arXiv subjects

Rajesh Ganapathy

Publications and source records attributed to Rajesh Ganapathy.

At least 19 recordsLinked to original sources

arXiv2025

Reentrant melting of scarred odd crystals by self-shear

Spatial confinement can induce geometrical frustration in condensed phases, giving rise to topological defects that confer materials with new and exotic properties. Here, we experimentally uncover the remarkable effect of confinement-induced defect strings termed `grain boundary scars' on the behavior of dense two-dimensional assemblies of granular spinners, a canonical odd elastic solid. We show that the spatial arrangement of these scars fundamentally reshapes the flows triggered by chiral activity. Specifically, they cause the topologically protected edge flows - a ubiquitous feature of confined spinner assemblies - to decouple from the bulk. Strikingly, increasing the net chiral activity of the system by tuning the ratio of counterclockwise to clockwise spinners caused spontaneous self-shearing. The resulting odd radial stresses led to a chiral activity-mediated reentrant melting transition at a fixed areal spinner density. Our findings open new avenues for exploiting geometrical frustration to elicit novel responses from odd elastic solids.

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arXiv2025

A Single-granule Stirling Heat Engine

Single-particle heat engines at atomic and colloidal scales obey the universal thermodynamic bounds on work and efficiency. Here, we translate these principles to the macroscale by building an athermal Stirling engine whose working medium is a millimeter-sized, vibrofluidized granule confined in a time-dependent magnetic trap. By embedding a rattler within the granule to inject noise, we engineer overdamped, Brownian-like dynamics in an otherwise inertial particle. This design enables independent control over the granule's effective temperature and spatial confinement. Our engine quantitatively reproduces the universal power-efficiency trade-offs of finite-time thermodynamics, achieving the Curzon-Ahlborn efficiency at maximum power. Strikingly, we uncover a control parameter-dependent damping that leads to an unexpected dissipation mechanism - the losses in the compression stroke rival or even exceed those during expansion. Our work establishes an accessible experimental platform to study small-system thermodynamics in intrinsically athermal systems.

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arXiv2024

A shape-driven reentrant jamming transition in confluent monolayers of synthetic cell-mimics

Many critical biological processes, like wound healing, require confluent cell monolayers/bulk tissues to transition from a jammed solid-like to a fluid-like state. Although numerical studies anticipate changes in the cell shape alone can lead to unjamming, experimental support for this prediction is not definitive because, in living systems, fluidization due to density changes cannot be ruled out. Additionally, a cell's ability to modulate its motility only compounds difficulties since even in assemblies of rigid active particles, changing the nature of self-propulsion has non-trivial effects on the dynamics. Here, we design and assemble a monolayer of synthetic cell-mimics and examine their collective behaviour. By systematically increasing the persistence time of self-propulsion, we discovered a cell shape-driven, density-independent, re-entrant jamming transition. Notably, we observed cell shape and shape variability were mutually constrained in the confluent limit and followed the same universal scaling as that observed in confluent epithelia. Dynamical heterogeneities, however, did not conform to this scaling, with the fast cells showing suppressed shape variability, which our simulations revealed is due to a transient confinement effect of these cells by their slower neighbors. Our experiments unequivocally establish a morphodynamic link, demonstrating that geometric constraints alone can dictate epithelial jamming/unjamming.

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arXiv2023

Intermediate-range order governs dynamics in dense colloidal liquids

The conventional wisdom is that liquids are completely disordered and lack non-trivial structure beyond nearest-neighbor distances. Recent observations have upended this view and demonstrated that the microstructure in liquids is surprisingly rich and plays a critical role in numerous physical, biological, and industrial processes. However, approaches to uncover this structure are either system-specific or yield results that are not physically intuitive. Here, through single-particle resolved three-dimensional confocal microscope imaging and the use of a recently introduced four-point correlation function, we show that bidisperse colloidal liquids have a highly non-trivial structure comprising alternating layers with icosahedral and dodecahedral order, which extends well-beyond nearest-neighbor distances and grows with supercooling. By quantifying the dynamics of the system on the particle level, we establish that it is this intermediate-range order, and not the short-range order, which has a one-to-one correlation with dynamical heterogeneities, a property directly related to the relaxation dynamics of glassy liquids. Our experimental findings provide a direct and much sought-after link between the structure and dynamics of liquids and pave the way for probing the consequences of this intermediate-range order in other liquid state processes.

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arXiv2022

Motile topological defects hinder dynamical arrest in dense liquids of active ellipsoids

Recent numerical studies have identified the persistence time of active motion as a critical parameter governing glassy dynamics in dense active matter. Here we studied dynamics in liquids of granular active ellipsoids with tunable persistence and velocity. We show that increasing the persistence time at moderate supercooling is equivalent to increasing the strength of attraction in equilibrium liquids and results in reentrant dynamics not just in the translational degrees of freedom, as anticipated, but also in the orientational ones. However, at high densities, motile topological defects, unique to active liquids of elongated particles, hindered dynamical arrest. Most remarkably, for the highest activity, we observed intermittent dynamics due to the jamming-unjamming of these defects for the first time.

