Size is important!
Surface area effects
As the size of an object falls, its surface area (which is a function of the square of its radius) falls a lot less quickly than its volume (which is a function of the cube of its radius).
A α r2 whereas V α r3
This has two consequences:
Small cars don't work
The internal combustion engine works by confining a mixture of burning fuel and air in a piston. As the fuel burns the gases get hot and expand, forcing the piston to move. This movement is converted into useful work through a crankshaft or other arrangement.
As we make smaller and smaller pistons, however, the burning gases come into more contact with the piston walls. The walls conduct the heat away from the gases, so they don't expand as much. As a result the engine becomes less efficient...
All heat engines suffer from this problem as they get smaller, and we need to use different types of power sources to drive motion at the nano-scale. Many scientists are looking at the types of motors cells use - cilia and flagella driven by tiny rotory chemical motors.
...but inkjet printers do!
The problem with a small heat engine is that heat escapes too quickly for us to get any work out of it. This problem is turned to useful effect in many inkjet printers, however...
The printer heads for thermal inkjets consist of very fine channels that wick ink in from the reservoir. Around the channels are heating elements that can be switched on and off by the printer. When the printer needs to put some ink on the page the heating element is switched on.
Because the channel is very narrow, it has a large surface area to volume ratio. As a consequence the ink in the channel heats up very quickly, and almost instantly vapourises to form a bubble of steam. This bubble expands, pushing the ink in front of it out of the nozzle at very high velocity. This ink lands on the page as a very fine dot.
Small guys stick together
For things to hold together as liquids or solids, there must be a force of attraction between the atoms or molecules that make them up. After all, when this force isn't big enough to overcome the thermal motion of the atoms or molecules, the material becomes a gas!
While atoms or molecules in the middle of an object are surrounded by other atoms or molecules to which they are attracted, those on the surface are not. These surface atoms therefore have a higher energy than those in the core, and this is called the surface energy.
As particles become smaller the surface area to volume ratio becomes larger, and this surface energy term becomes more and more important. Small particles tend to stick together to form larger particles or aggregates.
Nanotechnologists can work either with or against this tendency:
- If you want to keep particles small, as in emulsions (e.g. many water-based paints), you usually have to add a surfactant to stop the particles sticking together.
- Nanotechnologists are looking for ways to control the tendency of
particles to stick together, so that they self-assemble into large data arrays
or useful devices.
Return to: Size is important!
