Zooplankton Forces and Moments
Many aquatic animals are microscopic and interact with the water around them at a range of velocities in which both viscous and inertial forces are important. In spite of their biological importance, hydrodynamic forces on bodies in this size and velocity range are poorly understood. We studied how the morphology and orientation of a variety of ecologically-important microscopic marine animals (copepod, veliger larva, barnacle nauplius and cyprid larvae) affect the forces they experience while swimming in the water column, and while on surfaces (e.g. predator tentacles, benthic substrata). We measured drag, lift, and side forces as well as moments about three axes for dynamically-scaled physical models of each animal. These forces and moments can reorient swimming animals, or push, lift, peel, or shear animals off surfaces. We found that body shape, orientation, and proximity to a surface had significant effects on the magnitudes of the forces and moments on the animals, particularly during extreme physical events. Our findings stress the importance of twisting and tumbling in conjunction with drag and lift forces.
Powdery Mildew Sporocarp Liberation and Transport
Much of fungal spore wind dispersal consists of spores that are 1-10 um in diameter, and which have terminal velocities on the order of mm/s. Such spores that are plant pathogens tend to be dispersed from sporophores that grow on the top of leaves. Conversely, there are some fungal plant pathogens that have evolved wind dispersal mechanisms of relatively massive entire sporocarps that are over 100um in diameter. Some species of powdery mildew are examples of this phenomenon. I am studying the mechanics and fluid dynamics that may explain liberation of these sporocarps, that, unlike much smaller spores, are found on the bottom of leaves. Possible hypotheses that explain massive spore liberation include unsteady, and steady forces and torques exerted by wind; aeroelastic shedding forces produced my turbulence of that wind or rain drop impacts; and visco-elastic properties of the sporocarp's structure. I am using high speed video of the organism, and physical modeling to determine which means of liberation these spores employ.
Furthermore, these sporocarps have dramatic shapes that seem to serve an aerodynamic purpose during the airborne transport stage of their dispersal. In fact, Phyllactinia, a particular genus I am studying resembles a tiny badminton birdie. Past scientific literature has equated this resemblance to a similarity in function, i.e. these spores rotate during free-fall and reach a aerodynamically stable orientation, thus always landing in the same orientation. I have confirmed with high speed video that these sporocarps indeed rotate and reach a stable state, and in the first 30ms of their descent. I have also measured that their terminal velocity is a function of their morphology. Further modeling experiments and video will elucidate precisely how this, the world's smallest shuttlecock, functions. The high-drag aerodynamically stable morphology of Phyllactinia also has some important implications for the mildew's ecology, such as a potential ability to aid in long distance dispersal during extreme wind events.
- Gonzalinajec, T. and Koehl, M. A. R. (2014) Hydrodynamic forces and moments on microscopic aquatic animals. Journal of Experimental Marine Biology and Ecology. (in press)