Active Matter

We study the asymptotic response of polar ordered active fluids ("flocks") to small external aligning fields h. The longitudinal susceptibility χ diverges, in the thermodynamic limit, like h -ν as h → 0. In finite systems of linear size L, χ saturates to a value ∼ L γ . The universal exponents ν and γ depend only on the spatial dimensionality d, and are related to the dynamical exponent z and the "roughness exponent" α characterizing the unperturbed flock dynamics. Using a well supported conjecture for the values of these two exponents, we obtain ν = 2/3, γ = 4/5 in d = 2 and ν = 1/4, γ = 2/5 in d = 3. These values are confirmed by our simulations.

We undertake a numerical study of the ordering kinetics in the two-dimensional ($2d$) active Ising model (AIM), a discrete flocking model with a conserved density field coupled to a non-conserved magnetization field. We find that for a quench into the liquid-gas coexistence region and in the ordered liquid region, the characteristic length scale of both the density and magnetization domains follows the Lifshitz-Cahn-Allen (LCA) growth law: $R(t) \sim t^{1/2}$, consistent with the growth law of passive systems with scalar order parameter and non-conserved dynamics. The system morphology is analyzed with the two-point correlation function and its Fourier transform, the structure factor, which conforms to the well-known Porod's law, a manifestation of the coarsening of compact domains with smooth boundaries. We also find the domain growth exponent unaffected by different noise strengths and self-propulsion velocities of the active particles. However, transverse diffusion is found to play the most significant role in the growth kinetics of the AIM. We extract the same growth exponent by solving the hydrodynamic equations of the AIM.

The effect of a diffusivity edge is studied in a system of scalar active matter confined by a periodic potential and driven by an externally applied force. We find that this system shows qualitatively distinct stationary regimes depending on the amplitude of the driving force with respect to the potential barrier. For small driving, the diffusivity edge induces a transition to a condensed phase analogous to the Bose–Einstein-like condensation reported for the nondriven case, which is characterized by a density-independent steady state current. Conversely, large external forces lead to a qualitatively different phase diagram since in this case condensation is only possible beyond a given density threshold, while the associated transition at higher densities is found to be reentrant.

This thesis presents a comprehensive numerical investigation into the aggregation phenomena and collective dynamics of active particle systems, aiming to elucidate the fundamental principles governing self-organization far from thermal equilibrium. While equilibrium statistical mechanics provides a robust framework for passive matter, active systems, which continuously convert energy into directed motion, pose significant theoretical challenges. This work systematically explores how breaking fundamental symmetries—translational, rotational, and parity—dictates the nature of phase transitions, the morphology of emergent structures, and the kinetics of their formation.

The first part of the thesis focuses on Motility-Induced Phase Separation (MIPS) in systems of isotropic Active Brownian Particles (ABPs). Through large-scale simulations, we reveal a complex morphology of the dense phase, characterized by a micro-phase separation of hexatic domains coexisting with a macro-phase separation of dense and dilute regions. We demonstrate that the coarsening kinetics, while exhibiting a growth exponent consistent with equilibrium universality classes, is driven by a mechanism beyond classical Ostwald ripening, dominated by the anomalous diffusion and collision of active clusters.

Next, we investigate the role of broken rotational symmetry by studying anisotropic active particles, modeled as dumbbells. We find that particle shape profoundly alters aggregation. In two dimensions, MIPS is enhanced, but the growth dynamics and cluster morphology differ significantly from the isotropic case. In three dimensions, we show that motility-induced aggregation is suppressed for purely repulsive particles. By introducing attraction, we map a rich phase diagram exhibiting gel, phase-separated, percolating network, and disordered states, and characterize the unique helical motion of self-assembled clusters.

Finally, we explore the consequences of breaking parity and time-reversal symmetry by introducing non-conservative transverse forces in a chiral Lennard-Jones fluid. We establish the non-equilibrium phase diagram, demonstrating that chirality modifies both liquid-gas coexistence and fluid-solid transitions. The system exhibits robust edge currents at the interface of chiral liquid droplets, from which we extract the rotational viscosity, a transport coefficient unique to chiral fluids. Our results show that concepts from equilibrium thermodynamics can be successfully extended to characterize phase coexistence in this non-equilibrium context.

Collectively, this research provides a deeper, particle-level understanding of aggregation in active matter, linking microscopic properties to macroscopic collective behavior and highlighting the distinct physical principles that emerge from different forms of symmetry breaking.

This is the first of two linked articles which draws on emerging understanding in the field of biology and seeks to communicate it to those of construction, engineering and design. Its insight is that nature 'works' at the process level, where neither function nor form are distinctions, and materialisation is both the act of negotiating limited resource and encoding matter as 'memory', to sustain and integrate processes through time. It explores how biological agents derive work by creating 'interfaces' between adjacent locations as membranes, through feedback. Through the tension between simultaneous aggregation and disaggregation of matter by agents with opposing objectives, many functions are integrated into an interface as it unfolds. Significantly, biological agents induce flow and counterflow conditions within biological interfaces, by inducing phase transition responses in the matter or energy passing through them, driving steep gradients from weak potentials (i.e. shorter distances and larger surfaces). As with biological agents, computing, programming and, increasingly digital sensor and effector technologies share the same 'agency' and are thus convergent.

