arXiv subjects

Ranjini Bandyopadhyay

Publications and source records attributed to Ranjini Bandyopadhyay.

At least 19 recordsLinked to original sources

arXiv2026

From Coffee Rings to Self-Driven Assembly: Active Matter Enabled Design of Drying Droplets

Evaporating colloidal droplets have long been used as model systems to understand capillarity, interfacial transport, and particle assembly, most prominently through the coffee ring effect. In classical descriptions, suspended particles are treated as passive tracers carried by evaporation-driven capillary flow, with additional influence from Marangoni stresses, wettability, and contact line pinning. More recent studies, however, show that this picture changes significantly when the particles themselves are active. Systems containing motile microorganisms, chemically active colloids, or externally driven particles can continuously inject energy or generate gradients within the droplet, leading to self-driven flows, modified interfacial stresses, and dynamic contact line behavior. In this Perspective, we bring together these developments, identify the key mechanisms governing active droplets, highlight the role of bubble-mediated flows, and outline strategies for controlled deposition and functional interface design.

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arXiv2026

Influence of Bubble Lifetime on the Drying of Catalytically Active Sessile Droplets

When colloidal droplets evaporate, suspended particles are redistributed by a competition between evaporation-driven capillary advection, interfacial Marangoni stresses and particle mobility, leading to diverse deposition patterns relevant to coating and self-assembly. While these mechanisms are well understood for passive suspensions, their interplay in chemically active colloidal systems remains less explored. Here, we investigate the drying dynamics of droplets containing catalytic polystyrene-platinum (PS-Pt) Janus particles in the presence of hydrogen peroxide (H2O2) fuel. H2O2 undergoes catalytic decomposition at the Pt hemisphere, resulting in the formation of oxygen (O2). By systematically varying H2O2 concentration, surface wettability and open versus confined drying conditions, we identify distinct transport regimes governed by the relative magnitudes of capillary flow and gas bubble-induced Marangoni convection. While time-resolved contact-angle measurements reveal substrate-dependent evaporation modes, an increase in catalytic activity promotes O2 bubble generation that locally reverses or disrupts outward particle transport. Closed drying conditions further modify evaporation rates and prolong bubble residence times, leading to transitions from peripheral accumulation to spatially uniform or centrally concentrated deposits. Bubble-induced Marangoni flow, controlled here by tuning substrate wettability and environmental conditions, therefore emerges as the dominant mechanism governing the evaporation dynamics and dried morphologies of catalytically active Janus particle droplets.

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arXiv2026

Elasticity-mediated Morphogenesis in Interfacial Colloidal Assemblies

We study the self-assembly of colloidal microgel particles at a quasi-two-dimensional air-water interface of a drying droplet. Using bright-field microscopy, we demonstrate that increasing particle elasticity drives interfacial organization from repulsion-stabilized crystallization to attraction-dominated gelation, via diverse metastable structures including clusters, voids and anisotropic aggregates. Molecular dynamics simulations using an effective potential that captures the interplay between hydrophobic, capillary, steric and dipolar interactions, reproduce the overall phenomenology of the observed colloidal morphogenesis. Our findings establish particle elasticity as a key parameter governing non-equilibrium structural organization of colloids at an interface.

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arXiv2025

Bubble-Driven Flow Transitions in Evaporating Active Droplets on Structured Surfaces

The evaporation of particle-laden droplets on engineered surfaces underpins a wide range of technologies, from printed electronics to biosensing. While the influence of substrate topography on passive particle deposition is well established, the combined effects of active matter dynamics, catalytic gas generation, and surface structuring remain unexplored. Here, we investigate the drying of aqueous droplets containing Janus particles (polystyrene-platinum, PS-Pt) on topographically patterned substrates in the presence of hydrogen peroxide (H2O2) fuel. The catalytic decomposition of H2O2 produces oxygen bubbles within the droplet, introducing strong, transient hydrodynamic perturbations that compete with evaporation driven capillary flows and contact line interactions. We show that bubble activity alters particle transport, leading to distinct and tunable final morphologies not achievable with passive suspensions. This study demonstrates how bubble-induced flow coupled with substrate topography determines deposition patterns during droplet evaporation. Our findings open a route to harnessing active matter and reaction driven flows for directed particle assembly.

