Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G30: Self-Limiting Assemblies III: Soft Assemblies and In and Out of Equilibrium |
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Sponsoring Units: DSOFT DPOLY DBIO Chair: Gregory Grason, Univ of Mass - Amherst Room: 502 |
Tuesday, March 3, 2020 11:15AM - 11:27AM |
G30.00001: Pitch-balanced helical motifs in tightly packed lamellar structures Efi Efrati Many biological and manmade lamellar structures form minimal surfaces due to surface area or bending energy minimization (or both). Recently, the endoplasmic reticulum sheets were shown to contain both right and left-handed helical motifs at equal amounts. In this talk, I will show that the lamellar structures that house photosynthesis in some plants, the thylakoid, also display right and left-handed motifs. The oppositely handed motifs, unlike those of the endoplasmic reticulum, differ in pitch and diameter, yet the stoichiometry is such that pitch is balanced between the differently handed motifs. We argue that the optimal packing of helical motifs in lamellar structures requires global pitch balance. We support this by a new constructive recipe that allows us to generate exact minimal surfaces with any arrangement of helical motifs. |
Tuesday, March 3, 2020 11:27AM - 11:39AM |
G30.00002: Colloidal membrane thickness sets critical surface area for vesicle formation Joanna Robaszewski, Leroy Jia, Thomas R Powers, Robert Alan Pelcovits, Zvonimir Dogic The malleable size and shape of biological membranes allow them to act in a wide range of cellular functions. Vesicles in particular have different uses, from transport to compartmentalization, depending on their size and shape. However, the rapidity with which bio-membranes bend and change structure on the nanoscale makes studying the physics that drive these changes difficult. We use colloidal membranes as experimental models to study the forces governing vesicle formation. Due to the larger length and time scales present in our colloidal system, we directly observe membranes undergoing lateral growth and then bending into vesicles. Both theory and experiment indicate that the size of the final vesicle structure depends on the thickness of the membrane. By tuning the thickness of our colloidal membranes, we can set the critical surface area at which a flat sheet will transition into a vesicle. Our system therefore allows for direct probing of vesicle formation physics, measurement of physical properties, and control over membrane curvature and structure size. |
Tuesday, March 3, 2020 11:39AM - 11:51AM |
G30.00003: How Curvature and Tension Direct Morphology and Interactions of Solid Membrane Domains in Fluid Vesicles Maria Santore, Weiyue Xin, Hao Wan Multi-phospholipid giant unilamellar vesicles containing at least one high melting lipid (such as DPPC) exhibit the coexistence of fluid and solid membrane domains. To the extent that the inner and outer leaflets of the membrane are matched in composition and molecular number, symmetry dictates flat solid domains. The curvature imposed we show that at sufficiently high tensions, the curvature imposed by global vesicle shape influences the growth of solid domains, producing deviations from hexagon domain shape to include edge instabilities that appear as flowers. At high tensions domains appear to fracture. At lower tensions, exclusion of curvature into the fluid membrane region produces reversible interactions between solid domains that give rise to aggregation or long range order, depending on the relative sizes of vesicle domains, curvature, and excess membrane area. These behaviors are demonstrated in a model experimental vesicle system. |
Tuesday, March 3, 2020 11:51AM - 12:03PM |
G30.00004: Collapse and folding of flexible colloidal polymers Angus McMullen, Jasna Brujic The self-assembly of colloids can yield novel materials with unusual optical or mechanical properties. Unlike periodically repeating structures, here we introduce a new paradigm of self-assembly in which flexible colloidal polymers find a 'fold', similar to the way polypeptides fold into proteins. We have previously shown how emulsion droplets decorated with DNA sticky ends self-assemble into freely-jointed polymers. Here, we demonstrate the addition of secondary, switchable interactions along the length of a self-assembled polymer. These interactions are triggered by a temperature quench, whose rate governs the folding pathways from the extended to the collapsed states. The folding time scales with the length of the chain, in agreement with the increase in entropy. Using the simplest form of sequence control, we show that labeling alternating particles with secondary interactions eliminates some folding pathways visited when all droplets along the chain interact, narrowing the ensemble of folded configurations. This result opens up the prospect of programmable sequence design to achieve a unique stable structure, as the Anfinsen's dogma for protein folding proposes. This work was supported by the NSF MRSEC Program (DMR-0820341). |
Tuesday, March 3, 2020 12:03PM - 12:15PM |
