Active Matter at Surfaces and in Complex Environments

Bacteria form biofilms in porous environments with external flows such as soil or filtration systems. To understand the initial stages of biofilm formation, one needs to study the interactions between cells, the fluid and the confining geometries. Here we present an agent based numerical model for bacteria that takes into account the planktonic stage of motile cells as well as surface attachment and biofilm growth in a lattice Boltzmann fluid. In the planktonic stage we show the importance of the interplay between complex flow and cell motility when determining positions of surface attachment. In the growth stage we show the applicability of our model by investigating how external flow and biofilm stiffness determine qualitative colony morphologies as well as quantitative measurements of,e.g., permeability. Another point of importance are concerning the optimal motility strategies in a porous environment. Microrobots for, e.g. biomedical applications, need to be equipped with motility strategies that enable them to navigate through complex environments. Inspired by biological microorganisms we have investigated various motility patterns applied to agent-based active rodlike particles in silicio and have explored their capabilities to efficiently explore disordered porous environments with various porosities and mean pore sizes ranging down to the scale of the active particle. By calculating the effective diffusivity for the different patterns, we can predict the optimal one for each porous sample geometry. We find that providing the agent with the ability to sense position for a certain time and to make a decision based on the observation yields a motility pattern outperforming the biologically inspired patterns for all investigated porous samples.

The flagellar motility of bacteria is a prime example of the engineering power of evolution. It consists of an electric rotary nanomotor, an optimally crafted propeller, and a simple and robust sensing and control network. The discovery of light-driven proton pumps in bacteria adds to this toolbox of biological components a solar nano-cell, proteorhodopsin, which allows optical control of swimming speed with high spatial and temporal resolution. Using structured light patterns for rapid and precise remote control, we can establish feedback loops in which computer programs can dynamically modulate cell motility. With this approach, we can rectify and confine swimming bacteria, switch from photophobic to photophilic behavior, transport colloidal cargoes by shaping the active pressure of a bacterial bath, and program biohybrid microbots to cooperate on a common task.

Biological cells are fascinating micromachines capable of moving and performing various tasks. Simple engineered cell-mimicking systems help us not only in learning about their natural counterparts, but also in designing soft-matter microrobots capable of performing cell-like and beyond-nature activities. One interesting model is an active vesicle which combines a soft membrane with enclosed self-propelled particles (SPPs). Due to the forces exerted by SPPs on the membrane, this system exhibits a variety of different non-equilibrium shapes with tether-like protrusions and highly branched, dendritic structures. However, this system does not seem to exhibit a directed motility. An alteration to the system design through the attachment of active particles to the membrane from outside leads to the generation of directed motion. We will discuss the behaviour of active vesicles and physical mechanisms involved. Furthermore, possible strategies for their motility control will be suggested.

Broken action-reaction symmetry has been recently explored in active matter in the context of non-equilibrium phoretic interactions between catalytically active colloids and enzymes, and shown to lead formation of self-propelled active molecules that break time-reversal symmetry, oscillating active complexes that break time-translation symmetry, chiral bound-states, and active phase separation with specified stoichiometry. Non-reciprocal interactions have been found to lead to rich physical phenomena involving various forms of spontaneous symmetry-breaking in other related non-equilibrium contexts. Recent applications of non-reciprocal active matter have revealed exotic behaviour such as the appearance of effervescent travelling patterns and shape-shifting multifarious self-organization, as well as implications of the physics of non-reciprocal interactions on the origin of life.

Active matter interacts with its environment. It thereby can perform useful work and exhibit complex dynamics. Controlling active matter and its dynamic structure formation has also come into focus of current research. We use the paradigmatic system of active Brownian particles to discuss how active Szilard engines use information to perform work [1] and explore the unusual dynamic properties of an active bath using microrheology [2]. In the third part, we rely on a continuum description of active nematics and show how active turbulence can be controlled by tuning activity locally. [1] P. Malgaretti and H. Stark, Szilard engines and information-based work extraction for active systems, Phys. Rev. Lett. \textbf{129}, 228005 (2022).\\ [2] M. Knezevic, L.E. Aviles Podgurski, and H. Stark, Oscillatory active microrheology of active suspensions, Sci. Rep. \textbf{11}, 22706 (2021).

