Capillary Filling at the Microscale: Control of Fluid Front Using Geometry

Area of Science:

• Fluid Dynamics
• Microfluidics
• Rheology

Background:

• Capillary filling is crucial in microfluidic devices and porous media.
• Understanding non-ideal flow regimes beyond the Washburn model is essential for accurate characterization.
• Characterizing fluid properties at the micro-scale requires specialized experimental setups.

Purpose of the Study:

• To establish an experimental and theoretical framework for studying micro-scale capillary filling.
• To investigate viscous, non-inertial, non-Washburn flow regimes.
• To analyze the behavior of immiscible Newtonian liquids in microcapillaries and develop a predictive theoretical model.

Main Methods:

• Development of a microfluidic apparatus enabling controlled fluid flow regimes.
• Experimental investigation of capillary filling with Newtonian fluids over a wide shear-rate range.
• Formulation of a theoretical model to explain observed non-Washburn regimes.

Main Results:

• Observation of a linear fluid front advancement in a viscous, non-inertial, non-Washburn regime.
• Analysis of accelerating or decelerating fluid fronts in systems of two immiscible Newtonian liquids, dependent on viscosity contrast.
• Validation of a theoretical model capable of explaining and predicting non-Washburn regimes.

Conclusions:

• The proposed framework accurately characterizes micro-scale capillary filling and fluid properties.
• The theoretical model provides insights into controlling non-Washburn flow regimes through geometrical parameters.
• The methodology facilitates the design and calibration of a constant-pressure micro-viscometer.