Session S07: Programmable Self-Assembly: Particle, Interaction and Pathway Design (Magnetic Interactions)

Live

Magnetic Handshake Materials is a recently developed materials platform that can encode specific interactions by printing distinct magnetic dipole patterns onto a substrate. Using this platform, we have achieved controlled polymerization, complementary binding strands, and 3D folding from 2D nets by self-assembly [1]. These results all built on the core principle of creating specific bindings, but they showcased only a tiny fraction of the programmability of the platform. In this talk, I will discuss how we can achieve complicated self-assembly behaviors in experiment by designing building block interactions from scratch. First, I will present a simple design criterion that enables high specificity (low-crosstalk) in building block interactions and a theoretical framework utilizing information theory that can measure the performance of a collection of designed building blocks. Second, I will show simulation results for our targeted self-assembly tasks using the designed building blocks compared to the randomly generated ones. Third, I will describe how we can realize the same self-assembly behaviors in experiment using our designed building blocks.

[1] R. Niu, C. X. Du, E. Esposito, J. Ng, M. P. Brenner, P. McEuen, and I. Cohen, PNAS (2019)   

Live

The potential energy surface (PES) describing the interactions between two or more atoms, molecules, or materials is a fundamental construct that can be used to predict the structures, properties, and dynamics of complex systems throughout biology, chemistry, physics, and materials science. In this work, we explore how patterned magnetostatic interactions can be used in the rational design of PES with targeted features. We first explore the PES design space that is accessible with small patterned magnetic arrays via forward and exhaustive enumeration, and characterize the resulting PES by the number, locations, and depths of the PES critical points. This is followed by a detailed investigation into the inverse problem—identification of magnetic patterns that correspond to PES with predefined features—using simulated annealing Monte Carlo (SA-MC) methods. In doing so, we demonstrate a robust theoretical and conceptual paradigm that enables forward and inverse PES engineering with precise control over the critical points and other salient surface features, thereby paving the way towards directed self-assembly using programmable magnetic interactions.   

Live

Manipulating the way in which colloidal particles self-organise is a central principle in the design and realisation of contemporary soft matter materials. In this work we demonstrate, by using easily accessible magnetic colloids, that we can create a variety of building blocks suitable for hierarchical self-organisation. Using computer simulations, we have investigated the suitability of magnetic colloids, spherical and cubic in shape, to form small clusters with reproducible structural and magnetic properties. We find that, while the structure of these clusters is highly reproducible, their magnetic character is dependent on the shape of the constituent magnetic particles. Cubic particle shape frustrates the minimisation of the cluster energy, while spherical particles have the rotational degrees of freedom to produce equivalent magnetic configurations. Building upon these results, we validate the ability of magnetic trimers to form hierarchical assemblies, proving that the spherical magnetic particles presented here can offer a route to effectively design a viable approach for novel self-assembly processes.   

Live

Magnetic handshake materials [1] are new self-assembling platforms that use magnetic dipole patterns to create lock and key interactions between building blocks to guide assembly. Because the technology is scale invariant, building blocks can range in size from centimeters to microns. Here, we show this platform can be used to design the persistence length of digital magnetic polymers. Specifically, we use a shaker to agitate chains of millimeter sized panels, each containing a 2x2 square array of magnets. By changing the contact surfaces between panels from flat to hemispherical the persistence length changes from ~10^4 to ~10^2 panel lengths for the same shaking settings (amp & freq). Similarly, keeping a hemispherical contact but changing the dipole pattern again results in a 2 order of magnitude change in persistence length. Combining the approaches enable us to change the persistence length by almost 4 orders of magnitude. This system constitutes a new type of self-healing granular polymer, which in addition to modeling molecular polymeric analogues may find uses in its own right. Finally, I will describe our fabrication efforts to make the panels microns in size where fluctuations in such polymers would be thermal.

References:
[1] Niu, et al. PNAS, 116 (2019)   

Live

Studies of short, self-assembling chains have uncovered many heuristic design principles for the formation of a target configuration. Among these principles is the fundamental trade-off between free energy and kinetic accessibility; high stability typically leads to low error correcting, and high error correcting typically leads to low stability. We quantitatively study this trade-off for the self-assembly of colloidal chains of 6 or 7 particles. We construct a model that allows for the efficient computation of ground state equilibrium probabilities and transition rates, for any set of design parameters. A genetic algorithm is then used to identify Pareto fronts for each ground state; parameter sets where neither equilibrium probability nor rate can be increased without decreasing the other. By examining the shape of the Pareto fronts, the minimal requirements for efficient self-assembly of a target state can be determined. We also discuss a sampling approach to extend these results to larger systems.   

Live

Incredible progress has been made in self-assembling complex structural features. Ranging from DNA origami to patchy colloidal particles, intricate static structures have been assembled at nearly every length scale. However, there have been few efforts to self-assemble dynamical features. Many biological systems rely heavily on precise dynamical control: protein folding, for instance, relies on careful aversion of kinetic traps. Additionally, non-biological processes such as crystal nucleation are largely controlled by dynamics. We present a method for designing the kinetic features of self-assembled systems and demonstrate the method by tuning the crystallization rate of a 2D honeycomb lattice.   