Absolute negative mobility (ANM) refers to the situation where the average velocity of a driven tracer is opposite to the direction of the driving force. This effect was evidenced in different models of nonequilibrium transport in complex environments, whose description remains effective. Here, we provide a microscopic theory for this phenomenon. We show that it emerges in the model of an active tracer particle submitted to an external force, and evolving on a discrete lattice populated with mobile passive crowders. Resorting to a decoupling approximation, we compute analytically the velocity of the tracer particle as a function of the different parameters of the system, and confront our results to numerical simulations. We determine the range of parameters where ANM can be observed, characterize the response of the environment to the displacement of the tracer, and clarify the mechanism underlying ANM and its relationship with negative differential mobility (another hallmark of driven systems far from the linear response). Reference: Absolute negative mobility of an active tracer in a crowded environment P. Rizkallah, A. Sarracino, O. Bénichou, P. Illien arXiv:2212.01216

In this talk, I will introduce experiments and simulations of a new type of droplet swimmer that is based on a mixture of polyelectrolytes and slowly solidifies at the surface of a Petri dish containing weakly acidic water. During solification, the droplet emits polymers into the surrounding solvent, leading to self-generated concentration gradients that evoke Marangoni flows and induce self-propulsion through spontaneous symmetry breaking. The emitted polymers diffuse slowly, leading to a complex environment made of long-lived "trails", with which the swimmer interacts. As time evolves, these interactions and the associated memory effects become more and more important and at some point, they induce a transition from ballistic to chiral motion (in 2D bulk), showing that an isotropic droplet in an isotropic environment can acquire chiral self-propulsion. The new swimmer can efficiently navigate through complex environments, such as mazes, and can be used e.g. for collecting uranium as we will show.

I will give examples of symmetry breakìng in epitheliums on substrates with several different topographies.

Plasmodium sporozoites are the highly motile forms of the malaria parasite that during a blood meal are injected from the salivary glands of infected mosquitos into the skin of the hosts. While they accumulate as collectives in the salivary glands of mosquitos, they proceed as single cells in the skin of the host. Their unusually high speed is based on actomyosin gliding motility, which together with the curved shape of the sporozoites leads to chiral motion. Using models for active Brownian particles with both propulsion forces and torques, we first discuss the motion trajectories of single sporozoites in two and three dimensions. We then use agent-based modelling to explain the experimentally observed formation of rotating vortices for sporozoite collectives dissected out of salivary glands. We find that the mechanical flexibility of the sporozoites leads to breathing motion of the rotating vortices. This suggests that also single sporozoites use elasticity to mechanically interact with their host environment.

Understanding the way motile micro-organisms explore their environment is central to many ecological, medical and biotechnological questions. Here, I will present some recent advances on the actual spatial exploration processes undertaken by flagellated bacteria such as E.coli, undergoing a run and tumble exploration process, eventually leading to a random-walk. The extreme sensitivity of the motor rotation switch (CCW/CW) to the presence of a phosphorylated protein (CheYP) in its vicinity, leads to a behavioral variability of run-times, characterized by a large distribution in run-times and memory effects. We show that this internal mechanism has important consequences on the residence times at surfaces as well as the large scale transport and dispersion properties in confined environments.

In this talk, we present a simulation study of self-propelled hard spheres, for which we find that with increasing activity the relaxation dynamics can be sped up by orders of magnitude [1]. As a consequence, the glass transition shifts to higher packing fractions upon increasing the activity, allowing the study of sphere packings with fluid-like dynamics close to random close packing. We apply these findings to devitrify glassy systems consisting of mixtures of active and passive hard spheres [2]. We show that the crystallization of hard-sphere glasses can be dramatically promoted by doping the system with small amounts of active particles. Our results suggest a novel way of fabricating crystalline materials from (colloidal) glasses. This is particularly important for materials that get easily kinetically trapped in glassy states, and the crystal nucleation hardly occurs. In addition, we show that grain boundaries can be removed in a polycrystalline material by the addition of active particles [3,4]. [1] R. Ni, M.A. Cohen-Stuart, M. Dijkstra, Nature Communications 4, 2704 (2013). [2] R. Ni, M.A. Cohen Stuart, M. Dijkstra and P.G. Bolhuis, Soft Matter 10, 6609 (2014). [3] B. van der Meer, L. Filion, and M. Dijkstra, Soft Matter 12, 3406 (2016). [4] B. van der Meer, M. Dijkstra and L. Filion, Soft Matter 12, 5630-5635 (2016).

