Physicists create first-ever mechanical device that measures the mass of a single molecule: The device which is only a couple millionths of a meter in size consists of a tiny, vibrating bridge-like structure. When a particle or molecule lands on the bridge, its mass changes the oscillating frequency in a way that reveals how much the particle weighs...
To do so, the researchers analyzed how a particle shifts the bridge's vibrating frequency. All oscillatory motion is composed of so-called vibrational modes. If the bridge just shook in the first mode, it would sway side to side, with the center of the structure moving the most. The second vibrational mode is at a higher frequency, in which half of the bridge moves sideways in one direction as the other half goes in the opposite direction, forming an oscillating S-shaped wave that spans the length of the bridge. There is a third mode, a fourth mode, and so on. Whenever the bridge oscillates, its motion can be described as a mixture of these vibrational modes.
The team found that by looking at how the first two modes change frequencies when a particle lands, they could determine the particle's mass and position, explains Mehmet Selim Hanay, a postdoctoral researcher in Roukes's lab and first author of the paper. "With each measurement we can determine the mass of the particle, which wasn't possible in mechanical structures before."
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