In this article, I am building on an emerging 'process view of nature' and how biological membranes emerge through the combined action of (locally) autonomous construction agents. In Part 1, we considered the simultaneous aggregation and disaggregation of matter around embedded processes, used to create, sustain and regulate matter, energy and information gradients from which 'work' is derived for the benefit of the agents or organisms present in the system. In Part 2, I intend to demonstrate that emerging digital design, simulation and fabrication techniques, when linked to sensory and effector feedback, memory and actions, directed by pre-encoded objectives (as rules or algorithms), produce the same fundamental unit of 'agency' as biological agents possess. By understanding how biological membranes emerge in nature, as the outcome of 'negotiated agency', to regulate matter, energy and information exchange between adjacent spaces, we can begin to consider the building envelope as a biological interface or membrane from which 'work' can be derived from the environment we inhabit, as a physiological extension of ourselves.

This is the first of two linked articles which draws on emerging understanding in the field of biology and seeks to communicate it to those of construction, engineering and design. Its insight is that nature 'works' at the process level, where neither function nor form are distinctions, and materialisation is both the act of negotiating limited resource and encoding matter as 'memory', to sustain and integrate processes through time. It explores how biological agents derive work by creating 'interfaces' between adjacent locations as membranes, through feedback. Through the tension between simultaneous aggregation and disaggregation of matter by agents with opposing objectives, many functions are integrated into an interface as it unfolds. Significantly, biological agents induce flow and counterflow conditions within biological interfaces, by inducing phase transition responses in the matter or energy passing through them, driving steep gradients from weak potentials (i.e. shorter distances and larger surfaces). As with biological agents, computing, programming and, increasingly digital sensor and effector technologies share the same 'agency' and are thus convergent.

In this article, I am building on an emerging 'process view of nature' and how biological membranes emerge through the combined action of (locally) autonomous construction agents. In Part 1, we considered the simultaneous aggregation and disaggregation of matter around embedded processes, used to create, sustain and regulate matter, energy and information gradients from which 'work' is derived for the benefit of the agents or organisms present in the system. In Part 2, I intend to demonstrate that emerging digital design, simulation and fabrication techniques, when linked to sensory and effector feedback, memory and actions, directed by pre-encoded objectives (as rules or algorithms), produce the same fundamental unit of 'agency' as biological agents possess. By understanding how biological membranes emerge in nature, as the outcome of 'negotiated agency', to regulate matter, energy and information exchange between adjacent spaces, we can begin to consider the building envelope as a biological interface or membrane from which 'work' can be derived from the environment we inhabit, as a physiological extension of ourselves.

Statistical-mechanical models frequently exhibit a dimension-dependent solvability: in 1D, exact solutions are straightforward; in 2D, solutions are exact but may require nontrivial derivations; and in 3D, closed-form solutions are typically unavailable. This logic is repeated for a simple model of self-propelled particles, run-and-tumble particles (RTPs) in a harmonic trap, confirming the assertion that the system in 2D enjoys special status. This study revisits the RTP-harmonic-trap model using a recently introduced integral-equation formulation based on reinterpreting RTP motion as a jump process (Frydel 2024 Phys. Fluids 36 011910). The key quantity of the formulation is the probability distribution of jumps G(x, x ′) of an auxiliary system. The stationary distribution is subsequently obtained from the integral equation ρ(x) = ´dx ′ ρ(x ′)G(x, x ′). In 2D, G(x, x ′) is found to be reversible, implying that the auxiliary system satisfies the detailed balance condition, ρ(x ′)G(x, x ′) = ρ(x)G(x ′ , x). This condition is then used to calculate the stationary distribution ρ(x) which significantly reduces the complexity of the problem. It is determined that G(x, x ′) is reversible only if the probability distribution of the waiting times, that determine the duration on the 'run' stage of the RTP motion, has an exponential form.

Activity and autonomous motion are fundamental in living and engineering systems. This has stimulated the new field of ‘active matter’ in recent years, which focuses on the physical aspects of propulsion mechanisms, and on motility-induced emergent collective behavior of a larger number of identical agents. The scale of agents ranges from nanomotors and microswimmers, to cells, fish, birds, and people. Inspired by biological microswimmers, various designs of autonomous synthetic nano- and micromachines have been proposed. Such machines provide the basis for multifunctional, highly responsive, intelligent (artificial) active materials, which exhibit emergent behavior and the ability to perform tasks in response to external stimuli. A major challenge for understanding and designing active matter is their inherent nonequilibrium nature due to persistent energy consumption, which invalidates equilibrium concepts such as free energy, detailed balance, and time-reversal symmetry. Unraveli...

