Active Matter at Surfaces and in Complex Environments

Much attention is currently being given to the problem of manipulating fluids at the microscale, with successful applications to fields such as 3D fabrication and biomedical research. Often micropumps are a fundamental component of these microfluidic systems. An intriguing technique to manipulate fluid flows in a channel is diffusioosmosis. Fluid flow is obtained upon imposing an inhomogeneous concentration of some solute, which generates flow in a boundary layer around the channel walls. These inhomogeneities may be achieved via an inhomogenous catalytic coating of the channel walls. For channels with lengths in the hundreds of micrometers, both diffusion and advection play a role in the transport of solute. Indeed, when advection dominates, a corrugated catalytic channel can act as a micropump even when it is fore-aft symmetric. We show this both numerically and analytically. Furthermore, the channel can either exhibit steady pumping (with a flow rate that is constant in time), or unsteady pumping with sustained oscillations in the flow rate. We show how both the flow rates and the frequency of the oscillations may be tuned via the geometrical and chemical properties of the channel.

Bacteria are found in confined environments, wether in the porosity of natural media (soils, mucus) or in biomedical application (tubing, micro-fluidic channels). We aim at understanding quantitatively the large scale transport properties of the run & tumble micro-swimmer E. coli in such environments. In our model experiment, the confinement is induced by varying the distance between the two glass slides in which the object can swim freely. The impact of this distance on the exploration is revealed, explained and recovered in a numerical model (behavioral variability model) with no adjustable parameter. In this model, bacteria are shown to auto-organize : the runs are statistically longer on the surface than in the bulk, which is not the case in the classical model from Berg. A link with exploration in porous media is proposed.

We study the behavior of a pair of chiral swimmers, which are hydrodynamically interacting in the low Reynolds number regime. We consider both the near and far-field interactions for the pair swimming. To study this, we use the chiral squirmer model, a spherically shaped body with non-axisymmetric surface slip velocity, which generalizes the well-known squirmer model. When they are close, we calculate the lubrication force and torque between the squirmers. By varying the slip coefficients and the initial configuration of the squirmers, we investigate the hydrodynamic behavior of the squirmers. In the presence of lubrication force, the squirmers either repel each other or exhibit bounded motion where the distance between the squirmers alters periodically. The influence of external chemical gradient in the hydrodynamic behavior of chiral squirmers is also investigated. Interestingly, the lubrication and chemical gradient favor the bounded motion in some parameter regimes. This study helps to understand the collective behavior of dense suspension of self-propelled swimmers.

Soft active particles, which have the ability to deform under external forces, behave differently than the hard particles. The collective effects of such particles in a box geometry with periodic boundary conditions have been numerically studied using Langevin simulations. It has been found that the softness of particles disfavors MIPS and brings outs interesting structural and dynamical properties in the system[1]. In the present study, we investigate the effect of confining geometry on the collective effects of soft particles by invoking a corrugated channel as our confinement system. We find that the presence of boundaries plays a crucial role in allowing the manipulation of active particles. We also explore interesting aspects of this study, such as the change in the dynamics when considering soft and hard walls[2], with no explicit alignment or reorientation among the particles[3]. Several of these insights into the behavior of soft active particles in confined spaces and the potential for the manipulation of their motion through the properties of the walls will be presented in this work. We believe that our study has several implications such as drug delivery, understanding cell movement through pores and veins in biological systems, and separation of particles among others. References : [1] Collective behavior of soft self-propelled disks with rotational inertia, Soumen De Karmakar, Anshika Chugh and Rajaraman Ganesh, Scientific Reports 12, 22563 (2022) [2] Active Particles with Soft and Curved Walls: Equation of State, Ratchets, and Instabilities, Nikolai Nikola et al, Phys. Rev. Lett. 117, 098001 (2016) [3] Reentrant phase separation of a sparse collection of nonreciprocally aligning self-propelled disks, Soumen De Karmakar and Rajaraman Ganesh, Phys. Rev. E. 106, 044607 (2022)

Efficient nutrient mixing is crucial for the survival of bacterial colonies and other living systems. This raises the question of whether the optimization of this feature through the bacterial motion was a factor in the evolution of bacterial shapes. In this work, we couple the hydrodynamic equations for active nematics with the advection-diffusion equation where the advected solute is the activity (representing the nutrients). In addition to molecular diffusion, the activity is transported by the flow and modifies it by means of the active stress. We find that the dispersion of activity is subdiffusive due to the movement of defects in both directions of the gradient. In addition, we found a non-monotonic behaviour of the subdiffusion coefficient as a function of the aligning parameter, which is related to the shape of the particles. Our simulations suggest that there is a shape that optimizes the dispersion of activity in active nematics.

