Symposium NM02 : Active Colloids with Order

We present a class of electro-active colloidal composite based on polymer-encapsulated and polymer-stabilized blue phase liquid crystal (PE-PSBPLC) droplets. Encapsulated BPLC droplets are formed in a block copolymer shell via a microfluidic device or emulsification formed by mechanical shearing of a polyvinyl alcohol solution. The encapsulated BPLC in the droplets are stabilized via the polymerization of reactive monomers to extend the BP temperature range. Polymer-stabilized droplets display widened blue phase temperature range from 53°C to below 0°C. The effects of fabrication process and composition on droplet formation and electro-optical behavior as well as the morphological properties of these droplets are reported.

The laws of equilibrium statistical mechanics impose severe constraints on the properties of conventional materials assembled from inanimate building blocks. Consequently, such materials cannot exhibit autonomous motion or perform macroscopic work. Inspired by the remarkable properties of the biological cytoskeleton which is driven away from equilibrium by a conserved set of protein nanomachines, our goal is to develop soft activematerials assembled from the bottom-up, using animate energy-consuming building blocks such as molecular motors and microtubule filaments. Released from the constraints of the equilibrium, these internally driven gels, liquid crystals and emulsions are able to change-shape, crawl, flow, swim, and exert forces on their boundaries to produce macroscopic work. In particular, we describe properties of an active isotropic fluid that upon confinement, transitions from a quiescent to a spontaneously flowing state. We characterize the properties of the emergent self-organized flows as well as how the transition to a flowing state depends on the properties of the confining geometry. Our results illustrate how active matter can serve as a platform for testing theoretical models of non-equilibrium statistical mechanics, developing a new class of soft machines and potentially even shedding light on self-organization processes occurring in living cells.

We will discuss our recent results with active nematics on toroidal surfaces and show how, despite the intrinsic activity and out-of-equilibrium character of our system, we still observe remnants of the expected curvature-induced defect unbinding prediceted for regular nematics. In our experiments, however, the number of defects is far larger than what one would expect for regular nematics. In addition, these defects move throughout the toroidal surface and explore "phase space", bringing about interesting analogies with what we could call the high-temperature limit of regular nematic liquid crystals. By comparing the experiments with numerical simulations, we unravel the role of activity and perform defect microrheology, which enables us to extract the material properties of the active nematic liquid crystal.

Stimuli-responsive liquid crystal elastomers (LCEs) with a strong coupling of orientational molecular order and rubber-like elasticity, show a great potential as working elements in soft robotics, sensing, transport and propulsion systems. We demonstrate a dynamic thermal control of the surface topography of LCE coatings achieved through pre-designed patterns of in-plane molecular orientation. These patterns determine whether the LCE coating develops elevations, depressions, or in-plane deformations. The deterministic dependence of the out-of-plane dynamic surface profile on the in-plane orientational pattern is explained by activation forces. These forces are caused by two factors: (i) stretching-contraction of the polymer networks driven by temperature; (ii) spatially varying orientation of the LCE. The activation force concept brings the responsive LCEs into the domain of active matter. The demonstrated relationship can be used to design programmable coatings with functionalities that mimic biological tissues such as skin. The work was supported by NSF grant DMR-1507637, the Netherlands Organization for Scientific Research (NWO; TOP PUNT grant 10018944) and the European Research Council (Vibrate ERC, grant 669991).

Assemblies of functional nanoparticles will lead to unique optical properties for potential applications in sensing, imaging, and light modulation. Liquid crystals (LCs), an active anisotropic soft matter, are sensitive to surface chemistry and surface topography. Their assemblies directed by surface cues (chemistry, topographies, and topologies) in the microscales could lead to a rich library of topological defects in nanoscale. These topological defects in turn offer new active media to trap, transport, and actuate the assembly and disassembly of nanoparticles, leading to dynamic and reversible switching of optical properties near LC phase transition temperatures. Here, by interplay of surface cues at the microscale to create topological defects and nanoscaled LC defect core and related core energy, we show dynamic tuning the (dis)assembly of gold nanorods (AuNRs) and nanoprisms in topological defects of nematic and semectic LCs near the phase transition temperature. Due to their own shape anisotropy, the nanorods can form side-to-side, end-to-end, or face-to-face assemblies. In turn, the localized surface plasmon resonance peak wavelength can be shifted more than 100 nm.

