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Bubble-based microfluidic devices have been proven to be useful for many biological and chemical studies. These bubble-based microdevices are particularly useful when operated at the trapped bubbles' resonance frequencies. In this work, we present an analytical expression that can be used to predict the resonant frequency of a bubble trapped over an arbitrary shape. Also, the effect of viscosity on the dispersion characteristics of trapped bubbles is determined. A good agreement between experimental data and theoretical results is observed for resonant frequency of bubbles trapped over different-sized rectangular-shaped structures, indicating that our expression can be valuable in determining optimized operational parameters for many bubble-based microfluidic devices. Furthermore, we provide a close estimate for the harmonics and a method to determine the dispersion characteristics of a bubble trapped over circular shapes. Finally, we present a new method to predict fluid properties in microfluidic devices and complement the explanation of acoustic microstreaming. V C 2013 AIP Publishing LLC. [http://dx

We investigated bubble oscillation and its induced enhancement of mass transfer in a liquid-liquid extraction process with an acoustically-driven, bubble-based microfluidic device. The oscillation of individually trapped bubbles, of known sizes, in microchannels was studied at both a fixed frequency, and over a range of frequencies. Resonant frequencies were analytically identified and were found to be in agreement with the experimental observations. The acoustic streaming induced by the bubble oscillation was identified as the cause of this enhanced extraction. Experiments extracting Rhodanmine B from an aqueous phase (DI water) to an organic phase (1-octanol) were performed to determine the relationship between extraction efficiency and applied acoustic power. The enhanced efficiency in mass transport via these acoustic-energy-assisted processes was confirmed by comparisons against a pure diffusion-based process.

The acoustic scattering characteristics of ∼10 μm-long microfibers of Parylene C embedded in water were investigated, towards the eventual goal of designing polymeric sculptured thin films for biomedical applications. The chosen microfibers were upright circular-cylindrical, slanted circular-cylindrical, chevronic, and helical in shape. A combination of numerical and analytical techniques was adopted to examine the scattering of plane waves in a spectral regime spanning the lower few eigenfrequencies of the microfibers. Certain maximums in the spectrums of the forward and back scattering efficiencies arise from the phenomenon of creeping waves. The same phenomenon affects the total scattering efficiency in some instances. The spectrums of all efficiencies exhibit the geometric symmetry of a microfiber in relation to the direction of propagation of the incident plane wave. Similarities in the shapes of the slanted circular-cylindrical and the chevronic microfibers are reflected in the spectrums of their scattering efficiencies. A highly compliant microfiber has shorter and broader peaks than a less compliant microfiber in the spectrums of the total scattering efficiency. The proper design of polymeric sculptured thin films will benefit from the knowledge gained of the directions of maximum scattering from individual microfibers.

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