Showing papers by "Ali Beskok published in 2017"

Electric field controlled transport of water in graphene nano-channels

Abstract: Motivated by electrowetting-based flow control in nano-systems, water transport in graphene nano-channels is investigated as a function of the applied electric field. Molecular dynamics simulations are performed for deionized water confined in graphene nano-channels subjected to opposing surface charges, creating an electric field across the channel. Water molecules respond to the electric field by reorientation of their dipoles. Oxygen and hydrogen atoms in water face the anode and cathode, respectively, and hydrogen atoms get closer to the cathode compared to the oxygen atoms near the anode. These effects create asymmetric density distributions that increase with the applied electric field. Force-driven water flows under electric fields exhibit asymmetric velocity profiles and unequal slip lengths. Apparent viscosity of water increases and the slip length decreases with increased electric field, reducing the flow rate. Increasing the electric field above a threshold value freezes water at room temperature.

Dielectrophoresis assisted loading and unloading of microwells for impedance spectroscopy.

Abstract: Dielectric spectroscopy (DS) is a noninvasive, label-free, fast, and promising technique for measuring dielectric properties of biological cells in real time. We demonstrate a microchip that consists of electro-activated microwell arrays for positive dielectrophoresis assisted cell capture, DS measurements, and negative dielectrophoresis driven cell unloading; thus, providing a high-throughput cell analysis platform. To the best of our knowledge, this is the first microfluidic chip that combines electro-activated microwells and DS to analyze biological cells. Device performance is tested using Saccharomyces cerevisiae (yeast) cells. DEP response of yeast cells is determined by measuring their Clausius-Mossotti factor using biophysical models in parallel plate microelectrode geometry. This information is used to determine the excitation frequency to load and unload wells. Effect of yeast cells on the measured impedance spectrum was examined both experimentally and numerically. Good match between the numerical and experimental results establishes the potential use of the microchip device for extracting subcellular properties of biological cells in a rapid and nonexpensive manner.

Accuracy of the Maxwell–Wagner and the Bruggeman–Hanai mixture models for single cell dielectric spectroscopy

Abstract: Dielectric spectroscopy (DS) is a non-invasive, label-free, and promising technique for measuring dielectric properties of biological cells. Recent developments in microfabrication techniques made it possible to perform DS measurements with minute volume of cell suspensions. However, when the cell size approaches the size of the measurement chamber, especially, for single cell measurements, the analytical models [Maxwell–Wagner and Bruggeman–Hanai (BH) mixture models] to extract cell parameters lose their accuracy. Moreover, variations in the cell position relative to the measurement electrodes decrease the accuracy of the analytical solutions. Impedance spectrum of a typical eukaryotic mammalian cell is generated for different geometrical configurations using finite element. The generated data are fitted to the analytical models and extracted cell parameters are compared with the original values. The results show that BH model works more effectively when chamber to cell radius ratio is <3.5 and chamber height to cell radius ratio is <3. Moreover, it is observed that analytical models estimate cell parameters with major errors when the cells are in the vicinity of the electrodes. However, for high-volume fraction simulations, the BH model was able to predict cell parameters better even in the vicinity of the electrodes.

Enhancement of dielectrophoresis using fractal gold nanostructured electrodes

Abstract: Dielectrophoretic motions of Saccharomyces cerevisiae (yeast) cells and colloidal gold are investigated using electrochemically modified electrodes exhibiting fractal topology. Electrodeposition of gold on electrodes generated repeated patterns with a fern-leaf type self-similarity. A particle tracking algorithm is used to extract dielectrophoretic particle velocities using fractal and planar electrodes in two different medium conductivities. The results show increased dielectrophoretic force when using fractal electrodes. Strong negative dielectrophoresis of yeast cells in high-conductivity media (1.5 S/m) is observed using fractal electrodes, while no significant motion is present using planar electrodes. Electrical impedance at the electrode/electrolyte interface is measured using impedance spectroscopy technique. Stronger electrode polarization (EP) effects are reported for planar electrodes. Decreased EP in fractal electrodes is considered as a reason for enhanced dielectrophoretic response.

