Search

Self-propelling active particles are an exciting and interdisciplinary emerging area of research with projected biomedical and environmental applications. Due to their autonomous motion, control over these active particles that are free to travel along individual trajectories, is challenging. This work used optically patterned electrodes on a photoconductive substrate using a digital micromirror device (DMD) to dynamically control the region of movement of self-propelling particles (i.e. metallo-dielectric Janus particles (JPs)). This extends previous studies where only a passive micromotor was optoelectronically manipulated with a translocating optical pattern that illuminated the particle. In contrast, the current system used the optically patterned electrode merely to define the region within which the JPs moved autonomously. Interestingly, the JPs avoided crossing the optical region edge, which enables constraint of the area of motion and to dynamically shape the JP trajectory. Using the DMD system to simultaneously manipulate several JPs enabled to self-assemble the JPs into stable active structures (JPs ring) with precise control over the number of participating JPs and passive particles. Since the optoelectronic system is amenable to closed-loop operation using real-time image analysis, it enables exploitation of these active particles as active microrobots that can be operated in a programmable and parallelized manner

This article presents the implementation of on-chip iontronic circuits via small-scale integration of multiple ionic logic gates made of bi-polar polyelectrolyte diodes. These ionic circuits are analogous to solid-state electronic circuits, with ions as the charge carriers instead of electrons/holes. We experimentally characterize the responses of a single fluidic diode made of a junction of oppositely charged polyelectrolytes (i.e., anion and cation exchange membranes), with a similar underlying mechanism as a solid-state p- and n-type junction. This served to carry out pre-designed logical computations in various architectures by integrating multiple diode-based logic gates, where the electrical signal between the integrated gates was transmitted entirely through ions. The findings shed light on the limitations affecting the number of logic gates that can be integrated, the degradation of the electrical signal, their transient response, and the design rules that can improve the performance of iontronic circuits.

Herein we demonstrate the ability to optically tune the mobility of electrically powered Janus particles (JP) that are half coated with various Zinc Oxide (ZnO) semiconducting layers, i.e. polycrystalline, amorphous and amorphous with a SiO2 passivation layer. The ZnO semiconductor photo-response enables increase in its electrical conductivity with light having wavelengths of sufficient photon energy with respect to the semiconductor bandgap. This effect, termed optically modulated electrokinetic propulsion (OMEP), can be harnessed to increase the contrast in polarizability between the dielectric and semiconducting hemispheres, which in turn, results in an increased electrokinetic mobility. The addition of optical activation to the electrical field enables an additional degree of control of JP mobility. We also demonstrate optical control of collective behavior and particle-particle interactions for dense semi-conducting Janus particle populations.

Microfluidic channels with embedded ion permselective medium under the application of electric current are commonly used for electrokinetic processes as on-chip ion concentration polarization (ICP) and bioparticle preconcentration to enhance biosensing. Herein, we demonstrate the ability to dynamically control the electrically driven ion transport by integrating individually addressable microvalves. The microvalves are located along a main microchannel that is uniformly coated with a thin layer of an ion-exchange membrane (IEM). The interplay of ionic transport between the solution within the microchannel and the thin IEM, under an applied electric current, can be locally tuned by the deformation of the microvalve. This tunability provides a robust and simple means of implementing new functionalities into lab-on-a-chip devices, e.g., dynamic control over multiple ICP layers and their associated preconcentrated molecule plugs, multiplex sensing, suppression of biofouling as well as plug dispersion, while maintaining the well-known application of microvalves as steric filtration.

