# Self-assembly and complex manipulation of colloidal mesoscopic particles by active thermocapillary stress

## TL;DR

This paper explores how active thermocapillary stresses from microbubbles can be used to self-assemble and manipulate colloidal particles in controlled ways, enabling complex arrangements and rotations.

## Contribution

It introduces a novel method of using thermocapillary flows from microbubbles for directed self-assembly and micromanipulation of colloidal particles.

## Key findings

- Microbubbles induce particle self-assembly in rings.
- Asymmetric temperature profiles cause particle accumulation and repulsion.
- Flow-induced vorticity enables particle rotation, first demonstrated with optical tweezers.

## Abstract

We demonstrate that the active thermocapillary stresses induced by multiple microbubbles offer simple routes to directed self-assembly and complex but controllable micromanipulation of mesoscopic colloidal particles embedded in a liquid. The microbubbles are nucleated on a liquid-glass interface using optical tweezers. The flow around a single bubble causes self-assembly of the particles in rings at the bubble-base, while an asymmetric temperature profile generated across the bubble interface breaks the azimuthal symmetry of the flow, and induces simultaneous accumulation and repulsion of particles at different axial planes with respect to the bubble. The flows due to two adjacent bubbles lead to more diverse effects including the sorting of particles, and to local vorticity that causes radial and axial rotation of the particles - the latter being obtained for the first time using optical tweezers. The sorting is enabled by nucleating the bubbles on spatially discrete temperature profiles, while the vorticity is generated by nucleating them in the presence of a temperature gradient which once again causes a strong symmetry-breaking in the azimuthal flow. The flow profiles obtained in the experiments are explained by analytical solutions or qualitative explanations of the associated thermocapillary problem employing the Stokes and heat equations.