Current projects: 3. ICEP micromotors
These active colloids are meta-dielectric Janus microspheres that upon applying an AC electric field move towards the dielectric hemisphere. Their directional motion is enabled by an asymmetric surface electroosmotic flow, resulting from an asymmetric electrical polarization of the two hemispheres under an electric field. This propulsion, called induced charge electrophoresis (ICEP), has been theoretically and experimentally studied for more than a decade. Active colloids moving via ICEP are easy to control in both speeds and inter-particle interactions, and exhibit interesting collective behaviors. We are particularly interested in increasing the degree of complexity in the system, by exposing ICEP swimmers to confinements, or by superimposing additional energy sources.
1. Zuyao Xiao, Shifang Duan, Pengzhao Xu, Jingqin Cui, Hepeng Zhang, and Wei Wang, Synergistic Speed Enhancement of an Electric-Photochemical Hybrid Micromotor by Tilt Rectification, ACS Nano, Accepted (published on June 12, 2020)
A hybrid micromotor is an active colloid powered by more than one power source, often exhibiting expanded functionality and controllability than those of singular energy source. However, these power sources are often applied orthogonally, leading to stacked propulsion that is just a sum of two independent mechanisms. Here, we report that TiO2-Pt Janus micromotors, when subject to both UV light and AC electric fields, moved up to 90% faster than simply adding up the speed powered by either source. This unexpected synergy between light and electric fields, we propose, arises from the fact that an electrokinetically powered TiO2-Pt micromotor moves near a substrate with a tilted Janus interface that, upon the application of an electric field, becomes rectified to be vertical to the substrate. Control experiments with magnetic fields and three types of micromotors unambiguously and quantitatively show that the tilting angle of a micromotor correlates positively with its instantaneous speed, reaching maximum at a vertical Janus interface. Such “tilting induced retardation” could affect a wide variety of chemically powered micromotors, and our findings are therefore helpful in understanding the dynamics of micromachines in confinement.
2. Dong, R#., Wang, W#., and Granick, S., Colloidal Flatlands Confronted with Urge for the Third Dimension, ACS Nano, 2019, 13(8), 9442-9448 (published on July 26, 2019)
Two-dimensional sheets are a relatively neglected form of soft matter, interesting because of their capability to deform into the third dimension with little energy cost. Here, we confront colloidal sheets with an abruptly imposed potential tending to produce strings normal to the plane. Experimentally, this is implemented first by using ultrasound-induced acoustic levitation to produce planar sheets and then by abruptly imposing AC electric fields that introduce dipolar interactions. Seeking to identify the microscopic mechanisms underlying the observed collective behavior, we find that the patterns quantified from our fast confocal experimental imaging are reproduced by our Brownian dynamics simulations. We follow the evolution of these patterns, including their structure factor, from start to final steady state, and from successful parametrization we predict simulation phases not yet observed in experiment. The transient-state evolution toward final outcome includes monocrystalline hexagonal lattice, polycrystalline body-centered tetragonal lattice with grain boundaries, interconnected rings, serpentine zigzag chains, and columns vertical to the plane, and a “fat worm” serpentine pattern. To explain the counterintuitive findings presented here, we map dependence on softness of the confining potential.
3. Zhang, L. #., Xiao, Z. #., Chen, X., Chen, J., and Wang, W.*, Confined 1D Propulsion of Metallodielectric Janus Micromotors on Microelectrodes Under Alternating Current Electric Fields, ACS Nano, 2019, 13(8), 8842-8853 (published on July 2, 2019)
Inspired by an earlier work with magnetic domains on a garnet film as predefined tracks, we present an interdigitated microelectrodes (IDE) system where, upon the application of AC electric fields, metallodielectric (e.g., SiO2–Ti) Janus particles are hydrodynamically confined and electrokinetically propelled in one dimension along the electrode center lines with tunable speeds. In addition, comoving micromotors moved in single files, while those moving in opposite directions primarily reoriented and moved past each other. At high particle densities, turbulence-like aggregates formed as many-body interactions became complicated. Furthermore, a micromotor made U-turns when approaching an electrode closure, while it gradually slowed down at the electrode opening and was collected in large piles. Labyrinth patterns made of serpentine chains of Janus particles emerged by modifying the electrode configuration. Most of these observations can be qualitatively understood by a combination of electroosmotic flows pointing inward to the electrodes, and asymmetric electrical polarization of the Janus particles under an AC electric field. Emerging from these observations is a strategy that not only powers and confines micromotors on prefabricated tracks in a contactless, on-demand manner, but is also capable of concentrating active particles at predefined locations. These features could prove useful for designing tunable tracks that steer synthetic microrobots, as well as to enable the study of single file diffusion, active turbulence, and other collective behaviors of active matters.
