Generation and rotation of a single water droplet by hypodermic needle in ultrasonic vibration

We show that a hypodermic needle can generate and rotate a small droplet of water around its needle tip when the hypodermic needle is mechanically driven by an acoustic needle. The water droplet increases in size with the driving time, and begins to rotate around the tip of the hypodermic needle when the diameter of the droplet is large enough. The rotating droplet falls off when its diameter is larger than a critical value of several millimeters. For a given operating voltage, the revolution speed, maximum diameter and lifetime of a droplet can be controlled by the operating frequency of the ultrasonic transducer. A revolution speed up to 16.7 rev/s has been achieved.