The spontaneous formation of collective states in animal groups is often explained in terms of social interaction rules where individuals adjust their behaviors in accordance with their neighbors. Opposed to this, it has been suggested that a selfish motivation where individuals try to gain an advantage compared to their peers may also lead to collective states. In my talk, I will compare both approaches using experiments and numerical simulations and demonstrate that selfishness may even lead to fair distributions within the group. S. Monter, V.-L. Heuthe, E. Panizon, C. Bechinger J. Theor. Biol. 562, 111433 (2023).

Xiangzun Wang1 , Pin-Chuan Chen2, Klaus Kroy2, Viktor Holubec3 and Frank Cichos1 1 Peter Debye Institute for Soft Matter Physics, Leipzig University, Leipzig, Germany 2 Institute for Theoretical Physics, Leipzig University, Leipzig, Germany 3 Department of Macromolecular Physics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic The ability of living systems to perceive signals, process information, and actively respond yields emerging collective dynamical states, which are inaccessible to inanimate matter controlled by time-local physical forces. The complexity of many signaling processes requires to provide effective descriptions that are commonly heavily coarse-grained neglecting internal degrees of freedom. Such effective descriptions may require the introduction of time delays or retardations, which makes interaction non-local in time. We report on experiments with self-thermophoretic active microparticles, which are equipped with delayed attraction to a target using a computer controlled optical feedback mechanism. We observe a spontaneous symmetry breaking to a transiently chiral dynamical state and concomitant critical behavior that does not rely on many-particle cooperativity. The observed bifurcation is controlled by the propulsion speed and the delay. Ensembles of swimmers follow the single particle dynamics but synchronize due to steric and hydrodynamic interactions and yield complex dynamical phases. This demonstrates that even most simple retarded non-reciprocal interactions can foster emergent complex adaptive behavior in relatively small ensembles.

In a first part, I will present our study on active colloids and show how wall-enhanced polarity makes active particles climb up. For that, we characterize experimentally a sediment of self-propelled active colloids in contact with a wall. Although the passive system does not wet the wall, we observe a particle-thick wetting layer that rises against gravity with increasing self-propulsion. Using Brownian dynamics simulations, we reproduce this phenomenon and disentangle the respective effects of activity-dependent adhesion that is specific to our system (i.e. phoretic or hydrodynamic) and of wall accumulation generic to any self-propelled particle system. We also find that wall alignment plays a complex role, either decreasing or increasing the height of the wetting layer depending on the absence or presence of wall adhesion. In a second part, I will present our studies on the dynamics of interfacial swimmers, self-propelled objects lying between two fluid phases, and more precisely, the spontaneous motion of camphor particles at a liquid-gas interface.

Biological microswimmers such as bacteria and sperm cells often encounter complex biological fluid environments. Here we use the well-known squirmer microswimmer model to show the importance of the local fluid microstructure and non-continuum effects which determine the swimming speed of squirmers in different polymeric and filamentous fluids [1]. Surprisingly, we find that different squirmer types move at considerably different speed in filamentous fluids which cannot be explained by existing continuum models, but by considering the local fluid and polymer properties around the squirmers. Furthermore, neglecting hydrodynamic flow fields by using a simple Active Brownian Particle model strongly decreases the speed compared to squirmers. [1] A. Zöttl, Dynamics of squirmers in explicitly modeled polymer solutions, under review (EPL), (2023).

Microscale active systems such as swarms of swimming bacteria and cell tissues demonstrate fascinating dynamics that can potentially be used in applications ranging from micro-robotics to regenerative medicine. Control of this dynamics in isotropic media such as water is difficult. We describe an approach in which instead of an isotropic medium, the dynamics of micro-organisms is guided by a liquid crystal. An example with a droplet of an active bacterial suspension shows an immediate benefit of such a replacement: when placed in an isotropic fluid, the droplet experiences random Brownian motion, but once the medium becomes a nematic liquid crystal, the droplet acquires an ability to swim unidirectionally along a prescribed trajectory. Other examples of liquid crystal control over the dynamics of microscopic objects include shear-alignment of a living liquid crystal representing a dispersion of swimming bacteria in a lyotropic liquid crystal. Director gradients and topological defects impact the biological microstructures most strongly, causing spatial variation of bacterial orientation and concentration. The physical mechanisms are shaped by the nontrivial effect of the orientational order of a liquid crystal on the interactions of dynamic active units. The work is supported by NSF grants DMS-2106675 and DMR-1905053.

