Brownian assembly is a powerful technique with substantial difficulties and limitations

Brownian motion (the motion of small particles in a fluid, resulting from thermal agitation) can be used to put parts in place. The fundamental advantage of Brownian assembly is that 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, Brownian assembly is a powerful techique. It is the basis for most molecular assembly in biology and for an increasing range of artificial nanostructures. Brownian assembly of artificial molecular parts is a leading candidate among techniques for implementing first-generation molecular machine systems, including machines that can perform assembly under flexible, programmable control.

Brownian assembly is commonly called “self assembly”, but this term has a range of other meanings and tends to create confusion. In manual or robotic assembly, manual or robotic motions put parts in place; in Brownian assembly, Brownian motion puts parts in place. The term “self assembly” suggests that a product somehow acts to build itself before it itself exists, a concept as mysterious as it is inaccurate.