Abstract
The mesoscopic collective behavior of a thermophoretic-type active particle suspension under a spatial–temporal modulated excitation field is experimentally studied. By using a digital light processing chip, a sinusoidal-like spatial–temporal varying laser pattern (intensity wave) is projected on the particle suspension to modulate the particle propulsion strength through the thermophoretic effect and to exert spatial confinement through the optical trapping effect. For static and slow (wave speed ≪ particle propulsion speed) modulations, in contrast to the passive particles trapped around the wave crest, the self-propulsion strength splits the particle distribution into two branches adjacent to the edges of confinement similar to the bacteria system, and the profile of particle distribution moves coherently with the modulation wave. Once the wave speed increases to the order of the particle propulsion speed, the splitting of particle distribution disappears. Furthermore, it is remarkable that the forward propagating modulation-induced certain fraction of backward streaming particles are observed. This is a generic behavior of a self-propelling active particle suspension, which is not found in the bacteria system. This finding should be useful for particle manipulation at the mesoscopic scale.
Highlights
The suspension of artificial active particles is a good platform to mimic the bacteria system for understanding the generic collective behaviors,1,2 and a good experimental system bridging the equilibrium and non-equilibrium states by controlling the particle propulsion strength.3–14 It is very interesting that in the homogeneous background, with increasing particle self-propulsion strength, the active particle suspension can self-organize to far-from-equilibrium heterogeneous structures with the coexistence of fast propelling isolated particles and the less mobile jamming clusters, the so-called motility-induced phaseseparation (MIPS)
By using a digital light processing (DLP) chip to generate a sinusoidal-like laser pattern for exerting spatial–temporal varying confinements, we experimentally investigate the mesoscopic scale collective behavior of a thermophoretic-type active particle suspension
The propulsion free diffusivity (D0) is first scitation.org/journal/adv measured from particle mean square displacement (MSD) according to ⟨ΔR2⟩ = 4D0Δt
Summary
The suspension of artificial active particles is a good platform to mimic the bacteria system for understanding the generic collective behaviors, and a good experimental system bridging the equilibrium and non-equilibrium states by controlling the particle propulsion strength. It is very interesting that in the homogeneous background, with increasing particle self-propulsion strength, the active particle suspension can self-organize to far-from-equilibrium heterogeneous structures with the coexistence of fast propelling isolated particles and the less mobile jamming clusters, the so-called motility-induced phaseseparation (MIPS). It is very interesting that in the homogeneous background, with increasing particle self-propulsion strength, the active particle suspension can self-organize to far-from-equilibrium heterogeneous structures with the coexistence of fast propelling isolated particles and the less mobile jamming clusters, the so-called motility-induced phaseseparation (MIPS). This is a general behavior of active particles and has been widely demonstrated in many theoretical and experimental studies. The boundary-induced active particle accumulation, obstacle array-induced particle motility sorting, cone-shaped obstacle capturing active particles, harmonic trap-induced climbing-orbiting motion, and even confinement speeding up particle propulsion are some interesting and unique behaviors of an active particle suspension under the interplay between the particle self-propulsion and the boundary or obstacle. Some extended studies of active nematic liquid crystal systems with elongated particle shape-induced orientational interactions exhibit unique and interesting behaviors.
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