Micron scale active Janus colloids produce propulsive motion at low Reynolds numbers by employing asymmetric catalytic decomposition of dissolved ‘fuels’. Many applications have been proposed for small scale motile devices such as drug delivery within the body, and mass transport in microfluidic devices.
A current limitation to practical use is the low yielding fabrication methods employed, typically involving physical vapour deposition (PVD) to deposit a thin layer of platinum catalyst. PVD is limited by its line of sight nature to planar coatings and defects arising from colloids shadowing close contact neighbours limits the packing density of the colloids and therefore the resulting yield.
Here we describe a scalable, solution based synthesis to producing large quantities of catalytic Janus colloids by the seeding and growth of the platinum metal catalyst, utilising a Pickering emulsion technique to control the asymmetric catalyst distribution.
We compare the morphology and phoretic motion between the solution based method and the traditional PVD prepared Janus colloids. Our results indicate that the solution based synthesis produces active Janus colloids with comparable propulsive velocities and trajectories to the traditionally prepared active colloids by PVD in aqueous solutions of hydrogen peroxide and is therefore a viable synthetic route. Importantly the solution based method can be easily scaled to produce gram scale quantities of active Janus colloids.
Our results also indicate that morphological differences between the two metal deposition results in less platinum being required to produce equivalent propulsion for the solution prepared active colloids.
Finally we discuss the potential for the solution based synthesis to enable coatings of alternate metal oxide catalysts which are difficult to deposit through PVD.

In this talk, I will describe a system of catalytically self-propelled particles that self-assemble in a “contactless” manner (A. Nourhani, D. Brown, N. Pletzer, and J. G. Gibbs, Adv. Mater. (2017) – DOI: 10.1002/adma.201703910). By combining catalytic and magnetic properties, the particles are attracted to one another yet do not come into direct contact. Thus, the particles are seen to self-assemble into clusters that easily rearrange into various dynamic morphologies. New modes of motion arise in this system including predator-prey type chasing, as well as contactless cargo delivery. This system may prove to be a new paradigm for investigating collective behavior of active colloids.

We develop a model to describe the behavior of a system of active and passive particles in solution that can undergo spontaneous self-organization and self-sustained motion. The active particles are uniformly coated with a catalyst that decomposes the reagent in the surrounding fluid. The resulting variations in the fluid density give rise to a convective flow around the active particles. The generated fluid flow, in turn, drives the self-organization of both the active and passive particles into clusters that undergo self-sustained propulsion along the bottom wall of a microchamber. This propulsion continues until the reagents in the solution are consumed. Depending on the number of active and passive particles and the structure of the self-organized cluster, these assemblies can translate, spin, or remain stationary. We also illustrate a scenario where the geometry of the container is harnessed to direct the motion of a self-organized, self-propelled cluster. The findings provide guidelines for creating autonomously moving active particles, or chemical “motors” that can transport passive cargo in microfluidic devices.

The targeted design of photoactive, swimming micro-particles and the effects of fuel concentration, pH of suspending media and particle geometry were investigated. Of particular interest is that the various TiO₂ particle geometries exhibit geometry-dependent motion. The individual swimmers were made from a common and inexpensive photocatalyst TiO₂, and when exposed to both hydrogen peroxide and ultraviolet (UV) light, the colloids self-propel. Thus, the swimmers can be respectively activated and de-activated by exposure to UV light and subsequent removal. In order to achieve a particular type of desired swimming behavior, we carefully constructed particles with specified morphology and material composition. The active TiO₂ particles serve as a basis for desired attributes such as long-term stability, tunability, energy efficiency, and controllable motion for applications in hydrogen fuel cells, targeted drug delivery systems, and environmental remediation.

In this talk, I will discuss how photocatalysts can be exploited in artificial active matter at the microscale. Over the past decade, catalyzed chemical reactions have been used extensively for this purpose, particularly in the case of platinum, but photocatalysts have several advantages over traditional materials. The major benefit from using photocatalysts is that the activity can be switched on-and-off by a light stimulus. Through co-deposition of multiple matterials we can alter the band-gap and can actuate complex particles in different ways by switching the light source. Using these and other techniques, we investigate novel ways to control the motion of such active matter systems at the microscale.

