Current MEMS microfilter improvement is driven by the need to perform absolute separation of micron-sized particles from milliliter-scale uid volumes. Such filtration is an essential step in many biological and medical applications. For example, routine blood tests require separating blood plasma from whole blood. Using microfabrication techniques it has recently become possible to create integrated miniaturized fluid-handling devices. Promising applications include miniaturized systems for the filtration, fractionation, and manipulation of biological cells and DNA. Before such promise can be realized, however, individual components of these systems must be developed.
This research focuses on the problem of filter clogging. Our intent was to show that acoustic forces generated by a exural plate wave (FPW) device act to reduce filter fouling. In an FPW-based assembly, acoustic agitation prevents particles from adhering to the filter element, while pumping sweeps away the particles that could potentially clog the filter. MEMS devices have already been shown to be effective for absolute filtration, but clogging has made them impractical for most applications. Also previously shown was the ability of FPW devices to move particles in a pumped liquid. Recognizing the FPW pumping as a self-cleaning mechanism for microfilters presents exciting possibilities for reduced filter fouling, allowing larger volumes of uid to be filtered and extending filter lifetime.
A typical FPW device consists of a piezoelectric film (ZnO) on a 1-µm thick silicon nitride membrane supported by a silicon frame. The ZnO is sandwiched between a thin polysilicon ground plane and patterned aluminum fingers. This structure is referred to as an interdigitated transducers (IDT). When driven with a sinusoidal voltage, the IDT generates acoustic waves that propagate across the membrane. The new FPW-based microfilters have unidirectional IDTs across half of the membrane and pores in the other half, and are suitable for in-line filtering. Lithographically defined pores allow for a well-defined size threshold for particle removal, while acoustic forces act to reduce filter fouling.
Both lateral and vertical forces acting on a particle were identified and evaluated for typical operating conditions for particles of radii 0.1, 1.0, and 10 µm. Calculations show that the Stokes drag due to pressure-driven flow is the largest lateral force for all particle sizes. Van der Waals forces (for 0.1, and 1.0 µm spheres) and the Newtonian "added-mass" force (for 10 µm spheres) were found to be the largest vertical forces.
A stroboscopic laser interferometer developed by Dr. C. Rembe was used to measure dynamic membrane displacement for 800-µm wavelength FPW devices. These measurements revealed the presence of large (60 nm peak-to-peak) standing waves. Acoustic damping material (RTV silicone) was placed at both ends of the device membrane and found to be very effective in reducing unwanted wave reections. It was demonstrated that a device with unidirectional IDTs and acoustic damping launches a wave traveling in a single direction, and that this direction may be reversed by changing the phasing of the drive signal. Several sets of tests were performed in order to characterize FPW filter performance. The first tests investigated the ability of various wavelength pumps to manipulate different-sized spheres. Pumping of 44 µm diameter spheres was demonstrated with an 800 µm wavelength device. Basic filter characterization was performed by measuring pressure drop vs. uid ow rate. Attempts were made to measure the effect that acoustic pumping has on pressure, but it was found that unwanted recirculation allowed pressure to equalize. In the most important set of tests, FPW filters were allowed to clog, and then were agitated with acoustic waves to demonstrate clearing of pores. Acoustic particle manipulation wasshown to free 2, 6, and 10 µm diameter spheres and sweep them away from once-blocked pores in both 4 µm and 8 µm filters.