Current projects : 1. Oscillating micromotors
Unlike most other types of active colloids that move at a more or less constant speed, colloidal particles containing Ag on their surfaces, when exposed to H2O2, Cl- and UV light, spontaneously oscillate between a fast episode of directional motion and a slow episode of Brownian like motion. This effect, first discovered and studied by the Sen lab since 2010 (ACS Nano 2010, 4 (8), 4845–4851), is believed to arise from an oscillating conversion on the particle surface between Ag and AgCl. The released chemical species during this conversion drives the particle via diffusiophoresis.
Inspired by the pioneering work from the Sen lab, we have embarked on a journey to explore the deeper beauty and hidden secrets of this system, by employing a polymer-Ag Janus microsphere as the model system. Compared with earlier work of pure Ag or AgCl microparticles, Janus spheres boast a much clearer interface and better controlled dynamics on both an individual and collective level. The overarching theme of our research with oscillating particles is to understand why single particles oscillate, how its oscillation changes with experimental parameters, how oscillating particles communicate and synchronize, and how their interaction leads to collective behaviors such as traveling waves and pattern formation. Finally, we use structural light to manipulate the position, speed and periods of oscillating particles, in an attempt to control them as precisely as possible.
Over the last few years, we have published a few papers on this topic, with a few more in the pipeline:
1. Zhou, C., Chen, X., Han, Z., Wang, W.*, Photochemically Excited, Pulsating Janus Colloidal Motors of Tunable Dynamics, ACS Nano, 2019, 13(4), 4064-4072 (published on March 27, 2019)
We report silver–poly(methyl methacrylate) microsphere Janus colloidal motors that moved, interacted with tracers, and exhibited negative gravitaxis all in an oscillatory fashion. Its dynamics, including pulsating speeds and magnitude, as well as whether moving forward in a pulsating or continuous mode, can be systematically modulated by varying chemical concentrations, light intensity, and the way light was applied. A qualitative mechanism is proposed to link the oscillation of Janus colloidal motors to ionic diffusiophoresis, while nonlinearity is suspected to arise from a sequence of autocatalytic decomposition of AgCl and its slow buildup in the presence of H2O2 and light. The generation of light-absorbing Ag nanoparticles is suspected to be the key. This study therefore establishes a robust model system of chemically driven, oscillatory colloidal motors with clear directionality, good tunability, and an improved mechanism, with which complex, emergent phenomena can be explored.
2. Xi Chen, Chao Zhou, Yixin Peng, Qizhang Wang and Wei Wang*, Temporal Light Modulation of Photochemically Active, Oscillating Micromotors: Dark Pulses, Mode Switching, and Controlled Clustering, ACS Appl. Mater. Interfaces 2020, 12, 10, 11843-11851 (published on Feb. 24, 2020)
Using Ag-based oscillating micromotors as a model system, a strategy is presented for the controlled modulation of their individual and collective dynamics via periodically switching illumination on and off. In particular, such temporal light modulation drives individual oscillating micromotors into a total of six regimes of distinct dynamics, as the light-toggling frequencies vary from 0 to 1000 Hz. On an ensemble level, toggling light at 5 Hz gives rise to controlled, reversible clustering of oscillating micromotors and self-assembly of tracer microspheres into colloidal crystals. A qualitative mechanism based on Ag-catalyzed decomposition of H2O2 is given to account for some, but not all, of the above observations. This study might potentially inspire more sophisticated temporal control of micromotors and the development of smart, biomimetic materials that respond to environmental stimuli that not only change in space but also in time.
3. Chao Zhou, Nobuhiko Jessis Suematsu, Yixin Peng, Qizhang Wang, Xi Chen, Yongxiang Gao, and Wei Wang*, Coordinating an Ensemble of Chemical Micromotors via Spontaneous Synchronization, ACS Nano, ASAP (published on April 9, 2020)
Spatiotemporal coordination of a nanorobot ensemble is critical for their operation in complex environments, such as tissue removal or drug delivery. Current strategies of achieving this task, however, rely heavily on sophisticated, external manipulation. We here present an alternative, biomimetic strategy by which oscillating Ag Janus micromotors spontaneously synchronize their dynamics as chemically coupled oscillators. By quantitatively tracking the kinetics at both an individual and cluster level, we find that synchronization emerges as the oscillating entities are increasingly coupled as they approach each other. In addition, the synchronized beating of a cluster of these oscillating colloids was found to be dominated by substrate electroosmosis, revealed with the help of an acoustic trapping technique. This quantitative, systematic study of synchronizing micromotors could facilitate the design of biomimetic nanorobots that spontaneously communicate and organize at micro- and nanoscales. It also serves as a model system for nonlinear active matter.
4. Chao Zhou #, Qizhang Wang #, Xianglong Lv, and Wei Wang*, Non-oscillatory Micromotors “Learn” to Oscillate On-the-fly from Oscillating Ag Micromotors, Chemical Communications, ASAP (published on April 23, 2020)
The ability to learn new functionalities on-the-fly is highly desired for micromotors operating in a changing environment. Here, we demonstrate how non-oscillatory micromotors transform into spontaneous oscillators, superficially resembling students learning from teachers, via the diffusion then deposition of Ag ions that act as a speed-booster that periodically turns on.
