I had some fun yesterday photographing oil and water mixed together. I used a shallow glass bowl with a flat bottom and filled it partially with water. Then I placed it about two feet above a piece of 8 1/2 x 11 compute paper on which I had smeared various colors of paint. I dropped a tiny amount of vegetable oil in the water, and positioned a single flash off-camera pointing to the paint. I triggered the flash with a wireless unit, the Pocket Wizard. With a 50mm macro lens and one extension tube, I took some shots at f/16 but didn't like the results because the paint in the background was too defined due to the extensive depth of field. When I used f/8, I liked the results much better.
I hand held the camera and made sure the back of the camera was as parallel as possible to the surface of the water. When the camera back (i.e. the plane of the sensor) is parallel, you can have complete depth of field of the water's surface at any lens aperture.
For a variation, I pointed the flash up to the water bowl from below and created space-like images that I thought were pretty cool.
I used 100 ISO for these pictures and a daylight white balance.