Satish Kumar, University of Minnesota
The displacement of one fluid by another lies at the core of two practical applications relevant to the petroleum industry and alternative energy technologies: liquid-applied coating processes and enhanced oil recovery. Successful displacement of one fluid by the other requires that the fluid-fluid interface maintain contact lines at the boundaries, i.e., wetting failure must not occur. Remarkably, despite the importance of successful fluid-fluid displacement to these applications, two major issues have received relatively little attention. These are (i) the role of the confining geometry, and (ii) the role of the viscosity of the displaced phase. Our research is addressing these issues using a combination of flow visualization experiments, lubrication-approximation-based models, and finite element simulations.
During the most recent grant reporting period, we have completed a study of how meniscus confinement influences dynamic wetting failure. A novel experimental system was designed to simultaneously view confined and unconfined wetting systems as they approach wetting failure. The experimental apparatus consists of a scraped steel roll that rotates into a bath of glycerol. Confinement is imposed via a gap formed between a coating die and the roll surface. Flow visualization is used to record the critical roll speed at which wetting failure occurs. Comparison of the confined and unconfined data shows a clear increase in the relative critical speed as the meniscus becomes more confined. A hydrodynamic model for wetting failure was developed and analyzed with (i) lubrication theory and (ii) a two-dimensional finite-element method (FEM). Both approaches do a remarkable job of matching the observed confinement trend, but only the two-dimensional model yields accurate estimates of the absolute values of the critical speeds due to the highly two-dimensional nature of the stress field in the displacing liquid. The overall success of the hydrodynamic model suggests a wetting failure mechanism primarily related to viscous bending of the meniscus. A paper describing this work is currently in press at the Journal of Fluid Mechanics.
Our results are important because they help rationalize why so-called "bead coating" methods work. These methods are used extensively in industry to delay wetting failure, and involve using a coating die to confine the wetting meniscus. However, how these methods work remained a puzzle until our work. In addition, our results demonstrate that all that is needed to understand the onset of wetting failure in these systems is a relatively simple mathematical model that involves hydrodynamics, a microscopic contact angle, and a Navier slip law. There does not appear to be a compelling reason to bring in additional physical processes, as others have speculated. Current work is focused on understanding the role of the fluid viscosity ratio on wetting failure, and incorporating more complex system geometries. Our research on this topic has already led to interactions with companies such as 3M and Corning, which has been an excellent educational experience for the student working on this problem. In addition, these interactions will help me to establish a longer-range research program on the fundamentals of dynamic wetting. Overall, our work will significantly advance fundamental understanding of the conditions under which wetting failure occurs, and open the door to major improvements in our ability to control wetting failure.