TIFR Centre for Interdisciplinary Sciences

This review summarizes theoretical progress in the field of active matter, placing it in the context of recent experiments. This approach offers a unified framework for the mechanical and statistical properties of living matter: biofilaments and molecular motors in vitro or in vivo, collections of motile microorganisms, animal flocks, and chemical or mechanical imitations. A major goal of this review is to integrate several approaches proposed in the literature, from semimicroscopic to phenomenological. In particular, first considered are ``dry'' systems, defined as those where momentum is not conserved due to friction with a substrate or an embedding porous medium. The differences and similarities between two types of orientationally ordered states, the nematic and the polar, are clarified. Next, the active hydrodynamics of suspensions or ``wet'' systems is discussed and the relation with and difference from the dry case, as well as various large-scale instabilities of these nonequilibrium states of matter, are highlighted. Further highlighted are various large-scale instabilities of these nonequilibrium states of matter. Various semimicroscopic derivations of the continuum theory are discussed and connected, highlighting the unifying and generic nature of the continuum model. Throughout the review, the experimental relevance of these theories for describing bacterial swarms and suspensions, the cytoskeleton of living cells, and vibrated granular material is discussed. Promising extensions toward greater realism in specific contexts from cell biology to animal behavior are suggested, and remarks are given on some exotic active-matter analogs. Last, the outlook for a quantitative understanding of active matter, through the interplay of detailed theory with controlled experiments on simplified systems, with living or artificial constituents, is summarized.

Longevity has been a long-standing concern for hybrid perovskite photovoltaics. We demonstrate high-resilience positive-intrinsic-negative perovskite solar cells by incorporating a piperidinium-based ionic compound into the formamidinium-cesium lead-trihalide perovskite absorber. With the bandgap tuned to be well suited for perovskite-on-silicon tandem cells, this piperidinium additive enhances the open-circuit voltage and cell efficiency. This additive also retards compositional segregation into impurity phases and pinhole formation in the perovskite absorber layer during aggressive aging. Under full-spectrum simulated sunlight in ambient atmosphere, our unencapsulated and encapsulated cells retain 80 and 95% of their peak and post-burn-in efficiencies for 1010 and 1200 hours at 60° and 85°C, respectively. Our analysis reveals detailed degradation routes that contribute to the failure of aged cells.

The beauty and complexity of the shapes and dynamics of bubbles rising in liquid have fascinated scientists for centuries. Here we perform simulations on an initially spherical bubble starting from rest. We report that the dynamics is fully three-dimensional, and provide a broad canvas of behaviour patterns. Our phase plot in the Galilei–Eötvös plane shows five distinct regimes with sharply defined boundaries. Two symmetry-loss regimes are found: one with minor asymmetry restricted to a flapping skirt; and another with marked shape evolution. A perfect correlation between large shape asymmetry and path instability is established. In regimes corresponding to peripheral breakup and toroid formation, the dynamics is unsteady. A new kind of breakup, into a bulb-shaped bubble and a few satellite drops is found at low Morton numbers. The findings are of fundamental and practical relevance. It is hoped that experimenters will be motivated to check our predictions. The motion of gas bubbles in liquid is highly relevant to heat and mass transfer in our daily lives. Here, Tripathi et al.simulate a bubble rising under gravity in three-dimensions and find sharp boundaries in parameter space between different types of dynamics, and the coupled evolution of shape.

The study of systems with sustained energy uptake and dissipation at the scale of the constituent particles is an area of central interest in nonequilibrium statistical physics. Identifying such systems as a distinct category -- Active Matter -- unifies our understanding of autonomous collective move- ment in the living world and in some surprising inanimate imitations. In this article I present the Active Matter framework, briefly recall some early work, review our recent results on single-particle and collective behaviour, including experiments on active granular monolayers, and discuss new directions for the future.

The creation of synthetic systems that emulate the defining properties of living matter, such as motility, gradient-sensing, signaling, and replication, is a grand challenge of biomimetics. Such imitations of life crucially contain active components that transform chemical energy into directed motion. These artificial realizations of motility point in the direction of a new paradigm in engineering, through the design of emergent behavior by manipulating properties at the scale of the individual components. Catalytic colloidal swimmers are a particularly promising example of such systems. Here we present a comprehensive theoretical description of gradient-sensing of an individual swimmer, leading controllably to chemotactic or anti-chemotactic behavior, and use it to construct a framework for studying their collective behavior. We find that both the positional and the orientational degrees of freedom of the active colloids can exhibit condensation, signaling formation of clusters and asters. The kinetics of catalysis introduces a natural control parameter for the range of the interaction mediated by the diffusing chemical species. For various regimes in parameter space in the long-ranged limit our system displays precise analogs to gravitational collapse, plasma oscillations, and electrostatic screening. We present prescriptions for how to tune the surface properties of the colloids during fabrication to achieve each type of behavior.

