Alicia Boymelgreen

Spinning Janus doublets driven in uniform ac electric fields.

We provide an experimental proof of concept for a robust, continuously rotating microstructure-consisting of two metallodielectric (gold-polystyrene) Janus particles rigidly attached to each other-which is driven in uniform ac fields by asymmetric induced-charge electro-osmosis. The pairs (doublets) are stabilized on the substrate surface which is parallel to the plane of view and normal to the direction of the applied electric field. We find that the radius of orbit and angular velocity of the pair are predominantly dependent on the relative orientations of the interfaces between the metallic and dielectric hemispheres and that the electrohydrodynamic particle-particle interactions are small. Additionally, we verify that both the angular and linear velocities of the pair are proportional to the square of the applied field which is consistent with the theory for nonlinear electrokinetics. A simple kinematic rigid body model is used to predict the paths and doublet velocities (angular and linear) based on their relative orientations with good agreement.

Observing Electrokinetic Janus Particle–Channel Wall Interaction Using Microparticle Image Velocimetry

Three-dimensional/two-component microparticle image velocimetry is used to examine the hydrodynamic flow patterns around metallodielectric Janus particles 15 μm in diameter adjacent to insulating and conducting walls. Far from the walls, the observed flow patterns are in good qualitative agreement with previous experimental and analytical models. However, close to the conducting wall, strong electrohydrodynamic flows are observed at low frequencies, which result in fluid being injected toward the particle. The proximity of the metallic hemisphere to the conducting wall is also shown to produce a localized field gradient, which results in dielectrophoretic trapping of 300 nm polystyrene particles across a broad range of frequencies.

Alternating current induced-charge electrophoresis of leaky dielectric Janus particles

We hereby provide a semi-analytic and numerical solution for the nonlinear, induced-charge electrophoretic motion of an electrically inhomogeneous Janus sphere—comprising two hemispheres with differing dielectric permittivities—under the application of a uniform, time-dependent (ac) electric field. No assumptions are made regarding the size of the electric double layer (EDL) and thus the analysis remains valid even in the case of nanoparticles where the particle radius can be of the same order as the EDL thickness. We consider a number of practical and realistic configurations of metallic and dielectric hemispheres and predict the variations in particle mobility as a function of the conductivity of the two hemispheres and the electrolyte, the frequency of the applied electric field and the EDL length. It is determined that there exist critical values for the conductivity of each hemisphere and the frequency of the applied field, which when exceeded, can cause the mobility to decay rapidly to zero.

The influence of flow intensity and field frequency on continuous-flow dielectrophoretic trapping

We examine the combined influence of the intensity of pressure driven background flow and the frequency of the applied field on the continuous-flow dielectrophoretic trapping behavior of micro-particles within a micro-channel. Using an embedded interdigitated electrode array, we find that the measured trapping percentage over a continuous frequency range exhibits several curious effects which are strongly dependent on the flow intensity, including an apparent shift of the cross-over frequency and low-frequency dispersion. A numerical and theoretical model accounting for the combined effects of pressure-driven flow, dielectrophoresis and alternating-current electro-osmosis on the equation of motion for the particle is used to qualitatively describe the main experimental results.

Active colloids as mobile microelectrodes for unified label-free selective cargo transport

Utilization of active colloids to transport both biological and inorganic cargo has been widely examined in the context of applications ranging from targeted drug delivery to sample analysis. In general, carriers are customized to load one specific target via a mechanism distinct from that driving the transport. Here we unify these tasks and extend loading capabilities to include on-demand selection of multiple nano/micro-sized targets without the need for pre-labelling or surface functionalization. An externally applied electric field is singularly used to drive the active cargo carrier and transform it into a mobile floating electrode that can attract (trap) or repel specific targets from its surface by dielectrophoresis, enabling dynamic control of target selection, loading and rate of transport via the electric field parameters. In the future, dynamic selectivity could be combined with directed motion to develop building blocks for bottom-up fabrication in applications such as additive manufacturing and soft robotics.Janus particles have a surface with two distinct physical or chemical properties; when one side is conducting and the other dielectric, the particles can be guided by an electric field. Here the authors polarize the metallic hemisphere of the particle which enables selective pickup and release of cargo.

Travelling wave dipolophoresis of ideally polarizable nano-particles with overlapping electric double layers in cylindrical pores

We provide a general integral formulation for the dipolophoretic transport of a polarizable colloid in a likewise polarizable nanochannel which takes into account electric double layer (EDL) overlap between the channel walls and resultant background flow as well as the overlap between the wall EDL and that of the particle. The analysis is based on extension of the Lorentz reciprocal theorem for Stokes flows and necessitates the solving of two auxiliary problems; the background induced-charge electroosmotic flow in the channel and the Stokesian motion of a nanoparticle under confinement. To demonstrate our general methodology, we provide a closed form analytical solution for the specific case of a polarizable spherical colloid, located at the axis of a cylindrical nanopore whose walls are subject to a travelling-wave alternating-current electric signal. We quantify the level of EDL overlap via the introduction of a new parameter, ξ which represents the undefined ionic density at the centerline under Boltzm...