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arXiv2025

Investigating the roles of hydrophobicity and electrostatics in the particle-scale dynamics and rheology of dense microgel suspensions

Colloidal microgel particles such as poly(N-isopropylacrylamide) (PNIPAM) shrink reversibly in an aqueous medium due to the expulsion of water at a volume phase transition temperature, VPTT $\sim$33$^\circ$C. Romeo et al. [Adv. Mater. 2010, 22, 3441-3445] had previously shown that dense aqueous PNIPAM suspensions transformed from one viscoelastic solid-like phase to another when suspension temperature was increased, with an intermediate viscoelastic liquid-like phase near the VPTT. They attributed this observation to a change in the inter-particle interaction from hydrophilic to hydrophobic. Here, we show using a combination of experimental techniques that particle hydrophobicity can become significant even below the VPTT. We achieve this by incorporating dissociating additives such as sodium chloride and potassium chloride, or non-dissociating additives such as sucrose, into the aqueous medium. Above the VPTT, we observe that suspension rigidity is the highest in the presence of salts because of the combined effects of electrostatic and hydrophobic attractions. In the presence of non-dissociating sucrose, in contrast, the inter-microgel interaction remains hydrophobic across the VPTT. Such easy tunability of interactions by incorporating commonly available chemicals into the suspension medium opens up new avenues for the synthesis of novel metamaterials.

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arXiv2025

Thermal History Asymmetry and Dissipation in Dense Colloidal Microgel Glasses

Microstructurally arrested matter, from molecular glasses to soft glassy materials, can retain a memory of their thermal or mechanical (shear) histories. Their history-dependent and nonlinear microstructural recoveries have been studied within the Kovacs framework. Here, we applied the temperature ramps of varying magnitudes to dense colloidal suspensions of thermoresponsive, deformable and compressible microgel particles should serve as an effective strategy to probe the nonlinear path-dependent structural recovery of these systems. We synthesised Poly (N-isopropyl acrylamide) (PNIPAM) microgel particles using the free radical precipitation polymerisation method. Using oscillatory rheology, we studied the relaxations of the viscoelastic moduli of dense PNIPAM suspensions that were heated and cooled at various temperature ramp rates. Path-dependent structural recovery was quantified by studying the asymmetric approach of the suspension elastic modulus toward the target temperature during the heating and cooling temperature ramps. The loss modulus peaks, observed at the times of initiation and termination of the temperature ramps, were understood to arise from energy dissipation due to microgel rearrangement events. The heights of the peaks were found to be inversely correlated with the asymmetry in the elastic response. Our work highlights the important role of energy dissipation through microgel rearrangements in eliminating path-dependent asymmetries in the storage moduli of dense PNIPAM suspensions subjected to thermal shocks. By tuning the applied temperature ramp rate and particle packing density, therefore, asymmetric storage modulus relaxations in dense systems can be modulated via adjustments of the accessible free volume.

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arXiv2025

Permeability heterogeneity and bulk linear elasticity of displaced clay suspensions determine interfacial pattern morphologies in Hele-Shaw experiments

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. In the present work, we controlled the mechanical properties of clay suspensions by incorporating additives and studied the interfacial instabilities that resulted when these 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 propagation mechanisms: pattern growth \textit{via} skewering and zig-zag finger propagation. These patterns have hitherto never been observed in experiments with colloidal systems. For moderate clay elasticities, we demonstrate here 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 characterized by the emergence of viscoelastic fractures. Our work demonstrates that incorporating additives is an effective strategy to manipulate the onset and growth of interfacial instabilities during the confined displacement of clay by miscible Newtonian fluids.

