Phong Thanh Nguyen

Anomalous ion effects on rupture and lifetime of aqueous foam films formed from monovalent salt solutions up to saturation concentration

We report the effects of ions on rupture and lifetime of aqueous foam films formed from sodium chloride (NaCl), lithium chloride (LiCl), sodium acetate (NaAc), and sodium chlorate (NaClO 3) using microinterferometry. In the case of NaCl and LiCl, the foam films prepared from the salt solutions below 0.1 M were unstable they thinned until rupturing. The film lifetime measured from the first interferogram (appearing at a film thickness on the order of 500 nm) until the film rupture was only a second or so. However, relatively long lasting and nondraining films prepared from salt solutions above 0.1 M were observed. The film lifetime was significantly longer by 1 to 2 orders of magnitude, i.e., from 10 to 100 s. Importantly, both the film lifetime and the (average) thickness of the nondraining films increased with increasing salt concentration. This effect has not been observed with foam films stabilized by surfactants. The film lifetime and thickness also increased with increasing film radius. The films exhibited significant surface corrugations. The films with large radii often contained standing dimples. There was a critical film radius below which the films thinned until rupturing. In the cases of NaAc and NaClO 3, the films were unstable at all radii and salt concentrations they thinned until rupturing, ruling out the effect of solution viscosity on stabilizing the films.

Synthetic Bandwidth Radar for Ultra-Wideband Microwave Imaging Systems

A synthetic bandwidth radar as an approach to build ultra-wideband (UWB) imaging systems is presented. The method provides an effective solution to mitigate the challenges of UWB antennas implementation with ideal performance. The proposed method is implemented by dividing the utilized UWB into several channels, or sub-bands, and designing an antenna array that includes a number of antennas equal to the number of channels. Each of those antennas is designed to have excellent properties across its corresponding channel. As part of the proposed approach, a two-stage calibration procedure is used to accurately estimate the effective permittivity of a heterogeneous imaged object at different angles and the phase center of each antenna for accurate delay time estimation. When imaging an object, each of the antennas transmits and captures signals only at its channel. Those captured signals are properly combined and processed to form an image of the target that is better than the current systems that use array of UWB antennas. The presented method is tested on breast imaging using the band 3-10 GHz via simulations and measurements on a realistic heterogeneous phantom.

Modeling Human Head Tissues Using Fourth-Order Debye Model in Convolution-Based Three-Dimensional Finite-Difference Time-Domain

A fourth order Debye model is derived using genetic algorithms to represent the dispersive properties of the 17 tissues that form the human head. The derived model gives accurate estimation of the electrical properties of those tissues across the frequency band from 0.1 GHz to 3 GHz that can be used in microwave systems for head imaging. A convolution-based three-dimensional finite-difference time-domain (3D-FDTD) formulation is implemented for modeling the electromagnetic wave propagation in the dispersive head tissues whose frequency dependent properties are represented by the derived fourth-order Debye model. The presented results show that the proposed 3D-FDTD and fourth-order Debye model can accurately show the electromagnetic interaction between a wide band radiation and head tissues with low computational overhead and more accurate results compared with using multi-pole Cole-Cole model.

Drainage, rupture, and lifetime of deionized water films: effect of dissolved gases?

Gas bubbles coalesce in deionized (DI) water because the water (foam) films between the bubbles are not stable. The so-called hydrophobic attraction has been suggested as the cause of the film instability and the bubble coalescence. In this work, microinterferometry experiments show that foam films of ultrapure DI water can last up to 10 s and the contact time between the two gas bubble surfaces at close proximity (approximately 1 microm separation distance) significantly influences the film drainage, rupture, and lifetime. Specifically, when the two bubbles were first brought into contact, the films instantly ruptured at 0.5 microm thickness. However, the film drainage rate and rupture thickness sharply decreased and the film lifetime steeply increased with increasing contact time up to 10 min, but then they leveled off. The constant thickness of film rupture was around 35 nm. Possible contamination was vigorously investigated and ruled out. It is argued that migration of gases inherently dissolved in water might cause the transient behavior of the water films at the short contact time. The film drainage rate and instability at the long contact time were analyzed employing Eriksson et al.s phenomenological theory of long-range hydrophobic attraction (Eriksson, J. C.; Ljunggren, S.; Claesson, P. M., J. Chem. Soc., Faraday Trans. 2 1989, 85, 163-176) and the hypothesis of water molecular structure modified by dissolved gases, and the extended Stefan-Reynolds theory by incorporating the mobility of the air-DI-water interfaces.

Detection and localization of brain strokes in realistic 3-D human head phantom

Stroke is the second leading cause of death worldwide. The treatment depends on the location and extent of stroke and it should be started within 4.5 hours of the attack. Thus rapid detection is a must. Existing imaging systems do not provide a solution to monitor the patient in real time and not portable, thus requires patients movement. Addressing these limitations, a portable diagnostic system is proposed in this paper employing monostatic radar approach using only one antenna covering 73% fractional bandwidth centered at 1.85 GHz. A realistic three-dimensional (3-D) head phantom is generated to simulate different stroke conditions. The phantom anatomically holds the structure of a real human head with frequency dispersive electrical properties of respective biological tissues of head that are rigorously fitted with fourth-order Debye model for the simulation environment. A confocal imaging technique is used to map the abnormalities inside the head. The obtained images indicate the potential of the presented technique to detect and locate the position of both hemorrhagic and ischemic strokes.