In order to fully realize the potential of active colloids there is an increasing drive to exploit complex behaviour, such as collective motion and self-organisation. In the main, these phenomena have been studied theoretically to date, although with sufficient experimental attention to show the viability of observing collective phenomena such as clustering. Models and simulations have also shown that the specific details of the active colloids propulsion mechanism critically alter predicted ensemble behaviour. One particular example is the link between mechanism, hydrodynamic flow field and the resulting inter-particular interactions that can lead to re-orientation and translation. In this context, here we focus on describing the way in which experimental characterisation of catalytic Janus colloids motion can provide critical information about mechanism, which in turn will assist predicting and controlling their collective self-organising structures.

In particular, we highlight a recently developed method where the combination of video microscopy analysis of tracer particles with image analysis algorithms allows the flow field around active colloids to be experimentally determined. These experimental observations are compared with predictions from theory, providing new insights into propulsion mechanism. The flow data and mechanistic implications are discussed in the context of previous observations for catalytic Janus colloid behaviour, and in respect to informing the development of more accurate predictions for collective behaviour.

However, catalytically powered motile Janus colloids present a particularly complex scenario when considering collective effects due to the ability for interactions mediated by chemical fields to re-orientate and translate neighbouring colloids, in addition to hydrodynamics. Consequently, we also discuss how analysis of tracer particle motion can allow chemical field induced effects to be assessed in certain scenarios, and describe the potential to develop new empirical models for collective motion based on these experimental data sets.

Finally, we offer a perspective on eventual routes by which the collective behaviour of active colloids may be controlled and exploited in the future to enable new applications.

The research field of micro-motors has become ever more important especially with its myriad of applications such as environmental remediation, micro-surgeries, controllable drug delivery and self-sorting logic systems. Photocatalytic micromotors actuated with multiple photocatalysts were investigated to determine how each material’s respective band-gap can affect their behavior of motion in a solution. Composite micromotors are specifically the interest of this research, as they are not only cost effective to fabricate, but they are tunable to different wavelengths of light for activation. Glancing angle deposition (GLAD) was used to synthesize these micromotors and their behavior was compared using mean square displacement and path velocity calculations.

Hollow nanospheres are peculiar nanoparticles featured by an inner cavity [1]. They are characterized by a high specific surface, a high porosity, a high mechanical stability, and nanocontainer-type functionalities. Based on these features, hollow nanospheres are very interesting for catalysis, gas sorption, drug delivery, etc. [1].

As a straightforward access to hollow nanospheres – especially with diameters of 10 to 50 nm – we could establish a microemulsion (ME)-based synthesis strategy [1a]. In fact, MEs turned out as ideal for obtaining nanosized hollow spheres since their size directly correlates to the micelle diameter. The thermodynamical stability of MEs and the uniformity of the micelle diameter are additional assets for hollow-nanosphere synthesis [1a]. To obtain hollow nanospheres, the reactants are separately added to the polar droplet phase and to the non-polar dispersant phase.

Based on the ME-based strategy, we could prepare a wide range of hollow nanospheres (e.g., g-AlO(OH), La(OH)₃, ZnO, Fe₂O₃, SnO₂, TiO₂, ZrO₂, CuS, Cu_1.8S, Cu₂S, MgCO₃, Gd₂(CO₃)₃, Ag₂S, Au, Ag) with outer diameters of 10–50 nm, a wall thickness of 2–10 nm and an inner cavity ranging from 5 to 30 nm in diameter [1a,2,3]. The as-prepared hollow nanospheres shown unique performance in view of catalyis (e.g. CO oxidation [2]), sensing (e.g. H₂ detection [3]) and drug delivery (e.g. tuberculosis therapy or synergistic chemical/physical tumor treatment [3]). This presentation will address the ME-based synthesis as well as the properties and performance of hollow nanospheres.


References
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Artificial opaline structure exhibits unique characteristics such as hexagonally-arranged pores and interconnected pore channels. Conventional fabrication routes involve gravitational and centrifugal sedimentation, as well as solvent evaporation of microspheres into an orderly colloidal template, followed by backfilling of materials into the interstitial voids among the closely-packed microspheres, and the selective removal of colloidal template. In this work, we adopt a sequential electrophoresis process in which a dc electrophoresis is used to direct the packing of polystyrene microspheres into a colloidal crystal with significantly-reduced crystallographic defects, followed by an ac electrophoresis to drive the oxide nanoparticles into the colloidal template. Relevant processing parameters are optimized and discussed. Structural observation and material characterization are performed using SEM, EDX, XRD, and XPS.

