The effect of charge separation on the phase behavior of dipolar colloidal rods

Electric field-induced clustering in nanocomposite films of highly polarizable inclusions

A nanocomposite film containing highly polarizable inclusions in a fluid background is explored when an external electric field is applied perpendicular to the planar film. For small electric fields, the induced dipole moments of the inclusions are all polarized in field direction, resulting in a mutual repulsion between the inclusions. Here we show that this becomes qualitatively different for high fields: the total system self-organizes into a state which contains both polarizations, parallel and antiparallel to the external field such that a fraction of the inclusions is counter-polarized to the electric field direction. We attribute this unexpected counter-polarization to the presence of neighboring dipoles which are highly polarized and locally revert the direction of the total electric field. Since dipoles with opposite moments are attractive, the system shows a wealth of novel equilibrium structures for varied inclusion density and electric field strength. These include fluids and solids with homogeneous polarizations as well as equilibrium clusters and demixed states with two different polarization signatures. Based on computer simulations of an linearized polarization model, our results can guide the control of nanocomposites for various applications, including sensing external fields, directing light within plasmonic materials, and controlling the functionality of biological membranes.

Field-Driven Reversible Alignment and Gelation of Magneto-Responsive Soft Anisotropic Microbeads

Magnetic fields offer untethered control over the assembly, dynamics, and reconfiguration of colloidal particles. However, synthesizing "soft" colloidal particles with switchable magnetic dipole moment remains a challenge, primarily due to strong coupling of the dipoles of the adjacent nanoparticles. In this article, we present a way to overcome this fundamental challenge based on a strategy to synthesize soft microbeads with tunable residual dipole moment. The microbeads are composed of a polydimethylsiloxane (PDMS) matrix with internally embedded magnetic nanoparticles (MNPs). The distribution and orientation of the MNPs within the PDMS bead matrix is controlled by an external magnetic field during the synthesis process, thus allowing for the preparation of anisotropic PDMS microbeads with internal magnetically aligned nanoparticle chains. We study and present the differences in magnetic interactions between microbeads containing magnetically aligned MNPs and microbeads with randomly distributed MNPs. The interparticle interactions in a suspension of microbeads with embedded aligned MNP chains result in the spontaneous formation of percolated networks due to residual magnetization. We proved the tunability of the structure by applying magnetization, demagnetization, and remagnetization cycles that evoke formation, breakup, and reformation of 2D percolated networks. The mechanical response of the microbead suspension was quantified by oscillatory rheology and correlated to the propensity for network formation by the magnetic microbeads. We also experimentally correlated the 2D alignment of the microbeads to the direction of earth's magnetic field. Overall, the results prove that the soft magnetic microbeads enable a rich variety of structures and can serve as an experimental toolbox for modeling interactions in dipolar systems leading to various percolated networks, novel magneto-rheological materials, and smart gels.

Development of a coarse-grained lipid model, LIME 2.0, for DSPE using multistate iterative Boltzmann inversion and discontinuous molecular dynamics simulations

We suggest an improved version of the intermediate resolution implicit solvent model for lipids, LIME, that was previously developed for use with discontinuous molecular dynamics (DMD) simulations. LIME gets its geometrical and the energy parameters between bonded and nonbonded pairs of coarse-grained (CG) sites from atomistic simulations. The improved model, LIME 2.0, uses multiple square wells rather than the single square well used in original LIME to obtain intermolecular interactions that more faithfully mimic those from atomistic simulations. The multi-state iterative Boltzmann inversion (MS-IBI) scheme is used to determine the interaction parameters. This means that a single set of interaction parameters between coarse-grained sites can be used to represent the lipid bilayers at different temperatures. The physical properties of CG DSPE lipid bilayer are calculated using CG simulations and compared to atomistic simulations results to verify the improved model. The phase transition temperature of the lipid bilayer is measured accurately and the lipid translocation phenomenon, " flip-flop" is observed through CG simulation. These results suggest that CG parameterization using multiple square-well and the MS-IBI scheme is well suited to the study of lipid bilayers cross a range of temperatures with DMD simulations.

Phase diagrams of mixtures of dipolar rods and discs

Self-assembly of binary mixtures that contain anisotropic, interacting colloidal particles have been proposed as a way to create new, multi-functional materials. We simulate binary mixtures of dipolar rods and dipolar discs in two-dimensions using discontinuous molecular dynamics to determine how the assembled structures of these mixtures differ from those seen in single component systems. Two different binary mixtures are investigated: a mixture of an equal number of dipolar rods and dipolar discs ("equal number"), and a mixture where the area fraction of dipolar rods is equal to the area fraction of dipolar discs ("equal area"). Phase boundaries between fluid, string-fluid, and "gel" phases are calculated and compared to the phase boundaries of the pure components. Looking deeper at the underlying structure of the mixture reveals a complex interplay between the rods and discs and the formation of states where the two components are in different phases. The mixtures exhibit phases where both rods and discs are in the fluid phase, where rods form a string-fluid while discs remain in the fluid phase, a rod string-fluid coexisting with a disc string-fluid, a "gel" that consists primarily of rods while the discs form either a fluid or string-fluid phase, and a "gel" that contains both rods and discs. Our results give insight into the general assembly pathway of binary mixtures, and how complex aggregates can be created by varying the mixture composition, strength of interaction between the two components, and the temperature. By manipulating the properties of one of the components it should be possible to fabricate bifunctional, thermally responsive self-assembled materials.