We carefully examine common measures of dynamical heterogeneity for a model polymer melt and test how these scales compare with those hypothesized by the Adam and Gibbs (AG) and random first-order transition (RFOT) theories of relaxation in glass-forming liquids. To this end, we first analyze clusters of highly mobile particles, the string-like collective motion of these mobile particles, and clusters of relative low mobility. We show that the time scale of the high-mobility clusters and strings is associated with a diffusive time scale, while the low-mobility particles' time scale relates to a structural relaxation time. The difference of the characteristic times for the high- and low-mobility particles naturally explains the well-known decoupling of diffusion and structural relaxation time scales. Despite the inherent difference of dynamics between high- and low-mobility particles, we find a high degree of similarity in the geometrical structure of these particle clusters. In particular, we show that the fractal dimensions of these clusters are consistent with those of swollen branched polymers or branched polymers with screened excluded-volume interactions, corresponding to lattice animals and percolation clusters, respectively. In contrast, the fractal dimension of the strings crosses over from that of self-avoiding walks for small strings, to simple random walks for longer, more strongly interacting, strings, corresponding to flexible polymers with screened excluded-volume interactions. We examine the appropriateness of identifying the size scales of either mobile particle clusters or strings with the size of cooperatively rearranging regions (CRR) in the AG and RFOT theories. We find that the string size appears to be the most consistent measure of CRR for both the AG and RFOT models. Identifying strings or clusters with the "mosaic" length of the RFOT model relaxes the conventional assumption that the "entropic droplets" are compact. We also confirm the validity of the entropy formulation of the AG theory, constraining the exponent values of the RFOT theory. This constraint, together with the analysis of size scales, enables us to estimate the characteristic exponents of RFOT.

Double perovskites have recently emerged as possible alternatives to lead-based halide perovskites for photovoltaic applications. In particular, Cs2AgBiBr6 has been the subject of several studies because of its environmental stability, low toxicity, and its promising optoelectronic features. Despite these encouraging features, the performances of solar cells based on this double perovskite are still low, suggesting severe limitations that need to be addressed. In this work we combine experimental and theoretical studies to show that the short electron diffusion length is one of the major causes for the limited performance of Cs2AgBiBr6 solar cells. Using EQE measurements on semitransparent Cs2AgBiBr6 solar cells we estimate the electron diffusion length to be only 30 nm and corroborated this value by terahertz spectroscopy. By using photothermal deflection spectroscopy and surface photovoltage measurements we correlate the limited electron diffusion length with a high density of electron traps. Our findings highlight important faults affecting this double perovskite, showing the challenges to overcome and hinting to a possible path to improve the efficiency of Cs2AgBiBr6 solar cells.

This review highlights the profound and unexpected ways in which viscosity varying in space and time can affect flow. The most striking manifestations are through alterations of flow stability, as established in model shear flows and industrial applications. Future studies are needed to address the important effect of viscosity stratification in such diverse environments as Earth's core, the Sun, blood vessels, and the re-entry of spacecraft.

Intrinsic organic–inorganic metal halide perovskites (OIHP) based semiconductors have shown wide applications in optoelectronic devices. There have been several attempts to incorporate heterovalent metal (e.g., Bi3+) ions in the perovskites in an attempt to induce electronic doping and increase the charge carrier density in the semiconductor. It has been reported that inclusion of Bi3+ decreases the band gap of the material considerably. However, contrary to the earlier conclusions, despite a clear change in the appearance of the crystal as observed by eye, here we show that the band gap of MAPbBr3 crystals does not change due the presence of Bi3+ in the growth solution. An increased density of states in the band gap and use of very thick samples for transmission measurements, erroneously give the impression of a band gap shift. These sub band gap states also act as nonradiative recombination centers in the crystals.

