Fluid flow is both beautiful and central to many natural sciences. For example, understanding water flow is fundamental to understanding swimming, feeding, and settlement in marine organisms. For land animals, flow is important for flight, the sense of smell, and heat balance (e.g. wind chill). One can visualize fluid flow easily with ink or dyes (e.g. food coloring or fluorescein) to get a qualitative picture of flow patterns, but how can one quantify them?
One simple and striking method is to use a sheet of light to illuminate a thin plane of the fluid, and take images of particle tracks through the plane (with the camera oriented perpendicular to the direction of illumination). In the image on the right, a laser light sheet shines perpendicularly on a colony of a marine organism called a bryozoan. The black-and-white images of the video were superimposed in different colors to leave rainbow streaks: faster particles produced long rainbows in the false colored image, slow particles produced short rainbows, and the still tentacles of the animals appear white because all the colors add to white . Because the light is in a single plane, one can determine the three dimensional position of each particle in the image.
Making a light sheet is quite easy. All one needs is a laser, a glass rod (acrylic or polycarbonate work fine too), and a way to mount the rod in the laser beam. A cheap laser pointer works fine for the laser, although a stronger, focusable laser, which one can plug into a wall socket is more convenient than one which runs on a battery. It is also preferable to set it up so one does not have to continually hold down a button. When one shines the laser at the center of the cylinder (with the beam perpendicular to the axis of the cylinder), the light gets spread out perpendicular to the axis of the cylinder, but doesn't get bent along the axis of the cylinder. Therefore one ends up with a light sheet. The main limitation is that the light sheet is brighter towards the center, but it is usually adequate. Because laser pointers are fairly weak, it helps to have a camera with high light sensitivity. Obviously you need to use be aware of the safety precautions to prevent eye damage with the laser.
The next step is adding particles to see the water flow. One needs particles that are close enough to neutral buoyancy, so that particle movements follow the fluid flow, and that are reflective enough to show up in the image. A particle size that is small relative to the size of the field of view, and a high particle concentration are also good. Among other things, people have used small microscopic glass spheres, brine shrimp eggs, starch, carmine particles, or just whatever suspended sediment is in the water naturally.
One can get lots of interesting information — and beautiful images — without sophisticated methods. For example, one can see how the flow around one organism affects the flow around its neighbor, or how organisms orient with respect to the flow. Depending on flow speeds, particle concentrations, and image quality, one can then use various methods such as manual particle tracking, automated particle tracking, or "particle image velocimetry" (PIV) to quantify the two dimensional flow in the plane. For the technically more sophisticated, one can often get some 3D flow information based on the fact that volume is conserved: if the flow converges on a point in the plane, there has to be flow out of the plane. Combining multiple planes allows one to build up 3D information.