S. M. Sohel Murshed

Temperature dependence of interfacial properties and viscosity of nanofluids for droplet-based microfluidics

Interfacial tension and viscosity of a liquid play an important role in microfluidic systems. In this study, temperature dependence of surface tension, interfacial tension and viscosity of a nanofluid are investigated for its applicability in droplet-based microfluidics. Experimental results show that nanofluids having TiO2 nanoparticles of 15?nm diameter in deionized water exhibit substantially smaller surface tension and oil-based interfacial tension than those of the base fluid (i.e. deionized water). These surface and interfacial tensions of this nanofluid were found to decrease almost linearly with increasing temperature. The Brownian motion of nanoparticles in the base fluid was identified as a possible mechanism for reduced surface and interfacial tensions of the nanofluid. The measured effective viscosity of the nanofluid was found to be insignificantly higher than that of the base fluid and to decrease with increasing fluid temperature. The dependence on the temperature of the droplet formation at the T-junction of a microfluidic device is also studied and the nanofluid shows larger droplet size compared with its base fluid.

Droplet formation and stability of flows in a microfluidic T-junction

Flow regimes obtained as a consequence of two immiscible fluids interacting at a T-junction are presented for high Capillary numbers and different flow rates of the continuous and dispersed phases. Through lattice Boltzmann based simulations, a regime map is created that distinguishes parallel flows from droplet flows. Simulations show the dependence of flow rates and viscosity ratio on the volume of droplets formed, which is compared with existing experimental data. At high Capillary numbers, the transition zone which separates parallel and droplet flows shrinks, and is influenced by the viscosity ratio as well.

Determination of the effective thermal diffusivity of nanofluids by the double hot-wire technique

This paper introduces a new transient double hot-wire technique for the direct measurement of the thermal diffusivity of nanofluids. A correlation to be used with the double hot-wire technique to calculate the effective thermal diffusivity of nanofluids is also developed. Several types of nanofluids were prepared by suspending different volume percentages (1–5%) of titanium dioxide (TiO2), aluminium oxide (Al2O3) and aluminium (Al) nanoparticles in ethylene glycol and engine oil. The thermal diffusivities of these nanofluids determined directly by this technique were found to increase substantially with the increased volume fraction of nanoparticles in base fluids. Based on the calibration results obtained for the base fluids, ethylene glycol and engine oil, the measurement error is estimated to be within 1.2%. The measured thermal diffusivities of nanofluids were found to be significantly higher than those calculated from the thermal diffusivity expression (i.e. αeff = keff/(ρ cp)eff) by using the effective thermal conductivities and volumetric specific heats obtained from the conventional hot-wire method and from the volume fraction mixture rule, respectively.

Thermally controlled droplet formation in flow focusing geometry : formation regimes and effect of nanoparticle suspension

This paper reports experimental investigations on the droplet formation of deionized water and a nanofluid in a heat-induced microfluidic flow focusing device. Besides the effect of temperature, the effects of nanoparticle suspension (nanofluid) and the flow rate of aqueous fluid on the droplet formation and size manipulation were studied. At constant flow rates of the two liquids, three different droplet breakup regimes were observed and their transition capillary numbers as well as temperatures were identified. The heat generated by an integrated microheater changes the droplet formation process. Increasing the temperature enlarges the size of the droplets significantly. These results also demonstrate that the titanium oxide (15 nm)/deionized water-based nanofluid exhibits similar characteristics in droplet formation at different temperatures and any small change in the flow rate of this nanofluid has little impact on the size of the droplets formed in a flow focusing geometry.

Thermally mediated control of liquid microdroplets at a bifurcation

The ability to precisely control the motion of droplets is essential in droplet-based microfluidics. It serves as the basis for various droplet-based devices. This paper presents a thermal control technique for microdroplets at a bifurcation. Control was achieved using an integrated microheater that simultaneously induces a reduction in fluidic resistance and thermocapillarity. The temperature of the heater was monitored by an integrated temperature sensor. At a bifurcation with symmetric branches, a droplet can be split into two daughter droplets of controllable sizes or entirely switched into a desired branch. The physics of this phenomenon was investigated with the help of a numerical model. Splitting and switching were demonstrated within an operational temperature range 25‐38 ◦ C. The relatively low operational temperature range allows this technique to be used for droplets containing biological samples. The present control concept is not limited to bifurcations, but can be employed in other geometries.

