Sylvain Coulombe

Small particles, big impacts: A review of the diverse applications of nanofluids

Nanofluids—a simple product of the emerging world of nanotechnology—are suspensions of nanoparticles (nominally 1–100 nm in size) in conventional base fluids such as water, oils, or glycols. Nanofluids have seen enormous growth in popularity since they were proposed by Choi in 1995. In the year 2011 alone, there were nearly 700 research articles where the term nanofluid was used in the title, showing rapid growth from 2006 (175) and 2001 (10). The first decade of nanofluid research was primarily focused on measuring and modeling fundamental thermophysical properties of nanofluids (thermal conductivity, density, viscosity, heat transfer coefficient). Recent research, however, explores the performance of nanofluids in a wide variety of other applications. Analyzing the available body of research to date, this article presents recent trends and future possibilities for nanofluids research and suggests which applications will see the most significant improvement from employing nanofluids.

Design and preliminary characterization of a miniature pulsed RF APGD torch with downstream injection of the source of reactive species

The design of a miniature low-power atmospheric pressure glow discharge torch (APGD-t) and the results of its preliminary electrical and spectroscopic characterization are presented. A capacitively-coupled pulsed RF (13.56 MHz) helium plasma jet is formed in a converging confinement tube and O2 is injected downstream in the plasma afterglow region through a capillary electrode. With 1 SLM He, the APGD-t produced a non-thermal plasma jet of 500 µm-diameter and ≈2.5 mm-long at power levels ranging from 1 to 5 W. At ≈1 W, the gas temperature and He excitation temperature near the nozzle exit were ≈50°C and slightly below 2000 K, respectively. The breakdown voltage in 1 SLM He is approximately 220 Vpk−to−0. Careful electric probe measurements and circuit analysis revealed the strong effect of the voltage probe on the total load impedance. The injection of 10 SCCM O2 through the capillary electrode led to the transport of atomic O further downstream in the plasma jet and to a slight increase of the He excitation temperature without significant effects on the electrical properties and jet length. Alternatively, the addition of an equivalent amount of O2 (1 v/v%) to the plasma-forming gas affected the electrical properties slightly, but led to a drastic contraction of the plasma jet. The atomic oxygen production and transport conditions provided by the APGD-t are promising for precise bio-applications such as the treatment of skin tissues and cells.

Cell treatment and surface functionalization using a miniature atmospheric pressure glow discharge plasma torch

A miniature atmospheric pressure glow discharge plasma torch was used to detach cells from a polystyrene Petri dish. The detached cells were successfully transplanted to a second dish and a proliferation assay showed the transplanted cells continued to grow. Propidium iodide diffused into the cells, suggesting that the cell membrane had been permeabilized, yet the cells remained viable 24 h after treatment. In separate experiments, hydrophobic, bacteriological grade polystyrene Petri dishes were functionalized. The plasma treatment reduced the contact angle from 93° to 35°, and promoted cell adhesion. Two different torch nozzles, 500 µm and 150 µm in internal diameter, were used in the surface functionalization experiments. The width of the tracks functionalized by the torch, as visualized by cell adhesion, was approximately twice the inside diameter of the nozzle. These results indicate that the miniature plasma torch could be used in biological micropatterning, as it does not use chemicals like the present photolithographic techniques. Due to its small size and manouvrability, the torch also has the ability to pattern complex 3D surfaces.

Miniature atmospheric pressure glow discharge torch (APGD- t ) for local biomedical applications

The operating parameters of a miniature atmospheric pressure glow discharge torch (APGD-t) are optimized for the production of excited atomic oxygen, and the effect of the plasma jet on endothelial cells grown in Petri dishes is studied. We first demonstrate the importance of accounting for the effect of the voltage probe used to measure the electrical parameters of the torch on its ignition and operation characteristics. When operated with a main plasma gas flow rate of 1 SLM He and a power level of ~1 W, the torch shows an optimum in the production of excited atomic oxygen for a O2 flow of ~3.5 SCCM injected downstream from the plasma-forming region through a capillary electrode (i.e., 0.35 v/v % O2/He). It is shown that endothelial cells are detached from the Petri dishes surface under the action of the optimized plasma jet and that this effect does not originate from heating and fluid shearing effects. It is postulated that the cell detachment is caused solely by plasma-induced biochemical processes taking place at the cell-substrate interface.

Feasibility of nanofluid-based optical filters

In this article we report recent modeling and design work indicating that mixtures of nanoparticles in liquids can be used as an alternative to conventional optical filters. The major motivation for creating liquid optical filters is that they can be pumped in and out of a system to meet transient needs in an application. To demonstrate the versatility of this new class of filters, we present the design of nanofluids for use as long-pass, short-pass, and bandpass optical filters using a simple Monte Carlo optimization procedure. With relatively simple mixtures, we achieve filters with <15% mean-squared deviation in transmittance from conventional filters. We also discuss the current commercial feasibility of nanofluid-based optical filters by including an estimation of todays off-the-shelf cost of the materials. While the limited availability of quality commercial nanoparticles makes it hard to compete with conventional filters, new synthesis methods and economies of scale could enable nanofluid-based optical filters in the near future. As such, this study lays the groundwork for creating a new class of selective optical filters for a wide range of applications, namely communications, electronics, optical sensors, lighting, photography, medicine, and many more.

