Kinetic Monte Carlo Simulations of Flow-Assisted Polymerization

Effect of size and charge asymmetry on aggregation kinetics of oppositely charged nanoparticles

We report a theoretical and experimental study of the aggregation kinetics of oppositely charged nanoparticles. Kinetic Monte Carlo simulations are performed for symmetric, charge-asymmetric and size-asymmetric systems of oppositely charged nanoparticles. Simulation results show that both the weight and number average aggregate size kinetics exhibit power law scaling with different exponents for small and intermediate time of evolution. The qualitative behavior of the symmetric and the size asymmetric system are the same, but the charge asymmetric system shows anomalous behavior for intermediate to high particle concentrations. We also observe a strong dependence of power law exponents on the particle concentration. Radius of gyration of the cluster that indicates how nanoparticles inside a cluster are distributed around the center of mass of the cluster shows a non-monotonic time evolution with pronounced peak at higher particle concentration. The dependence of particle concentration on aggregation kinetics as observed by predictive numerical simulation is further verified experimentally by monitoring the time evolution of aggregate size of nanoparticles assemblies of Poly (methacrylic acid) (PMMA) nanoparticles functionalized with oppositely charged ligands. These size and charge tunable asymmetric polymeric nanoparticles were synthesized by modified miniemulsion technique. The integrated approach for studying nanoparticles aggregation as described here renders new insights into super structure formation and morphology optimization which can be potentially useful in the design of new materials, such as organic photovoltaics.

Self-replication with magnetic dipolar colloids

Colloidal self-replication represents an exciting research frontier in soft matter physics. Currently, all reported self-replication schemes involve coating colloidal particles with stimuli-responsive molecules to allow switchable interactions. In this paper, we introduce a scheme using ferromagnetic dipolar colloids and preprogrammed external magnetic fields to create an autonomous self-replication system. Interparticle dipole-dipole forces and periodically varying weak-strong magnetic fields cooperate to drive colloid monomers from the solute onto templates, bind them into replicas, and dissolve template complexes. We present three general design principles for autonomous linear replicators, derived from a focused study of a minimalist sphere-dimer magnetic system in which single binding sites allow formation of dimeric templates. We show via statistical models and computer simulations that our system exhibits nonlinear growth of templates and produces nearly exponential growth (low error rate) upon adding an optimized competing electrostatic potential. We devise experimental strategies for constructing the required magnetic colloids based on documented laboratory techniques. We also present qualitative ideas about building more complex self-replicating structures utilizing magnetic colloids.

Self-replication in colloids with asymmetric interactions

Self-replication is a ubiquitous process in organisms. Understanding the key ingredients of self-replication is critical for developing self-sustaining systems in the laboratory. Moreover, finding the optimal conditions to generate accurate replicas and adequate output can accelerate industrial processes enormously. Here, we propose a scheme for self-replication where asymmetric interactions in colloids are used to find optimal self-replication conditions by controlling the input of energy. We generalize a recently developed kinetic Monte Carlo algorithm to treat both translational and rotational motions of Brownian anisotropic colloids. We report two main findings from our simulations: first, by fine tuning the particle interactions, highly accurate self-replication is achievable with a moderate sacrifice of reaction speed. Second, with the introduction of energy cycling to enable periodic assembly/disassembly of the system's components the replicator population grows exponentially. The exponential growth constant is a non-monotonic function of the period of the pulsed energy delivery.