| The field of microfluidics, having undergone a revolution at the hands of multitudes of researchers around the world, is at somewhat of an impasse in further penetration into applications. This impasse can be attributed to several factors such as device volume requirements, and inability to meet cost requirements for entire lab-on-a-chip systems. One of the ways to get beyond this impasse is to develop microfluidic technology that provides a wide host of microfluidic capabilities on one platform, with CMOS compatible electronics as the driver. Ultrasonic microfluidics, relying on nonlinear ultrasonic effects in fluids, is one possibility for the integration of many microfluidic functions on one chip. Ultrasonics allows one to project gradients of ultrasonic wave energy at a distance to realize non-contact steady forces. These forces can be used to mix, pump, and separate species. To keep operating power low and CMOS compatible voltages, one would prefer to use low frequency ultrasound (50 kHz to 5 MHz). However, at low frequencies, the large wavelength can lead to challenges in energy localization. In this thesis, the challenge of energy localization is investigated using embedded microstructures inside microchannels. Surface micromachined polysilicon microstructures are excited by bulk vibrations. Frequency addressability of acoustic streaming at surface microstructures is demonstrated by driving the bulk structure at the microstructure resonance frequencies. Frequency dependent fluid motion is qualitatively deduced and experimentally verified using dimensional analysis of micro-acoustic-streaming. Localized vortices are shown to exhibit particle trapping capabilities. In order to sense these particles, a process flow to integrate coaxial electrical interconnects has been developed, with promising results on sensing polystyrene beads that are trapped in the vortices. |
