|Abstract:||Here we use high-speed video to experimentally study yield-stress fluids impacting three types of surfaces that exhibit distinct and fascinating behaviors: coated surfaces, permeable surfaces, and hot surfaces. A variety of common materials such as peanut butter, toothpaste, paints, foams, printing ink, cement, and biological fluids can be described as “yield-stress fluids.” Their defining behavior is that they are effectively fluid at high stress and solid at low stress. Many applications take advantage of this duality, utilizing fluid-like behavior to distribute material (e.g. flowing paint through a spray nozzle), and solid-like behavior to hold material where it is placed (e.g. paint building up a coating layer that is stable under its own weight). In some applications, including firefighting and coating processes, the distribution of material involves drops of yield-stress fluids impacting surfaces. During these impact events several factors (material properties, drop size, drop speed, and surface properties amongst others) combine to cause a drop either to stick where it hits, or to display a range of diverse flow phenomena. For drop impacts on pre-coated, permeable, and heated surfaces we perform experiments at varying values of these parameters, and develop a dimensionless group that characterizes drop sticking behavior.
When impacting a solid surface pre-coated with the same material, a drop of yield-stress fluid can calmly deposit itself on the surface, or experience a large splash event. When incoming drop energy (set by drop size and speed) is small compared to arresting forces (set by the material yield stress and surface geometry) all motion is quickly halted, and the drop becomes a lump on the surface. When incoming drop energy is large compared to arresting forces the drop splashes, creating a long-lifetime, evolving ejection sheet that can breakup to eject interestingly shaped droplets. We study these extremes and the transition between them by observing impact events at varying drop size, drop speed, yield stress, and pre-coating thickness, and we develop a dimensionless group that characterizes stick/splash behavior.
When impacting permeable solid meshes (rigid surfaces with small, evenly spaced openings), yield-stress fluids can either stick to the mesh and accumulate large volumes, or pass through the mesh matrix. When incoming drop energy is small compared to arresting forces a drop will be completely stopped, as though it were hitting an impermeable solid surface. When incoming drop energy is large compared to arresting forces the drop can traverse the mesh, flowing through the openings and breaking into smaller fluid particles with varying shapes, sizes, and velocities in the process. We study these extremes and the transition between them by observing impact events at varying drop size, drop speed, yield stress, and mesh geometry. We find that the same dimensionless group that characterizes stick/splash behavior on coated surfaces also effectively predicts material transmission through permeable surfaces.
When impacting a dry, solid surface at sufficiently high temperatures, the Leidenfrost effect can be observed, wherein a layer of vapor is created between the material and the surface due to rapid boiling, which can prevent a drop from sticking to the surface. We report the unexpected result that at high temperatures yield-stress fluids are less prone to sticking than Newtonian fluids. As yield stress increases, the temperature required to prevent material adhesion decreases, and this critical temperature of all the aqueous yield-stress fluids we tested is lower than that of water. We study possible explanations for this counterintuitive trend using high-speed color interferometry. For all three types of surfaces our results and analysis characterize drop impact behavior as a function of a variety of input parameters, creating tools that enable design with and design of these complex materials for specific applications.
In addition, we more closely examine one specific application of these fluids: firefighting. We present experimental data for pressure drops and flowrates of yield-stress fluids in hose flow, and establish design criteria based on equipment specs. For the same materials we also discuss design criteria for forming and maintaining a surface coating. Finally, we expand on the author’s prior analytical work in thixotropic-viscoelastic constitutive modeling, which predicted a unique model signature in asymptotically-nonlinear large-amplitude oscillatory shear. Here we provide experimental data that demonstrates the predicted unique scaling