The (Anti) Gravity Machine: A Tracking Microscope to Study Biotic/Abiotic Systems in Gravitational Fields*

It is often desirable to observe freely moving microscale objects over macroscale length and time scales. For instance, microscale marine plankton exhibit vertical migrations over distances that are several orders greater than their individual size. Currently the problem of studying such objects at sub-cellular resolution while allowing free movement over macro-length scales is a fundamental challenge. We present a simple table-top solution to this problem for the case of an object with significant vertical movement due to the effects of motility, hydrodynamics and gravity. Our solution, a “hydrodynamic treadmill”, consists of an annular fluidic chamber with a horizontal rotation axis, which allows unrestricted vertical motion of the object. Object movements are compensated by rotation of the chamber using a closed-loop tracking system such that the object remains at a fixed point in the lab frame. We demonstrate this method by tracking the behavior of marine plankton swimming freely in the vertical direction over several hours and with displacements of tens of meters at high spatiotemporal resolution. We further use this method to observe abiotic systems such as sedimenting/rising microparticles and drops in an ambient fluid over long time scales.