Dynamically stable optical trapping of thermophoretically active Janus colloids

The ability to optically trap and manipulate artificial microswimmers such as active Janus particles (JPs) provides a breakthrough in active matter research and applications. However, it presents significant challenges because of the asymmetry in the optical properties of JPs and remains incomprehensible. Illustrating the interplay between optical and thermophoretic forces, we demonstrate dynamically stable optical trapping of Pt-silica JPs, where the force-balanced position evolves spontaneously within a localized volume around the focal point and in a vertically shifted annular confinement at low and high laser powers, respectively. Intriguingly, the orientational and orbital dynamics of JP remain strongly coupled in the delocalized confinement. Furthermore, we demonstrate simultaneous optical trapping of multiple JPs. This first report on thermophoresis of Pt-silica JPs and localized-to-delocalized crossover in the position distributions of an optically trapped active JP, verifying theoretical predictions, advances our understanding on confined active matter and their experimental realizations.

💡 Research Summary

In this work the authors investigate the optical trapping of half‑platinum‑coated silica Janus colloids (diameter ≈ 1.76 µm) using a tightly focused 1064 nm linearly polarized laser beam (NA = 1.4). The study focuses on how the interplay between optical forces (gradient Fg and scattering Fs) and thermophoretic force (Ft) generated by laser‑induced heating of the Pt cap determines the particle’s confinement.

First, the optical response of the Janus particle is analyzed. The transparent silica hemisphere experiences mainly a gradient force proportional to the intensity gradient, which pulls the particle toward the focal point. The Pt‑coated side reflects about 34 % and absorbs roughly 45 % of the incident light, producing a substantial scattering force and, more importantly, local heating. Consequently, the total optical force depends on both the particle’s position and its orientation vector n̂.

Second, the thermophoretic effect is quantified. Fluorescent thermometry with Rhodamine B shows a temperature rise on the Pt side under illumination. By tracking particle trajectories in the diverging part of the beam, the authors extract a propulsion speed V that scales linearly with laser power P above a threshold (~2 mW). The direction of motion is opposite to the Pt side (negative phototaxis), indicating a positive thermodiffusion coefficient D_th. The thermophoretic force is obtained from V via Stokes’ law (Ft = 6π η a V).

Third, the core discovery is the concept of “dynamically stable trapping.” Because Ft, Fg and Fs all vary with the instantaneous orientation, there is no single static equilibrium point. Instead, for any given orientation a set of instantaneous force‑balanced positions satisfies Fg + Fs + Ft = 0. The particle, undergoing rotational diffusion, continuously explores these points while remaining confined. At low laser powers the thermophoretic contribution is weak; the optical forces dominate and the particle stays in a three‑dimensional region around the focal point. Position histograms are single‑peaked and the effective potential U_eff(x) is well described by a harmonic (quadratic) form with an effective stiffness k_eff that decreases as P increases.

When the laser power is raised, Ft grows faster than the optical forces. The particle is expelled from the focal region until it reaches a radial distance where Ft is balanced by the combined optical forces. This balance occurs at a lower axial (z) plane, producing an annular confinement. The particle orbits in a ring while its Pt side continuously points outward, establishing a strong coupling between its orbital motion and orientation—a stochastic spin‑orbit coupling. Position histograms become bimodal, and U_eff(x) fits a quartic function with two minima, reflecting the annular potential.

The authors also demonstrate simultaneous trapping of multiple Janus particles. Each particle occupies a distinct annular region, showing that the delocalized confinement is robust against inter‑particle interactions.

To rationalize the observations, a simplified active Brownian particle model with a harmonic trap (HBABP) is employed. The Langevin equations include a linear restoring term (characterized by τ_k) and a constant propulsion speed V, both scaled linearly with laser power. Although the model does not capture the full orientation‑dependent optical scattering, it reproduces the transition from localized to delocalized steady‑state distributions, the shape of the effective potentials, and the autocorrelation functions of particle positions.

Overall, the paper provides the first experimental evidence of thermophoresis in Pt‑coated Janus colloids, validates theoretical predictions of a crossover from Boltzmann‑like localized confinement to a bimodal delocalized state in a harmonically bound active particle, and uncovers a stochastic spin‑orbit coupling in the delocalized regime. The work expands the toolbox for manipulating non‑spherical, self‑propelled microswimmers with optical tweezers and opens avenues for studying collective behavior, targeted delivery, and micro‑robotics using thermophoretically active particles.