1. Zuyao Xiao, Shifang Duan, Pengzhao Xu, Jingqin Cui, Hepeng Zhang, and Wei Wang, Synergistic Speed Enhancement of an Electric-Photochemical Hybrid Micromotor by Tilt Rectification, ACS Nano, Accepted (published on June 12, 2020)
A hybrid micromotor is an active colloid powered by more than one power source, often exhibiting expanded functionality and controllability than those of singular energy source. However, these power sources are often applied orthogonally, leading to stacked propulsion that is just a sum of two independent mechanisms. Here, we report that TiO2-Pt Janus micromotors, when subject to both UV light and AC electric fields, moved up to 90% faster than simply adding up the speed powered by either source. This unexpected synergy between light and electric fields, we propose, arises from the fact that an electrokinetically powered TiO2-Pt micromotor moves near a substrate with a tilted Janus interface that, upon the application of an electric field, becomes rectified to be vertical to the substrate. Control experiments with magnetic fields and three types of micromotors unambiguously and quantitatively show that the tilting angle of a micromotor correlates positively with its instantaneous speed, reaching maximum at a vertical Janus interface. Such “tilting induced retardation” could affect a wide variety of chemically powered micromotors, and our findings are therefore helpful in understanding the dynamics of micromachines in confinement.
2. Dong, R#., Wang, W#., and Granick, S., Colloidal Flatlands Confronted with Urge for the Third Dimension, ACS Nano, 2019, 13(8), 9442-9448 (published on July 26, 2019)
Two-dimensional sheets are a relatively neglected form of soft matter, interesting because of their capability to deform into the third dimension with little energy cost. Here, we confront colloidal sheets with an abruptly imposed potential tending to produce strings normal to the plane. Experimentally, this is implemented first by using ultrasound-induced acoustic levitation to produce planar sheets and then by abruptly imposing AC electric fields that introduce dipolar interactions. Seeking to identify the microscopic mechanisms underlying the observed collective behavior, we find that the patterns quantified from our fast confocal experimental imaging are reproduced by our Brownian dynamics simulations. We follow the evolution of these patterns, including their structure factor, from start to final steady state, and from successful parametrization we predict simulation phases not yet observed in experiment. The transient-state evolution toward final outcome includes monocrystalline hexagonal lattice, polycrystalline body-centered tetragonal lattice with grain boundaries, interconnected rings, serpentine zigzag chains, and columns vertical to the plane, and a “fat worm” serpentine pattern. To explain the counterintuitive findings presented here, we map dependence on softness of the confining potential.
3. Zhang, L. #., Xiao, Z. #., Chen, X., Chen, J., and Wang, W.*, Confined 1D Propulsion of Metallodielectric Janus Micromotors on Microelectrodes Under Alternating Current Electric Fields, ACS Nano, 2019, 13(8), 8842-8853 (published on July 2, 2019)
Inspired by an earlier work with magnetic domains on a garnet film as predefined tracks, we present an interdigitated microelectrodes (IDE) system where, upon the application of AC electric fields, metallodielectric (e.g., SiO2–Ti) Janus particles are hydrodynamically confined and electrokinetically propelled in one dimension along the electrode center lines with tunable speeds. In addition, comoving micromotors moved in single files, while those moving in opposite directions primarily reoriented and moved past each other. At high particle densities, turbulence-like aggregates formed as many-body interactions became complicated. Furthermore, a micromotor made U-turns when approaching an electrode closure, while it gradually slowed down at the electrode opening and was collected in large piles. Labyrinth patterns made of serpentine chains of Janus particles emerged by modifying the electrode configuration. Most of these observations can be qualitatively understood by a combination of electroosmotic flows pointing inward to the electrodes, and asymmetric electrical polarization of the Janus particles under an AC electric field. Emerging from these observations is a strategy that not only powers and confines micromotors on prefabricated tracks in a contactless, on-demand manner, but is also capable of concentrating active particles at predefined locations. These features could prove useful for designing tunable tracks that steer synthetic microrobots, as well as to enable the study of single file diffusion, active turbulence, and other collective behaviors of active matters.