G30.00005: Dissolving a DNA liquid through competition Gabrielle Abraham, Omar Saleh Coacervation plays a key role in spatial and temporal organization within the cell. Short nucleic acids are one mechanism that has been shown to control the intracellular dense liquids (e.g., introducing or removing short nucleic acids causes changes in the properties and appearance of droplets.) Here, we employ a model DNA liquid system and quantify the dissolution by short nucleic acids. Previous work has confirmed that this system is similar to intracellular droplets in terms of physical properties and spatial control (i.e., specific molecules are enriched or depleted in the droplet phase). The liquid is comprised of DNA in the shape of a 4-armed nanostar with each arm ending in a palindromic sequence that allows nanostars to interact and condense into droplets. By incorporating a toe-hold sequence on the nanostar, short hairpins are able to outcompete the nanostars for interaction sites causing the droplets to dissolve. Using confocal microscopy, we track DNA liquid break-up and compare the rate of decay to hairpin-nanostar interaction strength, concentration of the hairpin, and toehold location. This work will give insight into the different mechanisms that drive coacervate dissolution. |
Tuesday, March 3, 2020 12:15PM - 12:27PM |
G30.00006: Theory and simulation of reaction diffusion models of self-limiting droplet formation Trevor GrandPre, David T. Limmer Liquid droplets are a generic feature of biological systems. Within the cell, liquidlike substructures form that contain Cajal bodies, germ granules, and centrosomes. In the absence of surfactants or long ranged interactions, the thermodynamically stable phase separated state of a liquid will always be that which minimizes the surface area of the droplet resulting in macroscopic domains. For this reason, It remains unclear how cells mitigate the formation of such large scale structure. We discuss ways in which macroscopic domains can be avoided by slow coarsening dynamics accompanying gelation or active processes that modulate Ostwald ripening. Theory and computer simulations are used to understand the mechanisms of droplet formation and dynamics. Implications for T-cell signaling pathways are also discussed. |
Tuesday, March 3, 2020 12:27PM - 12:39PM |
G30.00007: Nonequilibrium Variational Control Forces for Self-Limited Colloidal Assembly Avishek Das, David T. Limmer Materials with tunable functionalities can be synthesized bottom-up by controlled self-assembly of colloidal particles. Design principles that are rooted in thermodynamic principles and optimize specific interactions are often thwarted by kinetic limitations and confined to compact surface energy minimizing structures. We discuss a variational principle for the optimization of interparticle and nonequilibrium driving forces, which lead to robust assembly of self-limited clusters. Molecular dynamics simulations and a novel optimization strategy are used to solve for these optimal control forces. Our results expand the design space outside thermal equilibrium and provide new principles for tuning the dynamics of self-limited assembly in colloidal systems. |
Tuesday, March 3, 2020 12:39PM - 12:51PM |
G30.00008: Kinetic entrapment: a mechanism for periodic one-dimensional growth Thomas Witten, Martin Lenz Globular proteins and other irregularly-shaped but identical molecular objects often self-assemble into fibers such as sickle-cell hemoglobin fibers. Such fibers are typically one-dimensional aggregates of fixed width, indefinitely long length and strong periodic order. A recent numerical study [Lenz, Witten 2017] gave strong evidence that such one dimensional fibers arise generically when a) the constituents are identical, b) their shape is asymmetric, so that they do not tile space, c) they aggregate irreversibly under the influence of a short-range attraction, and d) the energetic cost of distorting the constituents into bonding configurations is comparable to the attachment energy gained by this bonding. Here we propose a common kinetic mechanism in which the next growth site is a generic, deterministic function of the current aggregate configuration. We take the growth function to depend only on a local neighborhood of the previous growth site. A small bias favoring convex regions causes the growth site to revisit a given neighborhood of the deposit sufficiently often that the sequence of neighborhoods and growth sites reliably falls into a fixed, repeating cycle. This self entrapment does not occur when the bias parameter is halved. |
Tuesday, March 3, 2020 12:51PM - 1:03PM |
G30.00009: Pattern recognition through molecular self-assembly Constantine G Evans, Jackson O'Brien, Erik Winfree, Arvind Murugan The functional role of many weak promiscuous interactions among molecules in biology is not clear and is often assumed to be deleterious. Here, we exploit promiscuous interactions to engineer an experimental system of 917 single-stranded DNA molecules capable of associative pattern recognition on the high dimensional concentration patterns of these molecular assembly components. Such pattern recognition is achieved by exploiting a process of competitive nucleation between different polymorphic DNA structures that are predominantly made of the same molecules but co-localized in different combinations. We test the system with numerous concentration patterns and confirmed nucleation-based pattern recognition through Atomic Force Microscopy (AFM) and fluorescence measurements. We discuss how this system, in conjunction with additional enzymatic components (e.g. DNA ligase), can potentially learn the promiscuous interactions needed to perform unsupervised clustering of concentration patterns. |
Tuesday, March 3, 2020 1:03PM - 1:15PM |
