http://hdl.handle.net/2289/149 2026-03-09T08:49:01Z 2026-03-09T08:49:01Z Parmar, Vaibhav Raj Singh Bandyopadhyay, Ranjini http://hdl.handle.net/2289/8682 2026-02-27T11:28:13Z 2025-12-23T00:00:00Z

Title: Permeability heterogeneity and bulk linear elasticity of displaced clay suspensions determine interfacial pattern morphologies in Hele–Shaw experiments Authors: Parmar, Vaibhav Raj Singh; Bandyopadhyay, Ranjini Abstract: When a less viscous Newtonian fluid displaces an aging aqueous clay suspension in a confined space, a rich array of interfacial patterns emerges due to a predominantly viscous instability. Earlier studies on clay displacement identified macroscopic morphologies such as fingering and fracturing, but the local finger propagation modes did not receive much attention. For the present work, we controlled the mechanical properties of clay suspensions by incorporating additives and observed a range of local finger propagation modes when the suspensions were radially displaced by water in a Hele–Shaw cell. When the elasticity of clay was low, the interfacial dynamics exhibited features of nonlinear viscous fingering in heterogeneous media. By tuning the nature and content of additives that delay clay aging, we uncovered two novel finger propagation mechanisms: skewering and zig–zag growth, both characterised by tortuous trajectories and thick finger profiles. These patterns have hitherto never been observed in experiments with colloidal systems. For moderate clay elasticities, we report that shear-thinning-induced flow anisotropy leads to the formation of dendrites with dominant side branches. As clay elasticity increases due to the incorporation of salts, the energy required to create fractures becomes smaller than that for system-wide yielding. This scenario is characterised by the emergence of viscoelastic fractures. Our work demonstrates that incorporating additives in the suspension medium significantly modifies clay elasticity and permeability and is an effective strategy to manipulate the growth and morphologies of instabilities at clay-water interfaces. Description: Open Access.

2025-12-23T00:00:00Z Parmar, Vaibhav Raj Singh Bandyopadhyay, Ranjini http://hdl.handle.net/2289/8674 2026-02-27T11:14:07Z 2024-11-11T00:00:00Z

Title: Manipulating crack formation in air-dried clay suspensions with tunable elasticity Authors: Parmar, Vaibhav Raj Singh; Bandyopadhyay, Ranjini Abstract: Clay, the major ingredient of natural soils, is used as a rheological modifier while formulating paints and coatings. When subjected to desiccation, colloidal clay suspensions and clayey soils crack due to the accumulation of drying-induced stresses. Even when desiccation is suppressed, aqueous clay suspensions exhibit physical aging, with their elastic and viscous moduli increasing over time as the clay particles self-assemble into gel-like networks due to time-dependent inter-particle screened electrostatic interactions. The rate of evolution of the suspension structures and therefore of the mechanical moduli can be controlled by changing clay concentration or by incorporating additives. Since physical aging and desiccation should both contribute to the consolidation of drying clay suspensions, we manipulate the desiccation process via alterations of clay and additive concentrations. For a desiccating sample with an accelerated rate of aging, we observe faster consolidation into a semi-solid state and earlier onset of cracks. We estimate the crack onset time, tc⁠, in direct visualization experiments and the elasticity of the drying sample layer, E, using microindentation in an atomic force microscope. We demonstrate that tc ∝ √Gc/E⁠, where Gc⁠, the fracture energy, is estimated by fitting our experimental data to a linear poroelastic model that incorporates the Griffith's criterion for crack formation. Our work demonstrates that early crack onset is associated with lower sample ductility. The correlation between crack onset in a sample and its mechanical properties as uncovered here is potentially useful in preparing crack-resistant coatings and diverse clay structures. Description: Open Access.

