Active Atoms and Interstitials published in Phys. Rev. Lett.

Active Atoms and Interstitials in Two-dimensional Colloidal Crystals

Active Atoms and Interstitials in Two-dimensional Colloidal Crystals
Kilian Dietrich, Giovanni Volpe, Muhammad Nasruddin Sulaiman, Damina Renggli, Ivo Buttinoni & Lucio Isa
Physical Review Letters 120(26), 268004 (2018)
DOI: 10.1103/PhysRevLett.120.268004
arXiv: 1710.08680

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.

2D-Nature of Active Brownian Motion at Interfaces published in New J. Phys.

Two-dimensional nature of the active Brownian motion of catalytic microswimmers at solid and liquid interfaces

Two-dimensional nature of the active Brownian motion of catalytic microswimmers at solid and liquid interfaces
Kilian Dietrich, Damian Renggli, Michele Zanini, Giovanni Volpe, Ivo Buttinoni & Lucio Isa
New Journal of Physics 19, 065008 (2017)
DOI: 10.1088/1367-2630/aa7126

Colloidal particles equipped with platinum patches can establish chemical gradients in H2O2-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.