different mixing ratios. Specifically, our in situ measurements revealed that the supraparticle formation proceeds via the formation of a flexible shell of colloidal particles at the rapidly moving interfaces of the evaporating droplet. The time t c0 when the shell ceases to shrink and loses its flexibility is closely related to the size of particles. A lower t c0 , as observed for smaller colloidal particles, leads to a flat pancake-like supraparticle, in contrast to a more curved American football-like supraparticle from larger colloidal particles. Furthermore, using a mixture of large and small colloidal particles, we obtained supraparticles that display a spatial variation in particle distribution, with small colloids forming the outer surface of the supraparticle. Our findings provide a guideline for controlling the supraparticle shape and the spatial distribution of the colloidal particles in supraparticles by simply self-lubricating ternary drops filled with colloidal particles.

Surface Properties of Colloidal Particles Affect Colloidal Self-Assembly in Evaporating Self-Lubricating Ternary Droplets

“…Details of the experimental setup can be found in Raju et al 12 In short, the evaporating droplets were observed from the side using monochrome 8-bit CCD camera and from the top using a CMOS color camera. Both cameras were connected to a Navitar 12X adjustable zoom lens.…”

“…colloidal Ouzo droplets, is an efficient route to produce supraparticles. 11 , 12 Although it is known that the surface properties of colloidal particles can influence their assembly in droplets, 13 , 14 the role of the particles’ surface on the formation of supraparticles in Ouzo droplets is still unknown. Here we show that the surface properties of the colloidal particles affect and can even impede the formation of supraparticles in colloidal “Ouzo” droplets.…”

“… 15 − 19 This oil ring prevents pinning and leads to the formation of a supraparticle instead of the coffee ring. 11 , 12 …”

Influence of Surfactant-Mediated Interparticle Contacts on the Mechanical Stability of Supraparticles

“…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 self-assembled materials in the bulk.…”

“…Supraparticles are finite assemblies of primary colloidal particles with micron-scale dimensions fabricated by confined self-assembly processes, for example, within emulsion droplets. − 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, − the system size, , and the shape of the primary particle, − as well as the drying kinetics. , 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 self-assembled materials in the bulk. The size of the primary colloidal particles can be chosen to match the wavelengths of visible light, , in which case the defined structural arrangement in the supraparticles produces tunable structural color with an isotropic or anisotropic appearance. − The latter depends on whether their internal structure is amorphous, partially crystalline, − or completely crystalline .…”