We are also investigating how waves emerge from the interaction of oscillating microparticles, and how waves propagate. Building upon this knowledge, we use structural light to confine oscillating particles into patterns and study the fundamental properties of waves. In addition, we see labyrinth patterns spontaneously formed by a dense population of oscillating particles, presumably because of their periodic attraction and repulsion.
Inspired by the pioneering work from the Sen lab, we have embarked on a journey to explore the deeper beauty and hidden secrets of this system, by employing a polymer-Ag Janus microsphere as the model system. Compared with earlier work of pure Ag or AgCl microparticles, Janus spheres boast a much clearer interface and better controlled dynamics on both an individual and collective level. The overarching theme of our research with oscillating particles is to understand why single particles oscillate, how its oscillation changes with experimental parameters, how oscillating particles communicate and synchronize, and how their interaction leads to collective behaviors such as traveling waves and pattern formation. Finally, we use structural light to manipulate the position, speed and periods of oscillating particles, in an attempt to control them as precisely as possible.
Over the last few years, we have published a few papers on this topic, with a few more in the pipeline:
1. Zhou, C., Chen, X., Han, Z., Wang, W.*, Photochemically Excited, Pulsating Janus Colloidal Motors of Tunable Dynamics, ACS Nano, 2019, 13(4), 4064-4072 (published on March 27, 2019)
We report silver–poly(methyl methacrylate) microsphere Janus colloidal motors that moved, interacted with tracers, and exhibited negative gravitaxis all in an oscillatory fashion. Its dynamics, including pulsating speeds and magnitude, as well as whether moving forward in a pulsating or continuous mode, can be systematically modulated by varying chemical concentrations, light intensity, and the way light was applied. A qualitative mechanism is proposed to link the oscillation of Janus colloidal motors to ionic diffusiophoresis, while nonlinearity is suspected to arise from a sequence of autocatalytic decomposition of AgCl and its slow buildup in the presence of H2O2 and light. The generation of light-absorbing Ag nanoparticles is suspected to be the key. This study therefore establishes a robust model system of chemically driven, oscillatory colloidal motors with clear directionality, good tunability, and an improved mechanism, with which complex, emergent phenomena can be explored.
2. Xi Chen, Chao Zhou, Yixin Peng, Qizhang Wang and Wei Wang*, Temporal Light Modulation of Photochemically Active, Oscillating Micromotors: Dark Pulses, Mode Switching, and Controlled Clustering, ACS Appl. Mater. Interfaces 2020, 12, 10, 11843-11851 (published on Feb. 24, 2020)
Using Ag-based oscillating micromotors as a model system, a strategy is presented for the controlled modulation of their individual and collective dynamics via periodically switching illumination on and off. In particular, such temporal light modulation drives individual oscillating micromotors into a total of six regimes of distinct dynamics, as the light-toggling frequencies vary from 0 to 1000 Hz. On an ensemble level, toggling light at 5 Hz gives rise to controlled, reversible clustering of oscillating micromotors and self-assembly of tracer microspheres into colloidal crystals. A qualitative mechanism based on Ag-catalyzed decomposition of H2O2 is given to account for some, but not all, of the above observations. This study might potentially inspire more sophisticated temporal control of micromotors and the development of smart, biomimetic materials that respond to environmental stimuli that not only change in space but also in time.
3. Chao Zhou, Nobuhiko Jessis Suematsu, Yixin Peng, Qizhang Wang, Xi Chen, Yongxiang Gao, and Wei Wang*, Coordinating an Ensemble of Chemical Micromotors via Spontaneous Synchronization, ACS Nano, ASAP (published on April 9, 2020)
Spatiotemporal coordination of a nanorobot ensemble is critical for their operation in complex environments, such as tissue removal or drug delivery. Current strategies of achieving this task, however, rely heavily on sophisticated, external manipulation. We here present an alternative, biomimetic strategy by which oscillating Ag Janus micromotors spontaneously synchronize their dynamics as chemically coupled oscillators. By quantitatively tracking the kinetics at both an individual and cluster level, we find that synchronization emerges as the oscillating entities are increasingly coupled as they approach each other. In addition, the synchronized beating of a cluster of these oscillating colloids was found to be dominated by substrate electroosmosis, revealed with the help of an acoustic trapping technique. This quantitative, systematic study of synchronizing micromotors could facilitate the design of biomimetic nanorobots that spontaneously communicate and organize at micro- and nanoscales. It also serves as a model system for nonlinear active matter.
4. Chao Zhou #, Qizhang Wang #, Xianglong Lv, and Wei Wang*, Non-oscillatory Micromotors “Learn” to Oscillate On-the-fly from Oscillating Ag Micromotors, Chemical Communications, ASAP (published on April 23, 2020)
The ability to learn new functionalities on-the-fly is highly desired for micromotors operating in a changing environment. Here, we demonstrate how non-oscillatory micromotors transform into spontaneous oscillators, superficially resembling students learning from teachers, via the diffusion then deposition of Ag ions that act as a speed-booster that periodically turns on.
We are also investigating how waves emerge from the interaction of oscillating microparticles, and how waves propagate. Building upon this knowledge, we use structural light to confine oscillating particles into patterns and study the fundamental properties of waves. In addition, we see labyrinth patterns spontaneously formed by a dense population of oscillating particles, presumably because of their periodic attraction and repulsion.