Polar flocks in discrete active systems are often assumed to be robust, yet recent studies reveal their fragility under both imposed and spontaneous fluctuations. Here, we revisit the four-state active Potts model (APM) and show that its globally ordered phase is metastable across a broad swath of parameter space. Small counter-propagating droplets disrupt the flocking phase by inducing a persistent sandwich state, where the droplet-induced opposite-polarity lane remains embedded within the original flock, particularly pronounced at low noise, influenced by spatial anisotropy. In contrast, small transversely propagating droplets, when introduced into the flock, can trigger complete phase reversal due to their alignment orthogonal to the dominant flow and their enhanced persistence. At low diffusion and strong self-propulsion, such transverse droplets also emerge spontaneously, fragmenting the flock into multiple traveling domains and giving rise to a short-range order (SRO) regime. We further identify a motility-induced pinning (MIP) transition in small diffusion and low-temperature regimes when particles of opposite polarity interact, flip their state, hop, and pin an interface. Our comprehensive phase diagrams, encompassing full reversal, sandwich coexistence, stripe bands, SRO, and MIP, delineate how thermal fluctuations, self-propulsion strength, and diffusion govern flock stability in discrete active matter systems.

Self-propelling bacteria are a nanotechnology dream. These unicellular organisms are not just capable of living and reproducing, but they can swim very efficiently, sense the environment, and look for food, all packaged in a body measuring a few microns. Before such perfect machines can be artificially assembled, researchers are beginning to explore new ways to harness bacteria as propelling units for microdevices. Proposed strategies require the careful task of aligning and binding bacterial cells on synthetic surfaces in order to have them work cooperatively. Here we show that asymmetric environments can produce a spontaneous and unidirectional rotation of nanofabricated objects immersed in an active bacterial bath. The propulsion mechanism is provided by the self-assembly of motile Escherichia coli cells along the rotor boundaries. Our results highlight the technological implications of active matter’s ability to overcome the restrictions imposed by the second law of thermodynami...

We propose a method to modulate the drifting motion of overdamped circle swimmers in steady fluid flows by means of static sinusoidal potentials. Using Langevin formalism, we study drift velocity as a function of potential strength and wavelength with and without diffusional motion. Drift velocity is essentially quantized without diffusion, but in the presence of noise, the displacement per cycle has a continuous range. As a function of dimensionless potential wave number, domains of damped oscillatory and plateau regimes are observed in the drift velocity diagram. At weak potential and fluid velocity less than powered velocity, there is also a regime where drift velocity exceeds the fluid velocity. Methods based on these results can be used to separate biological and artificial circle swimmers based on their dynamical properties.

Life defies disorder, not just with molecules but with physics. We advance a substrate-independent, falsifiable criterion for life: a system is alive iff it sustains a positive rate of entropy resistance.

Classically, R(t) = −S(t) > 0, and in the quantum–informational regime, Rq(t) :=−d/dtTr [ρ(t) ln ρ(t)] > 0, with S the (coarse-grained or Shannon) entropy and ρ a density matrix (von Neumann entropy). The law reframes life as persistent resistance to uncertainty rather than a checklist of Earth-centric traits (DNA, metabolism, reproduction). We connect the criterion to non-equilibrium thermodynamics (entropy balance, housekeeping vs. excess production, fluctuation theorems), control theory (Lyapunov functions, dissipativity, requisite variety), information thermodynamics (Landauer cost, Maxwell demons with feedback), and quantum open-system dynamics (CPTP maps, Spohn’s inequality, GKSL/Lindblad semigroups, resource theories of coherence, quantum error correction). We provide operational estimators for R and Rq from time series, demonstrate falsifiability on canonical counterexamples, and propose mission-ready measurement protocols for astrobiology, synthetic life, and coherent biomolecular complexes.

Biological systems continuously acquire and use energy sources to perform various functions. This energy is, in part, transduced to generate the forces that control the mechanical behavior of the cell (e.g. cell shape and motion). In this case, the system is in the non-equilibrium state and the material may be called ''Active Soft Matter''. To investigate the mechanical properties of soft, active systems, we have synthesized an active gel with a well-known semi-flexible biopolymer, DNA, and DNA-associated motor proteins. We study the mechanics of this system using two kinds of microrheological techniques. First, we use a passive measurement in which the intrinsic fluctuation of embedded particles gives information on gel mechanics. Second, we use an active measurement utilizing the forced oscillating motion of embedded particles by an external magnetic field. We discuss these results in comparison to cytoskeletal systems, and seek to establish universal principles of motor-driven active gels.

Spatiotemporal chaos with shear banding in a driven nematogenic fluid DEBARSHINI CHAKRABORTY, CHANDAN DASGUPTA, SRIRAM RA-MASWAMY, AJAY SOOD, Indian Institute of Science -We present the results of a numerical study of a model of the hydrodynamics of a sheared nematogenic fluid in which spatial variations are allowed only in the gradient direction and the effects of order parameter stresses on the velocity profile are taken into account. When the value of a dimensionless viscosity parameter is so chosen that the order parameter stress is comparable to the bare viscous stress, the system exhibits steady states with the characteristics of shear banding, In addition, a non-zero choice of a parameter that governs the effect of the velocity field on stretching the nematic order parameter leads to the appearance of a new steady state in which the features of both spatiotemporal chaos and shear banding are present.