A MODEL FOR PREDICTING THE EFFECTIVE THERMAL CONDUCTIVITY OF NANOPARTICLE-FLUID SUSPENSIONS

The uniformity and homogeneously dispersed nanoparticles in base fluids contribute to enhanced thermal conductivity of the mixture. By considering the uniformity and geometrical structures (e.g., body-centered cubic) of homogeneously dispersed nanoparticles in base fluids, a model for determining the effective thermal conductivity (ETC) of such nanoparticle-fluid suspensions, commonly known as nanofluids is proposed in this study. The theoretical results of the effective thermal conductivities of TiO2/Deionized (DI) water and Al2O3/DI water-based nanofluids are presented, and they are found to be in good agreement with our experimental results and also with those reported in the literature. The new model presented in this study shows a better prediction of the effective thermal conductivity of nanofluids compared to other classical models attributed to Maxwell, Hamilton–Crosser, and Bruggeman.

An Experimental Study of Surface Tension-Dependent Pool Boiling Characteristics of Carbon Nanotubes-Nanofluids

This paper reports an experimental investigation of the pool boiling heat transfer characteristics of single-walled carbon nanotubes (SWCNTs)-nanofluids. Two main characteristics were studied to identify their influence on boiling heat transfer: one is the surface tension through the addition of surfactant and the other is the chemical treatment of nanotubes sidewalls (i.e. oxidized and untreated sidewalls). A Transmission Electron Microscope was used to study the morphology of the functionalized nanotubes and their deposition on heater wire. The maximum enhancement of both the critical and burnout heat fluxes of this nanofluid over those of the pure deionized water are found to be 492% and 265%, respectively at a surfactant to carbon nanotubes concentration ratio of 1:5. This indicates that high enhancement of heat flux is possible and would depend on the concentration of the surfactants. Present results also demonstrate that CNT-nanofluids in a pool boiling environment can extend the saturated boiling regime and the burnout of the heated surface. The burnout heat flux is found to be a strong function of the relaxation of nanofluid surface tension with the base fluid. Based on the best fit of experimental data, an empirical correlation between the burnout heat flux of nanofluid and its relaxation of surface tension is introduced.© 2009 ASME

Convective heat transfer characteristics of aqueous TiO2 nanofluid under laminar flow conditions International Journal of Nanoscience

This paper reports an experimental investigation into force convective heat transfer of nanofluids flowing through a cylindrical minichannel under laminar flow and constant wall heat flux conditions. Sample nanofluids were prepared by dispersing different volumetric concentrations (0.2–0.8%) of nanoparticles in deionized water. The results showed that both the convective heat transfer coefficient and the Nusselt number of the nanofluid increase considerably with the nanoparticle volume fraction as well as the Reynolds number. Along with the enhanced thermal conductivity of nanofluids, the migration, interactions, and Brownian motion of nanoparticles and the resulting disturbance of the boundary layer are responsible for the observed enhancement of heat transfer coefficients of nanofluids.

Determination of effective specific heat of nanofluids

The effective specific heat of several types of nanofluids are measured by transient double hot-wire technique. Sample nanofluids are prepared by suspending 1–5 volume percentages of titanium dioxide (TiO2), aluminium oxide (A12O3) and aluminium (Al) nanoparticles in various base fluids, such as deionised water, ethylene glycol and engine oil. The effective specific heats of these nanofluids were found to decrease substantially with increased volume fraction of nanoparticles. Besides particle volume fraction, particle materials and base fluids also have influence on the effective specific heat of nanofluids. Except Al/engine oil-based nanofluid, predictions of the effective specific heat of nanofluids by the volume fraction mixture rule-based model showed reasonably good agreement with the experimental results. Based on the calibration results obtained for the base fluids, the measurement error is estimated to be within 2.77%.

Thermal Conductivity of Nanoparticle Suspensions (Nanofluids)

This paper presents theoretical and experimental investigations on the enhanced thermal conductivity of nanofluids. The thermal conductivity of nanofluids is found to significantly increase with particle volume fraction. Taking into account the effects of the interfacial layer and particle size, two models (one for spherical nanoparticles and the other for cylindrical nanoparticles in base fluids) are developed to predict the effective thermal conductivity of nanofluids. The proposed models show good agreement with the experimental results and give better predictions of the effective thermal conductivity of nanofluids compared to existing models in the literature.