Conceptual design of a reversible micro-pump system is demonstrated by numerical simulations. Uns... more Conceptual design of a reversible micro-pump system is demonstrated by numerical simulations. Unsteady, incompressible Navier-Stokes equations in a moving boundary system are solved by ΝεκΤαr, a spectral element (high-order) algorithm employing an Arbitrary Lagrangian Eulerian (ALE) formulation on unstructured meshes. The performance of the micro-pump is evaluated as a function of the Reynolds number and the geometric parameters. The volumetric flowrate is shown to increase as a function of the Reynolds number. However, the efficiency of the micro-pump decreases with increased Reynolds number, due to the increased leakage effects.
A new micro heat spreader (MHS) concept for efficient dissipation of large, concentrated heat loa... more A new micro heat spreader (MHS) concept for efficient dissipation of large, concentrated heat loads is introduced. The MHS is a single-phase, closed micro-fluidic system, which utilize time-periodic forced convection cooling. We verified the MHS concept by numerically simulating its operation under various conditions. Our parametric studies have shown that, unlike the steady laminar forced convection, the Nusselt number for time-periodic forced convection laminar flows have strong dependence on the Reynolds and Prandtl numbers. The increase in the Nusselt number indicates enhanced cooling capability of the MHS device. Based on our parametric studies, we calculated the optimum operation conditions, device dimensions and the maximum heat-dissipation rates.
Gas flows through micro-filters are simulated in the continuum and slip flow regimes as a functio... more Gas flows through micro-filters are simulated in the continuum and slip flow regimes as a function of the Knudsen, Reynolds and Mach numbers. The numerical simulations are based on the spectral element formulation of compressible Navier-Stokes equations, which utilize previously developed high-order velocity slip and temperature jump boundary conditions. Both slip and no-slip simulations are used to identify the rarefaction effects. The simulation results show skin friction and form-drag reduction with increased Knudsen number. Pressure drops across the filters are compared against several empirical scaling laws, available in the literature. Compressibility becomes important for high-speed flows, creating large density fluctuations across the micro-filter elements. For high Mach number flows, interactions between thermal and kinetic energies of the fluid are observed. It is also shown that viscous heating plays a significant role for highspeed gas flows, impacting heat transfer char...
Slip lengths reported from molecular dynamics (MD) simulations of water flow in graphene nanochan... more Slip lengths reported from molecular dynamics (MD) simulations of water flow in graphene nanochannels show significant scatter in the literature. These discrepancies are in part due to the used water models. We demonstrate self-consistent comparisons of slip characteristics between the SPC, SPC/E, SPC/Fw, TIP3P, TIP4P, and TIP4P/2005 water models. The slip lengths are inferred using an analytical model that employs the shear viscosity of water and channel average velocities obtained from nonequilibrium MD simulations. First, viscosities for each water model are quantified using MD simulations of counterflowing, force-driven flows in periodic domains in the absence of physical walls. While the TIP4P/2005 model predicts water viscosity at the specified thermodynamic state with 1.7% error, the predictions of SPC/Fw and SPC/E models exhibit 13.9% and 23.1% deviations, respectively. Water viscosities obtained from SPC, TIP4P, and TIP3P models show larger deviations. Next, force-driven wa...
Water desalination using positively and negatively charged single-layer nanoporous graphene membr... more Water desalination using positively and negatively charged single-layer nanoporous graphene membranes.
Transport of saltwater through pristine and positively charged single-layer graphene nanoporous m... more Transport of saltwater through pristine and positively charged single-layer graphene nanoporous membranes is investigated using molecular dynamics simulations. Pressure-driven flows are induced by motion of specular reflecting boundaries at feed and permeate sides with constant speed. Unlike previous studies in the literature, this method induces a desired flow rate and calculates the resulting pressure difference in the reservoirs. Due to the hexagonal structure of graphene, the hydraulic diameters of nano-pores are used to correlate flow rate and pressure drop data. Simulations are performed for three different pore sizes and flow rates for the pristine and charged membrane cases. In order to create better statistical averages for salt rejection rates, ten different initial conditions of Na and Cl distribution in the feed side are used for each simulation case. Using data from 180 distinct simulation cases and utilizing the Buckingham Pi theorem, we develop a functional relationsh...