Within the overlap of physics, chemistry and biology, complex matter becomes ‘more deeply’ understood when high level mathematics converts regularities of experimental data into scientific laws, theories, and models (Krakauer et al., 2011. The challenges and scope of theoretical biology. J. Theoret. Biol. 276: 269-276). The simplest kinds of complex biological matter are bacterial cells; they appear complex–from a physicochemical standpoint–because they are multicomponent, multiphase, biomacromolecularly crowded, and re emergent; the property of re-emergence differentiates biological matter from complex chemical and physical matter. Bacterial cells cannot self-reassemble spontaneously from their biomolecules and biomacromolecules (via non-covalent molecular forces) without the action of external ‘drivers’; on Earth, such drivers have been diurnal (cycling) physicochemical gradients, i.e. temperature, water activity, etc. brought about by solar radiation striking the Earth’s rotating...

Synopsis Information, energy, and matter are fundamental properties of all levels of biological organization, and life emerges from the continuous flux of matter, energy, and information. This perspective piece defines and explains each of the three pillars of this nexus. We propose that a quantitative characterization of the complex interconversions between matter, energy, and information that comprise this nexus will help us derive biological insights that connect phenomena across different levels of biological organization. We articulate examples from multiple biological scales that highlight how this nexus approach leads to a more complete understanding of the biological system. Metrics of energy, information, and matter can provide a common currency that helps link phenomena across levels of biological organization. The propagation of energy and information through levels of biological organization can result in emergent properties and system-wide changes that impact other hier...

The role of hydrodynamics and confinement on the collective behavior of active emulsions SHASHI THUTUPALLI, DELPHINE GEYER, HOWARD STONE, Princeton Univ -Active droplets i.e. emulsion droplets which exhibit self-propelled motion are of tremendous interest in understanding the collective dynamics of systems far from thermal equilibrium. A particularly appealing feature of these active droplet systems is that the coupling between the individuals is well-controlled and mediated by purely physical effects such as steric interactions and hydrodynamics. We create such active droplet systems using liquid crystalline emulsions in aqueous phases stabilized by surfactants. The propulsion of the individual droplets is fully 3 dimensional and occurs via a spontaneously broken symmetry (unlike colloidal swimmers which are often asymmetric by design) which is sustained via dissipation of chemical energy. Here, we show that hydrodynamics and geometry play a crucial role in the emergent self-organization of the active droplets. In particular, we describe the pair-correlations emerging in a population of active droplets confined in Hele-Shaw geometry. We further demonstrate that these interactions result in the formation of stable travelling bands in reduced confinement and self-organize into crystalline vortices at a single interface.

The field of synthetic active matter has, thus far, been led by efforts to create point-like, isolated (yet interacting) self-propelled objects (e.g. colloids, droplets, microrobots) and understanding their collective dynamics. The design of flexible, freely jointed active assemblies from autonomously powered components remains a challenge. Here, we report freely-jointed active polymers created using self-propelled droplets as monomeric units. Our experiments reveal that the self-shaping chemohydrodynamic interactions between the monomeric droplets give rise to an emergent rigidity (the acquisition of a stereotypical asymmetric C-shape) and associated ballistic propulsion of the active polymers. The rigidity and propulsion of the chains vary systematically with their lengths. Using simulations of a minimal model, we establish that the emergent polymer dynamics are a generic consequence of quasi two-dimensional confinement and auto-repulsive trail-mediated chemical interactions between the freely jointed active droplets. Finally, we tune the interplay between the chemical and hydrodynamic fields to experimentally demonstrate oscillatory dynamics of the rigid polymer propulsion. Altogether, our work highlights the possible first steps towards synthetic self-morphic active matter.

A dilute suspension of active Brownian particles in a dense compressible viscoelastic fluid, forms a natural setting to study the emergence of nonreciprocity during a dynamical phase transition. At these densities, the transport of active particles is strongly influenced by the passive medium and shows a dynamical jamming transition as a function of activity and medium density. In the process, the compressible medium is actively churned up – for low activity, the active particle gets self-trapped in a cavity of its own making, while for large activity, the active particle ploughs through the medium, either accompanied by a moving anisotropic wake, or leaving a porous trail. A hydrodynamic approach makes it evident that the active particle generates a long-range density wake which breaks fore-aft symmetry, consistent with the simulations. Accounting for the back-reaction of the compressible medium leads to (i) dynamical jamming of the active particle, and (ii) a dynamical non-recipro...

