This image shows the 2-D micromanipulation of single particles using fluid flow. (a) This is a sample trajectory of a single particle manipulated in two dimensions using the trap. A predetermined trajectory was programmed to spell the letter "C." (b) This shows dynamic control of the surrounding medium of a trapped particle. A single fluorescein-coated polystyrene bead is trapped, and the surrounding medium is periodically exchanged in a preprogrammed manner between high pH and low pH buffer solutions. The intensity of fluorescence emission from the pH sensitive dye fluctuates as the surrounding medium changes, demonstrating the effectiveness of the buffer exchange in the trap. Adapted with permission 2013 American Chemical Society.