Shirley C. Tsai

Role of Interparticular Van der Waals Force in Rheology of Concentrated Suspensions

This article presents the results of a study of the mechanisms that account for the pseudoplastic behavior of a concentrated suspension of noncolloidal particles. Specifically, the measured flow behavior index of the suspension is correlated with the calculated Hamaker constant for interparticular van der Waals attraction. As the interparticular attraction increases, the number of aggregated particles increases. In turn, the number of aggregated particles decreases as the shear rate increases. As a result, unless the interparticular attraction is negligible, the concentrated suspension is pseudoplastic at low and moderate shear rates, but becomes Newtonian at high shear rates when the hydrodynamic force predominates. The measured viscosities both for high shear Newtonian limits of polystyrene suspensions and for the Newtonian suspensions of glass beads in silicone oil are in excellent agreement with the predicted values of the rigid sphere model for colloidal suspensions. In addition, we found that while ...

The role of capillary waves in two-fluid atomization

A mechanistic study of two-fluid atomization has been carried out using a new spray technique called ultrasound-modulated two-fluid (UMTF) atomization. This technique is based on resonance between the liquid capillary waves generated by ultrasound and those generated by high-velocity air. Specifically, capillary waves are established on the surface of a liquid jet as it issues from a coaxial two-fluid atomizer, the nozzle tip of which vibrates at the same frequency as the ultrasound while the frequency of the capillary waves is only half of the ultrasound frequency. As these capillary waves travel downstream in the direction of air flow, their amplitude is further amplified by the air flowing around them. Atomization occurs when the wave amplitude becomes too great to maintain wave stability; the resulting drop sizes are proportional to the wavelength of the resonant capillary waves which is determined by the harmonic frequency of the ultrasound in accordance with the Kelvin equation. Theoretical calculations of the amplitude growth rate are based on two models of temporal instability of wind-generated capillary waves: Taylor’s dispersion relation and Jeffreys’ one-parameter (sheltering factor) model. Good agreements between the theoretical predictions by these models and the experimental results of how drop-size and size distributions are influenced by air velocity and surface tension led to the conclusion that Taylor-mode breakup of capillary waves plays a very important role in two-fluid atomization. Furthermore, all peak drop diameters can be accounted for by the harmonic frequencies of the ultrasound. Hence, it is further concluded that secondary atomization is negligible in co-flow two-fluid atomization of a water jet at air velocities up to 170 m/s and air-to-water mass ratio up to 5.6. In addition, uniform drops with diameters predetermined by the ultrasound frequency can be accomplished by adjusting the air velocity.

Flotation of ilmenite using benzyl arsonic acid and acidified sodium silicate

Abstract An ilmenite concentrate of 47.4% TiO2 has been obtained by froth flotation of an ilmenite ore containing 9% TiO2 using acidified sodium silicate as a gangue depressant and benzyl arsonic acid as the collector. The acidified sodium silicate forms negatively charged colloidal silica that is selectively adsorbed on the gangue minerals, but does not affect the adsorption of benzyl arsonic acid on ilmenite. The resulting TiO2 recovery reaches a maximum of 57% at the pH value that corresponds to the zero point of charge of ilmenite in the presence of benzyl arsonic acid.

High-frequency, silicon-based ultrasonic nozzles using multiple Fourier horns

This paper presents the design, simulation, and characterization of microfabricated 0.5 MHz, silicon-based, ultrasonic nozzles. Each nozzle is made of a piezoelectric drive section and a silicon resonator consisting of multiple Fourier horns, each with half wavelength design and twice amplitude magnification. Results of finite element three-dimensional (3-D) simulation using a commercial program predicted existence of one resonant frequency of pure longitudinal vibration. Both impedance analysis and measurement of longitudinal vibration confirmed the simulation results with one pure longitudinal vibration mode at the resonant frequency in excellent agreement with the design value. Furthermore, at the resonant frequency, the measured longitudinal vibration amplitude sit the nozzle tip increases as the number of Fourier horns (n) increases in good agreement with the theoretical values of 2/sup n/. Using this design, very high vibration amplitude gain at the nozzle tip can be achieved with no reduction in the tip cross-sectional area for contact of liquid to be atomized. Therefore, the required electric drive power should be drastically reduced, decreasing the likelihood of transducer failure in ultrasonic atomization.

Effects of Liquid Polarity on Rheology of Noncolloidal Suspensions

This paper presents the effects of liquid polarity on the rheology of suspensions with particle sizes ranging from 2 to 150 μm. We have found that liquid polarity has a substantial effect on the non‐Newtonian behavior of suspensions of graphite particles, but only a slight effect on that of polystyrene suspensions. This may be attributed to the greater interparticular van der Waals‐London dispersion force for graphite than for polystyrene. The suspensions of polystyrene spheres become Newtonian at very high shear rates when the interparticular van der Waals force is negligible as compared to the hydrodynamic force. This high shear Newtonian limit of relative viscosity is in excellent agreement with the value predicted by the rigid sphere model for colloidal suspensions.