Three-Dimensional Microwave Hyperthermia for Breast Cancer Treatment in a Realistic Environment Using Particle Swarm Optimization

In this paper, a technique for noninvasive microwave hyperthermia treatment for breast cancer is presented. In the proposed technique, microwave hyperthermia of patient-specific breast models is implemented using a three-dimensional (3-D) antenna array based on differential beam-steering subarrays to locally raise the temperature of the tumor to therapeutic values while keeping healthy tissue at normal body temperature. This approach is realized by optimizing the excitations (phases and amplitudes) of the antenna elements using the global optimization method particle swarm optimization. The antennae excitation phases are optimized to maximize the power at the tumor, whereas the amplitudes are optimized to accomplish the required temperature at the tumor. During the optimization, the technique ensures that no hotspots exist in healthy tissue. To implement the technique, a combination of linked electromagnetic and thermal analyses using MATLAB and the full-wave electromagnetic simulator is conducted. The technique is tested at 4.2 GHz, which is a compromise between the required power penetration and focusing, in a realistic simulation environment, which is built using a 3-D antenna array of 4 × 6 unidirectional antenna elements. The presented results on very dense 3-D breast models, which have the realistic dielectric and thermal properties, validate the capability of the proposed technique in focusing power at the exact location and volume of tumor even in the challenging cases where tumors are embedded in glands. Moreover, the models indicate the capability of the technique in dealing with tumors at different on- and off-axis locations within the breast with high efficiency in using the microwave power.

Realistic simulation environment to test microwave hyperthermia treatment of breast cancer

We report in this work, the generation of a realistic simulation environment to test breast cancer microwave hyperthermia treatment. We introduce firstly a realistic breast phantom created in the simulation environment CST Microwave Studio. The phantom is created from MRI images of real patient with resolutions of 0.5 mm × 0.5 mm × 0.5 mm. Then, a temperature simulation is carried out based on the breast phantom and a circular array of tapered slot antenna antennas operating across the band from 3 GHz to 6 GHz. Contrary to previous studies on microwave hyperthermia, this work shows a significant improvement in the accuracy of the thermal simulation using realistic phantom and directional antenna. With purpose of using the model for microwave breast cancer treatment, the temperature distribution in the model is calculated using the simulator CST Microwave Studio.

Wideband quasi-Yagi antenna with tapered driver

This paper presents a wideband quasi-Yagi antenna with a driven element in the form of tapered strip. The antenna is designed to be part of a circular array for a microwave imaging system. The feeding structure of the antenna includes a balun that is formed using a pair of microstrip to slotline transitions. The simulated results of the proposed antenna indicate more than 73% fractional bandwidth with more than 10 dB return loss and about 3 dBi to 5 dBi gain.

Ion Specific Electrolyte Effects on Thin Film Drainage in Nonaqueous Solvents Propylene Carbonate and Formamide

Electrolytes have been found to stabilize thin films in nonaqueous solvents propylene carbonate and formamide, in the absence of surfactant. The thin film balance microinterferometry technique has been used to measure film lifetimes, drainage kinetics, and rupture thicknesses for thin films between air-nonaqueous solution interfaces. Electrolytes that were previously found to inhibit bubble coalescence in bulk bubble column measurements also increase the lifetimes of individual thin films across a similar concentration range (from 0 to 0.3 M). We report that increasing the concentration of inhibiting electrolyte stabilizes the thin liquid film in two ways: the rate of film drainage decreases, and the film reaches a lower thickness before rupturing. In contrast, noninhibiting electrolyte shows little to no effect on film stability. We have here demonstrated that both drainage and rupture processes are affected by the addition of electrolyte and the effect on the thin film is thus ion specific.

Thermo-Dielectric Breast Phantom for Experimental Studies of Microwave Hyperthermia

An artificial breast phantom that has realistic dielectric and thermal properties is presented. The phantom aims at enabling the experimental validation of microwave hyperthermia treatment for breast cancer. To perform credible breast hyperthermia experiments, the phantom should emulate both of the dielectric (permittivity and conductivity) and thermal properties (specific heat capacity and thermal conductivity) of different breast tissues with the correct anatomy. The main challenge in fabricating such a phantom is in developing suitable mixtures of materials to emulate those properties across the frequency band of interest in hyperthermia and to fabricate the phantom with realistic anatomy. For anatomical accuracy, a patient-specific MRI model is utilized in a three-dimensional printer, which is used to cast molds of the different breast tissues. Those molds are then filled with tissue-mimicking chemicals, which are formed using low-cost and stable materials. Once fabricated, the dielectric properties are measured using a dielectric probe, while the thermal properties are measured using a differential scanning calorimeter. Our measurements confirm the suitability of the fabricated breast phantom across the band 3-5 GHz, which is the suitable band for microwave hyperthermia.