Amorphous solids are ubiquitous among natural and man-made materials. Often used as structural materials for their attractive mechanical properties, their utility depends critically on their response to applied stresses. Processes underlying such mechanical response, and in particular the yielding behaviour of amorphous solids, are not satisfactorily understood. Although studied extensively, observed yielding behaviour can be gradual and depend significantly on conditions of study, making it difficult to convincingly validate existing theoretical descriptions of a sharp yielding transition. Here we employ oscillatory deformation as a reliable probe of the yielding transition. Through extensive computer simulations for a wide range of system sizes, we demonstrate that cyclically deformed model glasses exhibit a sharply defined yielding transition with characteristics that are independent of preparation history. In contrast to prevailing expectations, the statistics of avalanches reveals no signature of the impending transition, but exhibit dramatic, qualitative, changes in character across the transition.

Graphene quantum dots (GQDs) are a promising category of materials with remarkable size dependent properties like tunable bandgap and photoluminescence along with the possibility of effective chemical functionalization. Doping of GQDs with heteroatoms is an interesting way of regulating their properties. Herein, we report a facile and scalable one-step synthesis of luminescent GQDs, substitutionally co-doped with N, F and S, of ∼2 nm average size by a microwave treatment of multi-walled carbon nanotubes in a customized ionic liquid medium. The use of an ionic liquid coupled with the use of a microwave technique enables not only an ultrafast process for the synthesis of co-doped GQDs, but also provides excellent photoluminescence quantum yield (70%), perhaps due to the interaction of defect clusters and dopants.

We report computer simulations of oscillatory athermal quasistatic shear deformation of dense amorphous samples of a three-dimensional model glass former. A dynamical transition is observed as the amplitude of the deformation is varied: For large values of the amplitude the system exhibits diffusive behavior and loss of memory of the initial conditions, whereas localization is observed for small amplitudes. Our results suggest that the same kind of transition found in driven colloidal systems is present in the case of amorphous solids (e.g., metallic glasses). The onset of the transition is shown to be related to the onset of energy dissipation. Shear banding is observed for large system sizes, without, however, affecting qualitative aspects of the transition.

Regulating the emergence of leaders is a central aspect of collective cell migration, but the underlying mechanisms remain ambiguous. Here we show that the selective emergence of leader cells at the epithelial wound-margin depends on the dynamics of the follower cells and is spatially limited by the length-scale of collective force transduction. Owing to the dynamic heterogeneity of the monolayer, cells behind the prospective leaders manifest locally increased traction and monolayer stresses much before these leaders display any phenotypic traits. Followers, in turn, pull on the future leaders to elect them to their fate. Once formed, the territory of a leader can extend only to the length up-to which forces are correlated, which is similar to the length up-to which leader cells can transmit forces. These findings provide mechanobiological insight into the hierarchy in cell collectives during epithelial wound healing.

Determining a suitable noble-metal-free catalyst for hydrogen evolution reaction (HER) by photoelectrocatalytic (PEC) water splitting is an enduring challenge. Here, the molecular origin of number of layers and stacking sequence-dependent PEC HER performance of MoS2/graphene (MoS2/GR) van der Waals (vdW) vertical heterostructures is studied. Density functional theory (DFT) based calculations show that the presence of MoS2 induces p-type doping in GR, which facilitates hydrogen adsorption in the GR side compared to the MoS2 side with ΔGH closer to 0 eV in the MoS2/GR bilayer vertical stacks. The activity maximizes in graphene with monolayer MoS2 and reduces further for bilayer and multilayers of MoS2. The PEC HER performance is studied in various electrodes, namely, single-layer graphene, single- and few-layered MoS2, and their two different types of vertical heterojunctions having different stacking sequences. The graphene on top of MoS2 sequence showed the highest photoresponse with large reaction current density and lowest charge-transfer resistance toward HER, in aggrement with the DFT calculations. These findings establish the role of stacking sequence in the electrochemistry of atomic layers, leading to the design of new electrocatalysts by combinatorial stacking of a minimal number of layers.

The question of whether the dramatic slowing down of the dynamics of glass-forming liquids near the structural glass transition is caused by the growth of one or more correlation lengths has received much attention in recent years. Several proposals have been made for both static and dynamic length scales that may be responsible for the growth of timescales as the glass transition is approached. These proposals are critically examined with emphasis on the dynamic length scale associated with spatial heterogeneity of local dynamics and the static point-to-set or mosaic length scale of the random first-order transition theory of equilibrium glass transition. Available results for these length scales, obtained mostly from simulations, are summarized, and the relation of the growth of timescales near the glass transition with the growth of these length scales is examined. Some of the outstanding questions about length scales in glass-forming liquids are discussed, and studies in which these questions may be addressed are suggested.

