  In this work, we unravel the role of surface properties of colloidal
particles on the formation of supraparticles (clusters of colloidal particles)
in a colloidal Ouzo droplet. Self-lubricating colloidal Ouzo droplets are an
efficient and simple approach to form supraparticles, overcoming the challenge
of the coffee stain effect in situ. Supraparticles are an efficient route to
high-performance materials in various fields, from catalysis to carriers for
therapeutics. Yet, the role of the surface of colloidal particles in the
formation of supraparticles using Ouzo droplets remains unknown. Therefore, we
used silica particles as a model system and compared sterically stabilized
versus electrostatically stabilized silica particles - positively and
negatively charged. Additionally, we studied the effect of hydration. Hydrated
negatively charged silica particles and sterically stabilized silica particles
form supraparticles. Conversely, dehydrated negatively charged silica particles
and positively charged amine-coated particles form flat film-like deposits.
Notably, the assembly process is different for all the four types of particles.
The surface modifications alter (a) the contact line motion of the Ouzo droplet
and (b) the particle-oil and particle-substrate interactions. These alterations
modify the particle accumulation at the various interfaces, which ultimately
determines the shape of the final deposit. Thus, by modulating the surface
properties of the colloidal particles, we can tune the shape of the final
deposit, from a spheroidal supraparticle to a flat deposit. In the future, this
approach can be used to tailor the supraparticles for applications such as
optics and catalysis, where the shape affects the functionality.
