Eric B. Cummings

Laser-induced thermal acoustics (LITA) signals from finite beams

Laser-induced thermal acoustics (LITA) is a four-wave mixing technique that may be employed to measure sound speeds, transport properties, velocities, and susceptibilities of fluids. It is particularly effective in high-pressure gases (>1 bar). An analytical expression for LITA signals is derived by the use of linearized equations of hydrodynamics and light scattering. This analysis, which includes full finite-beam-size effects and the optoacoustic effects of thermalization and electrostriction, predicts the amplitude and the time history of narrow-band time-resolved LITA and broadband spectrally resolved (mulitplex) LITA signals. The time behavior of the detected LITA signal depends significantly on the detection solid angle, with implications for the measurement of diffusivities by the use of LITA and the proper physical picture of LITA scattering. This and other elements of the physics of LITA that emerge from the analysis are discussed. Theoretical signals are compared with experimental LITA data.

Dielectrophoretic trapping without embedded electrodes

We observe dielectrophoretic effects in mixed electrokinetic and dielectrophoretic flows in uniform arrays of posts. Above a threshold applied electric field, flowing filaments of concentrated and rarefied particles appear in the flow. Above a higher-threshold applied field, zones of highly concentrated, immobilized particles appear. At the lower and higher thresholds, dielectrophoresis apparently begins to dominate diffusion and advection/electrokinesis, respectively. The patterns of filaments and trapped zones depend dramatically on the angle of the array with respect to the mean applied electric field and the shape of the posts in the array.

Measurement of gas-phase sound speed and thermal diffusivity over a broad pressure range using laser-induced thermal acoustics

We report on the detection and analysis of signals generated from gas-phase laser-induced gratings over a large range of static pressure (0.04-100 atm). We employed the experimental technique of laser-induced thermal acoustics and performed measurements on mixtures of NO(2) in air and CO(2) as a function of pressure. Accurate analysis of the acquired data was obtained from a full theory, including beam size effects. The theory fully reproduces the observed data for a ratio of molecular mean free path to grating wavelength extending from 1 to 4 x 10(-4). Nonlinear, least-squares f its between modeled and experimental signals provided accurate values of the sound speed and thermal diffusivity.

Laser-induced thermal-acoustic velocimetry with heterodyne detection.

Laser-induced thermal acoustics (LITA) was used with heterodyne detection to measure simultaneously and in a single laser pulse the sound speed and flow velocity of NO>(2) -seeded air in a low-speed wind tunnel up to Mach number M =0.1 . The uncertainties of the velocity and the sound speed measurements were ~0.2 m/s and 0.5%, respectively. Measurements were obtained through a nonlinear least-squares fit to a general, analytic closed-form solution for heterodyne-detected LITA signals from thermal gratings. Agreement between theory and experiment is exceptionally good.

Beam misalignments and fluid velocities in laser-induced thermal acoustics

Beam misalignments and bulk fluid velocities can influence the time history and intensity of laser-induced thermal acoustics (LITA) signals. A closed-form analytic expression for LITA signals incorporating these effects is derived, allowing the magnitude of beam misalignment and velocity to be inferred from the signal shape. It is demonstrated how instantaneous, nonintrusive, and remote measurement of sound speed and velocity (Mach number) can be inferred simultaneously from homodyne-detected LITA signals. The effects of different forms of beam misalignment are explored experimentally and compared with theory, with good agreement, allowing the amount of misalignment to be measured from the LITA signal. This capability could be used to correct experimental misalignments and account for the effects of misalignment in other LITA measurements. It is shown that small beam misalignments have no influence on the accuracy or repeatability of sound speed measurements with LITA.

Irrotationality of uniform electro-osmosis

Steady electroosmotic flow of uniform liquids in uniform media is irrotational provided the electric double layers adjacent to surfaces are negligibly thin, the surfaces are non-conducting and impermeable, and the total pressure imposed at inlets and outlets is uniform. Because many microfluidic devices employing electroosmosis approximately satisfy these requirements, this ideal electrosmosis is a limiting case with considerable practical significance. In ideal electroosmosis, fluid motion follows current lines. Flow-fields have no Reynolds number dependence and are everywhere proportional to the electric field. Both fields may be obtained by a single solution of the Laplace equation. In this paper, we discuss these features of ideal electroosmotic flows and present particle-image derived velocity fields that confirm ideal flow conditions in glass microchannel networks.

