# Orientation control and nonlinear trajectory tracking of colloidal particles using microfluidics

## TL;DR

This paper presents a method to precisely control the position and orientation of anisotropic colloidal particles in microfluidic flows, enabling advanced manipulation for applications in materials science and biomedicine.

## Contribution

It introduces a path-following model predictive control scheme for 2D position and orientation control of colloidal particles using fluid flow, without external fields.

## Key findings

- Successful control of particle trajectories and orientations in experiments.
- Analytical modeling of orientation dynamics in extensional flow.
- Potential for fundamental studies using flow-only manipulation.

## Abstract

Suspensions of anisotropic Brownian particles are commonly encountered in a wide array of applications such as drug delivery and manufacturing of fiber-reinforced composites. Technological applications and fundamental studies of small anisotropic particles critically require precise control of particle orientation over defined trajectories and paths. In this work, we demonstrate robust control over the two-dimensional (2D) center-of-mass position and orientation of anisotropic Brownian particles using only fluid flow. We implement a path-following model predictive control scheme to manipulate colloidal particles over defined trajectories in position space, where the speed of movement along the path is a degree of freedom in the controller design. We further explore how the external flow field affects the orientation dynamics of anisotropic particles in steady and transient extensional flow using a combination of experiments and analytical modeling. Overall, this technique offers new avenues for fundamental studies of anisotropic colloidal particles using only fluid flow, without the need for external electric or optical fields.