Self-propelled nanorobots, as one of representative smart materials, attract more and more interests in the past decade for its potential application in biomedicine and environmental remediation as well as for a research model to study dynamic system. Here a hierarchical nano swimmer is fabricated in a large scale by top down etching in conjunction with bottom up synthesis, integrating n-type titanium dioxide (TiO₂) with p-type silicon (Si). The swimmer owns a sophisticated tree structure with TiO₂ nanowires as the branches and Si nanowire as the trunk. Powered by near ultraviolet light, the tree-structured swimmer could move spontaneously in dilute hydrogen peroxide solution through completing a photoelectrochemical reaction happening on separate tree surface. The self-locomotion is verified to be electrophoresis by light intensity manipulation, surface modification and other control experiments. What’s more, the tree-like swimmer could be aligned autonomously and tracking the incident light direction. The direction-sensitive nano swimmers imitates natural microorganisms capable of phototactic migration, either at individual level or at colonial level. Therefore, the nano swimmer could be well controlled externally including swimming speed and direction by regulating the incident light. Besides, efforts are also taken to address biocompatible issues, like hydrogen peroxide replaced by redox couple hydroquinone/p-benzoquinone and dye sensitization to enhance swimmer’s response to visible light. These concepts are successfully demonstrated by fuel replacement and delicately selected dyes like N719, D5 and SQ2. Furthermore, dye sensitized swimmers selectively respond to monochromic light, which could be exploited to develop a synergistic functional group. Hence the sensitization provides a promising scenario to make a multicomponent nanorobot with complex functions.

Microbot propulsion is challenging due to the reversible nature of microscale fluid transport. Recently, it has been shown that methods that break flow-field symmetry using a nearby surface can lead to net translation. Here, we demonstrate that coupling between rotating wheel-shaped bots and nearby walls can be enhanced through surface topography. In this, magnetic microwheels are assembled from individual colloidal particles via an in-plane rotating magnetic field. Upon reorientation of the magnetic field rotation plane, microwheels roll along flat walls, translating at differing velocities depending on wheel size, symmetry, and rotational frequency. We find here that frequencies associated with surface spatial periodicity during microwheel rolling on textured surfaces can lead to significant (~5x) microwheel translation velocity enhancements. These are observed when the spatial and rotational frequencies approach each other, a phenomenon that can be described with a simple hydrodynamic mobility model.

Magnetic active matter constitutes a particularly interesting class of systems, not least because of its versatility in terms of the fabrication of artificial swimmers and their control, as well as its potential for technological applications. We investigate the collective behavior of magnetic swimmers, which are suspended in a Poiseuille flow and placed under an external magnetic field, using analytical techniques and Brownian dynamics simulations. We find that the interplay between intrinsic activity, external alignment, and magnetic dipole-dipole interactions leads to longitudinal structure formation. Our work sheds light on a recent experimental observation of a clustering instability in this system.

Microswimmers show strikingly different emergent collective behavior than passive systems in equilibrium. The talk reviews some of our work, where we study how hydrodynamic flow fields and diffusiophoretic coupling via self-generated chemical fields influence the collective motion of active particles.

Using multi-particle collision dynamics, we simulate hydrodynamic flow fields generated by squirmer model swimmers. In quasi-two-dimensional geometry they show motility-induced phase separation [1,2]. However, the binodals depend on the mean squirmer density, a clear signature of the non-equilibrium. Under gravity squirmers exhibit a very dynamic sedimentation profile with dense layering at the bottom and exponential decay towards the top, where large-scale convective flow arises [3]. Finally, a single layer of squirmers under strong gravity exhibits different collective structures including "hydrodynamic Wigner crystals'' and swarming.

I also present results of Brownian dynamic simulations of active colloids interacting via self-generated chemical fields. Translational and rotational diffusiophoretic coupling gives rise to dynamic clustering, pulsating clusters, and a chemotactic collapse [4,5].

[1] A. Zöttl and H. Stark, Phys. Rev. Lett. 112, 118101 (2014).
[2] J. Blaschke, M. Maurer, K. Menon, A. Zöttl, and H. Stark, Soft Matter 12, 9821 (2016).
[3] J.-T. Kuhr, J. Blaschke, F. Rühle, and H. Stark, Soft Matter 13, 7548 (2017).
[4] O. Pohl and H. Stark, Phys. Rev. Lett. 112, 238303 (2014).
[5] O. Pohl and H. Stark, Eur. Phys. J. E 38, 93 (2015).