Pressure-driven water flow through hydrophilic alumina nanomembranes

Abstract: We present an experimental study that focuses on pressure-driven flow of distilled water through γ alumina membranes with 5, 10 and 20 nm pore radii. The nanopore geometry, pore size and porosity are characterized using scanning electron microscopy images taken pre- and post-flow experiments. Comparisons of these images have shown reduction in the pore size, which is attributed to precipitation of hydroxyl groups on alumina surfaces. Measured flowrates compared with the Hagen–Poiseuille flow relations consistently predict 2.2 nm reductions in the pore size for three different membranes. This behavior can be explained by the formation of a thick stick layer of water molecules over hydroxylated alumina surfaces, evidenced by water droplet contact angle measurements that exhibit increased hydrophilicity of alumina surfaces. Other possible effects of the mismatch between theory and experiments such as unaccounted pressure losses in the system or the streaming potential effects were also considered, but shown to be negligible for current experimental conditions.

Self-Similar Interfacial Impedance of Electrodes in High Conductivity Media.

Abstract: Electrode polarization (EP) happening due to accumulation of ions at the electrode/electrolyte interface is an inevitable phenomenon while measuring impedance spectrum in high conductivity buffers and at low RF spectrum. Well-characterized time scales elucidating the EP effect are important for the rational design of microfluidic devices and impedance sensors. In this Article, interfacial impedance at the electrode/electrolyte interface is investigated considering channel height and Debye length effects on characteristic time scale in a binary electrolyte solution using parallel plate electrode configuration. Experimental results reveal self-similarity of normalized electrical impedance as a function of the normalized frequency. The experimental results also match with numerical solutions obtained by finite element simulation of unsteady fully coupled Poisson–Nernst–Planck (PNP) equations. Furthermore, fractal shaped gold nanostructured electrodes are examined, and it has been proven that electric double ...

Pressure driven water flow through hydrophilic alumina nanomembranes

Abstract: We present an experimental study that focuses on pressure-driven flow of distilled water through γ alumina membranes with 5, 10 and 20 nm pore radii. The nanopore geometry, pore size and porosity are characterized using scanning electron microscopy images taken pre- and post-flow experiments. Comparisons of these images have shown reduction in the pore size, which is attributed to precipitation of hydroxyl groups on alumina surfaces. Measured flowrates compared with the Hagen–Poiseuille flow relations consistently predict 2.2 nm reductions in the pore size for three different membranes. This behavior can be explained by the formation of a thick stick layer of water molecules over hydroxylated alumina surfaces, evidenced by water droplet contact angle measurements that exhibit increased hydrophilicity of alumina surfaces. Other possible effects of the mismatch between theory and experiments such as unaccounted pressure losses in the system or the streaming potential effects were also considered, but shown to be negligible for current experimental conditions.

Temperature profiles and heat fluxes observed in molecular dynamics simulations of force-driven liquid flows

Abstract: This paper concentrates on the unconventional temperature profiles and heat fluxes observed in non-equilibrium molecular dynamics (MD) simulations of force-driven liquid flows in nano-channels. Using MD simulations of liquid argon flows in gold nano-channels, we investigate the manifestation of the first law of thermodynamics for the MD system, and compare it with that of the continuum fluid mechanics. While the energy equation for the continuum system results in heat conduction determined by viscous heating, the first law of thermodynamics in the MD system includes an additional slip-heating term. Interaction strength between argon and gold molecules is varied in order to investigate the effects of slip-velocity on the slip-heating term and the resulting temperature profiles. Heat fluxes and temperature profiles from "continuum", "continuum augmented with slip-heating", and "heat conduction due to the power input by the driving force" are modeled and compared with the MD results. The continuum model can neither predict the heat fluxes nor the temperature profiles from MD simulations. While the continuum model augmented with slip-heating matches the MD heat fluxes, the resulting temperature profiles do not agree with the MD predictions. Overall the analytical model based on "heat conduction due to power input by the driving force" matches the heat fluxes from MD simulations, while the temperature profiles match MD predictions using an effective thermal conductivity that is about 70% of the thermodynamic value. Using different liquid-wall pairs affects the slip velocity, temperature jump, and the resulting thermal conductivity of the fluid, but results in similar physical observations. The inability of the MD method in mimicking continuum fluid mechanics in energy transport for force-driven liquid flows is scale independent, and it is more likely a numerical artifact.