2024-11-11T00:00:00Z Misra, Chandeshwar Kawale, Sonali Vasant Behera, Sanjay Kumar Bandyopadhyay, Ranjini http://hdl.handle.net/2289/8672 2026-02-27T11:10:05Z 2024-10-18T00:00:00Z

Title: Effect of particle stiffness on microgel self-assembly and suspension phase behavior over a broad temperature range Authors: Misra, Chandeshwar; Kawale, Sonali Vasant; Behera, Sanjay Kumar; Bandyopadhyay, Ranjini Abstract: We synthesized thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) colloidal microgel particles of different stiffnesses by controlling the concentration of a polar crosslinker in a precipitation polymerization synthesis method. When suspended in an aqueous medium, the particles collapsed by expelling water as the temperature was raised toward the volume phase transition temperature (VPTT) of ≈ 34 °C. We noted that the sizes of the stiffer particles, synthesized with higher crosslinker concentration, collapsed less abruptly. Using Fourier transform infrared spectroscopy, we observed enhanced particle dehydration with increasing temperature and decreasing particle stiffness. Oscillatory rheology experiments on dense aqueous PNIPAM suspensions, prepared at a fixed particle effective volume fraction ϕeff = 1.5 at 25°C, revealed that suspensions constituted by the stiffest particles are the most elastic over a broad temperature range. Above the VPTT, suspensions of particles of intermediate stiffnesses exhibited two-step yielding, a typical signature of fragile gel formation. Zeta potential measurements showed that PNIPAM particles of lower stiffnesses are rendered electrostatically unstable in aqueous suspension. Combining cryogenic scanning electron microscopy and rheology, we noted a glass–glass transition when the temperature of a dense suspension of stiff PNIPAM particles was raised across the VPTT. In contrast, suspensions of particles of the lowest stiffnesses showed a gel-viscoelastic liquid–gel transition during an identical temperature ramp experiment. Our study reveals that temperature-induced phase transformations in dense PNIPAM suspensions depend sensitively on the stiffness of the constituent particles and can be explained by considering amphiphilicity-driven morphological changes in the suspension microstructures. Description: Open Access.

2024-10-18T00:00:00Z Biswas, Rajkumar Parmar, Vaibhav Raj Singh Thambi, Anson G Bandyopadhyay, Ranjini http://hdl.handle.net/2289/8671 2026-02-27T11:08:51Z 2023-03-10T00:00:00Z

Title: Correlating microscopic viscoelasticity and structure of an aging colloidal gel using active microrheology and cryogenic scanning electron microscopy† Authors: Biswas, Rajkumar; Parmar, Vaibhav Raj Singh; Thambi, Anson G; Bandyopadhyay, Ranjini Abstract: Optical tweezers (OTs) can detect pico-Newton range forces operating on a colloidal particle trapped in a medium and have been successfully utilized to investigate complex systems with internal structures. LAPONITE® clay particles in an aqueous medium self-assemble to form microscopic networks over time as electrostatic interactions between the particles gradually evolve in a physical aging process. We investigate the forced movements of an optically trapped micron-sized colloidal probe particle, suspended in an aging LAPONITE® suspension, as the underlying LAPONITE® microstructures gradually develop. Our OT-based oscillatory active microrheology experiments allow us to investigate the mechanical responses of the evolving microstructures in aging aqueous clay suspensions of concentrations ranging from 2.5% w/v to 3.0% w/v and at several aging times between 90 and 150 minutes. We repeat such oscillatory measurements for a range of colloidal probe particle diameters and investigate the effect of probe size on the microrheology of the aging suspensions. Using cryogenic field emission scanning electron microscopy (cryo-FESEM), we examine the average pore areas of the LAPONITE® suspension microstructures for various sample concentrations and aging times. By combining our OT and cryo-FESEM data, we report here for the first time to the best of our knowledge, an inverse correlation between the crossover modulus and the average pore diameter of the aging suspension microstructures for the different suspension concentrations and probe particle sizes studied here. Description: Restricted Access.

2023-03-10T00:00:00Z