We discuss a 2-dimensional vibration-fluidized system of macroscopic grains as a flexible model system for the study of active matter. Grains are driven by collisions with the floor and ceiling in the vertical direction. The collisional noise is rectified to produce directional motion in the horizontal plane by anisotropic shape or other features on the surface of the particle. A particular advantage of this system is that the shape of the particle – and therefore its interactions with other particles – can be kept fixed, while the anisotropy of its motion is varied. We show two examples where this flexibility leads to qualitatively new behaviour. First, we show that the melting of a crystallite of grains in noisy conditions can be strongly affected by the interplay of activity and the symmetry of the packing. Second, we demonstrate a geometry of particle where spontaneous gear-like motion of an assembly of particles is stably generated at the walls of a system.
We gratefully acknowledge support from NSF-DMR 1506750

Living nematic is a realization of active matter combining a nematic liquid crystal with swimming bacteria. The material exhibits a remarkable tendency towards spatiotemporal self-organization manifested in the formation of dynamic textures of self-propelled half-integer topological defects (disclinations).This living nematic demonstrates the coexistence of isotropic and nematic domains, or tactoids, in a certain range of temperatures. Our computational study and dedicated experiment revealed how the tactoids control organization of living nematic by targeting singularities in molecular alignment, or topological defects. We have established that tactoid's I-N interface spontaneously acquire negative topological charge which is proportional to the tactoid's size and depends on the concentration of bacteria. The observed negative charging is attributed to the drastic difference in the mobilities of 1/2 and -1/2 topological defects in active systems.The results hint into new strategies for control of active matter.

In this talk, I will discuss some recent theoretical and experimental work on active nematic liquid crystals confined on two-dimensional curved interfaces and highlight how the geometrical and topological structure of the environment can substantially affect collective motion in active materials, leading to spectacular life-like functionalities.

Motor-proteins are responsible for transport inside cells. Harnessing their activity is key towards developing new nano-technologies or functional biomaterials. Cytoskeleton-like networks, recently tailored in vitro, result from the self-assembly of subcellular autonomous units. Taming this biological activity bottom-up may thus require molecular level alterations compromising protein integrity.

We have taken a top-down perspective consisting on tuning the anisotropic viscosity of a contacting thermotropic liquid crystal oil. We show that the seemingly chaotic flows of a tubulin-kinesin aqueous active gel with nematic-like order [1] can be forced to adopt well-defined spatial directions that correlate with the structure of the responsive oil/water interface. Different configurations of the active material are realized, when the passive liquid crystal is either unforced [2] or commanded by a magnetic field [3]. The inherent instability of the extensile active fluid (active turbulence) is thus spatially regularized, leading to organized flow patterns, endowed with characteristic length and time scales whose role is redefined under the imposed geometrical confinements. Other control strategies, based in the preparation of active nematic droplets emulsified in liquid crystals, will be discussed

There is high interest in new colloidal and polymerization methods that will go beyond the traditional emulsion polymerized systems involving lithographic and also extrusion methods. The ability to control the microstructure and composition enables new functionality different from colloids prepared in solution. The ability to hierarchically prepare and process layers mean that new materials and material order can be produced during the processing stage. This results in a more quantitative and uniform behavior for actuation, controlled drug release, peristaltic motion, and flow behavior. Herein we report the use of layer-fabrication methods from the molecular all the way to macroscopic that results in hierarchical and high throughput methods. The advantage os such control also enables the use of isotropically melt polymer layers through a polymer extrusion process. This has proven to be versatile for the preparation of colloidal particles that are capable of stimuli response. These polymer particles can be made by reactive ion etching lithography or via roll-to-roll multilayer processing. This will enable in the future the high throughput fabrication of such particles with the built-in modified properties expected.

I will first show how to engineer spontaneously flowing colloidal liquids. Simply put our strategy consists in letting self-propelled colloids with velocity-alinement interactions to collide. After a short transient they self-assemble into liquids with emergent long-range orientational order which translates into spontaneous unidirectional flows. I will then devote most of my talk to the fluctuations and dynamical response of these intrinsically non equilibrium materials. (i) I will show that both density and velocity fluctuations almost freely propagate along all directions and exploit these sound modes to infer the analogous of the Navies Stokes equation for polar active liquids. (ii) I will finally discuss the robustness of their spontaneous flows to external pressure gradients. I will evidence that (french) colloids can be collectively very resistant when one tries to waive their privilege to freely choose their direction of motion.

