Surface Tension: Floating Paper Clip

The surface tension of water can be shown by suspending a paper clip on the surface of a beaker or petri dish filled with water. Ordinary tap water works just fine (at least in Philadelphia). The paper clip should be a small one made with smooth, not serrated wire. It should be clean of any surface dirt or chemicals, and dry. To facilitate handling, the inner end of the clip should be bent up, perpendicular to the plane of the clip. This should be done with care to keep the bottom side of the clip flat. That is, avoid kinking the first curve in the wire down as the end is bent up.

The clip is slowly lowered onto the water with its flat bottom parallel to the surface. This can be done by hand or by using tweezers or needle-nosed pliers. If a tool is used, check in advance for magnetism. A magnetic tool makes it very difficult to release the clip with no disturbance.

Once the clip is suspended, it is possible to go a step further. The water's surface tension can be broken with a drop of detergent, and the result is that the paper clip sinks immediately. This is done simply and dramatically by dipping a toothpick or thin rod into liquid dishwashing detergent, and then touching the bead of detergent to the surface of the water supporting the paper clip. The touch point can be on the far side of the beaker, but the clip drops to the bottom almost immediately. It is so fast that it has the appearance of an electric discharge effect. The use of a beaker makes the sinking most dramatic, but with a petri dish it is much easier to retrieve the paper clip to try again (after drying).