Active particles are capable of self-propelling by consuming energy from the environment. [1] The particles may be living, such as bacteria, or synthetic, such as bimetallic rods or spherical Janus particles. Due to persistent energy input, active materials are examples of out-of-equilibrium systems. They exhibit variety of intriguing phenomena such as the onset of collective behavior, [2,3] reduction of effective viscosity, [4-6] extraction of useful energy, [7-9] and enhanced mixing. [10-12] Typically, active microswimmers show a preferred orientation which determines the self-propulsion direction. Distribution of active particle orientation may have a significant impact on the macroscopic properties of the active material. It was shown in refs. [13-15] that reduction of effective viscosity in the suspension of active microswimmers, exemplified by bacteria, may be explained by a specific form of orientational distribution with respect to the background shear flow. In refs. [16,17] authors showed how the orientation of active microswimmers in the background shear flow leads to the formation of depletion regions, where particles' number density is significantly lower than the average value. Chemically-driven synthetic microswimmers, mimicking motility of living microorganisms , were first introduced by Paxton et al. [18] Since then the repertoire of synthetic microswimmers, as well as mechanisms which can be used to activate their self-propulsion, have significantly expanded. [1] The importance of synthetic microswimmers is twofold. On the one hand, their development leads to a variety of potential applications, for example in medicine [19,20] and materials science. [21] On the other hand, their study sheds new light on fundamental motility mechanisms and biological self-organization.

Suspensions of motile bacteria or synthetic microswimmers, termed active matter, manifest a remarkable propensity for self-organization, and formation of large-scale coherent structures. Most active matter research deals with almost homogeneous in space systems and little is known about the dynamics of strongly heterogeneous active matter. Here we report on experimental and theoretical studies on the expansion of highly concentrated bacterial droplets into an ambient bacteria-free fluid. The droplet is formed beneath a rapidly rotating solid macroscopic particle inserted in the suspension. We observe vigorous instability of the droplet reminiscent of a violent explosion. The phenomenon is explained in terms of continuum first-principle theory based on the swim pressure concept. Our findings provide insights into the dynamics of active matter with strong density gradients and significantly expand the scope of experimental and analytic tools for control and manipulation of active systems.

Synthetic self‐propelled nano and microparticles have a growing appeal for targeted drug delivery, collective functionality, and manipulation at the nanoscale. However, it is challenging to control their positions and orientations under confinement, e.g., in microchannels, nozzles, and microcapillaries. This study reports on the synergistic effect of acoustic and flow‐induced focusing in microfluidic nozzles. In a microchannel with a nozzle, the balance between the acoustophoretic forces and the fluid drag due to streaming flows generated by the acoustic field controls the microparticle's dynamics. This study manipulates the positions and orientations of dispersed particles and dense clusters inside the channel at a fixed frequency by tuning the acoustic intensity. The main findings are: first, this study successfully manipulates the positions and orientations of individual particles and dense clusters inside the channel at a fixed frequency by tuning the acoustic intensity. Second, when an external flow is applied, the acoustic field separates and selectively extrudes shape‐anisotropic passive particles and self‐propelled active nanorods. Finally, the observed phenomena are explained by multiphysics finite‐element modeling. The results shed light on the control and extrusion of active particles in confined geometries and enable applications for acoustic cargo (e.g., drug) delivery, particle injection, and additive manufacturing via printed self‐propelled active particles.