Feedback Controlled Colloidal Self‐Assembly

Abstract

Colloidal self‐assembly provides one promising route to fabricate spatially periodic meta‐materials with novel properties important to a number of emerging technologies. However, colloidal assembly is generally initiated via irreversible step‐changes and proceeds along unspecified, non‐equilibrium kinetic pathways with little opportunity to manipulate defects or reconfigure microstructures. Here, a conceptually new approach that enables the use of feedback control to quantitatively and reversibly guide the dynamic evolution of colloid ensembles between disordered fluid and crystalline configurations is demonstrated. The key to this approach is the use of free energy landscape models to inform feedback control laws that close the loop between real‐time sensing (via order parameters) and actuation (via tunable electrical potentials). This approach, which demonstrates controlled assembly to create ordered materials and perform active reconfiguration, is based on chemical physics that suggest it can be generalized to other microscopic processes.

Control of self-assembly in micro- and nano-scale systems

“…This inherent stochasticity can greatly impact the self-assembly of particles at these scales. For example, self-assembly of colloidal particles (at fixed conditions) can require excessively long periods of waiting time before initiation of the first step of the process (e.g., nucleation) needed to make a product, due to the first step having a high-energy activation barrier [5]. Another example is a microfluidic platform that uses evaporation to induce crystal nucleation of organic compounds such as amino acids and proteins [18,19].…”

“…Although most self-assembling systems have one lowest energy state (i.e., the state that is thermodynamically favored), systems can become trapped in "kinetically arrested" states associated with local minima in the free energy landscape, giving rise to various metastable configurations (e.g., glasses, gels, polycrystals) [5]. For example, suspensions of micrometer-sized paramagnetic colloidal spheres form disordered entangled chain-like structures in a steady magnetic field at high field strengths, where thermodynamic calculations indicate that the formation of well-ordered crystallization domains is favored.…”

“…In another example, the use of feedback to control the self-assembly of silica colloids was recently demonstrated experimentally [5], with the configuration shown in Fig. 10A.…”

“…Controlled self-assembly implies promoting or accelerating the organization of particles towards desired structures. Intervention is expected in a self-organizing process, for example, by changing the particle interactions [3,4] or by manipulating the environment (i.e., global system variables) in which self-assembly takes place [5][6][7]. This idea is often called "directed self-assembly" [8,9].…”

“…This idea is often called "directed self-assembly" [8,9]. The concept should be distinguished from "directed assembly", which refers to the precise manipulation of particles one-by-one during the construction of the structure (like a mason building a brick wall) [5]. Directed assembly at the microscale is now considered standard manufacturing technology such as in three-dimensional printing (see e.g., [10][11][12] and the citations therein).…”