Fluid Flows Act As Machines That Autonomously Assemble Hierarchical 3D Structures

The inherent coupling of chemical and mechanical behavior in fluid-filled micro-chambers enables the fluid to act as a “machine” as it autonomously directs the self-organization of objects immersed in the solution. Using theory and simulations, we show that the combination of diffusioosmotic and buoyancy mechanisms produce the respective, independently-controlled fluid flows: one generated by a confining surface and the other in the bulk of the solution. With both flows present, the fluid can autonomously join two-dimensional, disconnected pieces to a chemically active, “sticky” base and then fold the resulting layer into regular three-dimensional shapes (e.g., pyramids, tetrahedrons, and cubes). Here, the fluid itself performs the construction work and thus, this process does not require extensive external machinery. If several sticky bases are localized on the bottom surface, the process can be parallelized, with the fluid simultaneously forming multiple structures of the same or different geometries. Hence, this approach can facilitate the relatively low-cost, mass production of 3D micron to millimeter sized structures. Formed in an aqueous solution, the assembled structures could be compatible with biological environments, and thus, potentially useful in medical and biochemical applications.