Neural network data analysis for laser-induced thermal acoustics

A general, analytical closed-form solution for laser-induced thermal acoustic (LITA) signals using homodyne or heterodyne detection and using electrostrictive and thermal gratings is derived. A one-hidden-layer feed-forward neural network is trained using back-propagation learning and a steepest descent learning rule to extract the speed of sound and flow velocity from a heterodyne LITA signal. The effect of the network size on the performance is demonstrated. The accuracy is determined with a second set of LITA signals that were not used during the training phase. The accuracy is found to be better than that of a conventional frequency decomposition technique while being computationally as efficient. This data analysis method is robust with respect to noise, numerically stable and fast enough for real-time data analysis.

A Comparison of Theoretical and Experimental Electrokinetic and Dielectrophoretic Flow Fields (Invited)

Electrokinesis and dielectrophoresis, technologically important particle and fluid transport mechanisms in microscale flow channels, are respectively linear and (initially) quadratic in the electric eld applied along the flow channel. Ideal electrokinesis is a special case of general electrokinesis in which the flow velocity is everywhere proportional to the applied electric eld. Experiments to establish the validity and practicality of ideal electrokinesis employed a novel particle-image velocimetry (PIV) analysis methodology that provides accurate optical-diraction-limited velocity elds. This methodology extracts measurements from the particle-displacement histogram from a flow video, providing single-pixel-resolution measurements of mean velocity from an ensemble of O(10 3 ) images. The resulting PIV measurements validate the theory of ideal electrokinesis within 1% over complicated geometries, issuing a license to apply the theory for design and analysis. At a suciently high electric eld, dielectrophoretic transport of suspended particles overcomes electrokinetic transport in regions of the flow. This applied eld is a threshold between two technologically important flow regimes: lamentary and trapping dielectrophoresis. These flows are quantied and compared to simulations based on ideal electrokinesis and linear dielectrophoresis with good agreement. Simulations, analysis, and experimental observations are used to develop a practical continuous-flow particle concentrator design. Nomenclature t time delay between subimages in correlation pair x, y Cartesian independent variables of the lter basis function Dielectric constant of the fluid Electrostatic potential

Separation and concentration of water-borne contaminants utilizing insulator-based dielectrophoresis.

This report focuses on and presents the capabilities of insulator-based dielectrophoresis (iDEP) microdevices for the concentration and removal of water-borne bacteria, spores and inert particles. The dielectrophoretic behavior exhibited by the different particles of interest (both biological and inert) in each of these systems was observed to be a function of both the applied electric field and the characteristics of the particle, such as size, shape, and conductivity. The results obtained illustrate the potential of glass and polymer-based iDEP devices to act as a concentrator for a front-end device with significant homeland security and industrial applications for the threat analysis of bacteria, spores, and viruses. We observed that the polymeric devices exhibit the same iDEP behavior and efficacy in the field of use as their glass counterparts, but with the added benefit of being easily mass fabricated and developed in a variety of multi-scale formats that will allow for the realization of a truly high-throughput device. These results also demonstrate that the operating characteristics of the device can be tailored through the device fabrication technique utilized and the magnitude of the electric field gradient created within the insulating structures. We have developed systems capable of handling numerous flow rates and sample volume requirements, and have produced a deployable system suitable for use in any laboratory, industrial, or clinical setting.

Polymeric microfluidic devices for the monitoring and separation of water-borne pathogens utilizing insulative dielectrophoresis

We have successfully demonstrated selective trapping, concentration, and release of various biological organisms and inert beads by insulator-based dielectrophoresis within a polymeric microfluidic device. The microfluidic channels and internal features, in this case arrays of insulating posts, were initially created through standard wet-etch techniques in glass. This glass chip was then transformed into a nickel stamp through the process of electroplating. The resultant nickel stamp was then used as the replication tool to produce the polymeric devices through injection molding. The polymeric devices were made of Zeonor 1060R, a polyolefin copolymer resin selected for its superior chemical resistance and optical properties. These devices were then optically aligned with another polymeric substrate that had been machined to form fluidic vias. These two polymeric substrates were then bonded together through thermal diffusion bonding. The sealed devices were utilized to selectively separate and concentrate a variety of biological pathogen simulants and organisms. These organisms include bacteria and spores that were selectively concentrated and released by simply applying D.C. voltages across the plastic replicates via platinum electrodes in inlet and outlet reservoirs. The dielectrophoretic response of the organisms is observed to be a function of the applied electric field and post size, geometry and spacing. Cells were selectively trapped against a background of labeled polystyrene beads and spores to demonstrate that samples of interest can be separated from a diverse background. We have implemented a methodology to determine the concentration factors obtained in these devices.