Thermotaxis of Janus particles.

The interactions of autonomous microswimmers play an important role for the formation of collective states of motile active matter. We study them in detail for the common microswimmer-design of two-faced Janus spheres with hemispheres made from different materials. Their chemical and physical surface properties may be tailored to fine-tune their mutual attractive, repulsive or aligning behavior.

Self-Propulsion of Janus Particles near a Brush-Functionalized Substrate.

Thermophoresis is a common mechanism that can drive autonomous motion of Janus particles under the right environment. Despite recent efforts to investigate the mechanism underlying the self-propulsion of thermophoretic particles, the interaction of particles with the substrate underneath the particle has remained unclear. In this work, we explore the impact of poly(-isopropylacrylamide) (PNIPAM)-functionalized substrate with various chain lengths on the active motion of a single polystyrene particle half-coated with gold (Au-PS).

Theory for controlling individual self-propelled micro-swimmers by photon nudging II: confinement.

Photon nudging allows the manipulation and confinement of individual self-propelled micro-swimmers in 2D and 3D environments using feedback controls. Presented in this second part of a two-part contribution are theoretical models that afford the characterization for the positioning distribution associated with active localization. A derivation for the optimal nudging speed and acceptance angle is given for minimal placement uncertainty.

Theory for controlling individual self-propelled micro-swimmers by photon nudging I: directed transport.

Photon nudging is a new experimental method which enables the force-free manipulation and localization of individual self-propelled artificial micro-swimmers in fluidic environments. It uses a weak laser to stochastically and adaptively turn on and off the swimmer's propulsion when the swimmer, through rotational diffusion, points towards or away from its target, respectively. This contribution presents a theoretical framework for the statistics of both 2D and 3D controls.

Exact symmetries in the velocity fluctuations of a hot Brownian swimmer.

Symmetries constrain dynamics. We test this fundamental physical principle, experimentally and by molecular dynamics simulations, for a hot Janus swimmer operating far from thermal equilibrium. Our results establish scalar and vectorial steady-state fluctuation theorems and a thermodynamic uncertainty relation that link the fluctuating particle current to its entropy production at an effective temperature.

Thermo-Osmotic Flow in Thin Films.

We report on the first microscale observation of the velocity field imposed by a nonuniform heat content along the solid-liquid boundary. We determine both radial and vertical velocity components of this thermo-osmotic flow field by tracking single tracer nanoparticles. The measured flow profiles are compared to an approximate analytical theory and to numerical calculations.

Size dependent efficiency of photophoretic swimmers.

We investigate experimentally the efficiency of self-propelled photophoretic swimmers based on metal-coated polymer particles of different sizes. The metal hemisphere absorbs the incident laser power and converts its energy into heat, which dissipates into the environment. A phoretic surface flow arises from the temperature gradient along the particle surface and drives the particle parallel to its symmetry axis.

Single Molecules Trapped by Dynamic Inhomogeneous Temperature Fields.

We demonstrate a single molecule trapping concept that modulates the actual driving force of Brownian motion--the temperature. By spatially and temporally varying the temperature at a plasmonic nanostructure, thermodiffusive drifts are induced that are used to trap single nano-objects. A feedback controlled switching of local temperature fields allows us to confine the motion of a single DNA molecule for minutes and tailoring complex effective trapping potentials.

Stochastic localization of microswimmers by photon nudging.

Force-free trapping and steering of single photophoretically self-propelled Janus-type particles using a feedback mechanism is experimentally demonstrated. Realtime information on particle position and orientation is used to switch the self-propulsion mechanism of the particle optically. The orientational Brownian motion of the particle thereby provides the reorientation mechanism for the microswimmer.