We consider the dynamics of active nematics droplets on a planar surface, based on the continuum hydrodynamic theory. We investigate a wide range of dynamical regimes as a function of the activity, droplet size, surface anchoring and contact angle of the passive system. The activity was found to control a wide variety of dynamical regimes, including self-propulsion of droplets on surfaces, scission, active wetting and droplet evaporation. Furthermore, we found that these regimes may be controlled by the active capillary number introduced for wet systems, analogous to an active bond number for dry systems, providing an overarching description of the dynamics of active nematics. We also found that the nematic order parameter in the droplets varies with the activity, affecting the wetting behaviour weakly but ultimately driving the evaporation of the droplets. Our analysis provides an overview of the striking dynamical regimes reported for active droplets, ranging from self-propulsion to droplet evaporation and gives a unified description of active wetting for dry and wet systems.

Active Brownian particles (ABPs) accumulate at boundaries that confine them owing to their persistent self-propulsion. In this talk I will discuss several examples of boundary accumulation and its impact on active matter behavior. For example, two parallel plates immersed in a bath of active particles can experience an attractive force, known either as the Casimir effect or active depletion, because the plate separation limits the extent of the ABP’s random walk. In a pressure-driven flow in a channel, active particles can swim upstream owing to the confining effect of the channel coupled with the vorticity of the fluid motion. Finally, when active particles cross a boundary in which the resistivity changes, the direction of motion is refracted in a manner akin to ray optics and follows a variant of Snell’s law, allowing one to design lenses to control and focus active particles.

Active drops are synthetic, isotropic, micron-sized “swimmers” that emit or absorb chemical solutes. Solutal gradients drive interfacial flows and the solute’s own convective transport. If solute diffusion is small, the nonlinear coupling of the fluid flow and solute transport around the drop can cause a spontaneous symmetry-breaking, leading to sustained interfacial flows and “swimming” of the drop. Active drops are typically not neutrally-buoyant and evolve at small finite distances from rigid boundaries. Yet, existing theoretical models ignore this fundamental feature and systematically focus on unbounded flows. We bridge here this gap in understanding, to obtain a critical physical insight on the emergence of self-propulsion of active drops along a rigid wall. Specifically, by analyzing the linear stability and non-linear dynamics of the full hydro-chemical problem, we show that, and explain why, a reduction in the drop-wall separation actually promotes and enhances the drop’s self-propulsion.

Ionic and molecular selective transport is unique for the nanoscale (ion channels, nanofluidics) and not realizable at the micron scale due to the known scale-matching problem: a mismatch of the dimensions of ions and electrostatic potential screening lengths with the micron-sized confinements. In our recent work, we address this challenge and demonstrate that charged colloidal Janus micromotors can serve as a model system of ‘macro-ions’ moving in the microscopic assembly. While the motion of such spherical Janus micromotors is light activated, guidance of their motion is provided by the interaction of the particles with the complex chemical pattern possessing the surface charge distribution (see Figure, bottom left). These topographically flat charged patterns are realized on a chemically functionalized substrate containing regions of positive and negative Zeta potentials, which enable long-range (attractive and repulsive) potentials affecting the motion of charged Janus particles. Chemically flat charged patterns enable soft long-range potentials, leading to the formation of ‘soft walls’, affect the motion of the charged Janus particles, and reveal the ability of guiding, flow focusing, and 'filtering' Janus particles. The diverse behavior of Janus micromotor is based on the complex electro-osmotic flows arising between the Janus micromotors and the charged patterns. Our results deliver a new knowledge on the electro-kinetic transport of biochemical species, as well as on the control of species in fluidic samples in microscale confinement, relevant for the field of ionotronics.

