Stuart J. Williams

Electrokinetic patterning of colloidal particles with optical landscapes

We demonstrate an opto-electrokinetic technique for non-invasive particle manipulation on the surface of a parallel-plate indium tin oxide (ITO) electrode that is biased with an alternating current (AC) signal and illuminated with near-infrared (1064 nm) optical landscapes. This technique can generate strong microfluidic vortices at higher AC frequencies (>100 kHz) and dynamically and rapidly aggregate and pattern particle groups at low frequencies (<100 kHz).

Optically Modulated Electrokinetic Manipulation and Concentration of Colloidal Particles near an Electrode Surface

We study a recently demonstrated AC electrokinetic technique for manipulation and concentration of colloidal particles on an electrode surface. The technique uses indium tin oxide (ITO)-based parallel-plate electrodes on which highly localized infrared (1064 nm) laser illumination is shone. We show that the highly localized laser illumination leads to a highly nonuniform heating of the electrode substrate, which in turn drives an electrothermal microvortex resulting in a rapid transport of particles toward the illuminated site. Hundreds of polystyrene particles, with diameters ranging from 2.0 to 0.1 microm, suspended in a low conductivity solution (2.0 mS/m) could be aggregated at selected locations on the electrode by activating the laser illumination at suitable AC frequencies. Subsequent deactivation of the laser illumination causes the particles to scatter, and we explore this dynamical behavior for 1.0 microm particles using Delaunay tessellations and high-speed videography. We establish that drag from the electrothermal microvortex acts against a repulsive force, which decreases with increasing AC frequency, to create stable particle clusters. Moreover, experimentally we show that this particle capturing technique can be characterized by a critical frequency: a frequency at which the captured colloidal particle cluster becomes unstable and particles are carried away into the bulk by the electrothermal microvortex. This critical frequency increases with decreasing particle diameter for similar particles. For 0.1 microm particles, comparison of aggregation at different AC frequencies is achieved by the comparison of fluorescent intensity profiles of the aggregations.

Hybrid opto-electric manipulation in microfluidics—opportunities and challenges

Hybrid opto-electric manipulation in microfluidics/nanofluidics refers to a set of methodologies employing optical modulation of electrokinetic schemes to achieve particle or fluid manipulation at the micro- and nano-scale. Over the last decade, a set of methodologies, which differ in their modulation strategy and/or the length scale of operation, have emerged. These techniques offer new opportunities with their dynamic nature, and their ability for parallel operation has created novel applications and devices. Hybrid opto-electric techniques have been utilized to manipulate objects ranging in diversity from millimetre-sized droplets to nano-particles. This review article discusses the underlying principles, applications and future perspectives of various hybrid opto-electric techniques that have emerged over the last decade under a unified umbrella.

Optically induced electrokinetic concentration and sorting of colloids

We demonstrate an optically induced ac electrokinetic technique that rapidly and continuously accumulates colloids on a parallel-plate electrode surface resulting in a crystalline-like aggregation. Electrothermal hydrodynamics produce a microfluidic vortex that carries suspended particles toward its center where they are trapped by local ac electrokinetic hydrodynamic forces. We characterize the rate of particle aggregation as a function of the applied ac voltage, ac frequency and illumination intensity. Hundreds of polystyrene particles (1.0 mu m) suspended in a low conductivity solution (2.4 mS m(-1)) were captured at a range of voltages (5-20 V-pp) and frequencies (20-150 kHz) with an optical power of approximately 20 mW. This technique was not restricted to near infrared (1064 nm) illumination and was also demonstrated at 532 nm. The sorting capability of this technique was demonstrated with a solution containing 0.5 mu m, 1.0 mu m and 2.0 mu m polystyrene particles. This dynamic optically induced technique rapidly concentrates, sorts and translates colloidal aggregates with a simple parallel-plate electrode configuration and can be used for a variety of lab-on-a-chip applications.

Enhanced electrothermal pumping with thin film resistive heaters.

This work demonstrates the use of thin film heaters to enhance electrothermal pumping in microfluidic systems. Thin film heating electrothermal pumping is more efficient than Joule heating alone. Numerical simulations of an asymmetric electrode array are performed to demonstrate the advantages of incorporating thin film heaters. This specific simulation shows that thin film heater electrothermal pumping provides approximately two and one‐half times more volumetric flow than Joule heating alone for the same input power to both systems. In addition, external heating allows for electrothermal pumping to be applicable to low conductivity media.

Electrothermal pumping with interdigitated electrodes and resistive heaters.