Myxococcus xanthus is a soil-dwelling bacterium that exhibits several fascinating collective behaviors including streaming, swarming, and generation of fruiting bodies. A striking feature of M. xanthus is that it periodically reverses its motility direction. The first stage of fruiting body formation is characterized by the aggregation of cells on a surface into round mesoscopic structures. Experiments have shown that this aggregation relies heavily on regulation of the reversal rate and local mechanical interactions, suggesting motility-induced phase separation may play an important role. We have adapted self-propelled particle models to include cell reversal and motility suppression resulting from sporulation observed in aggregates. Using 2D molecular dynamics simulations, we map the phase behavior in the space of Péclet number and local density and examine the kinetics of aggregation for comparison to experiments.

Jyoti Prasad Banerjee, Rituparno Mandal, Deb Sankar Banerjee, Shashi Thutupalli, 4 and Madan Rao Simons Centre for the Study of Living Machines, National Centre for Biological Sciences (TIFR), Bangalore, India Institute for Theoretical Physics, Georg-August-Universität Göttingen, 37077 Göttingen, Germany Department of Physics, Carnegie Mellon University, Pittsburgh, USA International Centre for Theoretical Sciences (TIFR), Bangalore, India (Dated: October 14, 2021)

Active emulsions, i.e., emulsions whose droplets perform self-propelled motion, are of tremendous interest for mimicking collective phenomena in biological populations such as phytoplankton and bacterial colonies, but also for experimentally studying rheology, pattern formation, and phase transitions in systems far from thermal equilibrium. For fuelling such systems, molecular processes involving the surfactants which stabilize the emulsions are a straightforward concept. We outline and compare two different types of reactions, one which chemically modifies the surfactant molecules, the other which transfers them into a different colloidal state. While in the first case symmetry breaking follows a standard linear instability, the second case turns out to be more complex. Depending on the dissolution pathway, there is either an intrinsically nonlinear instability, or no symmetry breaking at all (and hence no locomotion).

We study water-in-oil emulsion droplets, running the Belousov-Zhabotinsky reaction, that form a new type of active matter unit. These droplets, stabilised by surfactants dispersed in the oil medium, are capable of internal chemical oscillations and also self-propulsion due to dynamic interfacial instabilities that result from the chemical reactions. The chemical oscillations can couple via the exchange of activator and inhibitor type of reaction intermediates across the droplets under precise conditions of surfactant bilayer formation between the droplets. Here we present the synchronization behaviour of networks of such chemical oscillators and show that the resulting dynamics depend on the network topology. Further, we demonstrate that the motion of droplets can be synchronized with the chemical oscillations inside the droplets, leading to exciting possibilities in future studies of active matter.

A distinguishing feature of active particles is the nature of the non-equilibrium noise driving their dynamics. Control of these noise properties is, therefore, of both fundamental and applied interest. We demonstrate emergent tuning of the active noise of a granular self-propelled particle by confining it to a quasi one-dimensional channel. We find that this particle, moving like an active Brownian particle (ABP) in two-dimensions, displays run-and-tumble (RTP) characteristics in confinement. We show that the dynamics of the relative orientation co-ordinate of the particle maps to that of a Brownian particle in a periodic potential subject to a constant force, in analogy to the dynamics of a molecular motor. This mapping captures the essential statistical characteristics of the one-dimensional RTP motion. Specifically, our theoretical analysis is in agreement with the empirical distributions of the relative orientation co-ordinate and the run-times (tumble-rates) of the particle. F...

The role of hydrodynamics and confinement on the collective behavior of active emulsions SHASHI THUTUPALLI, DELPHINE GEYER, HOWARD STONE, Princeton Univ -Active droplets i.e. emulsion droplets which exhibit self-propelled motion are of tremendous interest in understanding the collective dynamics of systems far from thermal equilibrium. A particularly appealing feature of these active droplet systems is that the coupling between the individuals is well-controlled and mediated by purely physical effects such as steric interactions and hydrodynamics. We create such active droplet systems using liquid crystalline emulsions in aqueous phases stabilized by surfactants. The propulsion of the individual droplets is fully 3 dimensional and occurs via a spontaneously broken symmetry (unlike colloidal swimmers which are often asymmetric by design) which is sustained via dissipation of chemical energy. Here, we show that hydrodynamics and geometry play a crucial role in the emergent self-organization of the active droplets. In particular, we describe the pair-correlations emerging in a population of active droplets confined in Hele-Shaw geometry. We further demonstrate that these interactions result in the formation of stable travelling bands in reduced confinement and self-organize into crystalline vortices at a single interface.

The series ''Springer Theses'' brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected for its scientific excellence and the high impact of its contents for the pertinent field of research. For greater accessibility to non-specialists, the published versions include an extended introduction, as well as a foreword by the student's supervisor explaining the special relevance of the work for the field. As a whole, the series will provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today's younger generation of scientists.

