Experimental investigation of mesoscopic heterogeneous motion of laser-activated self-propelling Janus particles in suspension

The mesoscopic collective motion of self-propelling active particle suspension is experimentally investigated. The active particles are silica micro spheres with Au hemisphere coating, and their propelling strength is activated by laser irradiation. The suspension is driven from equilibrium to near equilibrium and far from equilibrium by tuning the excitation laser intensity. By use of the long-term particle tracking technique, the time evolution of a large amount of active particles is resolvable. For low laser intensity, the suspension is driven to near equilibrium state with homogeneous superdiffusion motion. The strength of enhanced superdiffusion is monotonically related to the laser intensity. For high laser intensity, the motility-induced phase separation with the coexistence of dense cluster and very dilute individual particle are observed. It leads to highly heterogeneous dynamic with less mobile jammed cluster and fast-moving particles and subsequently suppresses the enhanced superdiffusion. Such heterogeneous dynamics is similar to many far from equilibrium systems. Finally, the degree away from equilibrium (Gaussian dynamics) triggered by propelling strength is quantified by non-Gaussian parameters.

Figure 1

The experimental setup of laser-activated micro JP suspension. (a) Enlarged view of a Janus particle. The dashed line separates the coated (darker side) and uncoated (lighter side) faces. (b) Diagram of the observation platform. (c) Sketch illustrating the laser-activated self-propulsion mechanism. Au film has a strong absorption of green laser light, which builds up a local temperature gradient from the ${\mathrm{SiO}}_2$ side to the Au side. The thermophoretic force drives the particle propelling against the temperature gradient.

Figure 2

The plot of 60-s particle trajectories for few particles. Initial positions are shifted to the origin of the plot. (a) The case without laser irradiation (Free). (b) Under 40-mW laser irradiation.

Figure 3

Self-propulsion-induced persistent motion under different laser intensities for area fraction $\varphi <1\%$, where the interparticle interaction is negligible. (a) Double-log plot of MSD verse $\tau$. The straight lines beside the curves indicating the slopes (scaling exponent $\beta$) of the MSD. (b) The plot of $\beta$ (dark opened circle) and $V$ (red opened diamond) versus laser intensity. The gray dashed line indicates the reference line of normal diffusion.

Figure 4

The self-propulsion induced persistent motion under different laser intensities for area fraction $\varphi \sim 7.5\%$, where the interparticle interaction cannot be neglected. (a) Double-log plot of MSD verse $\tau$. (b) The plot of $\beta$ verse laser intensity. The gray dashed line indicates the reference line of normal diffusion. $\beta$ shows nonmonotonic relation with laser intensity.

Figure 5

Mobility-induced mesoscopic phase separation associated with heterogeneous dynamics: The fast-propelling particles coexist with the jammed cluster. (a) A snapshot for the case under 150-mW laser irradiation. Dense clusters are formed (enclosed by dashed circle and ellipse). (b) The subsequent particle trajectories of panel (a) with 60-s exposure. Some particles are jammed locally (dashed ellipse), and some particles are still propelling persistently (the long and straight trajectories indicated by dark arrows). The jammed cluster is not always stationary. The cluster can move collectively (enclosed by dashed circle). (c) A reference case for the particle trajectories without laser irradiation. The trajectories is homogeneous and isotropic. (d) The case under 40-mW laser irradiation, where the system is slightly away from equilibrium. The trajectories are prolonged and feature less wiggle. The trajectories are still spatially homogeneous and isotropic.

Figure 6

Semilog plot of the probability distribution about the square of particle displacement, ${\left[\mathrm{\Delta }R\left(\tau \right)\right]}^2$. The straight dashed lines represent the best fit of the Gaussian distribution. The inset is the enlarged plot of the small ${\left[\mathrm{\Delta }R\left(\tau \right)\right]}^2$ regime.

Figure 7

Plot of normalized non-Gaussian parameter ($\alpha$) versus laser intensity. The monotonically relation between $\alpha$ and laser intensity has been found.