Capillary evaporation and the associated passive liquid flow are vital for numerous natural and a... more Capillary evaporation and the associated passive liquid flow are vital for numerous natural and artificial processes such as transpiration of water in plants [1], solar steam generation [2-3], water desalination [4], microfluidic pumping [5] and cooling of electronic and photonic devices [6]. Recent experiments conducted in nano- and angstrom-scale conduits have shown evaporation rates corresponding to heat fluxes that are one to two orders of magnitude larger than the kinetic theory limit [7,8]. The physical mechanism elucidating these enormous evaporation rates remains vague. Here we report the discovery of lateral momentum transport within and associated net evaporation from adsorbed liquid layers, which are long believed to be at the equilibrium established between equal rates of evaporation and condensation [9]. Contribution of evaporation from the adsorbed layer increases the effective evaporation area, rendering the excessively estimated evaporative heat flux values below the...
AC electrothermal flow (ACET) induced by Joule heating is utilized to transport biologically rele... more AC electrothermal flow (ACET) induced by Joule heating is utilized to transport biologically relevant liquids in microchannels using simple electrode designs. However Joule heating may cause significant temperature rises, which can degrade biological species, and hence, ACET may become impractical for biomicrofluidic sensors and other possible applications. In this study, the temperature rise at the electrode\electrolyte interface during ACET flow is measured using a high-resolution, non-invasive, thermoreflectance imaging method, which is generally utilized in microelectronics thermal imaging applications. The experimental findings reveal that Joule heating could result in an excessive temperature rise, exceeding 50°C at higher voltage levels (20 Vpp). The measured data are compared with the results of the enhanced ACET theoretical model, which predicts the temperature rise accurately, even at high levels of applied voltages. Overall, our study provides a temperature measurement technique, which is used for the first time for electrode/electrolyte systems. The reported results are critical in designing biomicrofluidic systems with significant energy dissipation in conductive fluids.
An ideal microelectrode array (MEA) design should include materials and structures which exhibit ... more An ideal microelectrode array (MEA) design should include materials and structures which exhibit biocompatibility, low electrode polarization, low impedance/noise, and structural durability. Here, the fabrication of MEAs with indium tin oxide (ITO) electrodes deposited with self-similar gold nanostructures (GNS) is described. We show that fern leaf fractal-like GNS deposited on ITO electrodes are conducive for neural cell attachment and viability while reducing the interfacial impedance more than two orders of magnitude at low frequencies (100-1000 Hz) versus bare ITO. GNS MEAs, with low interfacial impedance, allowed the detection of extracellular action potentials with excellent signal-to-noise ratios (SNR, 20.26 ± 2.14). Additionally, the modified electrodes demonstrated electrochemical and mechanical stability over 29 d in vitro.
Conceptual design of a reversible micro-pump system is demonstrated by numerical simulations. Uns... more Conceptual design of a reversible micro-pump system is demonstrated by numerical simulations. Unsteady, incompressible Navier-Stokes equations in a moving boundary system are solved by ΝεκΤαr, a spectral element (high-order) algorithm employing an Arbitrary Lagrangian Eulerian (ALE) formulation on unstructured meshes. The performance of the micro-pump is evaluated as a function of the Reynolds number and the geometric parameters. The volumetric flowrate is shown to increase as a function of the Reynolds number. However, the efficiency of the micro-pump decreases with increased Reynolds number, due to the increased leakage effects.
A new micro heat spreader (MHS) concept for efficient dissipation of large, concentrated heat loa... more A new micro heat spreader (MHS) concept for efficient dissipation of large, concentrated heat loads is introduced. The MHS is a single-phase, closed micro-fluidic system, which utilize time-periodic forced convection cooling. We verified the MHS concept by numerically simulating its operation under various conditions. Our parametric studies have shown that, unlike the steady laminar forced convection, the Nusselt number for time-periodic forced convection laminar flows have strong dependence on the Reynolds and Prandtl numbers. The increase in the Nusselt number indicates enhanced cooling capability of the MHS device. Based on our parametric studies, we calculated the optimum operation conditions, device dimensions and the maximum heat-dissipation rates.
Gas flows through micro-filters are simulated in the continuum and slip flow regimes as a functio... more Gas flows through micro-filters are simulated in the continuum and slip flow regimes as a function of the Knudsen, Reynolds and Mach numbers. The numerical simulations are based on the spectral element formulation of compressible Navier-Stokes equations, which utilize previously developed high-order velocity slip and temperature jump boundary conditions. Both slip and no-slip simulations are used to identify the rarefaction effects. The simulation results show skin friction and form-drag reduction with increased Knudsen number. Pressure drops across the filters are compared against several empirical scaling laws, available in the literature. Compressibility becomes important for high-speed flows, creating large density fluctuations across the micro-filter elements. For high Mach number flows, interactions between thermal and kinetic energies of the fluid are observed. It is also shown that viscous heating plays a significant role for highspeed gas flows, impacting heat transfer char...
