García-Rodríguez, William E.

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    Generation and dynamic control of air microbubbles subjected to a complex 3D acoustic field
    (2016) García-Rodríguez, William E.; Cancelos, Silvina; College of Engineering; Díaz, Ruben; Just, Frederick; Department of Mechanical Engineering; Santana, Damaris
    The purpose of this research is to verify that the translational movement of a microbubble in a complex three-dimensional geometry subjected to a standing acoustic field, can be predicted with the acoustic pressure gradient computed numerically using finite element method. To study the movement of a microbubble, it is necessary to know the forces at which the bubble is exposed. When it is subjected to an acoustic standing field, the strongest force is the Bjerknes force, which is caused by the acoustic pressure gradient in the fluid. This force, depending of the acoustic wave intensity, dominates over the drag force and buoyancy force (see Section 5). The Bjerknes force is calculated as the temporal average of the product of the microbubble volume and the pressure gradient. Therefore, to compute the force magnitude in all the fluid domain, it is necessary to know the acoustic pressure in all the domain. Experimentally, the pressure distribution can only be measured at some specific locations, where it is possible to introduce a pressure transducer, those limiting the analysis of the microbubble translational motion. This problem can be solved if a numerical solution of the pressure value in all the domain is obtained. For this purpose, an acoustic chamber named as DCSP (Decompression Sickness Prototype) was built and characterized. This acoustic chamber is a solid structure which contains a fluid, and is excited by a piezoelectric (PZT) transducer, yielding an acoustic standing wave in all the domain. The experimental results were compared with the numerical results obtained through a theoretical model of wave propagation in fluids coupled with the structure and PZT. The problem was numerically solved with the method of finite elements, using a commercial software Comsol Multiphysics 4.0. For the experimental characterization, an electrical and mechanical frequency response was made, in which the change in conductance in the PZT terminals and the pressure change in the fluid were measured respectively. These measurements were made in a range of frequencies which allowed to obtain the resonant conditions of DCSP, and the pressure distribution along one specific region, which is evaluated at resonant conditions. The data measured in the experimental test section was used to calibrate the theoretical model, which allowed to obtain the pressure distribution in all the fluid domain and the Bjerknes force, obtaining finally the effect of the acoustic three-dimensional field over a microbubble of a specific size. The principal objective of this research is to capture the microbubble translational motion for different sizes. The DCSP was filled with a gel which is obtained from the Sodium Hyaluronate powder dissolved in distilled water, yielding Hyaluronic Acid (HA). This gel is 99% distilled water, therefore the majority of the acoustic properties are those of water, except the viscosity, being 23473 times higher than water viscosity, and the surface tension being 30% higher than the surface tension of water. HA has excellent properties for the visualization of the microbubble, allowing to better capture the motion of the microbubble when it is exposed to an acoustic field. Furthermore, the air diffusion in HA is easily observed, this is principally caused by the high viscosity. As the diffusion coefficient is inversely proportional to the viscosity, the diffusion coefficient in HA is much lower than in water (1/23454 times), causing the time for one microbubble of 100 µm to be completely dissolved in HA at 50% O2 saturation in 9 weeks compared to 4 minutes in water at 50% O2 saturation. Finally, we captured images of microbubbles with different sizes, generated inside of the DCSP and its motion was correlated with the predicted one according to the Bjerknes force obtained numerically.
  • Thumbnail Image
    Publication
    Generation and dynamic control of air microbubbles subjected to a complex 3D acoustic field
    (2016) García-Rodríguez, William E.; Cancelos, Silvina; College of Engineering; Díaz, Ruben; Just-Agosto, Frederick; Department of Mechanical Engineering; Santana, Damaris
    The purpose of this research is to verify that the translational movement of a microbubble in a complex three-dimensional geometry subjected to a standing acoustic field, can be predicted with the acoustic pressure gradient computed numerically using finite element method. To study the movement of a microbubble, it is necessary to know the forces at which the bubble is exposed. When it is subjected to an acoustic standing field, the strongest force is the Bjerknes force, which is caused by the acoustic pressure gradient in the fluid. This force, depending of the acoustic wave intensity, dominates over the drag force and buoyancy force (see Section 5). The Bjerknes force is calculated as the temporal average of the product of the microbubble volume and the pressure gradient. Therefore, to compute the force magnitude in all the fluid domain, it is necessary to know the acoustic pressure in all the domain. Experimentally, the pressure distribution can only be measured at some specific locations, where it is possible to introduce a pressure transducer, those limiting the analysis of the microbubble translational motion. This problem can be solved if a numerical solution of the pressure value in all the domain is obtained. For this purpose, an acoustic chamber named as DCSP (Decompression Sickness Prototype) was built and characterized. This acoustic chamber is a solid structure which contains a fluid, and is excited by a piezoelectric (PZT) transducer, yielding an acoustic standing wave in all the domain. The experimental results were compared with the numerical results obtained through a theoretical model of wave propagation in fluids coupled with the structure and PZT. The problem was numerically solved with the method of finite elements, using a commercial software Comsol Multiphysics 4.0. For the experimental characterization, an electrical and mechanical frequency response was made, in which the change in conductance in the PZT terminals and the pressure change in the fluid were measured respectively. These measurements were made in a range of frequencies which allowed to obtain the resonant conditions of DCSP, and the pressure distribution along one specific region, which is evaluated at resonant conditions. The data measured in the experimental test section was used to calibrate the theoretical model, which allowed to obtain the pressure distribution in all the fluid domain and the Bjerknes force, obtaining finally the effect of the acoustic three-dimensional field over a microbubble of a specific size. The principal objective of this research is to capture the microbubble translational motion for different sizes. The DCSP was filled with a gel which is obtained from the Sodium Hyaluronate powder dissolved in distilled water, yielding Hyaluronic Acid (HA). This gel is 99% distilled water, therefore the majority of the acoustic properties are those of water, except the viscosity, being 23473 times higher than water viscosity, and the surface tension being 30% higher than the surface tension of water. HA has excellent properties for the visualization of the microbubble, allowing to better capture the motion of the microbubble when it is exposed to an acoustic field. Furthermore, the air diffusion in HA is easily observed, this is principally caused by the high viscosity. As the diffusion coefficient is inversely proportional to the viscosity, the diffusion coefficient in HA is much lower than in water (1/23454 times), causing the time for one microbubble of 100 µm to be completely dissolved in HA at 50% O2 saturation in 9 weeks compared to 4 minutes in water at 50% O2 saturation. Finally, we captured images of microbubbles with different sizes, generated inside of the DCSP and its motion was correlated with the predicted one according to the Bjerknes force obtained numerically.