We report on experiments investigating the locomotion of an active elastic sheet, which is confined to a curved surface. In the experiments, NIPA-BZ gel strips are confined to the curved interface (i.e. meniscus) between BZ (Belousov-Zhabotinsky) solution and (heavy) Perfluorodecalin liquid. The oscillating BZ reaction within the strips causes them to periodically vary their curvature. As a result, they move in periodic trajectories along the fluid interface, changing both their position and orientation. We present a model that successfully describes this locomotion. The key ingredient in the model is the elastic energy of the strip, which stems from the mismatch between the (varying) intrinsic curvature of the strip and the local curvature of the fluid interface. Spatial gradients of the energy provide the tangential force on the sheet and its derivatives with respect to the strip orientation provide the torques that rotate it. As the system is over damped, its linear and angular velocities are proportional to the force and torques respectively. We verify the model's predictions via experiments with sheets that have constant intrinsic curvature and via numerical integration of the equation of motion.

Microorganisms explore complex heterogeneous environments for food foraging or survival via various navigation mechanisms, including swimming, crawling, and flagellar motions. Despite the complexity of the surroundings, which would seem to only inhibit their movement, these microorganisms find a way to perform efficient locomotion. We aim to study the efficient movement of microorganisms in various complex environments by addressing two critical aspects of their locomotion. The first is to examine how morphological aspects, such as the shape or size of the organisms, dictate efficient navigation of their surroundings. The second objective is to focus on understanding how their interactions with various surroundings (porous medium, viscoelastic medium) result in efficient locomotion.

First, we study motility-induced surface wetting using a minimal model of bacteria that takes into account the intrinsic motility diversity of living matter. A mixture of “fast” and “slow” self-propelled Brownian particles is considered in the presence of a wall. The evolution of the wetting layer thickness shows an overshoot before stationarity and its composition evolves in two stages, equilibrating after a slow elimination of excess particles. Nonmonotonic evolutions are shown to arise from delayed avalanches towards the dilute phase combined with the emergence of a transient particle front. Second, we study a similar active mixture in the presence of large asymmetric obstacles which induce rectification currents. Speed diversity is observed to amplify rectification. This is shown to occur because the contribution of fast particles to rectification increases superlinearly with self-propulsion speed due to cooperative effects. Finally, we turn our attention to movement ecology scenarios where birth and death become important. We discuss diffusing individuals under confinement in the presence of a leaking toxic substance. We elucidate the role of chemotaxis in determining survival, particularly in the case of competition between chemotactic versus non-chemotactic strains. Also, we show in which circumstances chemotaxis can lead to higher competition due to confinement and therefore lower total populations.

Active matter is a class of nonequilibrium soft matter characterized by energy collection and conversion at the level of individual particles. Soft matter is bounded by surfaces and/or interfaces, the description of which requires the analysis of non-uniform systems. Even for scalar Active Soft Matter, surface and interfacial properties received little attention. Fundamental questions, such as how the interfacial fluctuations of systems at phase coexistence depend on the activity or how the roughness of the edge of a growing film depends on its thickness, are still elusive. In this work, a lattice model is used to study interfaces of repulsive active particles in the bulk and at a planar surface. Specifically, we study how the persistence length (related to the activity) affects the interface, measuring properties such as the power spectrum, roughness, and other structural characteristics of the interface. A rejection-free Kinetic Monte Carlo method is employed to access the relevant length and time scales. We also characterize the wetting behaviour. The model predicts a motility-induced phase separation of active particles, and the bulk coexistence of dense liquid-like and dilute vapour-like steady states is determined. A system, with a varying number of particles, mimicking the grand canonical ensemble at equilibrium, is introduced. The formation and growth of the liquid film between the solid surface and the vapour phase is investigated. At constant activity, as the system is brought towards coexistence from the vapour side, the thickness of the adsorbed film exhibits a divergent behaviour regardless of the activity. This suggests a complete wetting scenario along the full coexistence curve.

