# Controlling the Dynamic Behavior of Microposts in Solution via Diffusion–Convection

## TL;DR

Researchers show how to control tiny structures in fluid using chemical density differences, enabling self-powered microfluidic devices.

## Contribution

They demonstrate that buoyancy-driven flows can be generated without chemical reactions, using only density differences in solutions.

## Key findings

- Adding dense chemicals to fluid chambers can generate buoyancy-driven flows that move tethered posts.
- Controlling chemical release sequences allows programming collective motion of post arrays.
- Biomimetic motion and wave propagation can be achieved by toggling chemical influx and using cascade reactions.

## Abstract

Solutal buoyancy
forces in solution arise from density gradients,
which occur when the reactants and products of a chemical reaction
occupy different volumes in the fluid. These forces drive fluids to
spontaneously perform self-directed mechanical work such as shaping
and organizing flexible objects in fluid-filled microchambers. Here,
we use theory and simulation to show that chemical reactions are not
necessary to generate useful solutal buoyancy forces; it is sufficient
to just add reactants to aqueous solutions that have a different mass-to-volume
ratio than water to drive such spontaneous mechanical action. To demonstrate
this behavior, we model arrays of tethered, nonreactive posts in a
fluid-filled chamber. Relatively dense chemicals released from the
chamber’s side walls diffuse into the solution and generate
buoyancy-driven flows, which spontaneously trigger the posts to undergo
collective dynamics. The posts’ dynamics can be controllably
programmed by staging the sequence of chemical release from the different
walls. With chemically active posts within the array, turning on and
off the influx of chemicals from the side walls leads to propagating
waves that drive the posts to exhibit biomimetic coordinated motion.
The introduction of cascade reactions dynamically shifts the direction
of wave propagation. Our findings show how diffusion–convection
and diffusion–reaction–convection processes can be used
to regulate nonequilibrium spatiotemporal behavior in fluidic systems.
This level of control is vital for creating portable microfluidic
devices that operate without external power sources and thus function
in remote or resource-poor locations.