Evaporation-driven colloidal cluster assembly using droplets on superhydrophobic fractal-like structures.

Carola Seyfert,Erwin J W Berenschot,Niels R Tas,Arturo Susarrey-Arce,Alvaro Marin

doi:10.1039/d0sm01346c

Carola Seyfert, Erwin J W Berenschot + Show 3 more

Open Access

https://doi.org/10.1039/d0sm01346c

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Abstract

Microparticles can be considered building units for functional systems, but their assembly into larger structures typically involves complex methods. In this work, we show that a large variety of macro-agglomerate clusters ("supra-particles") can be obtained, by systematically varying the initial particle concentration in an evaporating droplet, spanning more than 3 decades. The key is the use of robust superhydrophobic substrates: in this study we make use of a recently discovered kind of patterned surface with fractal-like microstructures which dramatically reduce the contact of the droplet with the solid substrate. Our results show a clear transition from quasi-2D to 3D clusters as a function of the initial particle concentration, and a clear transition from unstable to stable 3D spheroids as a function of the evaporation rate. The origin of such shape transitions can respectively be found in the dynamic wetting of the fractal-like structure, but also in the enhanced mechanical stability of the particle agglomerate as its particle packing fraction increases.

Highlights

Agglomerating small solid particles in clusters of a particular shape and size has been an intriguing challenge of fundamental soft matter research for decades
We show that a large variety of macro-agglomerate clusters (‘‘supra-particles’’) can be obtained, by systematically varying the initial particle concentration in an evaporating droplet, spanning more than 3 decades
We demonstrate that, using a robust superhydrophobic substrate and a stable colloidal solution, one can obtain a vast variety of particle assemblies by controlling the initial particle concentration

Summary

Introduction

Agglomerating small solid particles in clusters of a particular shape and size has been an intriguing challenge of fundamental soft matter research for decades. To avoid the irreversible transport of particles away from the droplet centre, an unpinned contact line and high contact angles are essential requirements for a particle cluster formation. Both requirements can be met by evaporating the droplets on superhydrophobic substrates.[11,12,13,14,15,16,17] On a superhydrophobic substrate, water-based droplets minimize their contact area with the solid due to the considerably higher solid–liquid surface energy than the droplet’s liquid–gas energy. The increased energy difference is often induced by a 506 | Soft Matter, 2021, 17, 506--515

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