The ability to induce regions of high and low ionic concentration adjacent to a permeselective membrane or nanochannel subject to an externally applied electric field (a phenomenon termed concentration-polarization) has been used for a broad spectrum of applications ranging from on-chip desalination, bacteria filtration to biomolecule preconcentration. But these applications have been limited by the ability to control the length of the diffusion layer that is commonly indirectly prescribed by the fixed geometric and surface properties of the nanofluidic system. Here, we demonstrate that the depletion layer can be dynamically varied by inducing controlled electrothermal flow driven by the interaction of temperature gradients with the applied electric field. To this end, a series of microscale heaters, which can be individually activated on demand are embedded at the bottom of the microchannel and the relationship between their activation and ionic concentration is characterized. Such spatio-temporal control of the diffusion layer can be used to enhance on-chip electro-dialysis by producing shorter depletion layers, to dynamically reduce the microchannel resistance relative to that of the nanochannel for nanochannel based (bio)sensing, to generate current rectification reminiscent of a diode like behavior and control the location of the preconcentrated plug of analytes or the interface of brine and desalted streams.

With an eye towards next-generation, smart, micro/nanofluidic devices, capable of responding to external stimuli or changes in environment, we demonstrate a means to achieve dynamic control of the spatio-temporal properties of the electric field in a standardized microfluidic chamber. Typical top-down patterning, currently used to design the field distribution, is replaced by freely-suspended colloids which locally disturb the electric field from the bottom-up. Even under uniform forcing, polarization of the colloid induces the formation of strong, three-dimensional gradients at its surface - essentially repurposing it into a portable floating electrode whose precise location can be manipulated to reconfigure the electric field in real time. Focusing on active Janus colloids as a sample platform, we measure the strength of the induced gradients and highlight the advantages of a colloid-based system by revealing a prototype for an all-in-one cargo carrier, capable of on-demand, selective, label-free assembly and transport of micro/nano sized targets.

We demonstrate the role of selectivity variation in the structure of the non-equilibrium extended space-charge using 1D analytic and 2D numerical Poisson-Nernst-Planck models for the electro- diffusive transport of a symmetric electrolyte. This provides a deeper understanding of the underly- ing mechanism behind a previously-observed maximum in the resistance-voltage curve for a shallow micro-nanochannel interface device [Schiffbauer, Liel, Leibowitz, Park, and Yossifon, submitted to Phys. Rev. E.]. The current study helps to establish a connection between parameters such as the geometry and nanochannel surface-charge and the control of selectivity and resistance in the over-limiting current regime.

Induced-charge electro-osmosis (ICEO) is shown to control the length scale of the diffusion layer (DL), which in turn, affects the diffusion limited ion transport through the microchannel-Nafion membrane system. The ICEO vortices form at an interdigitated floating electrode array embedded within a microchannel interfacing a Nafion membrane and stir the fluid, arresting the diffusive growth of the depletion layer. By varying the spacing between the array and the membrane we are able to control the length of the DL. Activating the electrodes results in further enhancement of the fluid stirring due to the emergence of alternating-current electro-osmotic (ACEO) flow on top of the ICEO. Such a new method of controlling the DL with an electrode array is of great importance in many CP related realizations.

To bridge the gap between single/isolated pore systems to multi-pore systems, such as membranes/electrodes, we studied an array of nanochannels with varying interchannel spacing that controlled the degree of channel communication. Instead of treating them as individual channels connected in parallel or an assembly like a homogeneous membrane, this study resolves the pore-pore interaction. We found that increased channel isolation leads to current intensification whereas at high voltages electro-convective effects control the degree of communication via suppression of the diffusion layer growth, 4 figures,submitted to Phys. Rev. E

The microchannel corner is a common inherent component of most planar microfluidic systems and thus its influence on the channel flow is of significant interest. Application of an alternating current electric field enables quantification of the non-linear induced-charge electro-osmosis (ICEO) ejection flow effect by isolating it from linear electro-osmotic background flow which is present under dc forcing. The hydrodynamic flow in the vicinity of a sharp channel corner is analyzed using experimental micro-particle-image-velocimetry (PIV) and numerical simulations for different buffer concentrations, frequencies and applied voltages. Divergence from the purely ICEO flow with increasing buffer conductivity is shown to be a result of increasing electro-thermal effects due to Joule heating.