Thin-Film Deposition by a Confined Bubble Moving in Viscoelastic Boger Fluids

Thin-film deposition of fluids is widely applied in various engineering setups, such as surface coating, polymer processing, and biomedical device fabrication. While the viscous film deposition in Newtonian fluids has been extensively investigated, the dynamics in non-Newtonian complex fluids remain limited regarding experimentally validated scaling laws for the film thickness. Here we investigate thin film deposition by a confined bubble moving in a circular capillary tube filled with constant-viscosity viscoelastic (i.e., Boger) fluids, which allows us to quantify the influence of the fluid's viscoelasticity on the film deposition. We performed systematic experiments to measure the deposited film thickness across a wide range of bubble velocities and fluid rheology. Furthermore, we develop a scaling law based on the hydrodynamic lubrication theory to rationalize the experimental results with the Deborah number, which compares the viscoelastic relaxation timescale with the relevant flow timescale. The scaling relationship shows excellent agreement with the measured film thicknesses for all tested fluids and flow conditions. Our results may inform design principles for applications involving precision coating and microfluidic manufacturing with complex fluids.