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arXiv2024

Using optical tweezer electrophoresis to investigate clay nanoplatelet adsorption on Latex microspheres in aqueous media

The adsorption of charged clay nanoplatelets plays an important role in stabilizing emulsions by forming a barrier around the emulsion droplets and preventing coalescence. In this work, the adsorption of charged clay nanoplatelets on a preformed Latex microsphere in an aqueous medium is investigated at high temporal resolution using optical tweezer-based single-colloid electrophoresis. Above a critical clay concentration, charged clay nanoplatelets in an aqueous medium self-assemble gradually to form gel-like networks that become denser with increasing medium salinity. In a previous publication [R. Biswas et. al., Soft Matter, 2023, 19, 24007-2416], some of us had demonstrated that a Latex microsphere, optically trapped in a clay gel medium, is expected to attach to the network strands of the gel. In the present contribution, we show that for different ionic conditions of the suspending medium, the adsorption of clay nanoplatelets increases the effective surface charge on an optically trapped Latex microsphere while also enhancing the drag experienced by the latter. Besides the ubiquitous contribution of non-electrostatic dispersion forces in driving the adsorption process, we demonstrate the presence of an electrostatically-driven adsorption mechanism when the microsphere was trapped in a clay gel. These observations are qualitatively verified via cryogenic field emission scanning electron microscopy and are useful in achieving colloidal stabilisation, for example, during the preparation of clay-armoured Latex particles in Pickering emulsion polymerisation.

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arXiv2024

Manipulating crack formation in air-dried clay suspensions with tunable elasticity

Clay, the major ingredient of natural soils, is often 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 their microscopic structures evolve due to time-dependent inter-particle screened electrostatic interactions. The rate at which aging progresses is estimated from the rate of evolution of the mechanical moduli and can be controlled by changing clay concentration or by incorporating additives. Since physical aging and evaporation should both contribute to the consolidation of drying clay suspensions, we attempt to manipulate the desiccation process \textit{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, $t_c$, in direct visualization experiments and the elasticity of the drying sample layer, $E$, using microindentation in an atomic force microscope. We demonstrate that $t_c \propto \sqrt{\frac{G_c}{E}}$, where $G_c$, 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.

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arXiv2024

Influence of particle size polydispersity on dynamical heterogeneities in dense particle packings

The dynamics of dense particle packings near the jamming transition is characterized by correlated particle motion. The growth of dynamical heterogeneities, or strong spatial variations in the motion of the particles constituting the system, is a hallmark feature of slow glassy dynamics. We report here a systematic confocal microscopy study that characterizes the cooperative dynamics of fluorescently-labelled colloidal particles in dense aqueous suspensions. We demonstrate that jammed particulate suspensions can be fluidized by increasing the width of the particle size distribution. Our molecular dynamics simulations, performed to numerically investigate the effects of continuous-size polydispersity on dense particle packing dynamics, show an excellent match with our experimental results. Besides shedding light on the fundamental aspects of particle-scale dynamics at the jamming-unjamming transition, our findings are significant in the processing of commonly-encountered dense suspensions such as paints, cosmetics, and food.

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arXiv2024

Role of microgel stiffness in particle self-assembly and suspension rheology across the lower consolute solution temperature

We synthesize thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) colloidal microgel particles of different stiffnesses by controlling the concentration of crosslinker in a one-pot synthesis method. We employ oscillatory rheology and cryogenic scanning electron microscopy to study the temperature and stiffness-induced mechanical properties and microscopic structures of dense aqueous suspensions of the synthesized PNIPAM microgels. Using Fourier transform infrared (FTIR) spectroscopy, we show that particle hydrophobicity increases with increasing suspension temperature and decreasing particle stiffness. Our zeta potential measurements of soft PNIPAM particles and those of intermediate stiffnesses demonstrate that these particles are electrostatically unstable and prone to aggregation even at temperatures below the lower consolute solution temperature (LCST). In contrast, stiff PNIPAM particles in dilute aqueous suspensions are electrostatically stabilized at all temperatures explored in this study. Interestingly, our frequency and strain amplitude sweep rheology experiments reveal that the linear viscoelastic moduli and yield stresses of all the PNIPAM suspensions increase when the temperature is raised above the LCST. Combining cryogenic scanning electron microscopy (cryo-SEM) and rheology, we demonstrate that dense suspensions of soft PNIPAM microgels show a gel-liquid-gel transition with increase in temperature across the LCST. Suspensions of stiff particles, in contrast, exhibit a glass-glass transition under the same temperature sweep conditions and do not pass through an intermediate liquid state.