Atomization of coal-water slurry using twin-fluid jet atomizer

This paper presents both experimental and modelling results of airblast atomization of coal-water mixtures or slurries (CWMs) using a twin-fluid jet atomizer. Specifically, the mass median diameter (MMD) of atomized droplets is related to the slurry properties and atomization conditions through its power dependencies on the Weber number (We), the Ohnesorge number (Z) and liquid or fuel-to-air mass ratio (WFWA), namely, MMDDF = (A · We−x + B · Z2x) (1 + WFWA)y, where DF = the diameter of the fuel passage and A, B, x and y are constants determined by the least square method. The coal particle size and size distribution has substantial effects on both the droplet sizes and the exponents x and y. In addition, the presence of hydrophobic coal particles shifts the fundamental mechanisms of airblast atomization to the predominance of the fuel viscosity included in Z over the liquid surface tension included in We. Other factors such as particle packing efficiency also make significant contributions.

Silicon-based megahertz ultrasonic nozzles for production of monodisperse micrometer-sized droplets

Monodisperse ethanol droplets 2.4 mum and water droplets 4.5 mum in diameter have been produced in ultrasonic atomization using 1.5- and 1.0-MHz microelectromechanical system (MEMS)-based silicon nozzles, respectively. The 1.5- and 1.0-MHz nozzles, each consisting of 3 Fourier horns in resonance, measured 1.20 cm times 0.15 cm times .11 cm and 1.79 cm times 0.21 cm times 0.11 cm, respectively, required electrical drive power as low as 0.25 W and could accommodate flow rates as high as 350 mul/min. As the liquid issues from the nozzle tip that vibrates longitudinally at the nozzle resonance frequency, a liquid film is maintained on the end face of the nozzle tip and standing capillary waves are formed on the free surface of the liquid film when the tip vibration amplitude exceeds a critical value due to Faraday instability. Temporal instability of the standing capillary waves, treated in terms of the unstable solutions (namely, time-dependant function with a positive Floquet exponent) to the corresponding Mathieu differential equation, is shown to be the underlying mechanism for atomization and production of such monodisperse droplets. The experimental results of nozzle resonance and atomization frequencies, droplet diameter, and critical vibration amplitude are all in excellent agreement with the predictions of the 3-D finite element simulation and the theory of Faraday instability responsible for atomization.

Flow visualization of Taylor-mode breakup of a viscous liquid jet

We recently reported a new spray technique called ultrasound-modulated two-fluid (UMTF) atomization and the pertinent “resonant liquid capillary wave (RLCW) theory” based on linear models of Taylor-mode breakup of capillary waves. In this article, flow visualizations of liquid jets near the nozzle tip are presented to verify the central assumption of the RLCW theory that the resonant liquid capillary wave in UMTF atomization is initiated by the ultrasound at the nozzle tip. Specifically, a bright band beneath the nozzle tip was seen in ultrasonic and UMTF atomization separately, but not in two-fluid atomization. The bright band can be attributed to scattering of laser light sheet by the capillary waves generated by the ultrasound on the intact liquid jet. As the capillary wave travels downstream in the direction of airflow, it is amplified by the air blowing around it and eventually collapsed into drops. Therefore, the jet breakup time can be determined by dividing the measured band length with the capill...

Miniaturized multiple Fourier-horn ultrasonic droplet generators for biomedical applications

Here we report micro-electro-mechanical system (MEMS)-based miniaturized silicon ultrasonic droplet generators of a new and simple nozzle architecture with multiple Fourier horns in resonance but without a central channel. The centimetre-sized nozzles operate at one to two MHz and a single vibration mode which readily facilitates temporal instability of Faraday waves to produce monodisperse droplets. Droplets with diameter range 2.2-4.6 μm are produced at high throughput of 420 μl min(-1) and very low electrical drive power of 80 mW. We also report the first theoretical prediction of the droplet diameter. The resulting MHz ultrasonic devices possess important advantages and demonstrate superior performance over earlier devices with a central channel and thus have high potential for biomedical applications such as efficient and effective delivery of inhaled medications and encapsulated therapy to the lung.

Ultrasonic atomization using MHz silicon-based multiple-Fourier horn nozzles

Monodispersed droplets are produced in ultrasonic atomization using a microelectromechanical system-based three-Fourier horn 0.5MHz silicon nozzle 3.66×0.38×0.11cm3 in size. As water enters the 200μm×200μm central channel of the nozzle, a curved thin liquid film is maintained at the nozzle tip that vibrates at the resonance frequency of 486.5kHz, resulting in the formation of standing capillary waves on the free film surface. Temporal instability of these standing capillary waves occurs as the tip vibration amplitude exceeds a threshold, and a spray of droplets (mist) is produced. The measured droplet diameter of 7.0μm is in good agreement with the 6.7μm diameter calculated by 0.34 times the capillary wavelength.