Active particles as mechanical probes of glassy environments

Nature Materials (2019)

Understanding the complex entanglement between structural and dynamical properties of glasses is a great scientific challenge. A powerful technique to study such relations on a microscopic scale is microrheology, where one analyzes the translational dynamics of an externally driven probe particle. Here we experimentally investigate the mechanical properties of a colloidal glassy system by measuring simultaneously the translational and the rotational dynamics of an...

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Diffusing Wave Paradox of Active Particles in Traveling Light Pulses

Nature Communications (2019)

Despite lacking a brain, living amoebae are amazingly effective organisms when navigating through complex environments, solving maze problems or even mimicking the layout of man-made transportation networks. The underlying mechanism for such targeted motion is the emission of traveling chemical waves from a few leader cells which then triggers a collective behavior within entire colonies. Since amoebae exhibit positive chemotactic behavior, one would expect them...

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Orientational and directional locking of colloidal clusters driven across periodic surfaces

Nature Physics (2019)

When particles are driven across crystalline surfaces, their trajectories do not necessarily follow the applied force but become locked to the substrate lattice directions. Such directional locking, being relevant for bottom-up nanodevice assembly and particle sorting has been intensively studied for isolated or single particles. Here we experimentally study the motion of extended colloidal clusters sliding over a periodically corrugated surface. We observe that both...

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Group formation and cohesion of active particles with visual perception-dependent motility

Science, 364, 70 (2019)

Microscopic swimmers with visual perception of group members form stable swarms

Birds, fish and bacteria often gather into groups or swarms. This so-called collective behaviour requires all group members to continuously and reciprocally adapt their movements. It can be a challenging task, however, for researchers to ascertain the specific environmental stimuli that individuals respond to within the context of their group; in addition to optical and acoustic...

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Memory-Induced Transition from a Persistent Random Walk to Circular Motion for Achiral Microswimmers

Phys. Rev. Lett. 121, 078003 (2018)

We experimentally study the motion of light-activated colloidal microswimmers in a viscoelastic fluid. We find that, in such a non-Newtonian environment, the active colloids undergo an unexpected transition from enhanced angular diffusion to persistent rotational motion above a critical propulsion speed, despite their spherical shape and stiffness. We observe that, in contrast to chiral asymmetric microswimmers, the resulting circular orbits can...

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Experimental observation of the Aubry transition in two-dimensional colloidal monolayers

Physical Review X 8, 011050 (2018)

The possibility to achieve entirely frictionless, i.e. superlubric, sliding between solids, holds enormous potential for the operation of mechanical devices. At small length scales, where mechanical contacts are well-defined, Aubry predicted a transition from a superlubric to a pinned state when the mechanical load is increased. Evidence for this intriguing Aubry transition (AT), which should occur in one dimension (1D) and at zero temperature, was recently...

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Oscillating Modes of Driven Colloids in Overdamped Systems

Nature Communications 9, 999 (2018)

Microscopic colloidal particles suspended in liquids are a prominent example of an overdamped system where viscous forces dominate over inertial effects. Frequently, colloids are used as sensitive probes, e.g., in biophysical applications from which molecular forces are inferred. The interpretation of such experiments rests on the assumption that, even when the particles are driven, the liquid remains in equilibrium. Here we experimentally demonstrate that...

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