Slip lengths reported from molecular dynamics (MD) simulations of water flow in graphene nanochan... more Slip lengths reported from molecular dynamics (MD) simulations of water flow in graphene nanochannels show significant scatter in the literature. These discrepancies are in part due to the used water models. We demonstrate self-consistent comparisons of slip characteristics between the SPC, SPC/E, SPC/Fw, TIP3P, TIP4P, and TIP4P/2005 water models. The slip lengths are inferred using an analytical model that employs the shear viscosity of water and channel average velocities obtained from nonequilibrium MD simulations. First, viscosities for each water model are quantified using MD simulations of counterflowing, force-driven flows in periodic domains in the absence of physical walls. While the TIP4P/2005 model predicts water viscosity at the specified thermodynamic state with 1.7% error, the predictions of SPC/Fw and SPC/E models exhibit 13.9% and 23.1% deviations, respectively. Water viscosities obtained from SPC, TIP4P, and TIP3P models show larger deviations. Next, force-driven wa...
Water desalination using positively and negatively charged single-layer nanoporous graphene membr... more Water desalination using positively and negatively charged single-layer nanoporous graphene membranes.
Transport of saltwater through pristine and positively charged single-layer graphene nanoporous m... more Transport of saltwater through pristine and positively charged single-layer graphene nanoporous membranes is investigated using molecular dynamics simulations. Pressure-driven flows are induced by motion of specular reflecting boundaries at feed and permeate sides with constant speed. Unlike previous studies in the literature, this method induces a desired flow rate and calculates the resulting pressure difference in the reservoirs. Due to the hexagonal structure of graphene, the hydraulic diameters of nano-pores are used to correlate flow rate and pressure drop data. Simulations are performed for three different pore sizes and flow rates for the pristine and charged membrane cases. In order to create better statistical averages for salt rejection rates, ten different initial conditions of Na and Cl distribution in the feed side are used for each simulation case. Using data from 180 distinct simulation cases and utilizing the Buckingham Pi theorem, we develop a functional relationsh...
Capillary evaporation and the associated passive liquid flow are vital for numerous natural and a... more Capillary evaporation and the associated passive liquid flow are vital for numerous natural and artificial processes such as transpiration of water in plants [1], solar steam generation [2-3], water desalination [4], microfluidic pumping [5] and cooling of electronic and photonic devices [6]. Recent experiments conducted in nano- and angstrom-scale conduits have shown evaporation rates corresponding to heat fluxes that are one to two orders of magnitude larger than the kinetic theory limit [7,8]. The physical mechanism elucidating these enormous evaporation rates remains vague. Here we report the discovery of lateral momentum transport within and associated net evaporation from adsorbed liquid layers, which are long believed to be at the equilibrium established between equal rates of evaporation and condensation [9]. Contribution of evaporation from the adsorbed layer increases the effective evaporation area, rendering the excessively estimated evaporative heat flux values below the...
AC electrothermal flow (ACET) induced by Joule heating is utilized to transport biologically rele... more AC electrothermal flow (ACET) induced by Joule heating is utilized to transport biologically relevant liquids in microchannels using simple electrode designs. However Joule heating may cause significant temperature rises, which can degrade biological species, and hence, ACET may become impractical for biomicrofluidic sensors and other possible applications. In this study, the temperature rise at the electrode\electrolyte interface during ACET flow is measured using a high-resolution, non-invasive, thermoreflectance imaging method, which is generally utilized in microelectronics thermal imaging applications. The experimental findings reveal that Joule heating could result in an excessive temperature rise, exceeding 50°C at higher voltage levels (20 Vpp). The measured data are compared with the results of the enhanced ACET theoretical model, which predicts the temperature rise accurately, even at high levels of applied voltages. Overall, our study provides a temperature measurement technique, which is used for the first time for electrode/electrolyte systems. The reported results are critical in designing biomicrofluidic systems with significant energy dissipation in conductive fluids.
An ideal microelectrode array (MEA) design should include materials and structures which exhibit ... more An ideal microelectrode array (MEA) design should include materials and structures which exhibit biocompatibility, low electrode polarization, low impedance/noise, and structural durability. Here, the fabrication of MEAs with indium tin oxide (ITO) electrodes deposited with self-similar gold nanostructures (GNS) is described. We show that fern leaf fractal-like GNS deposited on ITO electrodes are conducive for neural cell attachment and viability while reducing the interfacial impedance more than two orders of magnitude at low frequencies (100-1000 Hz) versus bare ITO. GNS MEAs, with low interfacial impedance, allowed the detection of extracellular action potentials with excellent signal-to-noise ratios (SNR, 20.26 ± 2.14). Additionally, the modified electrodes demonstrated electrochemical and mechanical stability over 29 d in vitro.
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Papers by Ali Beskok