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arXiv2023

Interfacial instabilities in confined displacements involving non-Newtonian fluids

The growth of interfacial instabilities during fluid displacements can be driven by gradients in pressure, viscosity and surface tension, and by applying external fields. Since displacements of non-Newtonian fluids such as polymer solutions, colloidal and granular slurries are ubiquitous in natural and industrial processes, understanding the growth mechanisms and fully-developed morphologies of interfacial patterns involving non-Newtonian fluids is extremely important. In this perspective, we focus on {displacement experiments} wherein competition{s} between capillary, viscous, elastic and frictional forces drive the onset and growth of interfacial instabilities in confined geometries. We conclude by highlighting several exciting open problems in this research area.

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arXiv2023

Emergence of transient reverse fingers during radial displacement of a shear-thickening fluid

A highly sheared dense aqueous suspension of granular cornstarch particles displays rich nonlinear rheology. We had previously demonstrated the growth and onset of interfacial instabilities when shear-thinning cornstarch suspensions were displaced by a Newtonian fluid, and had suggested methods to maximise displacement efficiency [Palak, R. Sathayanath, S. K. Kalpathy and R. Bandyopadhyay, Colloids Surf. A Physicochem. Eng. Asp., 629 (2021) 127405]. In the present work, we explore the miscible displacement of a dense aqueous cornstarch suspension in its discontinuous shear-thickening regime in a quasi-two-dimensional radial Hele-Shaw cell. We systematically study the growth kinetics of the inner interface between water and the cornstarch suspension, and also of the outer interface between the suspension and air. In addition to the growth of interfacial instabilities at the inner interface, we observe a transient withdrawal of the suspension and the formation of fingering instabilities at the outer interface. We demonstrate that these `reverse fingering' instabilities are extremely sensitive to the injection flow rate of water, the gap of the Hele-Shaw cell and the concentration of the displaced cornstarch suspension, {and emerge irrespective of immiscibility between the fluid pair. We believe that as the cornstarch suspension dilates due to the high shear rate imposed by the displacing fluid, the outer suspension-air interface responds with a restoring force, resulting in the penetration of air into the suspension and the formation of reverse fingers. We note that the growth of reverse fingers significantly reduces the displacement efficiency of the suspension. Finally, we demonstrate a correlation in the growth of inner and outer interfacial patterns by computing the velocity with which stresses propagate in the confined dense suspension. Our findings are useful in understanding the flow of granular materials through constrained geometries and can be extended to study stress propagation in shear-thickening materials due to a sudden imposition of high shear rate, such as in impact behaviour.

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arXiv2023

Correlating microscopic viscoelasticity and structure of an aging colloidal gel using active microrheology and cryogenic scanning electron microscopy

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.

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arXiv2022

Growth kinetics of interfacial patterns formed by the radial displacement of an aging viscoelastic suspension

When a soft glassy colloidal suspension is displaced by a Newtonian fluid in a radial Hele-Shaw geometry, the pattern morphology that develops at the interface is determined by the complex rheology of the former. We had reported in an earlier work [Palak, V. R. S. Parmar, D. Saha and R. Bandyopadhyay, JCIS Open, 6 (2022) 100047] that a range of interfacial patterns can be formed by controlling the elasticity of the displaced suspension, the flow rate of the displacing fluid and the interfacial tension of the fluid pair. Interestingly, all the different morphological features can be distinguished in terms of their areal ratios, defined as the ratio of the areas occupied by the fully-developed pattern and the smallest circle enclosing it. In a significant advance to this earlier work, we show here that a systematic study of spatio-temporal pattern growth can reveal important information about pattern selection mechanisms. We analyse the time-evolution of the patterns to reveal interesting correlations between their growth mechanisms and fully-developed morphologies. We believe that such systematic identification of the unique temporal features characterising pattern growth at the interface between an aging viscoelastic clay suspension and a Newtonian fluid can be useful in predicting and suppressing the onset and evolution of interfacial instabilities in the displacement of mud and cement slurries.

