Linear-Motor from Carbon Nanotubes

A recent paper this week by Somada et al. regarding making a linear-motor from carbon nanotubes piqued my interest.[NanoLett] The general design idea is to encapsulate a piece of carbon nanotube material within a larger carbon nanotube. If done correctly you can end up with a configuration as shown below.

Reprinted with permission from American Chemical Society: Nano Letters (Nov. 2008).

The cargo, in yellow, transverses the carbon nanotube and rests in either position A or position B. An abridged summary of their observations is as follows: 1) From observing the system for 170 s the cargo traveled back-and-forth seven times; 2) The cargo was never filmed in between positions A and B, indicating the movement was less than the frame rate (0.5 s). From this information I can construct a likely energy landscape for this system.

Mitch’s hypothetical potential energy map for the linear-motor.

The diagram replicates the observation that the cargo at room temperature will be trapped at either position A or B. It also explains why it’s never seen between A or B, as there is no energy minimum for it to rest in. Lets assume every ~20 s there is randomly enough thermal energy to kick the system over the barrier, and that this accessible energy exists for less than 0.5 s. Then you would expect the cargo to be able to move either to A or B, and to do it faster than the shutter speed.

This is an interesting system for analysis, but it’s not a motor. Or conversely, it is as much a motor as ethane is a useful rotor. Just because thermal energy provides the means for things to happen it doesn’t mean it generates usable work. There is no way to construct a usable motor or any device from this system, but it’s a first step in that direction. I suspect if the authors raised the temperature they would see the cargo undergoes random walk motion. Thermal energy yields a random linear-motor.

Link to article: A Molecular Linear Motor Consisting of Carbon Nanotubes

Update 1: Tim Reid also covered it at Nature Chemistry — Nanotube motors: Sliding and spinning

Mitch

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