We study the active dynamics of self-propelled asymmetrical colloidal particles (Janus particles) fueled by an AC electric field. Both the speed and direction of the self-propulsion, and the strength of the attractive interaction between particles can be controlled by tuning the frequency of the applied electric field and the ion concentration of the solution. The strong attractive force at high ion concentration gives rise to chain formation of the Janus particles, which can be explained by the quadrupolar charge distribution on the particles. Chain formation is observed irrespective of the direction of the self-propulsion of the particles. When both the position and the orientation of the heads of the chains are fixed, they exhibit beating behavior reminiscent of eukaryotic flagella. The beating frequency of the chains of Janus particles depends on the applied voltage and thus on the selfpropulsive force. The scaling relation between the beating frequency and the self-propulsive force deviates from theoretical predictions made previously on active filaments. However, this discrepancy is resolved by assuming that the attractive interaction between the particles is mediated by the quadrupolar distribution of the induced charges, which gives indirect but convincing evidence on the mechanisms of the Janus particles. This signifies that the dependence between the propulsion mechanism and the interaction mechanism, which had been dismissed previously, can modify the dispersion relations of beating behaviors. In addition, hydrodynamic interaction within the chain, and its effect on propulsion speed, are discussed. These provide new insights into active filaments, such as optimal flagellar design for biological functions.
Electrorotations (EROTs) of the Pt-silica Janus particles are measured in different conditions under rotating electric fields. Unlike simple particles, we find that the rotation direction of a Janus particle is mainly opposite to the direction of the electric field (counter-field), which is similar to the metallic particles, while the rotation direction may reverse from counter-field to co-field at the low-frequency region (<1 kHz) and high-frequency region (>1 MHz), depending on the thickness of metallic coating and conductivities of solutions. We also find that EROT of a Janus particle reaches a maximum angular speed at a characteristic frequency, which increases with the thickness of metallic coating and can be one order higher than that of a fully metallic coated particle. These results suggest that the EROT responses of a Janus particle have both dielectric and metallic features and these responses are not simply averaged responses of its both sides. Half side metallic coating reduces the time of polarization due to the lack of fully electric field screening ability comparing with the metallic particles. The special properties of polarization of a Janus particle under electric fields may provide a method to create designable micro-rotors or active particles for applications.
The ICEP behavior of the metal-coated Janus particle is dominated by the thickness of its metallic coating and its orientation.
The authors present a method to control the conformation of DNA by using temperature gradient. The conformations of one end tethered and two ends tethered DNA are measured in different temperature gradients up to 3K∕μm. The results show that temperature gradient can exert force on a single DNA and create internal tension within it. The magnitude of the force is of the order of 0.1pN and is enough to manipulate and stretch DNA. This way of manipulating DNA requires no beads and provides local control, while none of the other methods can satisfy these two requirements at the same time.
We demonstrate a functional rotating electrothermal technique for rapidly concentrating and sorting a large number of particles on a microchip by the combination of particle dielectrophoresis (DEP) and inward rotating electrothermal (RET) flows. Different kinds of particles can be attracted (positive DEP) to or repelled (negative DEP) from electrode edges, and then the n-DEP responsive particles are further concentrated in the heated region by RET flows. The RET flows arise from the spatial inhomogeneous electric properties of fluid caused by direct infrared laser (1470 nm) heating of solution in a rotating electric field. The direction of the RET flows is radially inward to the heated region with a co-field (the same as the rotating electric field) rotation. Moreover, the velocity of the RET flows is proportional to the laser power and the square of the electric field strength. The RET flows are significant over a frequency range from 200 kHz to 5 MHz. The RET flows are generated by the simultaneous application of the infrared laser and the rotating electric field. Therefore, the location of particle concentrating can be controlled within the rotating electric field depending on the position of the laser spot. This multi-field technique can be operated in salt solutions and at higher frequency without external flow pressure, and thus it can avoid electrokinetic phenomena at low frequency to improve the manipulation accuracy for lab-on-chip applications.
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