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arXiv2021

Synergistic action in colloidal heat engines coupled by non-conservative flows

Collective operation of multiple engines to achieve a common objective is a vital step in the design of complex machines. Recent studies have reduced the length scales of engine design to micro and nanometers. While strategies to build complex machines from these remain to be devised, even the basic design principles remain obscure. Here, we construct and analyze the simplest collection of two engines from a pair of colloidal microspheres in optical traps at close separation. We demonstrate that at such proximity, non-conservative scattering forces that were hitherto neglected, affect the particle motion and hydrodynamics arising from dissipating these results in violating zeroth law of thermodynamics. Leveraging this in a manner analogous to microswimmers and active Brownian particles, we show that a collection of two interacting engines outperform those that are well separated. While these results explore the simplest case of two engines, the underlying concepts could aid in designing larger collections akin to biological systems.

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arXiv2021

Tuning the performance of a micrometer-sized Stirling engine through reservoir engineering

Colloidal heat engines are paradigmatic models to understand the conversion of heat into work in a noisy environment - a domain where biological and synthetic nano/micro machines function. While the operation of these engines across thermal baths is well-understood, how they function across baths with noise statistics that is non-Gaussian and also lacks memory, the simplest departure from equilibrium, remains unclear. Here we quantified the performance of a colloidal Stirling engine operating between an engineered \textit{memoryless} non-Gaussian bath and a Gaussian one. In the quasistatic limit, the non-Gaussian engine functioned like an equilibrium one as predicted by theory. On increasing the operating speed, due to the nature of noise statistics, the onset of irreversibility for the non-Gaussian engine preceded its thermal counterpart and thus shifted the operating speed at which power is maximum. The performance of nano/micro machines can be tuned by altering only the nature of reservoir noise statistics.

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arXiv2020

Emergent Stereoselective Interactions and Self-recognition in Polar Chiral Active Ellipsoids

In many active matter systems, particle trajectories have a well-defined handedness or chirality. Whether such chiral activity can introduce stereoselective interactions between particles is not known. Here we developed a strategy to tune the nature of chiral activity of 3D-printed granular ellipsoids without altering their shape or size. In vertically agitated monolayers of these particles, we observed two types of dimers form depending on the chirality of the pairing monomers. Heterochiral dimers moved collectively as a single achiral active unit, while homochiral ones formed a translationally immobile spinner. In active racemic mixtures, the former was more abundant than the latter indicating stereoselectivity. Through dimer lifetime measurements, we provide compelling evidence for chiral self-recognition in mixtures of particles with different chiral activities. We finally show that changing only the net chirality of a dense active liquid from a racemic mixture to an enantiopure liquid fundamentally alters its nature of collective relaxation.

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arXiv2019

Unraveling the Role of Frictional Contacts and Particle Orientational Order During Shear-thickening in Suspensions of Colloidal Rods

There is now convincing evidence that inter-particle frictional contacts are essential for observing shear-thickening in concentrated suspensions of compact particles. While this has inspired many strategies to tailor the rheology in these systems, in the more general case, viz-a-viz suspensions of anisotropic particles, the mechanism of shear-thickening remains unclear. Here through simultaneous measurements of the bulk viscosity and the first Normal stress difference, we show that strong shear thickening in suspensions of colloidal rods is accompanied by large positive normal stresses, indicating the formation of a system-spanning frictional contact network. We also find that flow in the shear-thickening regime is unsteady and shows a rather rich time-dependence. By carrying out single particle-resolved confocal rheology measurements, we provide compelling evidence that this rheological chaos arises from a strong coupling between the imposed flow and particle orientational order. Building on these observations, we designed colloidal rods with temperature-tunable tribological properties and demonstrate the feasibility of achieving in-situ control over suspension rheology. These findings show that the interplay between orientational order and frictional interactions plays a critical role in the shear thickening of dense suspensions of colloidal rods.

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arXiv2019

Long-wavelength fluctuations and anomalous dynamics in two-dimensional liquids

Long-wavelength Mermin-Wagner fluctuations prevent the existence of translational long-range order, in two-dimensional systems at finite temperature. Their dynamical signature, which is the divergence of the vibrational amplitude with the system size, also affects disordered solids and washes out the transient solid-like response generally exhibited by liquids cooled below their melting temperature. Through a combined numerical and experimental investigation, here we show that long-wavelength fluctuations are also relevant at high temperature, where the liquid dynamics does not reveal a transient solid-like response. In this regime, they induce an unusual but ubiquitous decoupling between long-time diffusion coefficient $D$ and structural relaxation time $τ$, where $D\propto τ^{-κ}$, with $κ> 1$. Long-wavelength fluctuations have a negligible influence on the relaxation dynamics only at extremely high temperatures, in molecular liquids, or extremely low densities, in colloidal systems.

