Intermittent Motility of a Synthetic Active Particle in Changing Environments

Abstract

We experimentally investigate the dynamics of synthetic active particles composed of gravitationally bouncing, superwalking droplets confined within an annular fluid bath. Driven by a topologically pumping dual-frequency waveform, the droplets exhibit alternating active (walking) and dormant (bouncing) phases, producing intermittent azimuthal motion. Tracking individual droplets reveals pseudolaminar chaotic dynamics in the time series of particle's angular position, characterized by laminar plateaus that are interrupted by short irregular bursts of activity. Increasing the driving amplitude induces a qualitative change in the active particle's intermittent dynamics, arising from a symmetry-breaking transition in its Faraday-wave field environment: continuous SO(2)-symmetric "channelling" waves give way to discrete "trapping" patterns. These findings demonstrate how environmental symmetry and spatiotemporal structure modulate motility and intermittency in synthetic active matter.

Figures (4)

• Figure 1: Illustration of intermittent motility of the active particle. Intermittent motility showing features of pseudolaminar chaos when the particle position is shown as a function of time. The time series comprises of consecutive low-motility, laminar plateaus $x_n$ and $x_{n+1}$ with $\delta x_{n}=x_{n+1}-x_{n}$, separated by high motility, irregular bursts.
• Figure 2: Geometric characteristics of the experiments. The droplet is confined within an annular region (a) of fluid bath with inner radius of 25.5 mm, channel width of 12.5 mm, and 2-4 mm depth of siloxane with 20 cSt kinematic viscosity. A typical droplet trajectory shown in (b) for $a_{\rm rms} =2.18g$ consists of small radial vibrations and azimuthal speed variations. By contrast, for $a_{\rm rms} =2.48g$ (c) strong radial vibrations take place with shorter strides in between. The 40Hz and 80Hz sinusoidal drive components, respectively, have independent Faraday thresholds and induce (d) radial (channelling) and (e) azimuthal (trapping) Faraday wave patterns when the respective thresholds are exceeded. The environments (d) and (e) lead to behaviours observed in (b) and (c), respectively.
• Figure 3: Level detection from the recorded droplet time series showing (a) the angular position $\theta(t)$ and (b) the angular speed $\dot{\theta}(t)$ of a droplet within the circular channel as functions of time. (c) angular position in a zoomed in region of panel (a) where the pink shaded regions indicate the detected time intervals during which the angular position of the droplet is not changing, corresponding to $\dot{\theta}(t)=0$ and constant $\theta(t)$.
• Figure 4: The first return maps of $\theta$ and $\dot{\theta}$ for increasing bath acceleration. The vertical bar indicates the threshold below which intermittent channelling Faraday waves, see Fig. \ref{['fig:PLC2']}(c), emerge in the annular fluid bath. Above this threshold, the intermittent trapping Faraday waves are observed, Fig. \ref{['fig:PLC2']}(d). The angular scales are different for different values of $a_{\rm rms}$ because the free walking speed of the droplet increases with increasing $a_{\rm rms}$.