G30.00010: Strain-Adaptive Self-Assembled Networks of Linear-Bottlebrush-Linear Copolymers Heyi Liang, Zilu Wang, Andrey Dobrynin We study the strain-adaptive behavior of the self-assembled networks of linear-bottlebrush-linear (LBL) triblock copolymers using a combination of analytical calculations and molecular dynamics simulations. Interactions between immiscible blocks result in microphase separation and formation of soft and strain-adaptive composite networks. Such unique network properties are manifestations of the architectural asymmetry of two blocks: (i) flexible linear chains that aggregate into domains and (ii) bottlebrush strands that form a soft matrix. The mechanical response of the networks is a two-stage process, which starts with the extension of the bottlebrush network strands (elastic regime) followed by the pulling out of the linear chains from L-domains (yielding regime). The two-stage network deformation process is incorporated into a unifying model of strain-adaptive network deformation. The model predictions are confirmed by molecular dynamics simulations of uniaxial deformation of self-assembled LBL copolymer networks and by experimental results for copolymers consisting of poly(dimethyl siloxane) bottlebrush block and two poly(methyl methacrylate) linear chain blocks with different compositions and block lengths. |
Tuesday, March 3, 2020 1:15PM - 1:27PM |
G30.00011: Self-directed Self-assembly of Block Copolymers Hejin Huang, Alfredo Alexander-Katz Directed self-assembly (DSA) of block copolymers (BCP) provides a powerful tool to fabricate complex thin film structures at small length scale. Despite its success in fabricating various 2D patterns, fabrication of complex 3D nanostructures remains a challenge. Here, we introduce a novel method, which enables self-directed self-assembly of 3D tailored nanostructures. Dissipative particle dynamics (DPD) is employed, which demonstrates that uniform multilayer nanostructures could be obtained through stacking two different block copolymers alternatively. By introducing graphoepitaxy or chemoepitaxy to the first layer and performing multilayer stacking, information propagates upwards. Different complex bilayer and trilayer structures have been achieved through carefully choosing the BCP used in each stacking layer. We will show several examples of the complex uniform multilayer structures. |
Tuesday, March 3, 2020 1:27PM - 1:39PM |
G30.00012: Identification of a Frank–Kasper Z phase from shape amphiphile self-assembly Stephen Cheng, Mingjun Huang, Zebin Su Frank–Kasper phases, a family of ordered structures formed from particles with spherical motifs, are found in a host of materials, such as metal alloys, inorganic colloids and various types of soft matter. All the experimentally observed Frank–Kasper phases can be constructed from the basic units of three fundamental structures called the A15, C15 and Z phases. The Z phase, typically observed in metal alloys, is associated with a relatively large volume ratio between its constituents, and this constraint inhibits its formation in most self-assembled single-component soft-matter systems. We have assembled a series of nano-sized shape amphiphiles that comprise a triphenylene core and six polyhedral oligomeric silsesquioxane cages grafted onto it through linkers to give a variety of unconventional structures, which include the Z phase. |
Tuesday, March 3, 2020 1:39PM - 1:51PM |
G30.00013: Theory of Complex Spherical Packing Phases in Surfactant Systems Jiayu Xie, Chi To Lai, Anchang Shi The emergence and stability of complex spherical phases from a vast number of soft matter systems have been attracting tremendous attention recently. In particular, recent experiments have demonstrated the formation of Frank-Kasper and Laves phases (A15, σ, C14 and C15) in surfactant systems. Specifically, it has been shown that micelles self-assembled from amphiphilic molecules in water could pack and form these complex spherical packing phases in the presence of hydrocarbon molecules. Theoretically, we model the surfactant molecules by short diblock copolymers composed of a hydrophilic head and a hydrophobic tail. The phase behaviour of the model system is examined by using self-consistent field theory applied to a model system of short diblock copolymers and homopolymers. Our results indicate that the complex spherical packing phases could become stable for a set of model parameters corresponding to surfactant systems. Phase diagrams containing a large number of complex phases are constructed. The theoretical results demonstrate that the emergence and stability of complex spherical packing phases could be regulated by surfactant composition and concentration, thus shedding light to the understanding of the formation mechanisms of complex phases in soft matter systems. |
Tuesday, March 3, 2020 1:51PM - 2:03PM |
G30.00014: Predictive Modeling of Dendrimer Directed Nanoparticle Self-Assembly Thi Vo, Katherine Elbert, Nadia Krook, William E Zygmunt, Jungmi Park, Kevin Yager, Russell Composto, Sharon C Glotzer, Christopher B Murray Traditional methods in nanocrystal self-assembly often rely on linear ligands isotropically grafted onto spherical cores. More recently, there has been a shift towards utilizing cores of varying shapes to expand on the current library of accessible morphologies. However, predictive simulations and designed experiments combining both the effect of ligand architecture as well as their anisotropic grafting onto nonspherical cores for self-assembly are still in their infancy. Here, we present a combined experimental and theoretical study in which a series of dendrimer ligands are used to direct the assembly of nanoplates into 2D and 3D geometries. We show that dendrimer ligands can be used to tune the degree of corona anisotropy about the nanoplates that then drives the formation of an off-set, layer-by-layer nanoplate architecture observed experimentally in 3D films. Our findings show that ligand architecture serves as a handle for layer specific, fine-tuning of self-assembly and provide a systematic approach to theoretically predict morphology purely from experimental design parameters. |
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