Colloidal (self-)assembly is a versatile route toward a wide variety of superstructures. Typically, the building blocks are designed such that the target structure is the thermodynamic equilibrium state. This implies that the assembly resides in the global minimum of the free-energy landscape. The associated stability of the final structure is inherently coupled to limited structural dynamics. This lack of dynamicity is in sharp contrast to many biological assembled structures, e.g., microtubules. These supramolecular systems form and operate far from equilibrium as a continuous input of energy is required to sustain them. By relying on kinetics of energy consumption rather than thermodynamics, these systems are endowed with dynamic properties unmatched in any synthetic system.
Inspired by these biological assemblies, we introduce two colloidal systems capturing the essence of out-of-equilibrium assembly. The first system displays transient clustering driven by a chemical fuel. The building blocks comprise colloids functionalized with fuel-responsive polymer brushes. In equilibrium, the brushes are charged, safeguarding colloidal stability. Addition of fuel forces the immobilized polymers into a metastable neutral and hydrophobic state causing non-equilibrium clusters to form. Fuel depletion causes spontaneous recharging of the brushes, guiding the system back to its stable equilibrium state. By leveraging the system’s cyclic nature, transient clustering can be induced repeatedly by sequential fuel addition.
The second system relies on colloids that assemble under the influence of time-dependent attractive depletion forces. These forces are generated by in situ formation of polymer chains in a colloid containing medium. The formed polymers impose an osmotic pressure onto the colloids, which drives their assembly. The strength and range of the generated attractions depend on the size of the polymer coils and hence on the polymerization time. As the system continuously evolves, non-equilibrium structures arise. By tuning the polymerization kinetics and coil size, structural control is anticipated.

Schematic representations of fuel-driven and polymerization-induced colloidal assembly.

Willem Kegel

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