Diffusioosmotic flow drives fluid-structure interaction and instability in microfluidic configurations

Diffusioosmotic flow arises in microfluidic devices due to solute concentration gradients. In soft microchannels, internal pressures generated by diffusioosmotic flow result in elastic deformation of the channel walls, triggering fluid-structure interaction. We analyze the fluid-structure interaction between diffusioosmotic flow and a deformable channel. We provide insight into the system’s physical behavior by developing a reduced-order model, in which a viscous film is confined between a rigid surface and an elastic substrate, modeled as a rigid plate connected to a linear spring. Applying lubrication theory, we derive a set of two-way coupled governing equations for the evolution of the gap height and the solute concentration. We show that above a certain concentration gradient threshold, negative pressures induced by diffusioosmotic flow give rise to fluid-structure instability, causing the elastic substrate to collapse onto the rigid surface. We elucidate the physical mechanisms for the onset of fluid-structure instability and identify three distinct dynamic modes. We validate our theoretical results with finite-element simulations, finding excellent agreement. Understanding this instability is important for the design of electrokinetic systems containing soft elements.