Session F17: Organization of Soft Materials Far from Equilibrium

Self-Assembly and Shear-Induced Long-Range Order of Nanorods in Wormlike Micelle Solutions.

Small angle x-ray scattering was employed to study the structural properties of nanorods within wormlike micelle (WLM) solutions. The gold rods (L$=$75 nm, D$=$14nm) were dispersed at a dilute concentration (0.003 percent by volume) in WLM solutions formed by the surfactant cetylpyridinium chloride (CPyCl) and counter-ion sodium salicylate (NaSaI) over CPyCl concentrations (112 to 400 mM), placing the solutions in the semi-dilute, entangled regime. In quiescent conditions, the SAXS profiles obtained for high CPyCl concentrations (higher than 200 mM) show the formation of powder-like Bragg peaks associated with a hexagonal nanorod arrangement, which develops over the course of tens of minutes. This isotropic self-assembly remarkably evolves into an anisotropic long range order upon shearing at rates between 2 and 5 s$^{-1}$, indicating a further shear-induced in-plane arrangement of the rods. This ordering, extending over macroscopic scales in the solutions, persists after the cessation of shear but is destroyed by strong shear (rates higher than 50 s$^{-1})$. At lower CPyCl concentrations, no nanorod assembly is observed under either quiescent conditions or steady shear, indicating the significance of the micelles in the nanoparticle ordering.   

Memory Dynamics in Cross-linked Actin Networks

Cells demonstrate the remarkable ability to adapt to mechanical stimuli through rearrangement of the actin cytoskeleton, a cross-linked network of actin filaments. In addition to its importance in cell biology, understanding this mechanical response provides strategies for creation of novel materials. A recent study has demonstrated that applied stress can encode mechanical memory in these networks through changes in network geometry, which gives rise to anisotropic shear response. Under later shear, the network is stiffer in the direction of the previously applied stress. However, the dynamics behind the encoding of this memory are unknown. To address this question, we explore the effect of varying either the rigidity of the cross-linkers or the length of actin filament on the time scales required for both memory encoding and over which it later decays. While previous experiments saw only a long-lived memory, initial results suggest another mechanism where memories relax relatively quickly. Overall, our study is crucial for understanding the process by which an external stress can impact network arrangement and thus the dynamics of memory formation.   

Self-assembly kinetics of DNA functionalised liposomes

DNA has been largely used to program state-dependent interactions between functionalised Brownian units resulting in responsive systems featuring complex phase behaviours. In this talk I will show how DNA can also be used to control aggregation kinetics in systems of liposomes functionalised by three types of linkers that can simultaneously bind. In doing so, I will present a general coarse-graining strategy that allows calculating the adhesion free energy between pairs of compliant units functionalised by mobile binders. I will highlight the important role played by bilayer deformability and will calculate the free energy contribution due to the presence of complexes made by more than two binders. Finally we will demonstrate the importance of explicitly accounting for the kinetics underlying ligand-receptor reactions when studying large-scale self-assembly.   

Active control of complex, multicomponent self-assembly processes

The kinetics of many complex biological self-assembly processes such as cytoskeletal assembly are precisely controlled by cells. Spatiotemporal control over rates of filament nucleation, growth and disassembly determine how self-assembly occurs and how the assembled form changes over time. These reaction rates can be manipulated by changing the concentrations of the components needed for assembly by activating or deactivating them. I will describe how we can use these principles to design driven self-assembly processes in which we assemble and disassemble multiple types of components to create micron-scale networks of semiflexible filaments assembled from DNA. The same set of primitive components can be assembled into many different, structures depending on the concentrations of different components and how designed, DNA-based chemical reaction networks manipulate these concentrations over time. These chemical reaction networks can in turn interpret environmental stimuli to direct complex, multistage response. Such a system is a laboratory for understanding complex active material behaviors, such as metamorphosis, self-healing or adaptation to the environment that are ubiquitous in biological systems but difficult to quantitatively characterize or engineer.   

Dynamic, Directed Self-Assembly of Nanoparticles via Toggled Interactions

Crystals self-assembled from nanoparticles have useful properties such as optical activity. During fabrication, gelation and glassification often leave these materials arrested in defective metastable states, a key difficulty preventing adoption of self-assembled nanomaterials at scale. Dynamic, directed self-assembly processes in which interactions are actuated temporally, offer a promising method to suppress kinetic arrest while accelerating growth of nanostructures. We show with simulation and theory how time-dependent, periodically toggled interparticle interactions can avoid kinetic barriers and yield large crystalline domains for a dispersion of nanoparticles. The growth mechanism and terminal structure are controlled by parameters of the toggling protocol, allowing for selection of processes that yield rapidly assembled, low defect crystals. Though toggled self-assembly forms dissipative materials inherently out-of-equilibrium, its outcome is predicted by a nonequilibrium thermodynamic theory, requiring equality of time-averaged pressure and chemical potential in coexisting phases of the dispersion. The predicted phase behavior agrees with that from simulations. We also present kinetic models to predict the rate of crystallization for several observed growth mechanisms.   