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arXiv2018

Strength of Mechanical Memories is Maximal at the Yield Point of a Soft Glass

We show experimentally that both single and multiple mechanical memories can be encoded in an amorphous bubble raft, a prototypical soft glass, subject to an oscillatory strain. In line with recent numerical results, we find that multiple memories can be formed sans external noise. By systematically investigating memory formation for a range of training strain amplitudes spanning yield, we find clear signatures of memory even beyond yielding. Most strikingly, the extent to which the system recollects memory is largest for training amplitudes near the yield strain and is a direct consequence of the spatial extent over which the system reorganizes during the encoding process. Our study further suggests that the evolution of force networks on training plays a decisive role in memory formation in jammed packings.

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arXiv2017

Growing Surface Tension of Amorphous-Amorphous Interfaces on Approaching the Colloidal Glass Transition

There is mounting evidence indicating that relaxation dynamics in liquids approaching their glass transition not only becomes increasingly cooperative (1,2) but the relaxing regions also become more compact in shape(3-7). While the surface tension of the interface separating neighboring relaxing regions is thought to play a crucial role in deciding both their size and morphology(8-10), owing to the amorphous nature of these regions, even identifying these interfaces has not been possible in bulk liquids. Here, by devising a scheme to identify self-induced disorder sites in bulk colloidal liquids, we directly quantified the dynamics of interfaces delineating regions of high and low configurational overlap. This procedure also helped unveil a non-monotonicity in dynamical correlations that has never been observed in bulk supercooled liquids. Using the capillary fluctuation method (11,12), we measured the surface tension of amorphous-amorphous interfaces with supercooling and find that it increases rapidly across the mode-coupling area fraction. Remarkably, a similar growth in the surface tension is also seen in the presence of a pinned amorphous wall. Our observations help prune theories of glass formation and opens up new research avenues aimed at tuning the properties of amorphous-amorphous interfaces, and hence the glass itself, in a manner analogous to grain boundary engineering in polycrystals (13).

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arXiv2016

Deconstructing the glass transition through critical experiments on colloids

The glass transition is the most enduring grand-challenge problem in contemporary condensed matter physics. Here, we review the contribution of colloid experiments to our understanding of this problem. First, we briefly outline the success of colloidal systems in yielding microscopic insights into a wide range of condensed matter phenomena. In the context of the glass transition, we demonstrate their utility in revealing the nature of spatial and temporal dynamical heterogeneity. We then discuss the evidence from colloid experiments in favor of various theories of glass formation that has accumulated over the last two decades. In the next section, we expound on the recent paradigm shift in colloid experiments from an exploratory approach to a critical one aimed at distinguishing between predictions of competing frameworks. We demonstrate how this critical approach is aided by the discovery of novel dynamical crossovers within the range accessible to colloid experiments. We also highlight the impact of alternate routes to glass formation such as random pinning, trajectory space phase transitions and replica coupling on current and future research on the glass transition. We conclude our review by listing some key open challenges in glass physics such as the comparison of growing static lengthscales and the preparation of ultrastable glasses, that can be addressed using colloid experiments.

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arXiv2016

Influence of an amorphous wall on the distribution of localized excitations in a colloidal glass-forming liquid

Elucidating the nature of the glass transition has been the holy grail of condensed matter physics and statistical mechanics for several decades. A phenomenological aspect that makes glass formation a conceptually formidable problem is that structural and dynamic correlations in glass-forming liquids are too subtle to be captured at the level of conventional two-point functions. As a consequence, a host of theoretical techniques, such as quenched amorphous configurations of particles, have been devised and employed in simulations and colloid experiments to gain insights into the mechanisms responsible for these elusive correlations. Very often, though, the analysis of spatio-temporal correlations is performed in the context of a single theoretical framework, and critical comparisons of microscopic predictions of competing theories are thereby lacking. Here, we address this issue by analysing the distribution of localized excitations, which are building blocks of relaxation as per the Dynamical Facilitation (DF) theory, in the presence of an amorphous wall, a construct motivated by the Random First-Order Transition theory (RFOT). We observe that spatial profiles of the concentration of excitations exhibit complex features such as non-monotonicity and oscillations. Moreover, the smoothly varying part of the concentration profile yields a length scale $ξ_c$, which we compare with a previously computed length scale $ξ_{dyn}$. Our results suggest a method to assess the role of dynamical facilitation in governing structural relaxation in glass-forming liquids.

