Chemically Controlled Spatiotemporal Oscillations of Colloidal Assemblies

Abstract

We report an autonomous oscillatory micromotor system in which active colloidal particles form clusters, the size of which changes periodically. The system consists of an aqueous suspension of silver orthophosphate microparticles under UV illumination, in the presence of varying concentrations of hydrogen peroxide. The colloid particles first attract each other to form clusters. After a short delay, these clusters abruptly disperse and oscillation begins, alternating between clustering and dispersion of particles. After a cluster oscillation initiates, the oscillatory wave propagates to nearby clusters and eventually all the clusters oscillate in phase‐shifted synchrony. The oscillatory behavior is governed by an electrolytic self‐diffusiophoretic mechanism which involves alternating electric fields generated by the competing reduction and oxidation of silver. The oscillation frequency is tuned by changing the concentration of hydrogen peroxide. The addition of inert silica particles to the system results in hierarchical sorting and packing of clusters. Densely packed Ag3PO4 particles form a non‐oscillating core with an oscillating shell composed largely of silica microparticles.

Coordinating an Ensemble of Chemical Micromotors via Spontaneous Synchronization

“…Tentatively, we hypothesize that such a size-dependence of the periods of oscillating clusters is related to the reaction-diffusion of certain chemicals involved in the oscillation and the recovery time for the refractory periods. For example, eq (the oxidation of Ag into AgCl) is believed to be the slow step, and the concentration of H 2 O 2 therefore regulates the period of the oscillation (as was shown in our study of an individual PMMA-AgCl oscillator) . A larger cluster consumes H 2 O 2 more and might lead to a low local concentration of H 2 O 2 that increases its period.…”

“…To reconcile this inconsistency, Altemose et al . proposed a hypothesis that the contraction or expansion of a cluster is dominated by an electroosmotic flow rather than particle phoresis . This arises because the cluster we have seen so far, and those reported in the literature, are always located very close (possibly within 1 μm) to a electrically charged glass substrate due to gravity. ,, The self-generated electric field from an active particle then induces substrate electroosmosis on top of the electrophoresis that we have described above.…”

“…A similar trend has been reported by Altemose et al . on oscillating clusters of Ag 3 PO 4 particles . Tentatively, we hypothesize that such a size-dependence of the periods of oscillating clusters is related to the reaction-diffusion of certain chemicals involved in the oscillation and the recovery time for the refractory periods.…”

“…Just like an oscillating PMMA-Ag motor, a beating cluster can also synchronize with its neighbors. This was first reported by Altemose et al ., who showed that a number of clusters of Ag 3 PO 4 microparticles scattered across the field of view oscillated in phase-shifted synchrony . However, the kinetics of synchronization as two asynchronized clusters are brought together remains unknown.…”

“…As will be eluded to later, our proposed strategy is based on the well-known principle of coupled oscillators and is preceded by pioneering studies of synchronized oscillators, ranging from mechanically coupled metronomes and crystal cavities and electrochemically coupled mercury hearts − to chemically coupled BZ solutions and gels, cardiac cells, and neurons . The particular micromotors we are interested in originated from the Sen group, who demonstrated that microparticles of silver (Ag), silver chloride (AgCl), or silver phosphate (Ag 3 PO 4 ), in the presence of certain chemicals and under illumination, form beating clusters that synchronize with each other. − These Ag-based oscillators could potentially serve as a prototype of coordinated microbots, yet their functionalities are unfortunately limited by a lack of understanding of their unique dynamics and interactions, an issue that we aim to address.…”