Microscale Mechanics of Actin Networks During Dynamic Assembly and Dissociation

Actin is one of the key components of the cytoskeleton, enabling cells to move and divide while maintaining shape by dynamic polymerization, dissociation and crosslinking. Actin polymerization and network formation is driven by ATP hydrolysis and varies depending on the concentrations of actin monomers and crosslinking proteins. The viscoelastic properties of steady-state actin networks have been well-characterized, yet the mechanical properties of these non-equilibrium systems during dynamic assembly and disassembly remain to be understood. We use semipermeable microfluidic devices to induce in situ dissolution and re-polymerization of entangled and crosslinked actin networks, by varying ATP concentrations in real-time, while measuring the mechanical properties during disassembly and re-assembly. We use optical tweezers to sinusoidally oscillate embedded microspheres and measure the resulting force at set time-intervals and in different regions of the network during cyclic assembly/disassembly. We determine the time-dependent viscoelastic properties of non-equilibrium network intermediates and the reproducibility and homogeneity of network formation and dissolution. Results inform the role that cytoskeleton reorganization plays in the dynamic multifunctional mechanics of cells.   

Liquid droplets of cross-linked actin filaments

Soft materials constructed from biomolecules self-assemble into a myriad of structures that work in concert to support cell physiology. One critical soft material is the actin cytoskeleton, a viscoelastic gel composed of cross-linked actin filaments. Although actin networks are primarily known for their elastic properties, which are crucial to regulating cell mechanics, the viscous behavior has been theorized to enable shape changes and flows. We experimentally demonstrate a fluid phase of cross-linked actin, where cross-linker condenses dilute short actin filaments into spindle-shaped droplets, or tactoids. Tactoids have shape dynamics consistent with a continuum model of liquid crystal droplets. The cross-linker, which acts as a long range attractive interaction, analogous to molecular cohesion, controls the tactoid shape and dynamics, which reports on the liquid's interfacial tension and viscosity. We investigate how the cross-linker properties and filament length influence the liquid properties. These results demonstrate a novel mechanism to control organization of the actin cytoskeleton and provide insight into design principles for complex, macromolecular liquid phases.   

Understanding electrostatic trapping of nanoparticles

In electrostatic trapping, nanoparticles are polarized by the nonuniform electric field between two nanoelectrodes. The resulting dielectrophoretic (DEP) force attracts the nanoparticles to locations where the field is maximum or minimum, depending on the nanoparticle permittivity. Coarse-grained molecular dynamics simulations are often used to study such electrokinetic effects, but face the challenge of fully resolving the polarization charges and DEP forces. We extend the iterative dielectric solver developed by Barros and Luijten [\emph{Phys.\ Rev.\ Lett.\ \textbf{113}, 017801 (2014)}] to simultaneously compute the polarization of both equipotential surfaces and dielectric objects. We apply our new algorithm to study the electrostatic trapping of nanoparticles with polarization effects resolved dynamically throughout the simulation. This method represents a new tool for design and study of collective motion of nanoparticles in DEP self-assembly.   

Non-equilibrium pseudo-crystal bandgap structures

Crystalline structures possess the ability to shape wave propagation, for example, through the formation of bandgaps. Such structures might be obtained either by top-down fabrication or by static self-assembly processes. However, these structures arise at thermodynamic equilibrium and are inherently rigid and, thus, difficult to reconfigure. Less rigid structures are obtained far from equilibrium through the continuous input of energy, which is collectively dissipated by the system and leads to spontaneous organization. Because of their many degrees of freedom, non-equilibrium structures are typically difficult to achieve artificially over large ranges. Here, we report the realization of a non-equilibrium bandgap structure composed of particles moving along a waveguide, driven by a coherent field. We observe the dynamic attraction of the particles towards a pseudo-crystalline order defined by many equivalent configurations. For an arbitrarily large set of particles, we analytically demonstrate the existence a unique phase distribution at steady state, which corresponds to the emergence of a transmission bandgap with an edge locked to $\lambda $. This work opens avenues for the realization of out-of-equilibrium bandgap structures, which can be dynamically self-assembled and reconfigured on-command at any scales.   

