Light-controlled Assembly of Active Colloidal Molecules (1801.06868v2)

Abstract: Thanks to a constant energy input, active matter can self-assemble into phases with complex architectures and functionalities such as living clusters that dynamically form, reshape and break-up, which are forbidden in equilibrium materials by the entropy maximization (or free energy minimization) principle. The challenge to control this active self-assembly has evoked widespread efforts typically hinging on engineering of the properties of individual motile constituents. Here, we provide a different route, where activity occurs as an emergent phenomenon only when individual building blocks bind together in a way that we control by laser light. Using experiments and simulations of two species of immotile microspheres, we exemplify this route by creating active molecules featuring a complex array of behaviors, becoming migrators, spinners and rotators. The possibility to control the dynamics of active self-assembly via light-controllable nonreciprocal interactions will inspire new approaches to understand living matter and to design active materials.

• The paper demonstrates that laser illumination induces nonreciprocal phoretic interactions, leading to the self-assembly of active colloidal molecules such as Janus dimers.
• It employs both experiments and simulations, using temperature gradients and effective Lennard-Jones potentials to model particle behavior.
• The results pave the way for designing responsive materials and exploring scalable applications in active matter systems.

Light-controlled Assembly of Active Colloidal Molecules: A Summary

In the paper "Light-controlled Assembly of Active Colloidal Molecules," the authors present a novel approach to the self-assembly of active colloidal molecules, leveraging photo-controlled interactions in a binary mixture. This research introduces a unique framework where nonreciprocal interactions among immotile particles are initiated using laser light, paving the way for the emergence of self-propelling structures without equilibrium constraints.

Key Findings

The paper explores the potential of self-assembly in active matter, a class distinct from equilibrium systems. Traditional self-assembly approaches focus on equilibrium states, constrained by classical thermodynamic principles like free energy minimization. Active matter, however, continuously injects energy into the system, thereby transcending the limitations imposed by thermal equilibrium.

The authors demonstrate that combining two species of immotile microspheres—light-absorbing and non-absorbing—can lead to the formation of active, motile entities when subjected to specific light conditions. This is achieved by creating Janus dimers, which are then capable of forming complex structures such as migrators, spinners, and rotators. The emergent motility in these systems arises due to nonreciprocal phoretic interactions, which become possible when these colloidal particles interact under laser illumination.

Methodology

This investigation utilized both experimental and simulation approaches. In both cases, suspensions of light-absorbing and non-absorbing colloidal particles were prepared in a near-critical fluid mixture. When illuminated, the light-absorbing particles increased the temperature in their vicinity. This temperature gradient did not result in self-propulsion but rather stimulated phoretic interactions and attractive forces between nearby colloids.

From a theoretical perspective, the authors developed a model incorporating Lennard-Jones potentials with effective screening to describe interactions among particles. The model adequately predicts the observed formation of colloidal structures and the associated behaviors such as linear and circular swimming. The approach taken was robust, quantitatively agreeing with experimental observations regarding the size, speed, and rotation frequencies of the self-assembled molecules.

Implications and Future Work

This control over light-induced self-assembly opens new avenues for research into nonreciprocal interaction-driven systems. The paper's findings offer a pathway for both fine-tuning the assembly of functional materials and for expanding our understanding of active matter systems. Further research can expand on the scalability of these systems and their applications in designing responsive materials and systems mimicking complex biological behaviors.

Potential future developments could involve exploring the interaction effects and stability of these colloidal structures under varying environmental conditions. Research could also explore the implications of this assembly approach in industrial applications, such as drug delivery, where self-propelled particles could navigate through biological environments.

Overall, this paper provides a thorough exploration of a nascent area in colloid science and self-assembly, offering insights that could significantly impact the design of future functional materials. The paper serves as a basis for further exploration into the phenomenon of emergent behavior in synthetic systems, with potential applications across chemical engineering, materials science, and nanotechnology.