Andreas P. Bregulla

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. The particle size dependence of the photophoretic propulsion velocity reveals that photon nudging provides an increased position accuracy for decreasing particle radius. The explored steering mechanism is suitable for navigation in complex biological environments and in-depth studies of collective swimming effects.

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. This new type of thermophoretic microbeaker even provides control over a well-defined number of single molecules and is scalable to large arrays of trapping structures.

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. From the measured slip velocity we deduce the thermo-osmotic coefficient for both bare glass and Pluronic F-127 covered surfaces. The value for Pluronic F-127 agrees well with Soret data for polyethylene glycol, whereas that for glass differs from literature values and indicates the complex boundary layer thermodynamics of glass-water interfaces.

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. A Markovian minimal model elucidates the underlying nonequilibrium physics.

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 swimmers 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. The main results are: the on- and off-time distributions for the controlling laser, the arrival time statistics for the swimmer to reach a remote target, and how the experimentally accessible control parameters influence the control, e.g., the optimal acceptance angle for directed transport. The results are general in that they are independent of the propulsion or the actuation mechanisms. They provide a concrete physical picture for how a single artificial micro-swimmer could be navigated under thermal fluctuations-insights that could also be useful for understanding biological micro-swimmers.

Polarization of thermophoretic swimmers in external temperature fields

We study the motion of a Janus particle in an inhomogeneous external temperature field generated by an optically heated gold nanoparticle. The Janus particle consists of a polystyrene particle covered on one hemisphere with a 50 nm gold film. The Janus particle is held in the vicinity of the immobilized gold nanoparticle by photon nudging, which actively propels the Janus particle towards a target. Close to the heat source, the propulsion is switched off. We find an angle dependent repulsion of the particle from the heat source. Further, an angular velocity of the Janus particle is measured, which results in an active polarization of the Janus particle in the temperature field.

Induvidually tunable Micromachines driven by laser induced self propelled thermophoresis

Trapping and steering of single photophoretically self-propelled Janus type particles using a feedback mechanism is experimentally demonstrated. The mechanism provides increased localization accuracy for decreasing particle radius and allows further the control of multiple particles independently

Photophoretic Trapping and Steering of Individual and Multiple Janus Particles

Trapping and steering of single photophoretically self-propelled Janus type particles using a feedback mechanism is experimentally demonstrated. The mechanism provides increased localization accuracy for decreasing particle radius and allows further the control of multiple particles independently.