In active matter, the lack of momentum conservation makes non-reciprocal interactions the rule rather than the exception. Non-reciprocity is responsible for a wealth of emerging behaviors that are hard to predict starting from the microscopic scale, due to the absence of a generic theoretical framework out of equilibrium. In this talk, we consider bacterial mixtures that interact via mediated, non-reciprocal interactions like quorum-sensing and chemotaxis. By explicitly relating microscopic and macroscopic dynamics, we show that non-reciprocity may fade as coarse-graining proceeds, leading to large-scale bona fide equilibrium descriptions. In turns, this allows us to account quantitatively, and without fitting parameters, for the rich behaviors observed in microscopic simulations including phase separation, demixing or multi-phase coexistence. We also derive the condition under which non-reciprocity is strong enough to survive coarse-graining, leading to a wealth of dynamical patterns. Again, the explicit coarse-graining of the dynamics allows us to predict the phase diagram of the system starting from its microscopic description. All in all, we show that the fate of non-reciprocity across scales is a subtle and important question.

Active matter can "communicate" through simple rules and exhibit interesting and complex clustering behaviors. Similar to active matter, artificial colloidal motors are able to convert the energy in the surrounding environment into self-drive, and can "communicate" through the interaction of fluid field, chemical field, electrostatic field, and so on, and cause rich clustering behaviors, thus receiving much attention from the academic community. Among them, light-driven colloidal motors have become one of the hot research focuses due to their unique feature of remote motion control. Currently, the group behaviors of light-driven motors are mainly studied to reveal the phenomena and elucidate the principles, but the growth process of clusters and cluster motion behaviors are still less studied. Based on the above problems, we choose titanium dioxide (TiO2) photoactive microspheres as the research model in this report. One exploits the photocatalytic performance of isotropic TiO2 microspheres, and the TiO2 micromotor exhibits good negative phototropism and start-stop controllability under the irradiation of patterned light sources. Relying only on water as a fuel, the TiO2 micromotor is driven away from the light source by generating an oxygen gradient through a local photochemical reaction. In addition, when a dynamic light spot is used, the TiO2 motor is then dynamically assembled according to a defined path. II describes the growth pattern and kinetic process of TiO2-Pt motor clusters, as well as the cluster motion pattern and characteristics, mainly from the kinetic and thermodynamic perspectives. When increasing the TiO2-Pt micromotors in the field of view, the clusters formed by the TiO2-Pt micromotors and the particle density of the motors have a strong correlation. When the particle density is larger the total cluster area is finally formed, the average cluster area, and the ratio of particles entering the cluster to the particles in space is also larger. The clusters formed in the two-dimensional plane tend to move in spirals, which is analyzed to be related to the configuration of the particle assembly. Finally, for the thermodynamic process of cluster growth, the influence of collision chance and escape factor on cluster growth is mainly considered. In summary, we found that TiO2 micromotors have the property of fast response to structured light spots, and the negative phototropism can be used to achieve structured assembly and immediate dynamic changes on the micro- and nano-scale, and good pattern erasability. We also elucidate the kinetic process of cluster growth and the motion pattern and characteristics of clusters in the group behavior of TiO2-Pt micromotors, which provides a new research perspective and direction for understanding the cluster behavior and cluster formation process of light-driven colloidal motors.

We demonstrate that the ubiquitous laboratory magnetic stirrer provides a simple passive method of magnetic levitation, in which the so-called “flea” levitates indefinitely. We study the onset of levitation and quantify the flea’s motion (a combination of vertical oscillation, spinning and “waggling”), finding excellent agreement with a mechanical analytical model. The waggling motion acts like a pump, driving recirculating flow and producing a centripetal reaction force that stabilizes the flea. Without these stabilising forces, the particle would self-propel through the fluid. We show with experiments and numerical calculations that the motion is sensitive to the fluid viscosity (through $Re_s$ the streaming Reynolds number) and switches direction from "pusher" to "puller" swimming behaviour around $Re_s \approx 100$. Our results have implications for how mechanical swimmers move through viscous fluids. KA Baldwin, J-B de Fouchier, PS Atkinson, RJA Hill, MR Swift, and DJ Fairhurst, Phys. Rev. Lett. 121, 064502

