If two fresh Cheerios are placed near each other while ﬂoating on milk, they will rapidly pull together. What force causes that attraction?
Jearl Walker - The Flying Circus of Physics
In Part One: The Natural World, we explored naturally occurring organic and inorganic systems that exhibit self-assembly. We saw that nature utilizes four key principles in her design of such systems: structured particles, binding forces, environment, and driving forces. These are listed and explained for reference in Table 4.1. In this, the second part of the text: Engineered Systems, we turn our atten-
tion to man-made self-assembling systems. Our goal is to answer the question: “What self-assembling systems can we build now and how do we do it?” In this and the next three chapters we will examine this question from the viewpoint of physics, chemistry, biology, and engineering. We will see that no matter what viewpoint we take, the four basic components of nature’s design are utilized. We will also see that from an engineering perspective, each of these principles provides an opportunity for design. However, we’ll also ﬁnd that the design of self-assembling systems presents
the designer with three major challenges. We call these the forward problem, the backward problem, and the yield problem. The forward problem is predictive. Given a set of particles, a binding force, an environment, and a driving force, what structures will the system produce? The backward problem1 goes the other way around. Given the desired structure, how do we choose a set of particles, a binding force, an environment, and a driving force to assemble this structure? The yield problem arises because of the stochastic nature of the driving force in self-assembly. Typical systems contain local energy minima and our actual output is diﬀerent from our desired output. The yield problem asks: How do we maximize our yield?