Digital Holography enables measurements of 3D locations and displacements of microscopic objects in space. It has the potential of revolutionizing microscopy, especially while studying multi-scale flow-structure interactions in engineering applications and hydrodynamic interactions of micro-organisms near an interface. The presentation introduces this technique, and then demonstrates its application variants in (a) performing 3D velocity measurement of turbulent shear flows near the wall, (b) measuring 3D structure deformation and fluid flows, and (c) tracking microorganisms including bacteria in 3D near a boundary and under unsteady flow stimuli.
In this talk, I will first focus on the simultaneous measurement of instantaneous wall stress and 3-D velocity distribution in the near-wall region of a turbulent boundary layer over a smooth wall, covering the viscous sublayer, buffer layer and lower portion of logarithmic layer (0<y+<150) at Re? = 50,000 and Re? = 1,470. The measurements are performed at a resolution of one wall unit (??=17 ?m) in all directions, allowing the wall stress measurement directly. Conditional sampling based on local stress maxima and minima reveals two types of 3-D buffer layer structures that generate extreme stress events (Fig. left). The characteristics of these structures and their implications will be discussed. I will conclude this section with our newly developed technique, Digital Holographic Virtual Object Removal (DiHVOR), which is capable of obtaining simultaneous 3-D structure deformation and flow around it. The simultaneous measurement of structural strain and near wall flow in an artificial artery facility will be presented.
The second part of the talk focuses on interaction, swimming and migration of micro-organisms. Firstly, aimed at understanding its shear responses, which has overarching implications in cell separation and thin layer formation, a model micro-algae, Dunaliella primolecta, is studied using a 3D digital holographic microscope and ?fluidics platform. We discover that when subject to high shear, D. primolecta “surfs” along the direction of positive flow vorticity. Unlike immobilized cells, motile D. primolecta does not rotate along Jeffrey orbits. We will show that by manipulating flow vorticity, we can guide cells to perform directional flocculation for efficient design of bioreactors for sustainable biofuel production and harvesting. Secondly, to demonstrate our capability, we have applied to measuring 3-D bacteria locomotion near a wall (Fig. right). Our measurement shows for the first time that a no-slip wall suppresses tumble and limits tumbling within an equatorial region parallel to the surface. Our theoretical and numerical analysis shows that the hydrodynamic forces necessary to initiate unbundling are reduced in the presence of wall. The model based on the proposed hydrodynamic hindrance principle predicts experimental observation well. This discovery opens a new paradigm of bio-fouling mitigation by mechanical control methods as well as of mechanistic investigations of microbial interactions at oil-water interfaces.
Dr. Jian Sheng received his doctorate from Laboratory of Experimental Fluid Dynamics at The Johns Hopkins University in 2007. He joined Department of Aerospace Engineering and Mechanics at Univ. of Minnesota as an assistant professor, 2008, after brief tenure as an assistant professor at Mechanical Engineering Department, U. of Kentucky. Since this fall, he joined Texas Tech University as the Edward Whitacre Endowed chair of Mechanical Engineering. He has established the laboratory for biological fluid mechanics and imaging. His research interests include flow structure interactions (FSI) in complex biomedical and energy systems; micro-/nano-scale interactions (nano-FSI) at biological and physical system interfaces; biological interactions in oil spill and development of experimental capabilities in multi-dimension, multi-component measurement. His research has been well supported by NIH, NSF, and GRI. He is the recipient of 2008 NSF Career Award. In the past few years, he has authored and co-authored 29 journal and conference papers with over 400 citations. The research on micro-organism locomotion and biophysical interactions under dynamical conditions has been published in Proceedings of Academy of Science (PNAS) and Annual Review of Fluid Mechanics (ARFM).