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Trapping, sorting, transportation, and manipulation of synthetic microparticles and biological cells enable investigations in their behavior and properties. Microfluidic techniques like rapid electrokinetic patterning (REP) provide a non-invasive means to probe into the nature of these micro and nanoparticles. The opto-electrically induced nature of a REP micro vortex allows tuning of the trap characteristics in real-time. In this work, we studied the effects of transient optical heating on the induced electrothermal vortex using micro-particle image velocimetry (μ-PIV) and computational modeling. A near infra-red (980 nm) laser beam was focused on a colloidal suspension of 1 μm polystyrene beads sandwiched between two parallel-plate electrodes. The electrodes were subjected to an AC current. The laser spot was scanned back-and-forth in a line, at different frequencies, to create the transient vortex. This phenomenon was also studied with a computational model made using COMSOL Multiphysics. We visualize fluid flow in custom-shaped REP traps by superposing multiple axisymmetric (spot) vortices and discuss the limitations of using superposition in dynamically changing traps.

This study used impedance spectroscopy measurements to extract the electrical properties of phytoplankton cells in suspension. Experimental measurements were acquired, and the single-shell model was applied to extract the specific membrane capacitance, cytoplasm permittivity, and conductivity of assumingly spherical cells in suspension utilizing Maxwell’s mixture theory of a controlled volume fraction of cells. The impedance of suspensions of algae was measured at different frequencies ranging from 3 kHz to 10 MHz and impedance values were compared to investigate differences between two types of cells by characterizing their change in cytoplasm permittivity and specific membrane capacitance. Differentiation between healthy control and nitrogen-depleted cultured algae was attempted. The extracted specific membrane capacitances of Chlamydomonas and Selenastrum were 15.5 ± 3.6 and 40.6 ± 12.6 mF m− 2 respectively. Successful differentiation based on the specific membrane capacitance of different algae species was achieved. However, no significant difference was noticed between nitrogen-abundant and nitrogen-depleted cultures.

Isomotive dielectrophoresis (isoDEP) is a unique electric field such that the gradient of the field-squared ([Formula: see text]) is constant, resulting a uniform dielectrophoretic force. The current status of isoDEP is presented in this review, and we will highlight the progress that has been achieved over the past 60 years in various avenues of isoDEP since H.A. Pohl initially described its premise. This article will discuss its applications and describe the various configurations of generating an isomotive force. Since H.A. Pohl introduced the theory of isoDEP, numerous authors have implemented isoDEP as a tool for the manipulation, sorting, separation, and characterization of polarizable particles without the need for biochemical labels or other bioengineered tagging. The growing field of microfluidics and electrokinetics has renewed interest in isoDEP, particularly for analytical characterization or separation of particles. Recent work has demonstrated that isoDEP can address some unmet needs for biomedical applications including single-cell analysis; moreover, advances in throughput as well as combining characterization and separation simultaneously will add significant value to isoDEP.

Two dielectrophoresis systems are introduced where the induced dielectrophoretic force is constant throughout the experimental region, resulting in uniform (isomotive) microparticle translation. Isomotive dielectrophoresis (isoDEP) is accomplished through a unique geometry where the gradient of the field-squared (∇Erms2) is constant, a characteristic that is otherwise highly nonuniform in traditional DEP platforms. The governing isoDEP equations were derived herein and applied to two different isoDEP prototypes: (i) one fabricated from deep reactive ion etching (DRIE) of a conductive silicon wafer (1-10 Ω-cm) whose patterned features served as electrodes and microchannel sidewalls simultaneously; (ii) a second where the electric field is applied lengthwise through a PDMS microchannel whose geometry follows a specific curvature. Both positive and negative dielectrophoresis was demonstrated with the isoDEP devices using silver-coated hollow glass spheres and polystyrene particles, respectively. Particle tracking was used to compare particle trajectory with the expected dielectrophoretic response; further, particle velocity was used to measure the Clausius-Mossotti factor of individual polystyrene particles (18-24.9 μm) in both devices with a value of -0.40 ± 0.063 (n = 110) and -0.48 ± 0.055 (n = 18) for the DRIE and PDMS isoDEP platforms, respectively. The isoDEP platform is capable of analyzing multiple particles simultaneously, providing greater throughput than traditional electrorotation platforms.

Isomotive dielectrophoresis (isoDEP) is a unique DEP geometrical configuration where the gradient of the field-squared ( ∇ E rms 2 ) is constant. IsoDEP analyzes polarizable particles based on their magnitude and direction of translation. Particle translation is a function of the polarizability of both the particles and suspending medium, the particles' size and shape, and the frequency of the electric field. However, other electrokinetics act on the particles simultaneously, including electrothermal hydrodynamics. Hence, to maximize the DEP force relative to over electrokinetic forces, design parameters such as microchannel geometry, fabrication materials, and applied electric field must be properly tuned. In this work, scaling law analyses were developed to derive design rules, relative to particle diameter, to reduce unwanted electrothermal hydrodynamics relative to DEP-induced particle translation. For a particle suspended in 10 mS/m media, if the channel width and height are below ten particle diameters, the electrothermal-driven flow is reduced by ∼500 times compared to a channel that is 250 particles diameters in width and height. Replacing glass with silicon as the device's underlying substrate for an insulative-based isoDEP reduces the electrothermal induced flow approximately 20 times less.

