The Effect of Viscosity Ratio and Peclet Number on Miscible Viscous Fingering in a Hele-Shaw Cell: A Combined Numerical and Experimental Study

Elucidating the roles of electrolytes and hydrogen bonding in the dewetting dynamics of the tear film

This study explores the impact of electrostatic interactions and hydrogen bonding on tear film stability, a crucial factor for ocular surface health. While mucosal and meibomian layers have been extensively studied, the role of electrolytes in the aqueous phase remains unclear. Dry eye syndrome, characterized by insufficient tear quantity or quality, is associated with hyperosmolality, making electrolyte composition an important factor that might impact tear stability. Using a model buffer solution on a silica glass dome, we simulated physiologically relevant tear film conditions. Sodium chloride alone induced premature dewetting through salt crystal nucleation. In contrast, trace amounts of solutes with hydroxyl groups (sodium phosphate dibasic, potassium phosphate monobasic, and glucose) exhibited intriguing phenomena: quasi-stable films, solutal Marangoni-driven fluid influx increasing film thickness, and viscous fingering due to Saffman-Taylor instability. These observations are rationalized by the association of salt solutions with increased surface tension and the propensity of hydroxyl-group-containing solutes to engage in significant hydrogen bonding, altering local viscosity. This creates a viscosity contrast between the bulk buffer solution and the film region. Moreover, these solutes shield the glass dome, counteracting sodium chloride crystallization. These insights not only advance our understanding of tear film mechanics but also pave the way for predictive diagnostics in dry eye syndrome, offering a robust platform for personalized medical interventions based on individual tear film composition.

Effects of fluid composition in fluid injection experiments in porous media

Experiments on fluid flow in porous media, using fluids loaded with solids of various grain sizes, have been conducted in a modified Hele-Shaw setup. This setup utilised weakly cemented porous media with specific hydraulic and mechanical properties. Fluid injection in coarse granular media with clean or low-concentration fine particles, results in infiltration only, with pressure close to the material tensile strength, while injection in finer granular material causes damage alongside infiltration, with the fluid pressure still close to the material tensile strength. When larger particle sizes or higher particle concentrations are used in the mixture, the fluid travels further within the porous medium, primarily influenced by the grain size of the granular medium. In the latter case, the Darcy flow equation with an effective permeability term can be employed to determine the pressure differential. For the largest particle sizes included in the fluid, the equation is still applicable, but the effective permeability requires adjustment for particle size within the fluid rather than the granular medium. This is crucial when the injection point is locally clogged. The experiments show that fracturing conditions are controlled by different mechanisms. Dimensional and statistical analysis was used to classify the injection pressures to regimes predicted by fracturing theory or by Darcy law with modified effective permeabilities. The findings show that both the material properties and fluid composition are important designing parameters.