Harnessing thermal fluctuations for purposeful activities: the manipulation of single micro-swimmers by adaptive photon nudging

Abstract

A simple scheme is presented for remotely maneuvering individual microscopic swimmers by means of ondemand photo-induced actuation, where a laser gently and intermittently pushed the swimmer along its body axis (photon nudging) through a combination of radiation-pressure force and photophoretic pull.The proposed strategy utilized rotational random walks to reorient the micro-swimmer and turned on its propulsion only when the swimmer was aligned with the target location (adaptive control). A Langevin-type equation of motion was formulated, integrating these two ideas to describe the dynamics of the stochastically controlled swimmer. The strategy was examined using computer simulations and illustrated in a proof-of-principle experiment steering a gold-coated Janus micro-sphere moving in three dimensions. The physical parameters relevant to the two actuating forces under the experimental conditions were investigated theoretically and experimentally, revealing that a $7 K temperature differential on the micro-swimmer surface could generate a propelling photophoretic strength of $0.1 pN. The controllability and positioning error were discussed using both experimental data and Langevin dynamics simulations, where the latter was further used to identify two key unitless control parameters for manipulation accuracy and efficiency; they were the number of random-walk turns the swimmer experienced on the experimental timescale (the revolution number) and the photonnudge distance within the rotational diffusion time (the propulsion number). A comparison of simulation and experiment indicated that a near-optimal micron-precision motion control was achieved.

Delay-induced transport in a rocking ratchet under feedback control

“…(16). On that level, the rate of change of the system entropy is given aṡ S(t) = −d/dt ln ρ(χ(t), t)σ , with .…”

“…In the "mean-field" limit, we can thus consider the feedback force as a conventional time-dependent force entering the "mean field" version of Eq. (16), that is, γχ(t) = −V (χ) + F fc (z(t − τ D )) + 2γk B T ξ(t) . (19) From Eq.…”

“…For example, for a classical ratchet model it has been shown that the energy input (work) is smaller with closed-than with open-loop control [3]. Exploring these ideas is fostered by recent advancements of experimental techniques for single-particle manipulation and electronic transport, which is of major relevance in various areas such as microfluidics [16], biomedical engineering [17], and quantum optics [2].…”

Trochoidal trajectories of self-propelled Janus particles in a diverging laser beam

“…Typically taking the form of bifunctional Janus particles [3], such colloidal swimmers are driven by gradients of concentration and temperature that result from chemical reactions [1,4]. Several examples of light-activated colloidal swimmers have been reported whose motions are driven by thermophoresis [5][6][7][8][9]. This mode of motion recently has been analyzed theoretically [10].…”

External control strategies for self-propelled particles: Optimizing navigational efficiency in the presence of limited resources

“…With experiments and simulations, it has been demonstrated that even in the presence of rotational diffusion, autonomous microswimmers can perform a directed motion when moving along planar or patterned walls, funnels, channels, or chemical gradients [9][10][11][12][13]. Alternatively, steering of active particles can also be achieved using feedback loops which control the particle propulsion depending on its position and orientation [14,15]. Employing optical and thermophoretic forces (photon nudging), it has been shown that high spatial localization of active particles can be achieved [15,16].…”