Natural flocks need to cope with various forms of heterogeneities, for instance, their composition, motility, interaction, or environmental factors. Here, we study the effects of such heterogeneities on the flocking dynamics of the reciprocal two-species Vicsek model [Phys. Rev. E 107, 024607 (2023)], which comprises two groups of self-propelled agents with anti-aligning inter-species interactions and exhibits either parallel or anti-parallel flocking states. The parallel and anti-parallel flocking states vanish upon reducing the size of one group, and the system transitions to a single-species flock of the majority species. At sufficiently low noise (or high density), the minority species can exhibit collective behavior, anti-aligning with the liquid state of the majority species. Unequal self-propulsion speeds of the two species strongly encourage anti-parallel flocking over parallel flocking. However, when activity landscapes with region-dependent motilities are introduced, parallel flocking is retained if the faster region is given more space, highlighting the role of environmental constraints. Under noise heterogeneity, the colder species (subjected to lower noise) attain higher band velocity compared to the hotter one, temporarily disrupting any parallel flocking, which is subsequently restored. These findings collectively reveal how different forms of heterogeneity, both intrinsic and environmental, can qualitatively reshape flocking behavior in this class of reciprocal two-species models.

The non-thermal nature of self-propelling colloids offers new insights into non-equilibrium physics. The central mathematical model to describe their trajectories is active Brownian motion, where a particle moves with a constant speed, while randomly changing direction due to rotational diffusion. While several feedback strategies exist to achieve position-dependent velocity, the possibility of spatial and temporal control over rotational diffusion, which is inherently dictated by thermal fluctuations, remains untapped. Here, we decouple rotational diffusion from thermal fluctuations. Using external magnetic fields and discrete-time feedback loops, we tune the rotational diffusivity of active colloids above and below its thermal value at will and explore a rich range of phenomena including anomalous diffusion, directed transport, and localization. These findings add a new dimension to the control of active matter, with implications for a broad range of disciplines, from optimal tran...

Dense assemblies of self propelled particles, also known as active or living glasses are abundant around us, covering different length and time scales: from the cytoplasm to tissues, from bacterial bio-films to vehicular traffic jams, from Janus colloids to animal herds. Being structurally disordered as well as strongly out of equilibrium, these systems show fascinating dynamical and mechanical properties. Using extensive molecular dynamics simulation and a number of different dynamical and mechanical order parameters we differentiate three dynamical steady states in a sheared model active glassy system: (a) a disordered phase, (b) a propulsion-induced ordered phase, and (c) a shear-induced ordered phase. We supplement these observations with an analytical theory based on an effective single particle Fokker-Planck description to rationalise the existence of the novel shearinduced orientational ordering behaviour in our model active glassy system that has no explicit aligning interactions, e.g. of Vicsek-type. This ordering phenomenon occurs in the large persistence time limit and is made possible only by the applied steady shear. Using a Fokker-Planck description we make testable predictions without any fit parameters for the joint distribution of single particle position and orientation. These predictions match well with the joint distribution measured from direct numerical simulation. Our results are of relevance for experiments exploring the rheological response of dense active colloids and jammed active granular matter systems.

We use numerical simulations to study the dynamics of dense assemblies of self-propelled particles in the limit of extremely large, but finite, persistence times. In this limit, the system evolves intermittently between mechanical equilibria where active forces balance interparticle interactions. We develop an efficient numerical strategy allowing us to resolve the statistical properties of elastic and plastic relaxation events caused by activity-driven fluctuations. The system relaxes via a succession of scale-free elastic events and broadly distributed plastic events that both depend on the system size. Correlations between plastic events lead to emergent dynamic facilitation and heterogeneous relaxation dynamics. Our results show that dynamical behaviour in extremely persistent active systems is qualitatively similar to that of sheared amorphous solids, yet with some important differences.

Information, energy, and matter are fundamental properties of all levels of biological organization, and life emerges from the continuous flux of matter, energy, and information. This perspective piece defines and explains each of the three pillars of this nexus. We propose that a quantitative characterization of the complex interconversions between matter, energy, and information that comprise this nexus will help us derive biological insights that connect phenomena across different levels of biological organization. We articulate examples from multiple biological scales that highlight how this nexus approach leads to a more complete understanding of the biological system. Metrics of energy, information, and matter can provide a common currency that helps link phenomena across levels of biological organization. The propagation of energy and information through levels of biological organization can result in emergent properties and system-wide changes that impact other hierarchical levels. Deeper consideration of measured imbalances in energy, information, and matter can help researchers identify key factors that influence system function at one scale, highlighting avenues to link phenomena across levels of biological organization and develop predictive models of biological systems.

We develop a general theory for active viscoelastic materials made of polar filaments. This theory is motivated by the dynamics of the cytoskeleton. The continuous consumption of a fuel generates a non equilibrium state characterized by the generation of flows and stresses. Our theory can be applied to experiments in which cytoskeletal patterns are set in motion by active processes such as those which are at work in cells.

Myxococcus xanthus is a soil-dwelling bacterium that exhibits several fascinating collective behaviors including streaming, swarming, and generation of fruiting bodies. A striking feature of M. xanthus is that it periodically reverses its motility direction. The first stage of fruiting body formation is characterized by the aggregation of cells on a surface into round mesoscopic structures. Experiments have shown that this aggregation relies heavily on regulation of the reversal rate and local mechanical interactions, suggesting motility-induced phase separation may play an important role. We have adapted self-propelled particle models to include cell reversal and motility suppression resulting from sporulation observed in aggregates. Using 2D molecular dynamics simulations, we map the phase behavior in the space of Péclet number and local density and examine the kinetics of aggregation for comparison to experiments.

