Session CP: Nanofluids II

Pump-Free Composite Nanochannels for Chip-Level Cooling

In this work, we propose a composite nanochannel system, where half of the channel is of low surface energy, while the other half has relatively high surface energy. It is shown that fluids in such channels can be continuously driven by a symmetric temperature gradient. In the low surface energy part, the fluid moves from high to low temperature, while the fluid migrates from low to high temperature in the part of high surface energy. The mechanisms that govern the flow are explained and the conditions required to guarantee the flow and the possible applications are discussed. One advantage about this system is the application for chip-level cooling, where the heat generated in the chip can be used to drive the liquid without using external pumps, which consume energy, occupy space, and therefore conflict with the miniaturization objectives of the next generation electronic devices.   

Field amplified sample stacking and focusing in nanofluidic channels

One major obstacle in the widespread adoption of nanofluidic technology for bioanalytical systems is efficient detection of samples due to the inherently low numbers of molecules present in small channels. This work explores one of the most common preconcentration techniques, field-amplified sample stacking (FASS), in nanofluidic systems in efforts to alleviate this obstacle. Holding the ratio of background electrolyte concentrations constant, the parameters of channel height, strength of electric field, and electrolyte concentration are experimentally varied. Although in micron scale systems these parameters have little or no effect on the final concentration enhancement achieved, nanofluidic experiments show strong dependencies on each of these parameters. Further, nanofluidic systems demonstrate an increased concentration enhancement over what is predicted and realized in micro-scale counterparts. Accordingly, a theoretical model is developed that explains these observations and furthermore predicts a novel focusing mechanism that can explain the observed increase in concentration enhancement. The simple model is capable of predicting key experimental observations, while a model that incorporates more detail provides good comparisons to the experiment.   

Investigation of flow velocity profile in a nanocapillary

In order to understand transport phenomena in nanofluidics, we study the flow velocity profile in a nanocapillary. Laser Induced Fluorescence Photobleaching Anemometer (LIFPA) and Stimulated Emission Depletion (STED) are combined to establish a far-field nanoscopic velocimeter for flow velocity measurement in a nanochannel. LIFPA uses molecular dye as tracer to avoid issues involved in particles as tracer in PIV, when one dimension of the channels is nearly in the same order of particle diameter. STED is applied to overcome conventional diffraction limit in physics to increase spatial resolution. To apply LIFPA, calibration is first required to establish the relationship between the fluorescence intensity signal and flow velocity. Current monitoring, time-fly of fluorescence and metering from syringe pump are used for the calibration respectively. Then the velocity profile is measured with a spatial resolution of about 70 nm in a nanocapillary with inner diameter of 360 nm. The conductivity influence on the velocity profile is investigated.   

Temperature and viscosity effects on the velocity profile of a nanochannel electro-osmotic flow

Significant temperature and viscosity effects on the electrokinetic transport in a nanochannel with a slab geometry are demonstrated using a molecular dynamics (MD) model. A previously studied system consisting of Na$^+$ and Cl$^-$ ions dissolved in water and confined between fixed crystalline silicon walls with negatively charged inner surfaces in an external electric field was investigated. Lennard-Jones (LJ) force fields and Coulomb electrostatic interactions with Simple Point Charge Extended (SPC/E) model were used to represent the interactions between ions, water molecules, and channel wall atoms. Dependence of the flow of water and ions on the temperature was examined. The magnitude of the water flux and even its direction are shown to be significantly affected by temperature. Temperature dependence of the flux was attributed to the charge redistribution and to the changes in viscosity of water. Using a simple inverse power approximation for water viscosity profile across the channel instead of constant viscosity, an improved prediction of MD electro-osmotic velocity profile from charge density by Stokes equation is demonstrated.   

Hydrodynamic rotational friction of single-wall carbon nanotubes in liquid suspension

The hydrodynamic resistance to rotation in liquid suspension of single-wall carbon nanotubes 1 nm in diameter was experimentally investigated and compared with theoretical predictions. Nanotubes were forced to rotate into alignment with an external electric field, and the rate of their alignment response was measured with laser polarimetry. The measured rates of change of the nematic-order parameter were approximately consistent with theoretical predictions based on classical, no-slip hydrodynamics. This implies that, despite the reduced resistance previously reported for internal flow through carbon nanotubes, and the fact that the nanotubes' diameter is of the same order as the size of the solvent molecules, classical continuum hydrodynamics essentially holds for external flow about individual single-wall carbon nanotubes in liquids.   

Nanofluid heat transfer enhancement in a developing laminar shear flow

The continuum conservation equations for nanofluid flow (J. Buongiorno 2006 J. Heat Transfer 128, 240-50) is applied to a two-dimensional channel entrance region, subjected to a Rayleigh approximation for the nonlinear advection. A perturbation expansion for very small nano-particle volume concentration is used to further simplify the system. The zeroth order similarity solution furnishes the input for the first order problem for nanofluid momentum, volume concentration and heat transport. The latter is cast into a form to show the effect of volume concentration as an effective heat source, thus promoting enhanced heat transfer. Similar solutions for the sequentially solvable first order system is sought to quantitatively describe the dynamics and thermodynamics of nanofluid flow in this much simplified shear flow problem.   

Evanescent wave based near-wall thermometry utilizing Brownian motion

Near wall velocity and temperature measurement is instrumental in research associated with convection heat transfer. Nano Particle Image Velocimetry (nPIV) technique is known to be an effective tool for near-wall velocity measurements. nPIV uses evanescent wave generated by total internal reflection of light to illuminate particles within few hundred nanometers of the wall. Furthermore, temperature measurement at micro scale using Brownian motion characteristics of sub-micron tracer particles used in Micro PIV is well established. This temperature measurement technique is based on the fact that a change in temperature affects Brownian motion that consequently affects the PIV cross-correlation characteristics. In this study the possibility of utilizing this effect for near-wall thermometry is investigated using synthetic nPIV images of 100nm diameter particles. In addition to Hindered Brownian motion, the numerical method includes other near wall forces on the particles such as shear induced lift, buoyancy, electrostatic repulsion, and van der Waals attraction. Simple experiments are carried out using stationary liquids at different temperatures to verify the obtained results.   

Hybrid Continuum and Molecular Modeling of Nano-scale Flows

A novel hybrid method combining the continuum approach based on boundary singularity method (BSM) and the molecular approach based on the direct simulation Monte Carlo (DSMC) is developed and then used to study viscous fibrous filtration flows in the transition flow regime, $Kn>0.25$. The DSMC is applied to a Knudsen layer enclosing the fiber and the BSM is employed to the entire flow domain. The parameters used in the DSMC and the coupling procedure, such as the number of simulated particles, the cell size and the size of the coupling zone are determined. Results are compared to the experiments measuring pressure drop and flowfield in filters. The optimal location of singularities outside of flow domain was determined and results are compared to those obtained by regularized Stokeslets. The developed hybrid method is parallelized by using MPI and extended to multi-fiber filtration flows. The multi-fiber filter flows considered are in the partial-slip and transition regimes. For Kn$\sim $1, the computed velocity near fibers changes significantly that confirms the need of molecular methods in evaluation of the flow slip in transitional regime.