Biological activity is highly concentrated on surfaces and interfaces, from ciliary arrays to sessile suspension feeders and thin layers – together they form the class of ‘active carpets’ [1]. We consider an active carpet made of actuators that generate flows at low Reynolds numbers near a planar surface, restricted to moving parallel to it. Even if the mean flow generated by the actuators is equal to zero, its variance at any one time is not. Hence, the flows can lead to ‘active fluctuations’ that induce non-Boltzmannian sedimentation profiles, with particles hovering a finite distance above the active carpets [2]. While the physics of active carpets has raised considerable interest, it remains unclear how these natural formations, collectively shape their environment beyond transport processes. Here we explore if active carpets, lying at fluid-fluid interfaces, can trigger aggregation process and fluid mixing in aquatic environments. Our results shed new light on the non-equilibrium properties of life at interfaces and new materials with active boundary conditions. Bibliography [1] Mathijssen, A. J., Guzmán-Lastra, F., Kaiser, A., & Löwen, H. (2018). Nutrient transport driven by microbial active carpets. Physical Review Letters, 121(24), 248101. [2] Guzmán-Lastra, F., Löwen, H., & Mathijssen, A. J. (2021). Active carpets drive non-equilibrium diffusion and enhanced molecular fluxes. Nature Communications, 12(1), 1906.

As an important part of micro and nanotechnology, micro and nanofabrication have received much attention from researchers in various countries and have a wide range of applications. However, the conventional micropatterning technology has prompted the need to develop novel techniques for preparing micropatterns due to its expensive process and sophisticated equipment. As a technology that can respond to external excitation and convert light energy into chemical energy and further into mechanical kinetic energy, the micro-nano pump system exhibits significant repulsion and clustering effects on colloidal particles. Therefore, the micro-nano pump system can offer a high potential in patterning structures. In this paper, we report a new method for the structured assembly of inert colloidal micro- and nanoparticles. Firstly, by preparing a photoactive substrate and then irradiating it with a structured light source, the substrate surface is activated to react. The advantage of the structured light source is that only the lighted region is excited about redox reactions, while the dark region is unresponsive. The difference based on the light spot leads to the generation of ionic or neutral substance concentration gradients on the surface of the photoactive substrate, which results in a structured assembly of inert colloidal particles subjected to diffusiophoresis and substrate electroosmotic flow. Further, we drive the targeted transport and dynamic assembly of microparticles by structurally programming the light source. In addition, the micropattern disappears immediately after the light source is turned off. Such a photoactive substrate system has good controllable, reversible assembly and disassembly properties, and compared to other techniques of photoinduced assembly, the technique is simple in the device without an auxiliary electric field, requires less light intensity from the light source, is clean without adding fuel, and has wide versatility.

Active suspensions can displace in a liquid medium in which they are suspended as a result of nonequilibrium processes, such as chemical reactions, or inhomogeneous thermal heating. These are intrinsically out of equilibrium systems, which makes them very versatile, with a natural tendency to self-assemble. Due to their small size, these out of equilibrium dynamical states generates flows that induce long range hydrodynamic interactions. These interactions have profound effects in the transport and assembly of colloidal suspensions. I will analyze the role that hydrodynamics play in the rectification mechanism that leads to their motion, as well as the possibility that hydrodynamic instabilities lead to novel estates of self-propulsion. I will discuss the impact that these mechanisms have in the emergence of patterns and different type of morhological structures in model active suspensions. I will combine theoretical simple models and computer simulations to gain insight in the role of hydrodynamics in these out of equilibrium system.

Equilibrium crystals are periodic structures formed by particles in equilibrium with the environment. As known in solid-state physics, these crystalline structures are described by collective excitations with thermal origins, the phonons. Here, we study the vibrational collective excitations of crystals that are intrinsically out of equilibrium and governed by entropy production. In particular, we focus on active crystals, consisting of self-propelled particles that take energy from the environment to produce directed motion. We discover that these crystals are described by novel vibrational excitations that we called “entropons” [1] because each of them represents a spectral contribution to entropy production. Entropons coexist with phonons but dominate over them for large activity when they are the most relevant thermodynamic modes. Entropons are at the basis of novel collective phenomena characterizing active systems at high density: even in the absence of alignment interactions, the particles of the solid locally align their velocity [2] showing collective motion and displaying spatial velocity correlations that increase with the activity [3]. The existence of entropons and spatial velocity correlations could be verified in experiments on cell monolayers at high density and active Janus colloids in crystalline structures. Here, we report experimental evidence of these phenomena by considering active granular particles, i.e. 3D-printed vibrobots that self-propel because of internal asymmetry in their body. [1] L. Caprini, U. Marini Bettolo Marconi, and A. Puglisi, H. Löwen, arXiv:2207.02369 (2022) [2] L. Caprini, U. Marini Bettolo Marconi, and A. Puglisi, Phys. Rev. Lett. 124 (7), 078001 (2020), [3] L. Caprini et al. Phys. Rev. Res. 2 (2), 023321 (2020), Soft Matter 17 (15), 4109 (2021).