Filamentous cyanobacteria are one of the world’s most ubiquitous types of organism, represent one of the earliest forms of multicellular organisation, and are considered as excellent candidates for biofuel production. These motile strands of bacteria, one cell wide and thousands of cells long, move continuously by a slow gliding motion. Here, we investigate the patterns and dynamics of cyanobacteria filaments grown in confined geometries, using microfluidic chambers with sizes comparable to the length of the filaments. We find that the cyanobacteria interact strongly with each other, and with the confining walls. They form a nematic texture, with a series of topological defects influenced by their container shape. We find that their density distribution is affected by the shape of their confinement: in chambers with acute angles (e.g. triangular), filaments spread more evenly, while filaments in circular or rectangular confinement grow more densely near the boundaries. The coupling of curvature with the biofilm growth architecture paves the way to the design of efficient microfuel cells to achieve a targeted filament density and distribution. These results are particularly useful in addressing the problem of self-shading of cyanobacteria which limits light exposure to other filaments, which in turn limits the efficiency of bioreactors. Growing cyanobacteria mats in confined chambers with specific geometries could control the thickness and distribution of the biofilm.

Confining in space the equilibrium fluctuations of statistical systems with long-range correlations is known to result into effective forces on the boundaries. Here we demonstrate the occurrence of these Casimir-like forces in the non-equilibrium context provided by flocking active matter. In particular, we consider a system of aligning self-propelled particles in two spatial dimensions in the bulk flocking phase and transversally confined by reflecting or partly repelling walls. We show that a finite-size contribution to the pressure on the wall emerges, which decays slowly and algebraically upon increasing the distance between the walls, with a certain degree of universality. We rationalize our findings within a hydrodynamic approach for the density and velocity field, formulating some conjectures based on our numerical data which need to be confirmed beyond the linear approximation.

Active turbulence is a peculiar phenomenon of collective motion present in active nonequilibrium systems. Different experimental studies have been focused on the study of turbulence in a fully developed state (1), or in presence of geometrical confinement achieved using microfluidics devices (2). In this poster, we will show the preliminary results of transient states of turbulence and the effects of spatial modulation of activity on the phenomenon. By using photokinetic E. coli bacteria and optical devices, it is possible to control with a precise spatiotemporal resolution the activity of the suspension. We investigate the response to these external stimuli and use them to validate the different models. References: [1] Jörn Dunkel et al. “Fluid Dynamics of Bacterial Turbulence”. en. In: Physical Review Letters 110.22 (May 2013), p. 228102. [2] Henning Reinken et al. “Organizing bacterial vortex lattices by periodic obstacle arrays”. en. In: Communications Physics 3.1 (Dec. 2020), p. 76.

The amoeboid motility mode is a fundamental way in which cells move, classically in cancer and immune cells. Experiments from our collaborators show that amoeboid cells migrate via this mode up a friction gradient, even when stiffness is uniform. We term this friction sensing as ‘frictiotaxis’. We present a simple active gel model of the cell cortex to explain frictiotaxis. Our numerical simulations show that a one-dimensional model with a linear friction gradient does not lead to frictiotaxis. Therefore, we suggest that frictiotaxis could be driven by fast contraction at the cell poles, where the cell cortex is not contact with the substrate, and thus friction is low. Overall, this work will demonstrate that an active gel model of the cell cortex can explain directed migration based on friction.

We investigate the motility of B. subtilis under different degrees of confinement induced by highly transparent porous hydrogel matrices by following bacterial movements through particle tracking applied to confocal microscopy measurements. The dynamical behavior of bacteria is linked to parameters describing the 2D and 3D hydrogel porosity extracted from the analysis of volume image stacks of fluorescently labeled hydrogels. Mean square displacements (MSDs) calculated from particle tracking reveal that the run-and-tumble motility of unconfined B. subtilis bacteria progressively turns into sub-diffusive motion with increasing confinement. Classifying single-trajectories into populations of active, diffusive, and sub-diffusive motions, we show that the average dynamical behavior is the result of changes in these populations. At moderate confinements, the reduction of the population of actively moving bacteria is balanced by the growth of the diffusive and sub-diffusive populations, while for stronger confinements the sub-diffusive trajectories become dominant. We interpret this transition as the effect of the increasingly prominent transient trapping imposed by the disordered porous network. This interpretation is confirmed by the excellent agreement between the effective diffusion coefficients estimated from the experimental MSDs under different confinement conditions, and those calculated using a recently proposed hopping and trapping model for bacterial motion in confinement. Within the model, we propose a more precise criterion to define the threshold velocity for distinguishing hopping and trapping intervals along a trajectory as the average velocity estimated from the ensemble of all trajectories. Additionally, we provide a phenomenological relation between the average velocity in confinement and the confinement length of the porous matrix. Our work provides new insights of bacterial motility in complex media that are relevant for applications and mimic natural environments.