Live

Biological systems have the unique ability to self-organize and generate autonomous motion and work. We investigate a colloidal network that can dynamically oscillate between crosslinked and unlinked states connected by bacterial clock proteins, KaiABC. By using Langevin dynamics, we tune properties like packing fractions, interaction forces, and crosslinking probabilities to produce desired mechanical responses. The particle bond average and bond length distributions, cluster sizes, and collective particle motion are used to assess the degree of order in the system. With spherical colloids, several orders of magnitude of separation exist between the colloid relaxation period and the crosslinker periods, producing systems that switch between more-ordered states to less-ordered-but still significantly connected-states. We are extending our model system to include rod-like colloids to mimic biological systems. Our results will establish appropriate material properties and aid in the experimental design of these smart active materials that can cycle between more-ordered and less-ordered states.   

Live

Understanding competitive nucleation between different crystal polymorphs is critical for the selective self-assembly of any particular structure. For crystals made of one or a few components, selectivity is often accomplished through carefully designed annealing protocols. In highly multi-component systems, we argue for a new winner-take-all mechanism that enhances selectivity through depletion. For such systems, nucleation and growth of one structure can deplete monomers in a way that dramatically reduces the nucleation rates of all other structures without significantly affecting its own nucleation rate. We identify regimes of winner-take-all depletion using theory and rare-event sampling methods in multi-component self-assembly with unequal component concentrations. We present experimental evidence for such winner-take-all dynamics in a system of single-stranded DNA molecules able to assemble into three distinct two-dimensional crystal configurations.   

Live

Measuring the intermolecular distance in supramolecular structures in solution is important as well as challenging, especially for distance below 1.0 nm. The fluorescence emission is helpful in indicating intermolecular distances due to its sensitivity to the microenvironment, and the aggregation-induced emission (AIE) luminophores are especially useful against fluorescence quench in assembly and aggregation states. Herein, polyhedral oligomeric silsesquioxane (POSS) macroionic hybrids were synthesized with AIE luminophores chemically incorporated. The charged macromolecules self-assemble into hollow spherical blackberry-type stable supramolecular structures through the regulation of the electrostatic interactions. Macromolecules in the single-layered shell are closer to each other when assembled into spheres of larger size. Due to the rotational restriction of phenyl rings, the fluorescence emission was enhanced when more hybrids were involved in the assembly. In addition, some hybrids exhibit emission wavelength shift when neighboring macromolecules are closer to each other in the assembly structures. The study of macroionic-AIE hybrids in mixed solvent enabled us to gain a more direct visualization of intermolecular distances from the fluorescence emission changes.   

Live

It is known that light can be used to manipulate and trap colloidal microspheres. What is less known is a second effect where two or more microspheres placed in a uniform field can manipulate and trap each other. This is effectively an inter-particle interaction mediated by a light field, which offers the ability to tune the strength of the interaction and shape of the potential by relatively simple means. We will share how we create “optically-bound” structures using this pathway of self-organization. We also share our attempts to understand the highly dynamical nature of these new structures; primarily, the emergence of driven behavior and spontaneous collapsing of stability.   

Not Participating

The ability to cyclically change structure and function with time is a property seen across the biological world, but this property has not yet been synthetically engineered as an autonomous regulatory process. Motivated by this, we construct and study a computational model of active colloids which undergo periodic crosslinking and unlinking as a function of time. The colloids are modeled as self-propelled particles which interact with each other via a Lennard Jones potential and whose dynamics follow the Vicsek model, while the crosslinking proteins are modeled as Hookean springs. Periodic crosslinking and unlinking on a microscopic scale leads to the emergence of cyclic macroscopic structural regimes with distinct material properties. Our results may provide insights into the design of autonomous active materials that can harness energy-driven, molecular-scale biological ratchets to perform large-scale motion and work.   

On Demand

A long-standing problem in nanoscale engineering is to mimic the capability of biological systems, such as DNA, to contain information and replicate. We approach this problem using building blocks based on magnetic handshake materials [1] - a scale-invariant platform using programmable lock-and-key interactions to guide self-assembly. Specifically, we have created centimeter-sized panels with programmed magnetic domains, where interactions analogous to Watson-Crick pairing and the modifiable DNA backbone bonding allow panels to attach linearly onto a base chain when agitated with a shaker table. After a new base is added, the application of an external magnetic field rotates magnetic dipoles, strengthening the backbone interaction. Repeating this cycle replicates the base chain. Applying a higher shaking amplitude separates the two chains, enabling repeated replication. Due to scale-invariance, technologies developed on the centimeter scale carry over to the micron scale. When combined with theory on the information capacity of magnetic handshake materials, there is large design potential; for example, this platform could be used to create polymers capable of metabolic function and much more.

References:
[1] Niu, et al. PNAS, 116 (2019)   

On Demand

Although less widely used then their optical counterparts, acoustic forces can be used to meaningfully control the assembly of particles at a variety of scales. In addition to the well known single particle gradient force, there is also an interparticle force which becomes comparable to other forces when the particle is of order one wavelength in size. As this size regime prevents the use of simple analytical models, we will present a robust numerical model which allows us to study these effects in numerical simulations. With it we elucidate some of the interesting dynamics present in multi-body acoustophoretic systems and demonstrate stable configurations in large groups.