Thermo-field emission: a comparative study

Thermo-field emission current densities calculated by the commonly used Richardson - Dushman equation corrected for the Schottky effect are compared with those calculated using the more accurate treatment of Murphy and Good for a wide range of temperatures (1000 - 5000 K), electric field strengths () and work functions (2 - 5 eV) encountered in numerical modelling of the thermal arc-cathode interactions. Results show that the emission current densities predicted by the Richardson - Dushman equation are always lower than those predicted by the more accurate treatment. The disagreement between the two predictions is in the range 20 - 30% for the conditions encountered in numerical modelling of the attachment of a thermal arc to a hot tungsten cathode whereas it is much larger when a cold copper cathode is considered (>175%). It is suggested that the more accurate treatment of Murphy and Good should be used in order to increase the accuracy of prediction of the numerical models.

A total internal reflection fluorescence microscopy study of mass diffusion enhancement in water-based alumina nanofluids

Mass diffusion of rhodamine 6G (R6G) in water-based alumina nanofluids is studied by means of total internal reflection fluorescence (TIRF) microscopy. We report a mass diffusivity enhancement that reaches an order of magnitude in a 2 vol % nanofluid when compared to the value in deionized water. Since experiments were performed with positively charged R6G, interfacial complexation between the dye and the nanoparticles was not observed. The effect of local density variations on mass diffusivity measurements is also addressed. An explanation for the enhancement of mass diffusion is presented using arguments based on dispersion, and it is shown that it correctly describes the order of magnitude differences between the thermal conductivity and mass diffusivity enhancements reported in the literature.

Atmospheric Pressure Plasma Jet Deposition of Patterned Polymer Films for Cell Culture Applications

A miniature nonthermal atmospheric plasma jet was used to deposit patterns of plasma polymers on standard Pyrex Petri dishes from an argon/acetylene gas mixture. The injection of the C2H2 monomer through the powered capillary electrode ending in the nozzle section allowed a stable operation of the plasma jet and the successful deposition of plasma-polymer tracks ap500 mum wide. The line-averaged gas temperature was measured from the OH molecular band emission originating from the entrainment of air into the plasma jet. The gas temperature decreased monotonically from ap440 K at the nozzle exit to ap385 K at 5 mm downstream. The emission profiles of the C2 (516 nm) band showed a peak ap0.5 mm downstream the nozzle exit. The maximum emission for the CH band (431 nm) was obtained at the torch exit. An ATR-FTIR spectroscopy analysis of the deposited plasma-polymer films revealed the presence of C-H, O-H, and C = O functional groups. Mammalian cells grown on the argon/acetylene plasma-treated dishes showed migration and greater density compared to the control and dishes treated with an argon plasma jet. This preliminary study opens the door to the localized enhancement of cell coverage on various substrates (shape and material).

Arc-cold cathode interactions: parametric dependence on local pressure

A numerical model describing the attachment of an electric arc on a vaporizing non-refractory cathode is developed and applied to a Cu cathode. The model describes the arc - cathode interaction zone by a combination of a quasi-stationary vacuum arc cathode spot model with a collisionless cathode sheath model for the current transfer in the cathode region. The conditions of pressure and electron temperature within the cathode spot plasma necessary to account for current densities ranging from A (upper limit for non-vaporizing cathode models) to A are presented. Results show that current densities higher than A can only be accounted for with metallic plasma pressures exceeding 35 atm and electron temperatures ranging from 1 to 2 eV within the cathode spots. The current transfer to the cathode is mainly assumed by the ions at low current densities ( A ) and by the thermo-field electrons for higher current densities. The heat flux to the cathode surface under the spots is mainly due to the flux of returning ions and ranges from to W for current densities ranging from to A . At low current densities (), the main heat loss is by conduction through the cathode while at high current densities, the Nottingham cooling associated with the thermo-field emission of electrons dominates. The model allowed us to define the upper and lower limits for the vacuum erosion rate by vaporization of the cathode. It is shown that the experimentally obtained vacuum erosion rate value for Cu falls between both limits for an electron temperature within the cathode spot of 1 to 2 eV.

A comparison of electron-emission equations used in arc - cathode interaction calculations

The predictions of a numerical model of the cathode-sheath region of high-pressure arcs using three different equations for the cathodic electron-emission current density are compared for two typical arc - cathode systems. These equations for cathodic electron emission include: (i) the field-enhanced thermionic emission (FEE) equation representative of the electron emission for high temperatures and moderate electric field strengths ; (ii) the Murphy and Good (MG) equation for thermo-field (T-F) emission valid for high temperatures and high surface electric field strengths in the range about - ; and (iii) the Murphy and Good equation for T-F emission enhanced by the presence of a high density of slowly moving ions in the cathode region (the MG+I equation). Results show that, in the case of a metal vapour arc (representative of vacuum arcs and high-pressure arcs on non-refractory cathodes) the use of the MG+I equation is always prescribed due to the formation of the high-local-pressure cathode-spot plasma. For high-pressure arcs on refractory cathodes the results show that, at atmospheric pressure, the generally used FEE equation introduces an underestimation by at least around 20% of the cathodic electron current density compared with the MG+I equation. The same comparison but for an ambient pressure of 50 atm shows that this underestimation becomes considerable.