Malaria is one of the most devasting infectious diseases and transmitted from mosquitos to humans by so-called Plasmodium sporozoites, which move by gliding motility. Myosin motors move actin filaments below the plasma membrane, which leads to surface flows of adhesins that are anchored into the plasma membrane. How this surface flow is converted into the complicated motility patterns observed in experiments is not clear. Here we introduce theoretical models to bridge this gap. The coupling between surface flow and substrate is modeled by a system of reversible adhesion bonds. We numerically solve the resulting system of ordinary differential equations and find a rich variety of motility patterns, including the circular and helical paths observed in experiments. This allows us to estimate likely patterns of surface flows, which are hard to measure experimentally.

We compute the response matrix for a tracer particle in a compressible fluid with odd viscosity living on a two-dimensional surface. Unlike the incompressible case, we find that an odd compressible fluid can produce an odd lift force on a tracer particle. Using a "shell localization" formalism, we provide analytic expressions for the drag and odd lift forces acting on the tracer particle in a steady state and also at finite frequency. Importantly, we find that the existence of an odd lift force in a steady state requires taking into account the non-conservation of the fluid mass density due to the coupling between the two-dimensional surface and the three-dimensional bulk fluid.

The majority of bacteria inhabit complex biological niches such as soil, inter-tissue pores, and mucus. These are granular and porous microenvironments, much unlike the traditionally used homogeneous liquid or flat-plate cultures. Such bacterial habitats exhibit highly variable mechanical regimes - for example, the properties of gut mucus are subject to infection, diet, and enzymatic activity. Similarly, moisture content of soil determines its porosity. To better understand the effect of microenvironmental properties on bacterial growth dynamics in 3D, we employ jammed packings of micron-scale polyelectrolyte hydrogel granules (microgels). These packings form a porous 3D growth media that match the viscoelastic properties of their natural habitats, such as the gut mucosal layer. We isolate several bacterial strains from the guts of flour beetles and directly culture them within this 3D growth media. We observe strong substrate stiffness-dependent growth responses under 3D confinement. Combining growth measurements with 3D confocal imaging and agent-based modelling, we find that the shape anisotropy of rod-shaped bacteria allows them to grow efficiently under increased confinement as opposed to spherical bacteria.

Group formation and collective motion in form of swarms or flocks is hallmark of living systems across different length scale. The behaviour often emerges without central control and is govern by the response of individual to the action of other group members. Important features of such systems are active non-equilibrium behavior, non-reciprocal interactions, information processing, and self-steering [1-2]. We study intelligent active Brownian particles (iABPS) which are capable of steering towards desired orientation based on visual information of peers. The dynamics of ABPs is extended by an orientational response with limited maneuverability to instantaneous visual input of positions of neighbours within the vision-cone and a cut-off radius, with a preferred reorientation towards the center of mass of detected particles. We obtain several non-equilibrium structures like worms, worm-aggregate coexistence, aggregate and, dilute phase depending upon the parameters. The strength of the response to the visual signal, vision angle, packing fraction, rotational diffusion, and activity (velocity $v_0$) determine the location and extent of these phases in the phase diagram. The radius-of-gyration tensor is used to distinguish between the worm and the aggregate phase. We find aggregates for high vision angle, worm-aggregate and worms at intermediate vision angle, and dilute phase at very low vision angle. The analysis of the particle's mean-square displacement shows ABP-like dynamics for dilute systems and the worm phase. Our results help to understand the collective behavior of cognitive self-propelled particles, like animal herds and micro-robotic swarms.\\ \noindent {\bf References:} 1. R. S. Negi, R. G. Winkler, and G. Gompper, Emergent collective behavior of active Brownian particles with visual perception, Soft Matter {\bf 18}, 6167 (2022). 2. S. Goh, R. G. Winkler, and G. Gompper, Noisy pursuit and pattern formation of self-steering active particles, New J. Phys. {\bf 24}, 093039 (2022).

