Large-scale optothermal assembly of colloids mediated by a gold microplate

TL;DR

This study demonstrates how a gold microplate can be used to optothermally assemble colloidal silica particles into two-dimensional crystals, with assembly characteristics influenced by excitation polarization and colloid material.

Contribution

It introduces a novel experimental platform using a gold microplate for controlled optothermal assembly of colloids, highlighting material and polarization effects.

Findings

Colloidal silica forms 2D poly-crystals near the microplate.

Assembly size varies with excitation polarization.

Material dependence observed: silica assembles, polystyrene does not.

Abstract

Light-activated colloidal assembly and swarming can act as model systems to explore non-equilibrium state of matter. In this context, creating new experimental platforms to facilitate and control two-dimensional assembly of colloidal crystals are of contemporary interest. In this paper, we present an experimental study of assembly of colloidal silica microparticles in the vicinity of a single-crystalline gold microplate evanescently excited by a 532 nm laser beam. The gold microplate acts as a source of heat and establishes a thermal gradient in the system. The created optothermal potential assembles colloids to form a two-dimensional poly-crystal, and we quantify the coordination number and hexagonal packing order of the assembly in such a driven system. Interestingly, we observe variation in assembly-size as a function of excitation-polarization. Furthermore, we observe that the assembly is colloidal-material dependent. Specifically, silica colloids assemble but polystyrene colloids do not, indicating an intricate behaviour of the forces under play. Our work highlights a promising direction in utilizing metallic, single crystalline microstructures that can be harnessed for optothermal colloidal crystal assembly and swarming studies. Our experimental system can be utilized to explore optically driven matter and photophoretic interactions in soft-matter including biological systems such as cells and micro organisms.