Self-assembly is a powerful technique with substantial difficulties and limitations
In assembling nanoscale systems, Brownian motion (motion of particles in a fluid caused by thermal agitation) can move parts into place, and selectively adherent surfaces can make them stay there. This process, sometimes termed Brownian assembly, is commonly (but confusingly) termed “self-assembly”*.
The fundamental advantage of self assembly is that Brownian motion comes for free. No motors or conveyance machines are needed to move parts, because thermal agitation moves them spontaneously. The fundamental disadvantage of Brownian assembly is that motion is uncontrolled. Every part moves at random, twisting, shifting, and bumping in all possible positions and orientations. Only the properties of the parts themselves can guide the process, biasing motion through electrostatic fields and selecting favorable positions through the selective fit and stickiness of interfaces (bumps matching hollows, positive charges opposite negative charges, and so forth).
This presents substantial difficulties and limitations. In ordinary engineering, parts must fit together and perform a function; they can be assembled under flexible, programmable control. In engineering for Brownian assembly, parts must do likewise, and in addition must suspend themselves in a fluid, stick to the proper place in the pre-existing structure, and fail to stick to any other surface. Assembly behavior is thus largely “hard wired” into the structure of the parts. These added requirements add to the difficulty of designing parts and of redesigning systems based on those parts. Moreover, the resulting structures have within them many weak, intricate interfaces that are a residue of the assembly process, serving no function in the product. Stronger, more compact systems must be built by other means.
Despite these difficulties and limitations, self assembly via Brownian motion is a powerful techique, as shown by nature, where it is responsible for the assembly of structures and mechanisms from molecular components. Self assembly is being used to fabricate an increasing range of artificial nanostructures, and self assembly of molecular and nanoscale components is an attractive way to implement early-generation molecular machine systems, including machines that can perform assembly under flexible, programmable control.
Constrained Brownian assembly
There is an important middle ground, however, in which Brownian motion subject to mechanical constraints is used to accomplish what in macroscale engineering practice would require motive power. This can greatly simplify the implemenation of mechanically directed assembly.
* Many people find the term “self assembly” confusing and paradoxical. In manual or robotic assembly, manual or robotic motions put parts in place; the term “self assembly” naturally suggests that a product somehow acts to build itself before it itself exists, a concept as mysterious as it is inaccurate. The term Brownian assembly is more descriptive, because it is Brownian motion that puts the components in place.