Understanding the flow of complex fluids through confined spaces and forces governing the flow is key to diverse fields, from blood flow to lubricant design. Studying such situations is difficult because typical devices cannot achieve the necessary degree of confinement. New experiments and simulations reveal flow behavior under different levels of confinement and show how this behavior can be tuned.
Complex fluids consisting of particles suspended in an ambient fluid — such as toothpaste, lubricants, and gels — are ubiquitous in everyday life. Understanding how these complex fluids flow through confined spaces, along with the underlying forces at play, are important; flows of confined suspensions have implications in areas ranging from blood flow in thin capillaries to the design of lubricants for micromachines. Studying these confined fluids is challenging, however, as traditional flow devices do not achieve such small gaps. Here we use a combination of experimental and simulation techniques to determine the flow behavior under extreme confinement and understand the forces that give rise to the resistance to flow, or viscosity.
We show that at small gaps, the suspension flow response is strongly coupled to the arrangements of the particles in the fluid. When a suspension is confined to a space between six and 15 times the particle diameter, the particles arrange themselves into layers, which gives the suspension a low viscosity. At gaps between three and six particle diameters, geometric effects such as the commensurability of the gap with the particle diameter become important, giving rise to fluctuations in the viscosity. Finally, at extreme confinements with gaps less than three particle diameters, the contact forces between particles become increasingly important and the particles form bridges between the plates that increase the viscosity by over an order of magnitude.