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arXiv2016

Site-Specific Colloidal Crystal Nucleation by Template-enhanced Particle Transport

The monomer surface mobility is the single most important parameter that decides the nucleation density and morphology of islands during thin film growth. During template-assisted surface growth in particular, low surface mobilities can prevent monomers from reaching target sites and this results in a partial to complete loss of nucleation control. While in atomic systems a broad range of surface mobilities can be readily accessed, for colloids, owing to their large size, this window is substantially narrow and therefore imposes severe restrictions in extending template-assisted growth techniques to steer their self-assembly. Here, we circumvented this fundamental limitation by designing templates with spatially varying feature sizes, in this case moire patterns, which in the presence of short-range depletion attraction presented surface energy gradients for the diffusing colloids. The templates serve a dual purpose, first, directing the particles to target sites by enhancing their surface mean free paths and second, dictating the size and symmetry of the growing crystallites. Using optical microscopy, we directly followed the nucleation and growth kinetics of colloidal islands on these surfaces at the single-particle level. We demonstrate nucleation control, with high fidelity, in a regime that has remained unaccessed in theoretical, numerical and experimental studies on atoms and molecules as well. Our findings pave the way for fabricating non-trivial surface architectures composed of complex colloids and nanoparticles.

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arXiv2016

A Micrometer-sized Heat Engine Operating Between Bacterial Reservoirs

Artificial micro heat engines are prototypical models to explore and elucidate the mechanisms of energy transduction in a regime that is dominated by fluctuations [1-2]. Micro heat engines realized hitherto mimicked their macroscopic counterparts and operated between reservoirs that were effectively thermal [3-7]. For such reservoirs, temperature is a well-defined state variable and stochastic thermodynamics provides a precise framework for quantifying engine performance [8-9]. It remains unclear whether these concepts readily carry over to situations where the reservoirs are out-of-equilibrium [10], a scenario of particular importance to the functioning of synthetic [11-12] and biological [13] micro engines and motors. Here we experimentally realized a micrometer-sized active Stirling engine by periodically cycling a colloidal particle in a time-varying harmonic optical potential across bacterial baths at different activities. Unlike in equilibrium thermal reservoirs, the displacement statistics of the trapped particle becomes increasingly non-Gaussian with activity. We show that as much as $\approx$ 85\% of the total power output and $\approx$ 50\% of the overall efficiency stems from large non-Gaussian particle displacements alone. Most remarkably, at the highest activities investigated, the efficiency of our quasi-static active heat engines surpasses the equilibrium saturation limit of Stirling efficiency - the maximum efficiency of a Stirling engine with the ratio of cold and hot reservoir temperatures ${T_C\over T_H} \to 0$. Crucially, the failure of effective temperature descriptions [14-16] for active reservoirs highlights the dire need for theories that can better capture the physics of micro motors and heat engines that operate in strongly non-thermal environments.

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arXiv2014

Shape of Dynamical Heterogeneities and the Stokes-Einstein and Stokes-Einstein-Debye Relations in Suspensions of Colloidal Ellipsoids

We examine the role of shape of dynamical heterogeneities on the validity of the Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations in quasi-two-dimensional suspensions of colloidal ellipsoids. For ellipsoids with repulsive interactions, although the orientational relaxation time remains coupled to the structural one, the SED relation by the Einstein formalism shows a breakdown. Strikingly, we find that it is the change in the shape of the dynamical heterogeneities from string-like to compact and not just their presence that results in the breakdown of both the SE and SED relations. On introducing a short-range depletion attraction between the ellipsoids, associated with the lack of morphological evolution of dynamical heterogeneities, the SE and SED relations remain valid even for deep supercooling. Our observations are consistent with numerical predictions.

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arXiv2014

Direct measurements of growing amorphous order and non-monotonic dynamic correlations in a colloidal glass-former

While the transformation of flowing liquids into rigid glasses is omnipresent, a complete understanding of vitrification remains elusive. Of the numerous approaches aimed at solving the glass transition problem, the Random First-Order Theory (RFOT) is the most prominent. However, the existence of the underlying thermodynamic phase transition envisioned by RFOT remains debatable, since its key microscopic predictions concerning the growth of amorphous order and the nature of dynamic correlations lack experimental verification. Here, by using holographic optical tweezers, we freeze a wall of particles in an equilibrium configuration of a 2D colloidal glass-forming liquid and provide direct evidence for growing amorphous order in the form of a static point-to-set length. Most remarkably, we uncover the non-monotonic dependence of dynamic correlations on area fraction and show that this non-monotonicity follows directly from the change in morphology of cooperatively rearranging regions, as predicted by RFOT. Our findings suggest that the glass transition has a thermodynamic origin.

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