Active Matter Research Articles

Irreversible dynamics of a continuum driven by active matter

Irreversible dynamics of a continuum driven by active matter

Self-Generated Ions Modify the Pair Interaction and the Phase Separation of Chemically Active Colloids.

Chemically active colloids that release/consume ions are an important class of active matter, and exhibit interesting collective behaviors such as phase separation, swarming, and waves. Key to these behaviors is the pair-wise interactions mediated by the concentration gradient of self-generated ions. This interaction is often simplified as a pair-wise force decaying at 1/r2, where r is the interparticle distance. Here, we show that this simplification fails for isotropic and immotile active colloids with net ion production, such as Ag colloids in H2O2. Specifically, the production of ions on the surface of the Ag colloids increases the local ion concentration, c, and attenuates the pair-wise interaction force that scales with ∇c/c. As a result, the attractive force between an Ag colloid and its neighbor (active or passive) decays at 1/r or 1/r2 for small or large r, respectively. In a population, the attraction of a colloid by a growing cluster also scales with ∇c/c, so that medium-sized clusters grow fastest, and that the cluster coarsening slows with time. These results, supported by finite element and Brownian dynamic simulations, highlight the important role of self-generated ions in shaping the collective behavior of chemically active colloids.

Dynamics of area-preserving curvature flow of convex plane curves with small area in an inhomogeneous medium

AbstractIn 1986, Gage studied area-preserving curvature flows in a two-dimensional homogeneous medium and proved that an initially convex closed curve remains convex and converges to a circle as time approaches infinity. However, a medium may not be homogeneous in many applications such as cell motility and motions of active matters. Therefore, this work introduces an area-preserving curvature flow in an inhomogeneous medium, which is a natural extension of an inhomogeneous medium. Through this extension, a closed curve numerically moves towards the higher medium by the flow. To show this fact, the case is considered in which the area enclosed by the closed curve is small. In this situation, it is shown that the dynamics of its center is approximated by a gradient flow determined by the distribution of the inhomogeneous medium and that its center moves toward the critical point of the inhomogeneous medium.

Transient transition from Stable to Dissipative Assemblies in Response to the Spatiotemporal Availability of a Chemical Fuel

AbstractThe transition from inactive to active matter implies a transition from thermodynamically stable to energy‐dissipating structures. Here, we show how the spatiotemporal availability of a chemical fuel causes a thermodynamically stable self‐assembled structure to transiently pass to an energy‐dissipating state. The system relies on the local injection of a weak affinity phosphodiester substrate into an agarose hydrogel containing surfactant‐based structures templated by ATP. Injection of substrate leads to the inclusion of additional surfactant molecules in the assemblies leading to the formation of catalytic hotspots for substrate conversion. After the local disappearance of the substrate as a result of chemical conversion and diffusion the assemblies spontaneously return to the stable state, which can be reactivated upon the injection of a new batch of fuel. The study illustrates how a dissipating self‐assembled system can cope with the intermittent availability of chemical energy without compromising long‐term structural stability.

Developing Biodegradable Liquid Soap Detergent Solution for Domestic and Industrial Applications

Liquid soap solution is a versatile and widely used personal care product, offers a convenient and hygienic way to maintain cleanliness. This abstract summarizes the key aspects of liquid soap. This is soap in a liquid form. Generally liquid detergent are used with soft water. Where as in case of detergent powder cloths can be washed in hard as well as soft water. Liquid soap for Industrial use as well as house hold use can be manufactured by employing a very simple process. A typical formula for 1 kg of product is given below. This is called 25% liquid soap, since the acid slurry (active matter) used is 25% For lesser Concentration the quantity of acid slurry may be decreased correspondingly. The main aim of this project is to develop the liquid soap solution.

Stochastically structured illumination microscopy scan less super resolution imaging

In super-resolution, a varying illumination image stack is required. This enriched dataset typically necessitates precise mechanical control and micron-scale optical alignment and repeatability. Here, we introduce a novel methodology for super-resolution microscopy called stochastically structured illumination microscopy (S2IM), which bypasses the need for illumination control exploiting instead the random, uncontrolled movement of the target object. We tested our methodology within the clinically relevant ophthalmoscopic setting, harnessing the inherent saccadic motion of the eye to induce stochastic displacement of the illumination pattern on the retina. We opted to avoid human subjects by utilizing a phantom eye model featuring a retina composed of human induced pluripotent stem cells (iPSC) retinal neurons and replicating the ocular saccadic movements by custom actuators. Our findings demonstrate that S2IM unlocks scan-less super-resolution with a resolution enhancement of 1.91, with promising prospects also beyond ophthalmoscopy applications such as active matter or atmospheric/astronomical observation.