Morphogenesis describes the emergence of form when an organism develops from a fertilized egg or a cell takes a complex shape. Morphogenetic processes are active mechanical processes which generate shape using dynamic chemical patterns. Such systems often rely on feedback where geometry influences chemical dynamics. Here, we study active surfaces, i.e. active processes confined to a surface of complex geometry. We show that active surfaces generically exhibit geometry sensing where surface curvature guides movements. We focus on active particles embedded in a curved fluid film with complex non-isotropic curvature. We describe the physics of these active elements as localized sources of active stress and active moment that can be characterized by multipoles, reflecting their symmetry. We consider translational and rotational dynamics of the particles governed by the emergent flow field. We show that the particles move towards specific points on the surface geometry because the flow field the particles generate depends on the surface geometry. We determine the flow field for a general surface geometry in a regime of small curvature gradients. In this scenario, we find that contractility monopoles are advected towards minimal Gaussian curvature, whereas isotropic moment monopoles are advected towards minimal mean curvature. Furthermore, we find that the movement of particles with broken rotational symmetry may be controlled by modulating the coupling to the flow field. Finally, we discuss biological implications of this effective curvature sensing.

Self-phoretic particles are capable of propulsion in liquid solution by catalyzing the decomposition of chemical “fuel.” Recent studies have focused on the self-organization of self-phoretic particles into larger assemblies , i.e., so-called “machines from machines.” For instance, Nasouri et al. (JFM 2020) have shown that two spheres can form stable bound pairs for specific choices of the particle design parameters. Particle shape and topology may hold the key for achieving greater control over pairwise interactions. In particular, increasing the topological genus of one of the particles could increase pair stability and even allow for a “lock-and-key” assembly mechanism. Here, we study the pairwise interaction between a self-phoretic torus and a self-phoretic sphere. We analytically calculate the concentration field for the torus-sphere pair and discuss the resulting motion of the pair. We consider both stable configurations as well as configurations in which the sphere passes through the torus.

Living organisms frequently move through inhomogeneous environments like the gradients in heat, light, nutrients, fluid viscosity or density. They respond to these inhomogeneities by adjusting their speed and orientation, often exhibiting a directed motion termed taxis. For example, E. coli bacteria reorients to swim up the nutrient gradients but down the light or viscosity gradients. Here we introduce the concept of “densitaxis”, which is the directed motion in response to density gradients. This type of taxis is influenced by whether the organism generates thrust in front (so-called pullers) or in the back (pushers). Pullers, such as Chlamydomonas reinhardtii, reorient to swim up or down the density gradients depending on their initial orientation, while pushers like E. coli rotate to swim normal to the gradients. This newly discovered taxis could provide insight into the motion of marine organisms in oceans where density gradients are prevalent or be exploited to sort and organize a suspension of organisms.

Jyoti Sharma1*, Lapo Corti2 and Stefano Palagi1 1) The Biorobotics Institute, Scuola Superiore Sant’Anna, Pontedera (Pisa), Italy 2) University of Pisa, Pisa, Italy Contact: Jyoti.Sharma@santannapisa.it, Stefano.Palagi@santannapisa.it The self-organization of active particles has been the subject of numerous studies 1,2,3. Particles confined in circular or square/rectangular geometry have gained much attention. However, in realistic scenarios, like particles inside a membrane, confinement in curved geometry is essential 4. Hence, we investigated the role of irregular-curvature-confinement in the collective dynamics of active particles. A suitable and simple geometry for such a case happens to be the ellipse. To this end, we present simulations (in Julia5) of active Brownian particles confined by hard elliptical boundaries. The particles are simulated by hard sphere correction and with a rigid boundary. We have studied different elliptical boundaries, various density of particles, and particles of different sizes. Our preliminary results indicate the curvature-dependent organization of the particles, wherein more particles tend to be at high curvature (equators) of the ellipse. Moreover, due to high rotational diffusion, the smaller radius particles display rich dynamics patterns. Lastly, we examine particles’ dynamics in more irregular confinements. We believe our results might apply to the design of active particles-based microrobots, where environment-induced curvature of the membrane boundary could guide confined active particles’ self-organization. References: 1) “Active particles in complex and crowded environments”, Rev. Mod. Phys. 88, 045006 (2016) 2) “Steering self-organization through confinement”, Soft Matter, 19.1695-1704 (2023) 3) “Active Brownian Motion with Orientation-Dependent Motility: Theory and Experiments”, Langmuir 36, 25, 7066–7073 (2020) 4) “Active particles induce large shape deformations in giant lipid vesicles”, Nature, volume 586, pages52–56 (2020) 5) https://julialang.org/ *Presenter