Aggregations of social animals are beautiful examples of self-organized behavior far from equilibrium. Understanding these systems, however, has proved to be quite challenging. Determining the rules of interaction from empirical measurements of animals is a difficult inverse problem. Thus, researchers tend to focus on the macroscopic behavior of the group instead. Because so many of these systems display large-scale ordered patterns, it has become the norm in modeling to focus on this order. Large-scale pattern alone, however, is not sufficient to characterize the dynamics of animal aggregations, and does not provide a stringent enough condition to benchmark models. Instead, I will argue that we should borrow ideas from materials characterization to describe the macroscopic state of an animal group in terms of its response to external stimuli. I will illustrate these ideas with recent experiments on swarms of the non-biting midge Chironomus riparius, where we have developed methods to apply controlled perturbations and measure the detailed swarm response. Our results allow us to begin to describe swarms in terms of state variables and response functions, bringing them into the purview of theories of active matter, and point towards new ways of characterizing and hopefully comparing collective behavior in animal groups.

Persistently moving self-propelled particles clump if they're isotropic, and flock or align if they're anisotropic. I will discuss experiments in which these tendencies occur together in the presence of obstacles that serve as traps, and offer a theoretical understanding

Active materials are composed of interacting particles individually powered by motors. In this talk, we focus on chiral active materials that violate parity and time reversal symmetry. First, we show how to generate topological sound in fluids of self-propelled particles exhibiting a spontaneous chiral active flow under confinement. These topological sound modes propagate unidirectionally, without backscattering, along either sample edges or domain walls and despite overdamped particle dynamics. Next, we discuss an exotic transport coefficient characteristic of quantum Hall fluids, called odd viscosity, which controls the hydrodynamics of classical fluids composed of active rotors. This odd viscosity couples pressure to vorticity leading to transverse flow in piston compression experiments. We envision that such transverse response may be exploited to design self-assembled hydraulic cranks that convert between linear and rotational motion in microscopic machines powered by active rotors fluids.

The propagation of waves is typically shaped through a spatial modulation of the environment. For instance, in devices achieved by repeatedly stacking layers with different properties, the propagation of waves can be forbidden over continuous frequency bandwidths, also known as bandgaps. Such structures are routinely obtained either through top-down fabrication—such as e-beam lithography—or bottom-up processes—such as self-assembly. However, these conventional materials are static and arise in thermodynamic equilibrium, which results in inherently rigid structures, thus difficult to reconfigure and very sensitive to inhomogeneity and imperfections. The observation of Nature teaches that complex materials and systems can be obtained through alternative mechanisms. Flocks of birds or schools of fish are examples of systems of high complexity that spontaneously self-organize, adapt to perturbations in their environment (e.g. predators), and collectively create synchronized motions despite important heterogeneity in their populations.

In this talk, we use Nature-inspired collective mechanisms to force the spontaneous emergence of wave devices. We report the realization of a non-equilibrium dynamic device composed of scatterers driven by a coherent field, which move along a waveguide and spontaneously organize onto a crystal-like order. Despite strong inhomogeneity in particles’ properties, the collective mechanism at play triggers the emergence of a bandgap in the transmission spectrum of the material. This work demonstrates the possibility of achieving dynamic wave materials that spontaneously organize and that are inherently robust to inhomogeneity and imperfections.

The locomotion of swimming bacteria in simple Newtonian fluids can successfully be described within the framework of low Reynolds number hydrodynamics. The presence of polymers in biofluids generally increases the viscosity, which is expected to lead to slower swimming for a constant bacterial motor torque. Surprisingly, however, several experiments have shown that bacterial speeds increase in polymeric fluids, and there is no clear understanding why. Therefore we perform extensive coarse-grained simulations of a bacterium swimming in explicitly modeled solutions of macromolecular polymers of different lengths and densities. We observe an increase of up to 60% in swimming speed with polymer density and demonstrate that this is due to a depletion of polymers in the vicinity of the bacterium leading to an effective slip. However this in itself cannot predict the large increase in swimming velocity: coupling to the chirality of the bacteria's flagella is also necessary.

Ensembles of self-propelled species have been shown to result in a variety of novel steady states and dynamic processes. We have been investigating several different systems of varying sizes and shapes including microtubule cytoskeletal filaments that glide along suraces powered by kinesin-1 motor proteins, droplets of liquid crystals that shed miscelles to propel and stir their insides, and recently individual self-propelled enzymes. These systems offer a variety of controls on the size, shape, aspect ratio, and tuneable interactions enabling us to inspect new phenomena of active systems.