Wicking with a yield stress fluid
1School of Engineering and Electronics, University of Edinburgh, King's Buildings, Mayfield Road, Edinburgh EH9 3JL, UK.
Summary
Capillary-driven flow of yield stress fluids in small tubes stops unexpectedly. Results challenge existing models, suggesting wall effects influence fluid behavior near surfaces.
Area of Science:
- Fluid dynamics
- Rheology
- Materials science
Background:
- Yield stress fluids exhibit a critical stress threshold before flow initiates.
- Capillary forces are crucial in microfluidic and porous media applications.
- Understanding fluid behavior near interfaces is vital for accurate modeling.
Purpose of the Study:
- To experimentally investigate capillary-driven flow of yield stress fluids in small-diameter tubes.
- To identify discrepancies between experimental observations and existing theoretical models.
- To propose a new scaling law for predicting flow behavior.
Main Methods:
- Experimental setup involving horizontal glass tubes (0.46–1.5 mm diameter).
- Utilized model yield stress fluids with varying yield stresses (5–21 Pa).
- Observed and measured liquid penetration distances.
Main Results:
- Liquid penetration consistently stopped after a few centimeters.
- Observed flow behavior deviated significantly from predictions based on capillary and frictional force balance.
- A new dimensionless number was proposed for scaling the results.
Conclusions:
- The standard yield stress fluid constitutive equation may be inadequate near solid walls.
- Wall effects play a critical role in capillary-driven flow of these fluids.
- The proposed scaling offers a better framework for understanding this phenomenon.
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