A single material that can perform water oxidation and oxygen reduction reactions (ORR), also called bifunctional catalyst, represents a novel concept that emerged from recent materials research and that has led to applications in new‐generation energy‐storage systems, such as regenerative fuel cells. Here, metal/metal‐oxide free, doped graphene derived from rhombohedral boron carbide (B 4 C) is demonstrated to be an effective bifunctional catalyst for the first time. B 4 C, one of the hardest materials in nature next to diamond and cubic boron nitride, is converted and separated in bulk to form heteroatom (boron, B) doped graphene (BG, yield ≈7% by weight, after the first cycle). This structural conversion of B 4 C to graphene is accompanied by in situ boron doping and results in the formation of an electrochemically active material from a non‐electrochemically active material, broadening its potential for application in various energy‐related technologies. The electrocatalytic efficacy of BG is studied using various voltammetric techniques. The results show a four‐electron transfer mechanism as well as a high methanol tolerance and stability towards ORR. The results are comparable to those from commercial 20 wt% Pt/C in terms of performance. Furthermore, the bifunctionality of the BG is also demonstrated by its performance in water oxidation.

Chromosomes within eukaryotic cell nuclei at interphase are not positioned at random, since gene-rich chromosomes are predominantly found towards the interior of the cell nucleus across a number of cell types. The physical mechanisms that could drive and maintain the spatial segregation of chromosomes based on gene density are unknown. Here, we identify a mechanism for such segregation, showing that the territorial organization of chromosomes, another central feature of nuclear organization, emerges naturally from our model. Our computer simulations indicate that gene density-dependent radial segregation of chromosomes arises as a robust consequence of differences in non-equilibrium activity across chromosomes. Arguing that such differences originate in the inhomogeneous distribution of ATP-dependent chromatin remodeling and transcription machinery on each chromosome, we show that a variety of non-random positional distributions emerge through the interplay of such activity, nuclear shape and specific interactions of chromosomes with the nuclear envelope. Results from our model are in reasonable agreement with experimental data and we make a number of predictions that can be tested in experiments.

Chronic diseases are becoming more prevalent, and the complexities of managing patients continue to escalate, since their care must be balanced between the home and clinical settings. Diabetes is the most advanced example, where self-monitoring has been shown to be necessary. Glucometers are point-of-care (POC) devices that have become standard platforms at home and clinical settings. Similarly, many other POC biosensors have also been developed. Enzymes are often used in these sensors because of their specificity and the reaction products can be electrochemically transduced for the measurement. When enzymes are immobilized to an electronically active substrate, enzymatic reactions can be transduced by direct electron transport. This paper describes an approach for the development of graphene-based POC devices. This includes modifying enzymes for improved performance, developing methods to bind them to the graphene surface, incorporation of the functionalized graphene on a field-effect transistor (FET), and integration into a microfluidic device suitable for home use. This paper describes an approach for the development of a graphene-based POC biosensor platform using glucose as an example of target molecule.

Deposition of amyloid-β (Aβ) in Alzheimer's disease (AD) is strongly correlated with the APOE genotype. However, the role of apolipoprotein E (apoE) in Aβ aggregation has remained unclear. Here we have used different apoE preparations, such as recombinant protein or protein isolated from cultured astrocytes, to examine the effect of apoE on the aggregation of both Aβ1-40 and Aβ1-42. The kinetics of aggregation, measured by the loss of fluorescence of tetramethylrhodamine-labeled Aβ, is shown to be dramatically slowed by the presence of substoichiometric concentrations of apoE. Using these concentrations, we conclude that apoE binds primarily to and affects the growth of oligomers that lead to the nuclei required for fibril growth. At higher apoE concentrations, the protein also binds to Aβ fibrils, resulting in fibril stabilization and a slower rate of fibril growth. The aggregation of Aβ1-40 is dependent on the apoE isoform, being the most dramatic for apoE4 and less so for apoE3 and apoE2. Our results indicate that the detrimental role of apoE4 in AD could be related to apoE-induced stabilization of the soluble but cytotoxic oligomeric forms and intermediates of Aβ, as well as fibril stabilization.

We show that memory can be encoded in a model amorphous solid subjected to athermal oscillatory shear deformations, and in an analogous spin model with disordered interactions, sharing the feature of a deformable energy landscape. When these systems are subjected to oscillatory shear deformation, they retain memory of the deformation amplitude imposed in the training phase, when the amplitude is below a "localization" threshold. Remarkably, multiple persistent memories can be stored using such an athermal, noise-free, protocol. The possibility of such memory is shown to be linked to the presence of plastic deformations and associated limit cycles traversed by the system, which exhibit avalanche statistics also seen in related contexts.