Capillary tubes are tiny glass tools built for tiny amounts of liquid. Their narrow openings use capillary action—the same force that helps water climb through plant stems—to draw fluid upward without a pump.

Their simplicity is part of their power. A capillary tube uses surface tension, viscosity, and attraction to glass to move liquid.

These tubes are mostly used to draw blood for laboratory tests, but they are useful throughout chemistry, microscopy, chromatography, and other scientific areas where a single drop of liquid can carry useful information.

Try it out: Place a drop of colored water, ink, or food coloring on a plate. Touch one end of a capillary tube to the liquid and watch it climb upward on its own. Try comparing different liquids. Which liquid rises fastest? Which rises highest? The differences reveal how viscosity and surface tension shape the behavior of fluids at a miniature scale.

100-pack of heparinized, glass capillary tubes.