Gravity-Driven Microfluidic Viscosity Measurement with a Small Capillary Radius and Strong Pinning Effect

Area of Science:

• Fluid Dynamics
• Rheology
• Materials Science

Background:

• Accurate microscale viscosity measurement is crucial for various scientific and industrial applications.
• Traditional methods often require complex procedures, such as direct contact angle determination, limiting their accessibility and cost-effectiveness.

Purpose of the Study:

• To introduce a novel, simplified method for microscale viscosity measurement.
• To eliminate the need for direct contact angle determination in viscosity measurements.
• To enable direct calculation of rheological parameters for power-law fluids.

Main Methods:

• Utilizing a capillary with a radius R < 0.2 mm to stabilize the apparent contact angle near 90°.
• Analyzing image sequences of a growing pendant droplet to determine volume flow rate (Q).
• Calculating rheological parameters (K and n) of power-law fluids directly from flow rate data.

Main Results:

• The method successfully measures viscosity for Newtonian (glycerol solutions) and non-Newtonian (xanthan gum solutions) fluids within a specific low-viscosity range.
• Experimental verification confirmed the apparent contact angle converges to 90° at small capillary radii.
• Reliable rheological parameters were obtained for low-viscosity and moderately non-Newtonian fluids, showing good agreement with reference values.

Conclusions:

• The developed technique offers a simple, reliable, and low-cost approach for microscale viscosity measurements of microliter-volume fluids.
• The method is effective for low-viscosity Newtonian and moderately non-Newtonian fluids.
• Current limitations exist for high-viscosity or highly non-Newtonian fluids due to increased measurement errors under gravity-driven flow.