Guiding nanocrystal organization within mesoscale lipid thin-film templates

Recently a great deal of interest has been established in the cooperative intermolecular interactions in hard and soft meso-structured composite materials. Much of this research has focused on the effects of nanoparticle incorporation into block copolymers that otherwise self-assemble into periodic mesostructures through microphase separation. Through careful selection of the polymer components the nanoparticles can be directed to also microphase separate and therefore exhibit symmetry induced by the block copolymers. Such systems are promising for enabling the organization of nanoparticle superstructures. Although this is useful in many applications such as in bottom-up assembly of opti-electronic materials, most of these applications would benefit from interplay between structure and dynamics. Much like block-copolymers, lipids can self-assembly into a variety of structures with 1D lamellar, 2D Hexagonal, and 3D cubic symmetry. However, unlike block-copolymers phase stabilization and conversion from one geometry to another happens under a minute. We will show our recent efforts into using lipid thin films to guide the assembly of nanoparticle superstructures resembling those displayed by lipid polymorphs and how they distort lipid equilibrium phase behavior.   

Hybrid films with phase-separated domains: A new class of functional materials

The cell membrane is highly compartmentalized over micro-and nano scale. The compartmentalized domains play an important role in regulating the diffusion and distribution of species within and across the membrane. In this work, we introduced nanoscale heterogeneities into lipid films for the purpose of developing nature-mimicking phase-separated materials. The mixtures of phospholipids and amphiphilic block copolymers self-assemble into supported 1D multi-bilayers. We observed that in each lamella, mixtures of lipid and polymer phase-separate into domains that differ in their composition akin to sub-phases in cholesterol-containing lipid bilayers. Interestingly, we found evidence that like-domains are in registry across multilayers, making phase separation three-dimensional. To exploit such distinctive domain structure for surface-mediated drug delivery, we incorporated pharmaceutical molecules into the films. The drug release study revealed that the presence of domains in hybrid films modifies the diffusion pathways of drugs that become confined within phase-separated domains. A comprehensive domain structure coupled with drug diffusion pathways in films will be presented, offering new perspectives in designing a thin-film matrix system for controlled drug delivery.   

Computational and theoretical modeling of pH and flow effects on the early-stage non-equilibrium self-assembly of optoelectronic peptides

Self-assembling $\pi$-conjugated peptides are attractive candidates for the fabrication of bioelectronic materials possessing optoelectronic properties due to electron delocalization over the conjugated peptide groups. We present a computational and theoretical study of an experimentally-realized optoelectronic peptide that displays triggerable assembly in low pH to resolve the microscopic effects of flow and pH on the non-equilibrium morphology and kinetics of assembly. Using a combination of molecular dynamics simulations and hydrodynamic modeling, we quantify the time and length scales at which convective flows employed in directed assembly compete with microscopic diffusion to influence assembly. We also show that there is a critical pH below which aggregation proceeds irreversibly, and quantify the relationship between pH, charge density, and aggregate size. Our work provides new fundamental understanding of pH and flow of non-equilibrium $\pi$-conjugated peptide assembly, and lays the groundwork for the rational manipulation of environmental conditions and peptide chemistry to control assembly and the attendant emergent optoelectronic properties.   

Molecular Simulation of Flow-Enhanced Crystal Nucleation in Alkane Melts

Under typical processing conditions, the crystallization of polymer materials occurs far from equilibrium. In particular, the application of a flow field is known to drastically accelerate the kinetics of crystallization, and in turn alter the morphology and properties of the resulting material. It remains a significant challenge to establish the processing-structure-property relationships associated with crystallization under flow. Non-equilibrium molecular dynamics simulation has proven to be a useful investigative tool for the study of the early stages of flow-induced crystallization, known as flow-enhanced nucleation. Using this technique, nucleation studies were performed under shear and uniaxial extension for monodisperse melts of short (C20) and long (C150) alkanes, as well as for bimodal mixtures composed of both short and long chains. Through the application of a mean first-passage time-based analysis method, the effect of flow on the nucleation kinetics was quantified and classified in terms of an increase in the driving force for crystallization, reflected in the reduction of the free energy barrier, as well as a diffusive enhancement, reflected in the increase of the monomer attachment pre-factor. In bimodal blends with C150 fractions of 3-9 wt{\%} chains, a drastic acceleration in the nucleation kinetics relative to monodisperse C20 was observed for strain rates intermediate to the inverse Rouse times of C20 and C150. For this range of strain rates, the long chains were found to preferentially participate in the formation of small clusters, and therefore serve as templates for the crystallization of the short chain fraction.