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arXiv2022

Dichotomous behaviors of stress and dielectric relaxations in dense suspensions of swollen thermoreversible hydrogel microparticles

Hypothesis: While the mechanical disruption of microscopic structures in complex fluids by large shear flows has been studied extensively, the effects of applied strains on the dielectric properties of macromolecular aggregates has received far less attention. Simultaneous rheology and dielectric experiments can be employed to study the dynamics of sheared colloidal suspensions over spatiotemporal scales spanning several decades. Experiments: Using a precision impedance analyzer, we study the dielectric behavior of strongly sheared aqueous suspensions of thermoreversible hydrogel poly(N-isopropylacrylamide) (PNIPAM) particles at different temperatures. We also perform stress relaxation experiments to uncover the influence of large deformations on the bulk mechanical moduli of these suspensions. Findings: The real parts of the complex dielectric permittivities of all the sheared PNIPAM suspensions exhibit distinct relaxation processes in the low and high frequency regimes. At a temperature below the lower consolute solution temperature (LCST), both real and imaginary parts of the permittivities of highly dense PNIPAM suspensions decrease with increase in applied oscillatory strain amplitudes. Simultaneously, we note a counter-intuitive slowdown of the dielectric relaxation dynamics. Contrary to our rheo-dielectric findings, our bulk rheology experiments, performed under identical conditions, reveal shear-thinning dynamics with increasing strain amplitudes. We propose the shear-induced rupture of fragile clusters of swollen PNIPAM particles to explain our observations. Keywords : Rheo-dielectric; Thermoresponsive hydrogels; Segmental motion; Counterion polarization; Interfacial polarization; Dense suspensions; Rheology; Shear-thinning.

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arXiv2022

Pattern selection in radial displacements of a confined aging viscoelastic fluid

Intricate fluid displacement patterns, arising from the unstable growth of interfacial perturbations, can be driven by fluid viscoelasticity and surface tension. A soft glassy suspension ages, $i.e.$ its mechanical moduli evolve with time, due to the spontaneous formation of suspension microstructures. The shear and time-dependent rheology of an aging suspension can be exploited to generate a wide variety of interfacial patterns during its displacement by a Newtonian fluid. Using video imaging, we report a rich array of interfacial pattern morphologies: dense viscous, dendritic, viscoelastic fracture, flower-shaped, jagged and stable, during the miscible and immiscible displacements of an aging colloidal clay suspension by Newtonian fluids injected into a radial quasi-two-dimensional geometry at different flow rates. We propose a new parameter, the areal ratio, which we define as the fully-developed pattern area normalized by the area of the smallest circle enclosing it. We show that the natural logarithms of the areal ratios uniquely identify the distinct pattern morphologies, such that each pattern can be segregated in a three-dimensional phase diagram spanned by the suspension aging time, the displacing fluid flow rate, and interfacial tension. Besides being of fundamental interest, our results are useful in predicting and controlling the growth of interfaces during fluid displacements.

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arXiv2021

Influence of dispersion medium structure on the physicochemical properties of aging colloidal suspensions investigated using the synthetic clay Laponite

Hypothesis: Aging in colloidal suspensions manifests as a reduction in kinetic freedom of the colloids. In aqueous suspensions of charged colloids, the role of inter-particle electrostatics interactions on the aging dynamics is well debated. Despite water being the dispersion medium, the influence of water structure on the physicochemical properties of aging colloids has never been considered before. Laponite, a model hectorite clay, could be used to evaluate the relative contributions of medium structure and electrostatics in determining the physicochemical properties of aging colloidal suspensions. Experiments: The structure of the dispersion medium is modified either by incorporating uncharged/charged kosmotropic (structure-inducing) or chaotropic (structure-disrupting) molecules or by changing suspension temperature. A new protocol, wherein the medium is heated before adding clay particles, is also introduced to evaluate the effects of hydrogen bond disruptions on suspension aging. Dynamic light scattering, rheological measurements and particle-scale imaging are employed to evaluate the physicochemical properties of the suspensions. Findings: A strong influence of medium structure is evident when inter-particle electrostatic interactions are weak. Enhancement and disruption of hydrogen bonds in the medium are, respectively, strongly correlated with acceleration and delay of suspension aging dynamics. The physicochemical properties of charged clay colloidal suspensions are therefore controlled by altering hydrogen bonding in the dispersion medium.

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