Evaluation of transition rates from nonequilibrium instantons

Equilibrium rate theories play a crucial role in understanding rare, reactive events. However, they are inapplicable to a range of irreversible processes in systems driven far from thermodynamic equilibrium like active and biological matter. Here we develop an efficient numerical method to compute the rate constant of rare nonequilibrium events in the weak-noise limit based on an instanton approximation to the stochastic path integral and illustrate its wide range of application. We demonstrate excellent agreement of the instanton rates with numerically exact results for a particle under a nonconservative force. We also study phase transitions in an active field theory. We elucidate how activity alters the stability of the two phases and their rates of interconversion in a manner that can be well described by modifying classical nucleation theory. Published by the American Physical Society 2024

ULTIMATE BOUND FOR AN ACTIVE LORENZ SYSTEM

Considering the Lorenz-like model [1] derived from Active Model H for scalar active matter introduced by Tiribocchi et al. [2], we investigate the ultimate bound and global attractive sets for the system, and obtain the ultimate bound characterized by an ellipsoid and a global exponential attractive set for the system.

Playing with active matter

In the past 20 years, active matter has been a very successful research field, bridging the fundamental physics of nonequilibrium thermodynamics with applications in robotics, biology, and medicine. Active particles, contrary to Brownian particles, can harness energy to generate complex motions and emerging behaviors. Most active-matter experiments are performed with microscopic particles and require advanced microfabrication and microscopy techniques. Here, we propose some macroscopic experiments with active matter employing commercially available toy robots (the Hexbugs). We show how they can be easily modified to perform regular and chiral active Brownian motion and demonstrate through experiments fundamental signatures of active systems such as how energy and momentum are harvested from an active bath, how obstacles can sort active particles by chirality, and how active fluctuations induce attraction between planar objects (a Casimir-like effect). These demonstrations enable hands-on experimentation with active matter and showcase widely used analysis methods.

Size-polydispersity-induced effects on the structure of active Brownian pseudo-hard disks

Studies of the effects of particle-size polydispersity or of particle interactions on active matter have been limited to determine and analyze the system short-time dynamics in the high-density regime. On the other hand, the effects of polydispersity on the structural behavior of active systems are of relevance and have received much less attention. In this paper, we comprehensively analyze the effects of size dispersion of pseudo hard-disk active Brownian particles, on its structural behavior at different system densities and different self-propelling velocities, thus elucidating the interplay of these features with the polydispersity. This is introduced into our analysis by well-known particle size distributions in such a manner that the average size is fixed, but with a variance that accounts for different dispersion of the particle size according to: Gaussian, Weibull, uniform and “two point” distribution. Local and global structural properties of the system are determined under these considerations. We observe that activity and size polydisperse effects become relevantly conspicuous at densities above the motility induced phase separation critical point of the monodispersal fluid. We notice that, while the activity promotes a more defined local and global structural arrangement, the polydispersity decreases such structure, observing the greatest effect when the particle size is defined by a uniform distribution.

Correlations between two vortices in dry active matter

It was recently shown that wet active matter can form synchronized rotating vortices in a square lattice, similar to an antiferromagnetic Ising model (by considering rotation direction as spin projection). In this study, we investigate whether such a correlated state occurs for a model of dry active matter. We achieve that by numerically simulating the dynamics of a system of active particles in the presence of two identical circular obstacles. Then, we measure the angular velocity correlation function of both vortices as a function of the obstacle diameter, their shortest separation (gap), and the particle density. When the correlation function is negative, both vortices rotate in contrary directions. They maintain this state by exchanging particles through the region between them, analogously to synchronized cogs. On the other hand, with a positive correlation function, a single rotating cluster emerges, and the particles move around the whole structure, similar to a belt strapped around the obstacles. Additionally, we observe the emergence of uncorrelated states at the transition between correlated states, in which only a single vortex is present, or in the large gap regime, in which the vortices are nearly independent on each other.

