High Resolution 3-D Measurements of Small-Scale Interactions and Transport near a Bio/physical Interface
Dr. Jian Sheng
11:00 AM on 2008-04-24
227 Akerman Hall
Digital Holographic Microscopy (DHM) enables measurements of 3D locations and displacements of microscopic objects in space. It has the potential of revolutionizing microscopy, especially while studying small-scale dynamic transport phenomena. The presentation introduces this technique, and then demonstrates its implementations in tracking microorganisms as well as in performing 3D velocity measurement of turbulent shear flows. 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 The talk will conclude with several recent biophysical interaction studies using newly developed cinematic DHM. I will focus on the investigation of prey-induced changes in swimming behavior of predatory dinoflagellates whose motility influences macro-assemblages of these cells into often-observed dense “blooms”. Cinematic DHM allows us, for the first time, track thousands of organisms per mm3 in 4-dimensional space and time over a volume with depth extending to more than 1000 times the organism size, without loss of resolution. Changes in swimming behavior are extracted from large populations of mixotrophic Karlodinium veneficum 2064 and heterotrophic Pfiesteria piscicida, respectively in the event of the introduction of their prey, Storeatula major and Rhodomonas salina. Nearest Neighbor Distance (cell-cell interaction) reveals that predator and prey cells are randomly distributed relative to themselves, but in mixed culture, each predator clusters around its respective prey. Both dinoflagellate species exhibit complex, highly variable swimming behavior as characterized by radius and pitch of helical swimming trajectories and by translational and angular velocity. K. veneficum moves in both left and right hand helices, while P. piscicida swims only in right hand helices. When presented with its prey (S. major), the slower K. veneficum reduces its velocity, radius and pitch, but increases its angular velocity, changes that reduce its hydrodynamic signature while still scanning its environment as "a spinning antenna". Conversely, the faster P. piscicida increases its speed, radius and angular velocity, but slightly reduces its pitch when exposed to prey (Rhodomonas), suggesting preferred predation tactics of an “active hunter”.