We study experimentally and numerically a (quasi) two dimensional colloidal suspension of selfpropelled spherical particles. The particles are carbon-coated Janus particles, which are propelled due to diffusiophoresis in a near-critical water-lutidine mixture. At low densities, we find that the driving stabilizes small clusters. At higher densities, the suspension undergoes a phase separation into large clusters and a dilute gas phase. The same qualitative behavior is observed in simulations of a minimal model for repulsive self-propelled particles lacking any alignment interactions. The observed behavior is rationalized in terms of a dynamical instability due to the self-trapping of self-propelled particles.PACS numbers: 82.70. Dd,64.60.Cn Following our physical intuition, "agitating" a system by, e.g., increasing the temperature also increases disorder. The most simple and paradigmatic example is the Ising model of interacting spins on a lattice, which, in two or more dimensions, displays a second-order phase transition from an ordered state to a disordered state as we increase the temperature [1]. Non-equilibrium driven systems, however, may defy our intuition and show the opposite behavior: increasing the noise strength leads to the emergence of an ordered state [2,3], for example the "freezing by heating" transition of oppositely driven particles in a narrow channel [4].One class of non-equilibrium systems that currently receives considerable attention are self-propelled, or "active", particles [5][6][7][8][9][10][11][12][13]. These are model systems for "living active matter" ranging from microtubules [14] to dense bacterial solutions [15][16][17] to flocks of birds [18]. A common feature of many of these models is that the particle orientations align, which leads to a multitude of collective phenomena such as swarming [19] and even micro-bacterial turbulence [20]. This alignment interaction can be either explicit (Vicsek-type models [21]) or indirect. For example, in dense granular systems of rods [22] and disks [23], the combination of hardcore repulsion and propulsion implies an effective alignment. Somewhat surprisingly, recently it has been found that also self-propelled suspensions lacking any alignment mechanism are able to show collective behavior. Specifically, simulations of a minimal model for a suspension of repulsive disks below the freezing transition [24] show phase separation into a dense large cluster and a dilute gas phase [25,26]. Phase separation due to a densitydependent mobility has been discussed theoretically in the context of run-and-tumble bacteria [27], and a link has been made recently to self-propelled Brownian particles [28].Experimentally, active clustering of spherical colloidal particles has been observed for sedimenting, platinumcoated gold particles [10] and colloidal particles with an embedded hematite cube [13], where platinum and hematite act as catalysts for the decomposition of water peroxide. In both studies, aggregation is attributed to attractive forces. In thi...

We demonstrate with experiments and simulations how microscopic self-propelled particles navigate through environments presenting complex spatial features, which mimic the conditions inside cells, living organisms and future lab-on-a-chip devices. In particular, we show that, in the presence of periodic obstacles, microswimmers can steer even perpendicularly to an applied force. Since such behaviour is very sensitive to the details of their specific swimming style, it can be employed to develop advanced sorting, classification and dialysis techniques.

Active Brownian particles are capable of taking up energy from their environment and converting it into directed motion; examples range from chemotactic cells and bacteria to artificial micro-swimmers. We have recently demonstrated that Janus particles, i.e. gold-capped colloidal spheres, suspended in a critical binary liquid mixture perform active Brownian motion when illuminated by light. In this paper, we investigate in more detail their swimming mechanism, leading to active Brownian motion. We show that the illumination-borne heating induces a local asymmetric demixing of the binary mixture, generating a spatial chemical concentration gradient which is responsible for the particle's self-diffusiophoretic motion. We study this effect as a function of the functionalization of the gold cap, the particle size and the illumination intensity: the functionalization determines what component of the binary mixture is preferentially adsorbed at the cap and the swimming direction (towards or away from the cap); the particle size determines the rotational diffusion and, therefore, the random reorientation of the particle; and the intensity tunes the strength of the heating and, therefore, of the motion. Finally, we harness this dependence of the swimming strength on the illumination intensity to investigate the behavior of a micro-swimmer in a spatial light gradient, where its swimming properties are space-dependent.

