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 ...

We apply laser light to induce the asymmetric heating of Janus colloids adsorbed at water-oil interfaces and realize active micrometric "Marangoni surfers." The coupling of temperature and surfactant concentration gradients generates Marangoni stresses leading to self-propulsion. Particle velocities span 4 orders of magnitude, from microns/s to cm/s, depending on laser power and surfactant concentration. Experiments are rationalized by finite elements simulations, defining different propulsion regimes relative to the magnitude of the thermal and solutal Marangoni stress components.

We study experimentally and numerically the motion of a self-phoretic active particle in two-dimensional loosely packed colloidal crystals at fluid interfaces. Two scenarios emerge depending on the interactions between the active particle and the lattice: the active particle either navigates throughout the crystal as an interstitial or is part of the lattice and behaves as an active atom. Active interstitials undergo a run-and-tumble-like motion, with the passive colloids of the crystal acting as tumbling sites. Instead, active atoms exhibit an intermittent motion, stemming from the interplay between the periodic potential landscape of the passive crystal and the particle's self-propulsion. Our results constitute the first step towards the realization of non-close-packed crystalline phases with internal activity.

Total internal reflection microscopy (TIRM) is a well-known technique to measure weak forces between colloidal particles suspended in a liquid and a solid surface by using evanescent light scattering. In contrast to typical TIRM experiments, which are carried out at liquid-solid interfaces, here we extend this method to liquid-liquid interfaces. Exemplarily, we demonstrate this concept by investigating the interactions of micrometer-sized polystyrene particles and oil droplets near a flat water-oil interface for different concentrations of added salt and ionic surfactant (SDS). We find that the interaction is well described by the superposition of van der Waals and double layer forces. Interestingly, the interaction potentials are, within the SDS concentration range studied here, rather independent of the surfactant concentration, which suggests a delicate counter play of different interactions at the oil-water interface and provides interesting insights into the mechanisms relevant for the stability of emulsions.

An RF body coil compatible with particle therapy was built for a clinical MR scanner at 1.5T with a rotatable patient capsule. The attenuation of 1H+ and 12C6+ ions due to inelastic scattering was calculated for different materials to estimate the detrimental effects of the RF coil on the particle beam. The imaging capabilities could be demonstrated with phantom measurements at different flip angles, and corresponding transmit and receive characteristics were analyzed and compared to electromagnetic field simulations for both a horizontal and a tilted position of a phantom.