Dynamically stable optical trapping of thermophoretically active Janus colloids

TL;DR

This work tackles the challenge of optically trapping asymmetric, thermophoretically active Pt-silica Janus particles by elucidating the interplay between optical forces ($\mathbf{F}_{\mathrm{g}}$, $\mathbf{F}_{\mathrm{s}}$) and thermophoretic force $\mathbf{F}_{\mathrm{t}}$. By focusing a linearly polarized laser at $\lambda=1064$ nm, the authors show dynamically stable confinement where force-balanced positions evolve with orientational diffusion, transitioning from localized trapping near the focal point to delocalized annular confinement as laser power increases, with the crossover reflected in the effective potential $U_{\text{eff}}(x)$. A key finding is the spin-orbit-like coupling between orientational $\phi(t)$ and orbital $\theta(t)$ dynamics in the delocalized state, indicating strong orientation–motion correlations that persist beyond the orientational diffusion timescale $\tau_{\mathrm{R}}$, supported by simulations of a harmonically confined ABP. The work further demonstrates simultaneous trapping of multiple JPs in different regions of the 3D trap, offering insights for controlled manipulation of confined active matter and potential microfluidic applications; all observations are anchored in thermophoresis of Pt-silica JPs and force-balance analysis under tight focusing.

Abstract

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.

Figures (4)

• Figure 1: Optical and thermophoretic forces experienced by Pt-silica Janus particles in a laser beam propagating along $\hat{z}$. (a-b) The generation of the gradient force $\mathbfit{F}_{\mathrm{g}}$ and scattering force $\mathbfit{F}_{\mathrm{s}}$ are pictorially explained by showing the path of a pair of typical incoming rays for a particle situated on the beam axis, below the focal point. (a) For a transparent dielectric particle, symmetric components and net $\mathbfit{F}_{\mathrm{g}}$ are exhibited; relatively weak $\mathbfit{F}_{\mathrm{s}}$ is not shown. (b) Axis-asymmetric $\mathbfit{F}_{\mathrm{g}}$ and $\mathbfit{F}_{\mathrm{s}}$ are shown for a Pt-coated (dark grey, along $\hat{n}$) silica Janus particle, whose FESEM image is exhibited in (d). (c) Schematic shows the temperature gradient $\mathbfit{\nabla} T$ (red gradient) generated across the Janus particle and resultant thermophoretic force $\mathbfit{F}_{\mathrm{t}}$. The inset shows the top view. (e) Brightfield micrograph (left) exhibits a monolayer of silica particles, where the bottom half of the surface is Pt-coated, under laser exposure (red overlay). Reduced fluorescence emission of Rhodamine B at the hotter region is shown in orange gradient (right). (f) Four typical trajectories of thermophoretically active Pt-silica Janus colloids (shown at the end points) in a defocused laser beam are shown for 100 at various laser power $P$ = 2.1 (blue), 4.1 (green), 6.3 (purple), and 8.8 (orange). (g) Corresponding MSDs, their fitting with Eq. \ref{['eq:msd-abp']}, and the crossover times ($\tau_{\mathrm{c}}$) are represented by open symbols, solid curves, and dashed vertical lines of the same color as those of the trajectories, respectively. The linear variation of propulsion speed $V$ with $P$ after a threshold value is shown in the inset.
• Figure 2: Localized (a – c) and delocalized (d – f) optical confinement of Pt-silica Janus colloids at low and high laser power, respectively. (a, d) Schematics exhibit the directions and relative strengths of the optical forces, $\mathbfit{F}_{\mathrm{g}}$ and $\mathbfit{F}_{\mathrm{s}}$, and thermophoretic force, $\mathbfit{F}_{\mathrm{t}}$, for two typical position-orientations of the Janus colloids in the laser field (orange gradient) propagating along $\hat{z}$, in each case. (b, e) A pair of typical trajectories recorded over 200 at 500 fps, with corresponding position distributions, $p(x)$ and $p(y)$, are shown, demonstrating (b) localized trapping near the focal point at lower values of $P$, and (e) delocalized confinement in annular regions at relatively higher values of $P$. (c, f) Effective potentials, $U_{\text{eff}}(x)$, experienced by the Janus particles and their fitting with (c) quadratic and (f) quartic functions are shown with open symbols and solid lines of the same color as those of the trajectories, respectively.
• Figure 3: Coupling between the orientational ($\phi(t)$) and orbital ($\theta(t)$) dynamics of an optically trapped Pt-silica Janus colloid at lower (a, c) and higher (b, d) laser powers. (a, b) Schematics exhibit typical stochastic positional and orientational evolution of a Janus particle passing over the force-balanced positions in the optical trap (Video S2, S3). $\phi$ and $\theta$ are pictorially defined in the inset. (c, d) Short parts of the time-series of $\phi(t)$ and $\theta(t)$ and their normalized cross-correlation (time-averaged over 200) are shown for the two laser powers.
• Figure 4: Simultaneous dynamically stable optical trapping of two Pt-silica Janus particles. (a) Snapshots at progressing times from Video S4 show the positions of the Janus particles in reference to the center of the trap (orange cross). The particles are tagged with green and red borders. (b) Annularly confined trajectories of the particles over 200 are exhibited on an orange gradient representing the laser intensity field. (c) Delocalized confinements, marked by white dotted circles, with typical position-orientations of simultaneously trapped Janus particles are shown schematically.