Micron-sized self-propelled (active) particles can be considered as model systems for characterizing more complex biological organisms like swimming bacteria or motile cells. We produce asymmetric microswimmers by soft lithography and study their circular motion on a substrate and near channel boundaries. Our experimental observations are in full agreement with a theory of Brownian dynamics for asymmetric self-propelled particles, which couples their translational and orientational motion. [9] driving forces lead to active motion of micron-sized objects. So far, most studies have concentrated on spherical or rod-like microswimmers whose dynamics is well described by a persistent random walk with a transition from a short-time ballistic to a long-time diffusive behavior [10]. Such simple rotationally symmetric shapes, however, usually provide only a crude approximation for selfpropelling microorganisms, which are often asymmetric around their propulsion axis. Then, generically, a torque is induced that significantly perturbs the swimming path and results in a characteristic circular motion.In this Letter, we experimentally and theoretically study the motion of asymmetric self-propelled particles in a viscous liquid. We observe a pronounced circular motion whose curvature radius is independent of the propulsion strength but only depends on the shape of the swimmer. Based on the shape-dependent particle mobility matrix, we propose two coupled Langevin equations for the translational and rotational motion of the particles under an intrinsic force, which dictates the swimming velocity. The anisotropic particle shape then generates an additional velocity-dependent torque, in agreement with our measurements. Furthermore, we also investigate the motion of asymmetric particles in lateral confinement. In agreement with theoretical predictions we find either a stable sliding along the wall or a reflection, depending on the contact angle.Asymmetric L-shaped swimmers with arm lengths of 9 and 6 µm were fabricated from photoresist SU-8 by photolithography [11]. In short, a 2.5 µm thick layer of SU-8 is spin coated onto a silicon wafer, soft-baked for 80 s at 95• C and then exposed to ultraviolet light through a photo mask. After a post-exposure bake at 95• C for 140 s the entire wafer with the attached particles is coated with a 20 nm thick Au layer by thermal evaporation. When the wafer is tilted to approximately 90• relative to the evaporation source, the Au is selectively deposited at the front side of the short arms as schematically shown in Figs. 1(a),(b). Finally, the coated particles are released from the wafer by an ultrasonic bath treatment. A small amount of L-shaped particles is suspended in a homogeneous mixture of water and 2,6-lutidine at critical concentration (28.6 mass percent of lutidine), which is kept several degrees below its lower critical point (T C = 34.1• C) [12]. To confine the particle's motion to two dimensions, the suspension is contained in a sealed sample cell with 7 µm height. The particles ar...

Researchers produce tailor-made colloidal molecules from a variety of materials using a simple sequential assembly process.

Active colloids, also known as artificial microswimmers, are self-propelled micro- and nanoparticles that convert uniform sources of fuel (e.g. chemical) or uniform external driving fields (e.g. magnetic or electric) into directed motion by virtue of asymmetry in their shape or composition. These materials are currently attracting enormous scientific attention as models for out-of-equilibrium systems and with the promise to be used as micro- and nanoscale devices. However, current fabrication of active colloids is limited in the choice of available materials, geometries, and modes of motion. Here, we use sequential capillarity-assisted particle assembly (sCAPA) to link microspheres of different materials into hybrid clusters of prescribed shapes ("colloidal molecules") that can actively translate, circulate and rotate powered by asymmetric electro-hydrodynamic flows. We characterize the active motion of the clusters and highlight the range of parameters (composition and shape) that can be used to tune their trajectories. Further engineering provides active colloids that switch motion under external triggers or perform simple pick-up and transport tasks. By linking their design, realization and characterization, our findings enable and inspire both physicists and engineers to create customized active colloids to explore novel fundamental phenomena in active matter and to investigate materials and propulsion schemes that are compatible with future applications.

Colloidal particles equipped with platinum patches can establish chemical gradients in H 2 O 2 -enriched solutions and undergo self-propulsion due to local diffusiophoretic migration. In bulk (3D), this class of active particles swim in the direction of the surface heterogeneities introduced by the patches and consequently reorient with the characteristic rotational diffusion time of the colloids. In this article, we present experimental and numerical evidence that planar 2D confinements defy this simple picture. Instead, the motion of active particles both on solid substrates and at flat liquid-liquid interfaces is captured by a 2D active Brownian motion model, in which rotational and translational motion are constrained in the xy-plane. This leads to an active motion that does not follow the direction of the surface heterogeneities and to timescales of reorientation that do not match the free rotational diffusion times. Furthermore, 2D-confinement at fluid-fluid interfaces gives rise to a unique distribution of swimming velocities: the patchy colloids uptake two main orientations leading to two particle populations with velocities that differ up to one order of magnitude. Our results shed new light on the behavior of active colloids in 2D, which is of interest for modeling and applications where confinements are present. Main textSelf-propelling colloidal particles, originally inspired to mimic living microorganims, offer exciting opportunities to engineer smart active materials [1]. Amongst them, catalytic microswimmers have for instance been realized using Janus particles [2][3][4][5]. These are colloidal particles (e.g., silica spheres) equipped with a surface patch (e.g., a platinum coating) that can catalyze the chemical reaction of a 'fuel' present in the medium (e.g., H 2 O 2 decomposed into H 2 O and O 2 ), leading to an asymmetric chemical gradient around the particles and subsequent propulsion by phoretic forces [6].The magnitude of the swimming velocity for a single particle, V, is given by the local fuel concentration [2]. The direction of motion is along the asymmetry axis of the particle (i.e. the axis that links the poles of the two different surface portions of a spherical Janus particle) and reorients with a characteristic time τ set by the particle size, the solvent viscosity and thermal energy [2,7]. Importantly, in the absence of gravitational effects [8] or interactions with confinements [9][10][11], the unit vector representing the direction of motion is allowed to freely diffuse on the surface of a unit sphere, so that reorientation occurs in 3D. Therefore, the resulting selfpropelled motion can be described by a 3D active Brownian motion model [12,13].V and τ are responsible for complex phenomena including clustering [14,15], active self-assembly [16, 17] and swarming [18], and can be altered using external fields (e.g. magnetic [19] and optical [20,21]) or by modifying the swimmer's geometry [22][23][24][25]. However, this simple picture is strictly valid only for freeswimming ...

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