Rheology of polymer solutions using colloidal-probe atomic force microscopy.

Area of Science:

• Rheology
• Polymer Science
• Surface Science

Background:

• Understanding the rheological behavior of confined fluids is crucial in various scientific and industrial applications.
• Polymer solutions exhibit complex flow properties, especially under confinement.
• Atomic Force Microscopy (AFM) offers a unique tool to probe fluid behavior at the nanoscale.

Purpose of the Study:

• To investigate the rheological properties of polymer solutions confined between a sphere and a flat surface.
• To determine how viscosity changes with confinement gap height for non-Newtonian fluids.
• To explore potential mechanisms behind observed viscosity variations.

Main Methods:

• Utilized a colloidal-probe atomic force microscope (AFM) setup.
• Measured hydrodynamic forces exerted by the polymer solution on a sphere.
• Calculated solution viscosity as a function of the gap distance between the sphere and a flat surface.

Main Results:

• Experimentally validated the method for Newtonian fluids where viscosity is gap-independent.
• Observed a significant decrease in viscosity with decreasing gap height for large molecular weight polymer solutions.
• The observed viscosity reduction is analogous to phenomena seen in colloidal suspensions.

Conclusions:

• The rheological behavior of confined polymer solutions is highly dependent on the gap height.
• Polymer depletion from the confinement region at high shear rates is a plausible explanation for the observed viscosity decrease.
• AFM is a suitable technique for studying confined fluid rheology, revealing non-Newtonian behaviors.