Negative normal stress differences N₁-N₂ in a low concentration capillary suspension

Area of Science:

• Rheology
• Colloid Science
• Fluid Dynamics

Background:

• Capillary suspensions are particle suspensions in a two-fluid system with strong capillary attractions.
• These suspensions exhibit complex behaviors influenced by particle interactions and fluid properties.
• Previous research suggested higher solid concentrations were needed to observe significant normal stress differences.

Purpose of the Study:

• To investigate the rheological properties of capillary suspensions at a lower-than-expected solid concentration.
• To determine the conditions under which negative normal stress differences occur in capillary suspensions.
• To understand the influence of different states (pendular and capillary) on rheological responses.

Main Methods:

• Rheological measurements were performed on capillary suspensions.
• Experiments were conducted at a solid concentration of 25%.
• The study examined both the pendular and capillary states of the suspensions.

Main Results:

• Negative normal stress differences were observed at 25% solid concentration, a level previously considered too low.
• Capillary suspensions exhibited shear-thinning behavior across all studied shear rates.
• In the pendular state, a transition from positive to negative normal stress difference occurred at low shear rates, indicating flexible floc reorientation.
• In the capillary state, only negative normal stress differences were observed, attributed to increased hydrodynamic interactions from droplet breakup.

Conclusions:

• The study demonstrates that negative normal stress differences can occur in capillary suspensions at lower solid concentrations than previously thought.
• The observed rheological behavior is linked to the flexibility of particle flocs and hydrodynamic interactions.
• The findings provide new insights into the complex rheology of capillary suspensions.