Interdigitated electrodes are used in electrokinetic lab‐on‐a‐chip devices for dielectrophoretic trapping and characterization of suspended particles, as well as the production of field‐induced fluid flow via AC electroosomosis and electrothermal mechanisms. However, the optimum design for dielectrophoresis, that if symmetrical electrodes, cannot induce bulk electrohydrodynamic pumping. In addition, the mechanism of intrinsic electrothermal pumping is affected by the properties of the fluid, with thermal fields being generated by Joule Heating. This work demonstrates the incorporation of an underlying thin film heater, electrically isolated from the interdigitated electrodes by an insulator layer, to enhance bulk electrothermal pumping. The use of integrated heaters allows the thermal field generation to be controlled independently of the electric field. Numerical simulations are performed to demonstrate the importance of geometrical arrangement of the heater with respect to the interdigitated electrodes, as well as electrode size, spacing, and arrangement. The optimization of such a system is a careful balance between electrokinetics, heat transfer, and fluid dynamics. The heater location and electrode spacing influence the rate of electrothermal pumping significantly more than electrode width and insulator layer thickness. This demonstration will aid in the development of microfluidic electrokinetic systems that want to utilize the advantages associated with electrothermal pumping while simultaneously applying other lab‐on‐a‐chip electrokinetics like dielectrophoresis.

Electrokinetic concentration, patterning, and sorting of colloids with thin film heaters.

Reliable and simple techniques for rapid assembly and patterning of colloid architectures advance the discovery and implementation of such nanomaterials. This work demonstrates rapid electrokinetic two-dimensional assembly of colloidal structures guided by the geometry of thin film heaters within a parallel-plate device. This system is designed to enable either independently addressable or massively parallel colloidal assembly. A combination of electrothermal hydrodynamics, particle-electrode, and particle-particle electrokinetic interactions governs their assembly. Concentration and patterning of structures are shown with 1.0 μm polystyrene particles and sorting between 1.0 μm and 2.0 μm particles is demonstrated.

Dielectrophoretic trapping of nanoparticles with an electrokinetic nanoprobe

A high aspect ratio 3D electrokinetic nanoprobe is used to trap polystyrene particles (200 nm), gold nanoshells (120 nm), and gold nanoparticles (mean diameter 35 nm) at low voltages (<1 Vrms). The nanoprobe is fabricated using room temperature self‐assembly methods, without the need for nanoresolution lithography. The nanoprobe (150–500 nm in diameter, 2–150 μm in length) is mounted on the end of a glass micropipette, enabling user‐specified positioning. The nanoprobe is one electrode within a point‐and‐plate configuration, with an indium–tin oxide cover slip serving as the planar electrode. The 3D structure of the nanoprobe enhances dielectrophoretic capture; further, electro‐hydrodynamic flow enhances trapping, increasing the effective trapping region. Numerical simulations show low heating (1 K), even in biological media of moderate conductivity (1 S/m).

Electrokinetic concentration and patterning of colloids with a scanning laser

Optically‐based lab‐on‐a‐chip systems have the distinct advantage of being dynamically controlled in real time, providing reconfigurable operations that can be tuned to perform a variety of tasks. This manuscript demonstrates the concentration of liquid‐suspended microparticles using a focused near‐infrared laser (980 nm) and a parallel‐plate electrode system. The parallel‐plate electrodes consisted of an indium tin oxide‐coated coverslip and a gold‐coated glass substrate. When the laser was applied at 36 mW, the indium tin oxide surface is locally heated creating sharp temperature gradients on the order of 0.07oC/μm. When an AC field was applied, electrothermal hydrodynamic forces generated microfluidic vortices. At an AC frequency of 40 kHz, the optically controlled electro‐hydrodynamics aggregated colloids at the center of fluid motion on the surface of the indium tin oxide coverslip. The nature of colloid aggregation, translation, and patterning was explored when the translational velocity of the laser spot was varied. This manuscript describes the design of the laser scanning system using commercially available components and the fabrication of the parallel‐plate chip. The effect that the laser scanning rate has on the heat transfer, fluid velocity, and colloid aggregation is discussed.

Comparison of Experiments and Simulation of Joule Heating in ac Electrokinetic Chips

ac electrokinetic manipulations of particles and fluids are important techniques in the development of lab-on-a-chip technologies. Most of these systems involve planar microelectrode geometries, generating high strength electric fields. When these fields are applied to a dielectric medium, Joule heating occurs. Understanding electrothermal heating and monitoring the temperature in these environments are critical for temperature-sensitive investigations including biological applications. Additionally, significant changes in fluid temperature when subjected to an electric field will induce electrohydrodynamic flows, potentially disrupting the intended microfluidic profile. This work investigates heat generated from the interaction of ac electric fields and water at various electrical conductivities (from 0.92 mS/m to 390 mS/m). The electrode geometry is an indium tin oxide (ITO) electrode strip 20 μm wide and a grounded, planar ITO substrate separated by a 50 μm spacer with microfluidic features. Laser-induced fluorescence is used to measure the experimental changes in temperature. A normalization procedure that requires a single temperature-sensitive dye, Rhodamine B (RhB), is used to reduce uncertainty. The experimental electrothermal results are compared with theory and computer simulations.