Despite the importance of bacterial dynamics in anisotropic environments, very common in biology, many important questions remain unresolved. Water-soluble liquid crystals have proven to be a model system to study the influence of anisotropy on the swimming and space exploration behavior of the micro-swimmers. Recent studies have showed that bacteria are forced to follow the nematic director, inhibiting their intrinsic run and tumble exploration dynamics. We show that, in these highly anisotropic media, bacteria can reverse their swimming direction by relocating at least one flagellum on the other side of their bodies. Liquid crystals also enable the emergence of new types of collective behavior, such as the spontaneous formation of self-propelled trains of bacteria. We compare these dynamic structures to those formed by the passive self-assembly of particles in liquid crystals and investigate the mechanism enabling their self-propulsion. Finally, we show that more complex collective behaviors emerge when the liquid crystal is confined to spherical liquid crystal droplets and shells, where bacteria are forced to interact with topological defects.

Persistent swimmers, when they encounter a rigid wall, stop until they reorient. But, depending on duration of this reorientation time, other particles can arrive, blocking the first arrivers, initiating the formation of a wetting film. Using simulations, we have shown that there are three well defined phases: dry, partially wet, and totally wet, with order parameters that present singular behaviors. Analyzing different models of active particles, in lattices and in continuous space, we have shown that the corresponding phase transitions are universal, with the same exponents for all cases. When studying the dynamics for the totally wet phase, we have shown that the temporal evolution is highly non-trivial being, for example, a non-monotonous for the layer thickness. A kinetic model appropriately captures the main features, demonstrating the relevance of avalanche events. We finally argue that the wetting phases are an excellent playground to test theories for active matter in complex environments. N Sepúlveda, RS, Physical review letters 119, 078001 (2017) N Sepúlveda, RS, Physical Review E 98, 052141 (2018) M Rojas-Vega, P de Castro, RS, Physical Review E 107, 014608 (2023)

Active matter describes systems comprising elementary units able to exert non-conservative forces on their environment. Activity leads to a fascinating variety of collective behaviours unmatched in passive systems, such as the transition to collective motion. The latter is arguably the most studied phase transition in active matter and the ordered phases emerging from the interplay between self-propulsion and aligning interactions have naturally attracted a lot of attention. In this talk, I will instead focus on the role of aligning interactions in the {\it disordered phase}. In particular, I will show that nematic alignment plays an unexpected role in the ‘high-temperature’ phase: it can induce or suppress phase separation, increase particle accumulation at boundaries, and suppress demixing in systems comprising active and passive particles. I will then show how all these phenomena can be understood by introducing a field-theoretical framework to go beyond the mean-field description of the system. In the presence of nematic torques, fluctuations are then shown to {\it enhance polar order}, leading to an increase in the particle persistence length. In turn, the latter accounts quantitatively for all the phenomena reported above. To show this, I will briefly describe a new theory for motility-induced phase separation in the presence of aligning torques.

A depinning transition occurs when a system in an immobile, pinned state is driven out of equilibrium by an external force. At a critical value of the driving strength, the system depins and starts to slide. This phenomenon appears in various contexts, it governs the onset of motion for, e.g., fronts, contact lines, magnetic skyrmions, and colloidal systems driven in disordered or ordered environments; for the latter, it is closely related to phenomena of dynamic mode locking and directional locking. In contrast to passive matter, self-propelled particles perform a directed motion also in the absence of any external driving; however, the direction is randomized by rotational diffusion. An intriguing and largely open question arises, whether and how such active motion affects the depinning transition. Based on the paradigm of the active Brownian particle, which is here driven over a periodic landscape, we show that the activity not only shifts the critical point but also modifies the nature of the transition, with the exponent switching from 1/2 for passive to 3/2 for active particles. Furthermore, this active depinning transition can be accompanied by an effective diffusivity that grows without bounds. On the contrary, the passive counterpart, known as giant diffusion, remains bounded as a function of the driving force.

