Raman Research Institute

Bengaluru

Pre-submission Thesis Presentation

Title: "Flow instabilities, mechanics and memory formation in soft and biological systems"

Speaker: Abhishek Ghadai

Date & Time: Thursday, 24 July 2025 at 11:00 AM

Venue: Auditorium

Abstract: Soft materials consist of diverse systems that exhibit properties between those of Newtonian fluids and crystalline solids. These materials are termed "viscoelastic" due to their combination of viscous and elastic characteristics, playing a crucial role in everyday life. This thesis focuses on the flow and deformation of soft materials, particularly wormlike micelles (WLM), biopolymer networks, and bio-composites. First, we investigate the origin of steady state stress fluctuations in a shear-thinning wormlike micellar fluid. We find large stress fluctuations during the elastic turbulent flow beyond a critical Weissenberg number (Wi) originating from the stick-slip and elastic recoil events. We observe that the characteristic persistent time of stress fluctuations matches well with the time scale of the stick-slip events, as well as the micellar breaking time, indicating a possible connection between the striking stress dynamics and the micellar kinetics. Next, we study the instability and stress fluctuation of a mm-sized probe driven through a wormlike micellar fluid. We measure the timedependent stress on the probe during the motion and find that the fluctuations have a sawtooth nature with a slow build-up and a sudden drop in stress. We correlate these fluctuations with the structural rearrangements of the wake behind the probe for varied temperature and salt concentration. Next, we study the Kovacs-like memory effect in strain stiffening biopolymer networks formed by collagen. We discover robust memory effects in the strain stiffening regime, which are absent in the linear response regime. The rearrangement of the network in the form of localized compression and decompression plays a crucial role. Further, we investigate network architecture-dependent mechanical response in temperature-responsive collagen-PNIPAM composites. These composites utilize thermo-responsive PNIPAM microgel particles to achieve reversible mechanical switching. We find that the shear modulus of the composite enhances when the microgel particle diameter changes across the system's volume phase transition temperature, with the degree of enhancement depending on the collagen network's architecture. Structural changes observed through confocal microscopy provided further insights into this behavior.

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