Rapid electrokinetic patterning (REP) is an emerging optoelectric technique that takes advantage of laser-induced AC electrothermal flow and particle-electrode interactions to trap and translate particles. The electrothermal flow in REP is driven by the temperature rise induced by the laser absorption in the thin electrode layer. In previous REP applications 350-700 nm indium tin oxide (ITO) layers have been used as electrodes. In this study, we show that ITO is an inefficient electrode choice as more than 92% of the irradiated laser on the ITO electrodes is transmitted without absorption. Using theoretical, computational, and experimental approaches, we demonstrate that for a given laser power the temperature rise is controlled by both the electrode material and its thickness. A 25-nm thick Ti electrode creates an electrothermal flow of the same speed as a 700-nm thick ITO electrode while requiring only 14% of the laser power used by ITO. These results represent an important step in the design of low-cost portable REP systems by lowering the material cost and power consumption of the system.

Rapid electrokinetic patterning (REP) is a technique for creating self-assembled monolayers (SAMs) of spherical particles in a liquid medium, and dynamically controlling them though the simultaneous application of an electric field and optically induced temperature gradients. Previous work has investigated and characterized REP axisymmetric aggregations generated from a focus laser within a uniform electric field; work herein characterizes line-shaped particle assemblies derived from the application of a linearly scanned laser. The resulting aggregations of spherical polystyrene particles (1 μm) suspended in low-conductivity aqueous potassium chloride solution (KCl, 2.5 mS/m) resembled elliptical-shaped crystalline geometries. The mean particle-to-particle spacing within the aggregation remained greater than 1.5 diameters for experiments herein (6.5 Vrms , 30 kHz) due to dipole-dipole repulsive forces. Interparticle spacing demonstrated a linear relationship (1.6-2.1 μm) with increasing scanning lengths (up to 83 μm), decreased from 1.9 to 1.7 μm with increasing scanning frequency (0.38-16 Hz) for a 53 μm scan length, and decreased from 2.0 to 1.6 μm with increasing laser power (11.9-18.8 mW) for a 59 μm, 16 Hz laser scan.

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.

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 0.07(o) C/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.

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.

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.

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 (

The ability to manipulate small amounts of fluid is important for a variety of lab-on-a-chip applications. Electrokinetic microfluidic actuation methods, such as electrothermal pumping, are popular due to their simple implementation and reliability from no moving parts. Electrothermal motion in aqueous solutions results from the electric field acting upon dielectric inhomogeneities in the liquid induced by temperature gradients. The most traditional method of inducing temperature gradients is through direct Joule heating of the fluid itself, which is a function of the electrical conductivity of the liquid. In this case, the microelectrode geometry controls both the location of the Joule heating and the nature of the electric field. Therefore, it is impossible to independently control temperature and electric field with a single set of electrodes. A novel electrothermal microfluidic pump is demonstrated herein. The distinguishing feature of this electrothermal pump is an independent heating source using resistive heating of microfabricated metal traces. A second set of electrodes for electrothermal pumping have been layered on top of the electrically isolated heating elements. An arrangement of coplanar electrode strips with an additional resistive heating electrode was used to demonstrate electrothermal pumping. Fluid motion results were compared to established theory.Copyright © 2011 by ASME

We demonstrate an optically induced electrokinetic technique that continuously concentrates nanoparticles on the surface of a parallel plate electrode that is biased with an AC signal. A highly focused beam of near-infrared light (1064 nm) was applied, inducing an electrothermal microfluidic vortex that carried nanoparticles to its center where they were accumulated. This technique was demonstrated with 49 nm and 100 nm fluorescent polystyrene particles and characterized as a function of applied AC frequency and voltage. With this technique the location and shape of colloidal concentration was reconfigured by controlling the optical landscape, yielding dynamic control of the aggregation. Colloidal concentration was demonstrated with a plain parallel plate electrode configuration without the need of photoconductive materials or complex microfabrication procedures.

AC electrokinetic manipulation 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 is 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 0.92–390 mS/m). The electrode geometry is an ITO electrode strip 20 μm wide and a grounded, planar ITO substrate separated by a 50 μm spacer with microfluidic features. Laser Induced Fluorescence (LIF) is used to measure the experimental changes in temperature. A normalization procedure that requires a single temperature-sensitive dye, Rhodamine B (RhB), is proposed to reduce uncertainty. The experimental electrothermal results are compared to theory and computer simulations.

New examples and sources of ghost peaks in reversed-phase gradient HPLC are described and related to previous publications to give a broad perspective of ghost peak problems. In one new example the ghost peak was found to be due to mixing problems caused by a period of non-delivery of the stronger “solvent B” in a stepped gradient system. In a second example the ghost peak was due to plasticizer contamination of the organic solvent. This paper includes tips and recommendations for the consistent running of ghost-peak-free reversed-phase gradient HPLC.

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.

We demonstrate an opto-electrokinetic technique for noninvasive 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).

Incubation of dilute polymer solutions with a KI–I2 standard shows the presence of a concentration dependent phase transition for the poly(oxyethylene)+ poly(oxypropylene)+ poly(oxyethylene) block copolymer F87 (P237) using diode-array UV spectrometry; the transition is shown to be reversible and reproducible and is in agreement with previous data obtained via high-sensitivity differential scanning microcalorimetry (HSDSC).