Anisotropic Self-Assembly from Isotropic Colloidal Building Blocks.

Marcel Rey,D Martin A Buzza,Nicolas Vogel,Adam D Law

doi:10.1021/jacs.7b08503

Abstract

Spherical colloidal particles generally self-assemble into hexagonal lattices in two dimensions. However, more complex, non-hexagonal phases have been predicted theoretically for isotropic particles with a soft repulsive shoulder but have not been experimentally realized. We study the phase behavior of microspheres in the presence of poly(N-isopropylacrylamide) (PNiPAm) microgels at the air/water interface. We observe a complex phase diagram, including phases with chain and square arrangements, which exclusively form in the presence of the microgels. Our experimental data suggests that the microgels form a corona around the microspheres and induce a soft repulsive shoulder that governs the self-assembly in this system. The observed structures are fully reproduced by both minimum energy calculations and finite temperature Monte Carlo simulations of hard core-soft shoulder particles with experimentally realistic interaction parameters. Our results demonstrate how complex, anisotropic assembly patterns can be realized from entirely isotropic building blocks by control of the interaction potential.

Highlights

The ability of colloidal particles to adsorb to liquid inter‐ faces is of fundamental importance for a range of scien‐ tific disciplines and applications [1,2]
We show that the attractive interactions between the two particle populations lead to the in‐situ formation of a soft, compressible microgel corona around the poly‐ styrene microspheres so that our binary system effectively acts as a one component core‐shell system
We observed close‐packed clusters even at very low surface pressures (π

Summary

Introduction

The ability of colloidal particles to adsorb to liquid inter‐ faces is of fundamental importance for a range of scien‐ tific disciplines and applications [1,2]. In contrast to atomic crystals where atoms interact via complex and predetermined potentials, colloidal interaction can often be described by simpler interaction potentials, which can be more readily controlled at the particle level or via external forces [9,10,11] These increased degrees of freedom, and the ability to directly observe the phase behaviour with an optical mi‐ croscope provide an ideal model system to fundamentally study self‐organization phenomena. If the size polydispersity is sufficiently low, colloidal par‐ ticles are able to form two‐dimensional crystals with long‐ range order 10 This property, coupled with the ability to control the periodicity via the size of the particles, pro‐ vides an experimentally simple, bottom‐up strategy to create nanoscale surface patterns with high fidelity over macroscopic areas, exploited in photonic [12,13], phononic 14 or lithographic [15,16] applications. Control over the available symmetries of such two‐dimensional colloi‐ dal crystals is currently limited

Results

Discussion

Conclusion