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

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.