Transition between solid and liquid state of yield-stress fluids under purely extensional deformations

Area of Science:

• Rheology and Material Science
• Fluid Dynamics
• Continuum Mechanics

Background:

• Elastoviscoplastic materials exhibit complex behaviors under deformation, combining elastic, viscous, and plastic properties.
• Understanding yielding and flow dynamics in extensional flows is crucial for material processing and performance.
• Standard viscoplastic theories often fail to capture the nuances of these materials due to factors like normal stress development.

Purpose of the Study:

• To experimentally measure and theoretically model the behavior of elastoviscoplastic materials under steady, planar elongation.
• To predict yielding and flow dynamics in pure extensional flows.
• To investigate the deviation from ideal viscoplastic theory and identify factors influencing yield stress ratios.

Main Methods:

• Utilized microfluidic measurements for experimental data acquisition.
• Developed and applied a theoretical model allowing deformation of the solid state.
• Analyzed velocity fields to derive a scaling law for elongational yield stress.

Main Results:

• Observed significant deviations in the ratio of elongational to shear yield stress compared to ideal viscoplastic predictions.
• Attributed these deviations to normal stresses developing in the solid state before yielding.
• Demonstrated that yield strain governs the transition dynamics from solid to liquid states.

Conclusions:

• The developed theory accurately predicts yielding and flow dynamics of elastoviscoplastic materials in extensional flows.
• Normal stresses in the solid state play a critical role in modifying yield stress behavior.
• A simple scaling law was identified to determine elongational yield stress from velocity fields, overcoming experimental quantification challenges.