Hyperuniformity in driven disordered systems and giant number fluctuations in active matter represent opposite ends in a spectrum of statistical correlations found in physical systems outside of thermal equilibrium. Despite both these phenomena representing milestones in our understanding of non-equilibrium physics -with potential applications ranging from optimal packing patterns and transport in biological matter to the fabrication of disordered materials with photonic bandgaps [7-10] -, the possibility of them being connected has remained so far elusive. Using a combination of experimental and theoretical work on active nematic defects, we demonstrate, through the notion of critical absorbing states , that such a spectrum is in fact closed, and that active matter can in fact be hyperuniform.

The dynamics of dry active matter have implications for a diverse collection of biological phenomena spanning a range of length and time scales, such as animal flocking, cell tissue dynamics, and swarming of inserts and bacteria. Uniting these systems are a common set of symmetries and conservation laws, defining dry active fluids as a class of physical system. Many interesting behaviours have been observed at high densities, which remain difficult to simulate due to the computational demand. Here, we show how two-dimensional dry active fluids in a dense regime can be studied using a simple modification of the lattice Boltzmann method. We apply our method on a model that exhibits motility-induced phase separation, and an active model with contact inhibition of locomotion, which has relevance to collective cell migration. For the latter, we uncover multiple novel phase transitions: two first-order and one potentially critical. We further support our simulation results with an analyti...

Self-organization is the generation of order out of local interactions. It is deeply connected to many fields of science from physics, chemistry to biology, all based on physical interactions. The emergence of collective animal behavior is the result of self-organization processes as well, though they involve abstract interactions arising from sensory inputs, information processing, storage, and feedback. Resulting collective behaviors are found, for example, in crowds of people, flocks of birds, and swarms of bacteria. Here we introduce interactions between active microparticles which are based on the information about other particle positions. A real-time feedback of multiple active particle positions is the information source for the propulsion direction of these particles. The emerging structures require continuous information flows. They reveal frustrated geometries due to confinement to two dimensions and internal dynamical degrees of freedom that are reminiscent of physically...

Although the mass of an object is defined to represent the equivalent of 3D matter it contains, it is often considered as the quantity of 3D matter contained in the object. Mass is the mathematical relation between an external linear effort on an object and the rate of the rate of its displacement in the direction of the external effort. This relationship is often ignored, and the value of a body's mass is regarded in almost all academic fields as the quantity of 3D matter present in the object. Loss (or gain) of mass of an object during its development stage or changes in its structure is understood as 'mass defect'. This is often related to an assumed phenomenon of binding energy required to stabilize the nucleons in the atoms in a body.

Controlling interfaces of phase-separating fluid mixtures is key to the creation of diverse functional soft materials. Traditionally, this is accomplished with surface-modifying chemical agents. Using experiment and theory, we studied how mechanical activity shapes soft interfaces that separate an active and a passive fluid. Chaotic flows in the active fluid give rise to giant interfacial fluctuations and noninertial propagating active waves. At high activities, stresses disrupt interface continuity and drive droplet generation, producing an emulsion-like active state composed of finite-sized droplets. When in contact with a solid boundary, active interfaces exhibit nonequilibrium wetting transitions, in which the fluid climbs the wall against gravity. These results demonstrate the promise of mechanically driven interfaces for creating a new class of soft active matter.

A classical particle in a harmonic potential gives rise to a continuous energy spectra, whereas the corresponding quantum particle exhibits countably infinite quantized energy levels. In recent years, classical non-Markovian wave-particle entities that materialize as walking droplets have been shown to exhibit various hydrodynamic quantum analogs, including quantization in a harmonic potential by displaying few coexisting limit cycle orbits. By considering a truncated-memory stroboscopic pilot-wave model of the system in the low dissipation regime, we obtain a classical harmonic oscillator perturbed by oscillatory nonconservative forces that display countably infinite coexisting limit-cycle states, also known as megastability. Using averaging techniques in the low-memory regime, we derive analytical approximations of the orbital radii, orbital frequency and Lyapunov energy function of this megastable spectrum, and further show average energy conservation along these quantized states. Our formalism extends to a general class of self-excited oscillators and can be used to construct megastable spectrum with different energy-frequency relations.

In an October issue of 1892, the German humorous weekly Fliegende Blätter printed a joke that has since become a staple of visual ambiguity, featured in different variants throughout publications of psychology and works on art and perception.¹ The small original engraving, since dubbed a "duckrabbit," presents a bi-stable image of a rabbit and a duck that plays with the ambiguous coexistence of its two alternative readings, conflated and congruent in one single form (fig. 1). Once recognized, neither figure can be erased again as present. Instead, the forms will become temporally unstable, irrevocably oscillating back and forth at some frequency between either representing a rabbit or a duck. At the same time, it is next to impossible to securely behold the image both as a rabbit's and as a duck's head at once. Debuting in the satiric magazine, the "duckrabbit" was embedded in a joke on likeness/ similitude, framed by the trick question: "Which animals are most alike?" and the par-Open Access.

