Transmutable Colloidal Crystals and Active Phase Separation via Dynamic, Directed Self-Assembly with Toggled External Fields

Abstract

A diverse set of functional materials can be fabricated by assembling dispersions of colloids and nanoparticles. Two principal engineering challenges prevent efficient production of these materials: first, scalable synthesis of particles with carefully tailored interactions required to generate complex structures, and second, the propensity of such materials to arrest in undesirable metastable states. Active assembly processes, such as dynamic, directed self-assembly in which the interactions among particles are externally controlled and vary over time, offer a promising method to address these challenges. For dispersions of polarizable dielectric or paramagnetic nanoparticles, an effective mode of active assembly can be achieved by toggling an external electric or magnetic field, which induces attractive particle interactions, on and off cyclically over time. Here, we develop computational and theoretical models for such active assembly processes and find that cyclically toggling the external field leads to growth of colloidal crystals at significantly faster rates and with many fewer defects than for assembly in a steady field. The active process stabilizes phases that are only metastable in steady fields, including a dense fluid phase and body-centered orthorhombic crystals. The growth mechanism and terminal structure of the dispersion are easily controlled by the toggling protocol, and the toggle parameters can be used to continuously transmute between crystal structures with different lattice parameters. Finally, we show how results from linear irreversible thermodynamics can be used to predict the dissipative terminal states of the active assembly process in terms of parameters of the toggling protocol.

Inverse methods for design of soft materials

“…Particularly challenging are fluid-solid and solid-solid transitions, which have many kinetic barriers but are essential for controlled manipulation of material properties. 139,140 Objective functions that vary during the optimization may help find solutions with transitions specifically embedded. For example, an objective function that periodically switched between two states of reconfigurable circuits 141 and of allosteric networks 142 found solutions prioritizing transitions between states.…”

Accelerated annealing of colloidal crystal monolayers by means of cyclically applied electric fields

“…Electrokinetic 2 – 7 , magnetic 8 , and photo-induced 9 methods have been used to drive colloidal self-assembly at accelerated rates. Yet these methods often confront a tradeoff between the rate of crystallization and the quality of the self-assembled crystal 10 . That is, colloidal crystals formed at faster rates are prone to include undesirable defect microstructures such as vacancies, dislocations, and grain boundaries, each of which has negative effects on the overall crystal quality 11 .…”

Enhanced diffusion and magnetophoresis of paramagnetic colloidal particles in rotating magnetic fields

“…This coupling can be problematic when designing processes involving field-induced forces. For example, large interparticle dipolar forces that drive self-assembly of photonic crystals also tend to cause kinetic arrest; 6,7 large field strengths that drive rapid magnetophoresis lead to particle aggregation that reduces mobility in porous media. 8 If energy is supplied to vary the field in time, the dispersion is driven out of equilibrium.…”

“…Unlike its equilibrium counterpart, the dispersion structure and dynamics can be tuned independently and optimized for a target application. 9 Several types of field modulation have been investigated including toggling the field on and off 6,7,[10][11][12][13][14][15][16] and switching the field's polarity, [17][18][19][20][21][22] but a particularly effective mode is rotating the field direction. 23 The dipolar interactions among particles drive them to align in the field direction, forming chains that rotate with the field.…”