Microliter viscometry using a bright-field microscope: η-DDM

Area of Science:

• Soft Matter Physics
• Rheology
• Colloidal Science

Background:

• Rheological properties are crucial for understanding material behavior.
• Microrheology infers bulk properties from tracer particle motion.
• Differential Dynamic Microscopy (DDM) analyzes particle motion via the intermediate scattering function.

Purpose of the Study:

• To present a novel method (η-DDM) for measuring steady-shear viscosity.
• To demonstrate the utility of DDM combined with the Cox-Merz rule for rheological measurements.
• To validate the method using diverse sample systems.

Main Methods:

• Utilizing Differential Dynamic Microscopy (DDM) to analyze Brownian motion of colloidal tracers.
• Calculating the mean-squared displacement (MSD) from the intermediate scattering function.
• Applying the empirical Cox-Merz rule to determine steady-shear viscosity (viscosity at zero frequency).

Main Results:

• DDM combined with the Cox-Merz rule accurately measures steady-shear viscosity.
• The method provides reliable data on long time and length scales, crucial for low frequencies.
• Successful validation across Newtonian fluids, protein suspensions, and polymer solutions.

Conclusions:

• The η-DDM method enables viscosity measurements using standard bright-field optical microscopes.
• This technique functions as a convenient and reliable microliter viscometer.
• Its low sample volume requirement makes it highly valuable for biological and soft matter applications.