Active matter modifies, and is oriented by, its environment

Active matter modifies, and is oriented by, its environment

Dispersionless Flat Mode and Vibrational Anomaly in Active Brownian Vibrators Induced by Stringlike Dynamical Defects.

In recent years, active Brownian particles have emerged as a prominent model system for comprehending the behaviors of active matter, wherein particles demonstrate self-propelled motion by harnessing energy from the surrounding environment. A fundamental objective of studying active matter is to elucidate the physical mechanisms underlying its collective behaviors. Drawing inspiration from advancements in molecular glasses, our study unveils a low-energy "flat mode" within the transverse spectrum of active Brownian vibrators-a nearly two-dimensional, bidisperse granular assembly. We demonstrate that this collective excitation induces an anomalous excess in the vibrational density of states (VDOS) beyond the phononic Debye contribution. We characterize the properties of this flat mode by exploring the parameter space of our experimental system and tuning the packing fraction, the vibrational frequency, the particle size ratio, and the mixture ratio. Additionally, we establish through empirical evidence that stringlike dynamical defects, discerned via the spatial distribution of each particle's contribution to the reduced transverse VDOS, serve as the microscopic origin of the flat mode and its associated anomalies.

Emergent behaviors of buckling-driven elasto-active structures

Active systems of self-propelled agents, e.g., birds, fish, and bacteria, can organize their collective motion into myriad autonomous behaviors. Ubiquitous in nature and across length scales, such phenomena are also amenable to artificial settings, e.g., where brainless self-propelled robots orchestrate their movements into spatial-temporal patterns via the application of external cues or when confined within flexible boundaries. Like their natural counterparts, these approaches typically require many units to initiate collective motion, so controlling the ensuing dynamics is challenging. Here, we demonstrate a simple mechanism that leverages nonlinear elasticity to tame near-diffusive motile particles in forming structures capable of directed motion and other emergent behaviors. Our elasto-active system comprises two centimeter-sized self-propelled microbots connected with elastic beams. These microbots exert forces that suffice to buckle the beam and set the structure in motion. We first rationalize the physics of the interaction between the beam and the microbots. Then we use reduced-order models to predict the interactions of our elasto-active structures with boundaries, e.g., walls and constrictions, and demonstrate how they can exhibit remarkable emergent behaviors such as maze navigation. These findings demonstrate that allowing and understanding changes in body morphology can enhance the capabilities of active matter systems and enable the design of robotic materials capable of space exploration, adaptation, and complex interactions with their surrounding environment.

Tuning collective behaviour in zebrafish with genetic modification.

Zebrafish collective behaviour is widely used to assess their physical and mental state, serving as a valuable tool to assess the impact of ageing, disease genetics, and the effect of drugs. The essence of these macroscopic phenomena can be represented by active matter models, where the individuals are abstracted as interactive self-propelling agents. The behaviour of these agents depends on a set of parameters in a manner reminiscent of those between the constituents of physical systems. In a few cases, the system may be controlled at the level of the individual constituents such as the interactions between colloidal particles, or the enzymatic behaviour of de novo proteins. Usually, however, while the collective behaviour may be influenced by environmental factors, it typically cannot be changed at will. Here, we challenge this scenario in a biological context by genetically modifying zebrafish. We thus demonstrate the potential of genetic modification in the context of controlling the collective behaviour of biological active matter systems at the level of the constituents, rather than externally. In particular, we probe the effect of the lack of col11a2 gene in zebrafish, which causes the early onset of osteoarthritis. The resulting col11a2 -/- zebrafish exhibited compromised vertebral column properties, bent their body less while swimming, and took longer to change their orientations. Surprisingly, a group of 25 mutant fish exhibited more orderly collective motion than the wildtype. We show that the collective behaviour of wildtype and col11a2 -/- zebrafish are captured with a simple active matter model, in which the mutant fish are modelled by self-propelling agents with a higher orientational noise on average. In this way, we demonstrate the possibility of tuning a biological system, changing the state space it occupies when interpreted with a simple active matter model.

