Calendar
Physical Chemistry seminar: Ricky F. López-Santiago
Seminarium
From:
2025-02-20 14:15
to
15:00
Place: Lecturehall KC:F
Contact: andrea [dot] scotti [at] fkem1 [dot] lu [dot] se
A Physical Chemistry seminar by Postdoc Ricky F. López-Santiago, RWTH Aachen University, Germany
When: 20 February, 14:15-15:00
Where: Lecturehall KC:F
Title: Entanglement of thread-like structures embedded in liquids and gels and its effect on linear and nonlinear viscoelasticity
Abstract: This talk discusses the effects of thread-like structures on their linear and nonlinear viscoelasticity. The study presents three different systems from an experimental physics perspective: Worm-like micelles solutions (WLMs) as a model of living polymers, gelatin solutions as a model of flexible biopolymers, and alginate and chitosan as models of charged biopolymers; all systems exhibit a phase transition under different conditions. WLMs show a transition from an isotropic to a nematic state when shear strain is applied, which is called “shear banding.” For biopolymers, the system undergoes a sol-gel transition due to modifications in the physicochemical environment. Linear and nonlinear rheological measurements were carried out using Small and Large Amplitude Oscillatory Shear (SAOS and LAOS). Diffusive wave spectroscopy micro-rheology was also employed to measure the mean square displacements of microspheres embedded in thread-like fluids, providing information about the relaxation mechanisms of chains and establishing a connection with their viscoelastic properties. This research has shown that the structure of the polymer matrix directly affects the nonlinear rheological response. The Lissajous curves serve as a fingerprint for each system, and their geometrical analysis aids in understanding how the network evolves with deformation. For WLMs, a significant discovery was defining the entanglement index as the ratio between the entanglement length and total contour length, an essential mesoscopic parameter for understanding the entanglement of the micellar network. This index is directly linked to the solution's elasticity from a dynamic fluid perspective. Studying the rheological behavior under large shear strain in biopolymer gels indirectly reveals the fractal dimension, which agrees with the dimension obtained from other experimental techniques. An important finding is that the Chambon & Winter criteria show a similar critical exponent (n ∼ 0.6) for all physical gels, indicating that the percolation threshold of the network is independent of the gelation mechanisms. The mixture of alginate and chitosan exhibits the same rheological signature as core-shell synthetic microgels, demonstrating that the gel preparation protocol forms these structures through self-assembly. Lastly, it introduces the launch of a new project discussing Pickering emulsions made with microgels: systems that exhibit a duality between thread-like structures and colloidal properties.