Filamentous cyanobacteria can show fascinating patterns of self-organization, which however are not well-understood from a physical perspective. We investigate the motility and collective organization of colonies of these simple multicellular lifeforms. As their area density increases, linear chains of cells gliding on a substrate show a transition from an isotropic distribution to bundles of filaments arranged in a reticulate pattern. Based on our experimental observations of individual behavior and pairwise interactions, we introduce a model accounting for the filaments' large aspect ratio, fluctuations in curvature, motility, and nematic interactions. This minimal model of active filaments recapitulates the observations, and rationalizes the appearance of a characteristic lengthscale in the system, based on the Peclet number of the cyanobacteria filaments. For more details, please see arXiv:2301.11667

From embryonic development to bacterial ecology, cell populations perform chemotaxis, mostly through complex environments like tissues, mucus, and soil. How do cells migrate collectively through such disordered media? I will propose that a possible strategy are chemotactic fronts moving up self-generated chemical gradients. I will theoretically show that the stability of such chemotactic fronts to morphological perturbations is determined by limitations in the ability of individual cells to sense and respond to the chemical gradient. Specifically, I will argue that cells at bulging parts of a front are exposed to a smaller gradient, which slows them down and promotes stability, but they also respond more strongly to the gradient, which speeds them up and promotes instability. We predict that this competition leads to chemotactic fingering when sensing is limited at too low chemical concentrations. Guided by this finding and by experimental data on E. coli chemotaxis, we suggest that the cells’ sensory machinery might have evolved to avoid these limitations and ensure stable front propagation. Finally, as sensing of any stimuli is necessarily limited, the principle of sensing-induced stability may operate in other types of directed migration such as durotaxis, electrotaxis, and phototaxis.

One of the ultimate goals in the field of self-propelled nanomotors is to use them in biomedical applications treating diseases. Yet, reaching that fascinating goal is not a trivial thing and several challenges need to be addressed. First, researchers need to incorporate efficient but also bio-friendly propulsion mechanisms into the nanobots. Our strategy comprises the use of biocatalysts such enzymes for converting biologically available fuels into a propulsive force. Secondly, nanoparticles’ chassis should be generally recognized as safe (GRAS) material, biocompatible and/or biodegradable. Then, millions of nanomotors or nanoparticles are actually needed in order to treat, for instance, a tumor. This can be only achieved if the move collectively in swarms and if they transport enough therapeutic material to the target site. In my talk, I will present how we engineer swarms of enzyme nanomotors, I will introduce some factors which affect the motion of these nanomotors individually and collectively. Moreover, I will present some of the recent proof-of-concept applications of swarms in vitro and in vivo settings.

Recent years witnessed a significant interest in physical, biological and engineering properties of self-propelled particles, such as bacteria or synthetic microswimmers. One of the most striking features of interacting microswimmers is the appearance of collective motion: at densities high enough, the system is characterised by jets and vortices comprising many individual swimmers. Although many experimental and theoretical works have shown the appearance of a length-scale intrinsic to the ensuing collective flow, its precise origin is not understood. In this work, we investigate the statistical properties of self-propelling particles with hydrodynamic interactions. Starting from the kinetic theory of microswimmers, we derive a closed set of mean-field moment equations. Performing large-scale pseudo-spectral simulations, we calculate the corresponding energy spectra, and spatial correlations for various values of the mean particle density. Our results demonstrate the emergence of a typical length-scale in the collective phase and we show that it is set by the microswimmer run-length and the inter-particle distance. We discuss the physical origins of this lengthscale and how confinment affects it. We conclude by comparing against recent experiments on lengthscale selection in active turbulence.

The motion of small active particles in homogeneous Newtonian fluids has received considerable attention, with interest ranging from phoretic propulsion to biological locomotion, whereas studies on active bodies immersed in inhomogeneous fluids are comparatively scarce. In this talk I will show how the dynamics of active particles can be dramatically altered by the introduction of fluid inhomogeneity or non-Newtonian fluid rheology, and discuss the effects of spatial variations of fluid density, viscosity, and other fluid complexity in the context of biological locomotion.