Controlled assembly of single colloidal crystals using electro-osmotic micro-pumps

TL;DR

This study demonstrates controlled assembly of colloidal crystals using electro-osmotic micro-pumps, revealing how boundary conditions influence crystal quality, size-based sorting, and defect-free single domain formation.

Contribution

It introduces a novel method for precise colloidal crystal assembly leveraging electro-osmotic micro-pumps and explores the underlying control parameters and mechanisms.

Findings

Crystal quality depends on assembly distance.

Size-based colloid sorting observed in binary mixtures.

Defect-free single domain crystals form around ion exchange particles.

Abstract

We assemble charged colloidal spheres at deliberately chosen locations on a charged unstructured glass substrate utilizing ion exchange based electro-osmotic micro-pumps. Using microscopy, a simple scaling theory and Brownian Dynamics simulations, we systematically explore the control parameters of crystal assembly and the mechanisms through which they depend on the experimental boundary conditions. We demonstrate that crystal quality depends crucially on the assembly distance of the colloids. This is understood as resulting from the competition between inward transport by the electro-osmotic pump flow and the electro-phoretic outward motion of the colloids. Optimized conditions include substrates of low and colloids of large electro-kinetic mobility. Then a sorting of colloids by size is observed in binary mixtures with larger particles assembling closer to the ion exchanger beads. Moreover, mono-sized colloids form defect free single domain crystals which grow outside a colloid-free void with facetted inner crystal boundaries centred on the ion exchange particle. This works remarkably well, even with irregularly formed ion exchange resin splinters.