Inspired by the motility of micro-biological structures, the design of self-propelled systems has become crucial to the material sciences for engineering novel technologies. Studies on external fields-activated colloids have been mainly done within simple liquids; however, we have gone one step forward by using a structured material that displays liquid crystalline mesophases as the host medium. In this work, we designed two self-propelled systems at different length scales in a nematic fluid. The first one consists of solid platelets with sizes of hundreds of microns, formed after drying droplets of a light-absorbing dye. The second one comprehends Janus silica particles half-coated with titanium. Both systems are immersed in a thermotropic liquid crystal (LC), and their mobility is triggered by light. The light-absorbing materials are heated, consequently inducing a localized LC nematic-isotropic (NI) phase transition. The inhomogeneous distribution of light-absorbing spots contributes to the unevenly formed NI interface. As a result, the platelets and Janus particles move. We show the differences between both systems’ dynamics and optical responses. A discussion of the characteristics that induce mobility in each case is also presented. This research helps to unveil the micro-swimmers’ dynamics at different length scales and different geometry, immersed in a highly structured media.

We numerically study, by means of lattice Boltzmann simulations, the physics of an active fluid droplet migrating through a constriction with adhesive properties. We report evidence of a large variety of dynamic regimes and morphological features, whose properties depend upon droplet speed and elasticity, degree of confinement within the constriction and adhesiveness to the pore. Our results support the view that that non-uniform adhesion forces are instrumental in enabling the crossing through narrow orifices, in contrast to larger gaps where a careful balance between speed and elasticity is sufficient to guarantee the transition. These observations may be useful for designing novel artificial micro-swimmers of potential interest in material science, pharmaceutics, and for cell sorting in microfluidic devices.

We study a quasi 2D colloidal gel doped with Janus particles, the activity of which is switched on using light once the gel is formed, kept constant for a while and switched off again. We monitor both the structure and dynamics, before, during and after the illumination period. The mobility of the passive particles increases and exhibits a characteristic scale-dependent response to the activation. Simultaneously, the gel reorganizes, with smaller strands coalescing with larger ones and leaving larger holes in the structure. Once activity is switched off, the gel keeps the structure inherited from the active phase; however, it does not adopt the slower thermal dynamics expected for a gel with such a structure. On the contrary, the motility remains larger than that of the gel, before the active period. The system has turned into a genuine different gel, the structure of which looks like that of an older gel, but the dynamics of which is actually that of a softer gel. We checked that the above conclusions remain valid long after the activity period.

Topological defects are important for regulating the structural, optical, and rheological properties of liquid crystals. Current research interests in liquid crystal defects are motivated by their potential applications as a template to direct self-assembly and their autonomous motion capabilities in active liquid crystals. However, our understanding of their structures in driven and active systems is limited by their microscopic size and transient behavior. Therefore, simulations can provide a convenient platform onto which to elucidate their emergence, structures, and dynamics. To demonstrate, we first use molecular dynamics to examine nanoscopic structures and thermodynamics of disclinations and find that the molecular system can be well understood by continuum theories. We next combine continuum simulation and experiment to study the super-elastic properties of disclinations and their emergent structures in tumbling, lyotropic liquid crystals under a pressure-driven flow. Lastly, we predict a new symmetry breaking mechanism for self-propelling disclinations in active chiral nematics. As such, our multiscale simulations have provided a deeper understanding of topological defects in out-of-equilibrium nematic liquid crystals, which could facilitate their applications in sensing, photonics, material transport, and rheology.

Active droplets swim autonomously in viscous fluids due to the nonlinear interplay between solute transport with self-generated Marangoni flows. This mechanism is also responsible for the spontaneous propulsion of disk-shaped camphor boats on a liquid-air interface. Here, we study the collective motion of isotropic autophoretic disks representing a paradigmatic system for suspensions of active droplets and camphor boats. We conducted extensive two-dimensional particle-resolved simulations considering full hydro-chemical interactions, spanning a two-parameter space of Péclet number and area fraction. Varying the two parameters, the disk suspensions exhibit multiple emerging states: triangular lattice crystal, liquid phase, gas of clusters, and active turbulence. A narrow range of hexatic phase between the liquid and solid phases has been identified, the emergence of which is captured by our far-field scaling theory. Our simulations have reproduced a few experimental observations, including the crossing and reflecting trajectories of two active droplets, and the stationary crystalline structure formed by or turbulent motion of camphor boats.