Understanding and modeling of complex interactions at the interface between a flowing fluid and a highly flexible solid boundary present major challenges with many applications in engineering and biology. This problem is particularly difficult to model when the bulk flow is time dependent or pulsatile. Flow control using biological surfaces or noise cancellation with compliant surfaces has received considerable attention from various research communities for studying cardiovascular disorders and Alzheimer neurological degeneration. Despite rapid advances in computational methods and theoretical development, complex interactions between deformable surfaces and near-wall flow remain only partially understood, hampering efforts to develop accurate models. Detailed experimental data are essential for guiding and validating modeling efforts. The PI's goal is to harness the momentum gathered during recent development of a cutting-edge 3-D imaging technique, digital holographic microscopy (DHM), to develop a technique to observe and quantify complex interactions at the interface directly. DHM will measure the wall deformation and 3-D velocity field near it simultaneously. The concurrent measurements are achieved by recording and tracking two groups of particles, one embedded in the deforming wall and the other located in the flow near it. DHM is uniquely capable of performing this task in 3-D. Subsequently, the technique will be extended to measure the distributions of pressure and stresses along the deforming wall. To provide unobstructed view on sample area, measurements will be performed in a special optically index-matched facility. This research includes development of instrumentation, an index-matched facility and methodology for all interfaces. This measurement technique will be applied to study three cases of interactions of a pulsating near wall flow with a deformable surface: Pulsating flow over homogeneous compliant surface intended to identify the similarities and differences in flow structure, wall stress and pressure distributions between rigid and compliant walls; a surface with a sudden change in compliance, intended to examine the role of compliance inhomogeneity in the generation of near wall flow structures and their impact on pressure and wall stress distribution; and motion of deformable droplets over a compliant surface, intended to quantify interactions among droplets, surrounding fluids and a deformable surface. Index-matched droplets with embedded particles with be developed for this phase. Conditional sampling over wide range of fluid features such as near wall coherent structure, wall stress distribution, pressure conditioned on wall deformation will obtain deformation-induced changes in flow characteristics. One can also assess the contribution of flow to wall deformation by conditioning upon certain flow characteristics, and from directly measuring surface forces. Analysis will provide two-way coupling mechanism between hydrodynamic loading and surface deformation. This measurement is expected to help understand this fundamental topic with wide range of applications in both engineering and biology. It will provide benefits to society, such as better healthcare through accurate decease diagnosis and patient specific risk assessment. The impacts of the research are expected to be broad. By extending DHM technique to fully quantify 3-D near wall flow and wall deformation, especially instantaneous pressure and wall stress distributions, the technique has a potential of changing the landscape of the research field and may lead to groundbreaking discoveries. The combination of research and educational activities will lay the foundations for a successful, well-rounded academic career. Undergraduate and graduate students will be involved in the research, and the interdisciplinary nature of this work will be reflected in the PI's approach to teaching and advising. The present educational plan is focused on expanding the popularity of digital holography by developing a kit that can record digital holograms using readily available cell-phone cameras. The software will be available for download, and will be advertised through teen popular websites like Facebook, MySpace, to reach a large audience and to recruit new diverse engineering students.