During this experimental exploration of acoustic mobility phenomena, I had the chance to work with wonderful scientists, in France but also across different countries, who have enriched me with their skills and insight in quite various domains. Prof. Thomas E. Mallouk has been extremely helpful in providing us with the metallic nanoparticles used along this PhD. In addition to his knowledge in chemistry fabrication processes, he has been an exquisite guest when I did come to Penn State University to work with his team, and an very helpful collaborator to talk with on scientific but also on everyday life topics. Next I will thank Prof. John Sader and all of his University of Melbourne team, for hosting me during a month and to be so engaging in building a collaboration on the nanorods propulsion. Dr. Jesse Collis and him have been more than pedagogic in providing me with insights about their models, and in building the right set of experiment to try to prove it right. My gratitude also goes to Prof. Philippe Marmottant and Dr. Jean-Louis Thomas for accompanying me along the project in my PhD committee. For those who have never set foot in the PMMH, they may not know what it is to be cocooned in a place filled to the brim with not only bright minds, but also genuinely creative personalities, able to build amazing experiments from pieces of cardboard as well as from extremely expensive microscopes. I would like to thank here all of the lab that contributed to this PhD, and especially Justine Laurent, for having helped so much with the design and production of these tiny shape controlled objects, and for having proofread almost all of the manuscript. Xavier Benoit-Gonin, without whom none of the microgravity experiments campaigns would have take place, also holds a special position in these acknowledgements. My gratitude also goes to all of my officemates, Yacine and the everlasting debates around a coffee cup, Piotr and his perpetual enthusiasm, Joachim and his unmoveable optimism. Such a lab is a shrine to conduct research in, and I am very happy to have been able to join and build a long lasting relationship with a lot of the people in there. I would thus like to thank Nathan for all the work he shed into the PDMS channels design exploited in the following pages, Ludovic for his help performing the acoustic energy measurements presented here, Angelica for her collaboration on the optoacoustic effect investigations, and finally Jean-Luc and Mauricio for allowing this PhD to take place. Funding, collaborators and a brilliant work environment allow to conduct great research. But it is the contribution of others that I would like to emphasize now, because they enabled the would-be doctor to achieve this goal. There are few things achievable alone, playing rugby not being one of them. I do not know much sports that reward as much as this one, and especially thanks to the team spirit it creates. For almost a decade now I have been sweating and spilling my blood with you, and I would like to give you many of my thanks for your undefeatable support in a lot of situations I have crossed along this PhD project. Having a close circle of friends from childhood is a rare thing that I can appreciate to ii its exact value; friends worth that much are quite uncommon. Whether it is through the reliability of Valentin, the english skills of David, the Florians the good plans, peachy Eliott and impeaching Sandy, they have also shaped the person I am today. Several things are notorious about the ESPCI Paris. It is obviously the best school in the world, but few know that it can be the place to meet some of the finest people in life. Where would I be without them? Probably somewhere between a false train luggage and a scooter. So thank you Hubert, for sharing so many good moments for eight years now, and to have allowed all of this to take place. Thanks to the Simons, each one incarnating prime values to me, finely knowledgeable and terribly practical, cultivated and rough, thin and thick made flesh. And how to evoke something weighty without thinking of something light? Thank you Dounia, for being able to lift my spirits each time we shared together, for being incredibly positive no matter the situation. I also sincerely believe I would not be the same person as I am today without Ambre, whom I truly thank by these words. I feel quite proud to be friend with a world class handball player in the person of Thomas Guyon, with a cook as gifted as Léo and with a phenomenal piper like Flavio. But true friends can also be found at the bottom of a mine, so thank you Guillaume and Wahbi, have lit my lantern. Likewise, I had the chance to share many road memories with Hugues, Cas and Conner, my steadfast travel companions. Speaking of this learning environment cannot be done without thinking of a family that was built there, perpetuated so far with close to no snag, the one to bring them all and in power bind them. This goes to these people, each one unique and special in their personalities and behaviours, but linked by this love we have for each other, that constitutes this unity we are so proud of. Particulars thoughts are going to Theo, whose light we try to keep shining on all our events. Fortifying the tree of my achievements now, my family would be at its deepest roots. It is quite a chance to grow up surrounded by characters so enriching. Everybody thanks their loved ones. I would like to thank my family for being different, for standing against the global tendencies of society, for moulding me as I am today through the interactions we had along all these years, together. A certain gipsyness may or may not come from this crucible, however it is an asset in a constant motion world to be able to effortlessly adapt to every situation. These acknowledgements finally go to all those who could not be cited in these lines, but that will know reading it that they matter the most to me, and without which I could not take this project to its end.