A hovering bubble with a spontaneous horizontal oscillation

Active matters, characterized by multi-mode motions, have been emerging for both engineering and biological applications. Generally, active objects rely on the symmetry-broken structures, compositions, or interfacial activities through a physical or chemical approach. Here, we report an active bubble spontaneously hovering with a horizontal oscillation at the solid/liquid interface by impacting a stationary laser beam into a liquid through a transparent solid cover. This spontaneous oscillation mode of the bubble synchronizes with that of the interfacial temperature and hydrodynamical flow. A physical mechanism is proposed, and the scaling analysis of the oscillation frequency agrees well with experiments in various liquids under different laser powers. Additionally, the bubble trajectory rotates azimuthally, arising from the symmetry breaking of the vortex pair accompanying the oscillation. Moreover, the double pendulum of oscillation bubbles has been demonstrated, achieving a preferable oscillation direction in a controllable way. These findings would not only advance our understanding of active matters but also shed light on the bubble-mediated technological applications, such as microrobots and drug deliveries.

Using Hexbugs to model active matter

The inexpensive toy can be used as a macro model for complex physical principles.

Learning protocols for the fast and efficient control of active matter

Exact analytic calculation shows that optimal control protocols for passive molecular systems often involve rapid variations and discontinuities. However, similar analytic baselines are not generally available for active-matter systems, because it is more difficult to treat active systems exactly. Here we use machine learning to derive efficient control protocols for active-matter systems, and find that they are characterized by sharp features similar to those seen in passive systems. We show that it is possible to learn protocols that effect fast and efficient state-to-state transformations in simulation models of active particles by encoding the protocol in the form of a neural network. We use evolutionary methods to identify protocols that take active particles from one steady state to another, as quickly as possible or with as little energy expended as possible. Our results show that protocols identified by a flexible neural-network ansatz, which allows the optimization of multiple control parameters and the emergence of sharp features, are more efficient than protocols derived recently by constrained analytical methods. Our learning scheme is straightforward to use in experiment, suggesting a way of designing protocols for the efficient manipulation of active matter in the laboratory.

Emergent cesarean section during active labor-does cervical dilatation matter?

To compare the immediate and late complications associated with emergent cesarean sections (CS) performed during the first and second stages of active labor. We conducted a retrospective analysis of electronic medical records from a single academic center, including data from 577 patients who underwent emergent cesarean sections at 4cm or more of cervical dilatation. Patients were divided into two groups: those who had CS during the first stage of labor (4-9cm dilatation) and those who had CS at complete dilatation (10cm). Maternal and neonatal outcomes were compared, including rates of complications such as uterine atony, post-partum hemorrhage, infection, and neonatal intensive care unit (NICU) admission. Of the 577 patients, 352 underwent CS during active labor and 255 at complete dilatation. The complete dilatation group exhibited significantly higher rates of uterine atony (19.6% vs. 11.6%, p = 0.009) and uterine incision extension (34.2% vs. 16.5%, p = 0.0001). In addition, they had longer hospital stays (4.8 vs. 4.25days, p = 0.003) and higher outpatient clinic visit rates (21.3% vs. 9.9%, p = 0.0001). Infection-related complications on readmission were more common in the complete dilatation group (20% vs. 9.7%, p = 0.001). Neonatal outcomes, including APGAR scores and NICU admissions, did not differ significantly between the groups. Emergent cesarean sections performed at complete cervical dilatation are associated with increased intra-operative and post-operative complications compared to those performed during active labor. These findings highlight the importance of considering the stage of labor when planning cesarean delivery to minimize risks and optimize outcomes for both mother and neonate.

Optimizing Circular Rotations in Confined Systems via Enhanced Self-Driven Speed of Active Nematics

Abstract Active matter exhibits collective motions at various scales. Geometric confinement has been identified as an effective way to control and manipulate active fluids, with much attention given to external factors. However, the impact of the inherent properties of active particles on collective motion under confined conditions remains elusive. Here, we use a highly tunable active nematics model to study active systems under confinement, focusing on the effect of self-driven speed of active particles. We identify three distinct states characterized by unique particle and flow fields within confined active nematic systems, among which circular rotation emerges as a collective motion involving rotational movement in both particle and flow fields. The theoretical phase diagram shows that increasing the self-driven speed of active particles significantly enhances the region of circular rotation state and improves its stability. Our results provide insights into the formation of high quality vortices in confined active nematic systems.