Optically driven nonlinear microrheology of gelatin

Area of Science:

• Biophysics
• Materials Science
• Rheology

Background:

• Understanding the mechanical properties of viscoelastic materials like gelatin is crucial for various applications.
• Macroscopic rheological measurements provide bulk material properties but lack microscale insights.
• Existing techniques struggle to probe yielding and nonlinear behavior at the microscopic level.

Purpose of the Study:

• To develop and demonstrate a microscopic rheological measurement technique.
• To investigate the yielding behavior of entangled viscoelastic biopolymers at the microscale.
• To correlate microscopic observations with macroscopic rheological responses.

Main Methods:

• Utilizing circularly polarized laser tweezers to apply optical torque to a birefringent wax microdisk.
• Embedding the microdisk within a gelatin matrix, a highly entangled viscoelastic biopolymer.
• Increasing laser power to control torque and measuring the microdisk's angular displacement.

Main Results:

• Successfully demonstrated the microscopic equivalent of a step-stress rheological measurement.
• Mapped the microscopic rheological response of presheared gelatin from linear to nonlinear regimes.
• Observed yielding behavior at the microscale, consistent with macroscopic observations.
• Extracted applied stress and deduced effective strain from the microscopic torque-angle relationship.

Conclusions:

• The developed technique provides a powerful tool for microscale rheology of soft materials.
• Microscopic rheology can effectively probe yielding and nonlinear mechanical responses.
• Findings validate the correlation between microscale and macroscale rheological behavior in complex fluids.