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2016, Bulletin of the American Physical Society
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Interplay of directional and isotropic interactions in self-assembly DEBRA AUDUS, National Institute of Standards and Technology, FRANCIS STARR, Wesleyan University, JACK DOUGLAS, National Institute of Standards and Technology -Patchy particle models, composed of hard spheres with decorated with attractive patches, have been introduced as models of micron-sized particles with anisotropic interactions, as well as solutions of globular proteins. Here, we extend the canonical model of the patchy particles to include a short-ranged isotropic interaction in order to probe of the coupling of the directional and isotropic interactions on the self-assembly process. In particular, we evaluate basic properties characterizing self-assembly including average cluster mass and the fraction of particles in the clustered state using both Monte Carlo simulation and analytic Wertheim theory. This combination allows for validation of the theory and for insight into analyzing experimental data. We also find that Flory-Stockmayer theory describes the cluster size distribution data found in our simulations remarkably well, despite its erroneous mass-scaling exponent. This result, coupled with Wertheim theory, predicts both a master curve for the average cluster mass and a method to parameterize patchy particle models using experimental data.
Bulletin of the American Physical Society, 2017
The canonical model of patchy particles, a hard sphere decorated with attractive patches, has been used to describe solutions of small globular proteins, as well as micron-size particles with attractive patches. Previously, we extended the canonical model by introducing an isotropic, attractive interaction. Using Monte Carlo simulations and an analytic, Wertheim based mean-field theory, we found that although the location of the self-assembly transition lines were dependent on the isotropic interaction strength, the nature of the self-assembly was unaffected. Specifically, we developed a formalism to describe a master curve for the average molecular mass by combining Flory-Stockmayer theory with an analysis of the thermodynamics of association. We also found that the self-assembled clusters have a fractal dimension of two; this value is consistent with randomly branched polymers swollen by repulsive self-excluded volume interactions. Extending this work, we consider the role of patch number and find that the formalism still holds but becomes dependent on the number of patches. We explore the experimental implications of this finding and investigate the role of patch number on cluster shape.
The Journal of Physical Chemistry B, 2007
Self-assembly is the mechanism that controls the formation of well-defined structures from disordered preexisting parts. Despite the importance of self-assembly as a manufacturing method and the increasingly large number of experimental realizations of complex self-assembled nano-aggregates, theoretical predictions are lagging behind. Here, we show that for a nontrivial self-assembly phenomenon, originating branched loopless clusters, it is possible to derive a fully predictive parameter-free theory of equilibrium self-assembly by combining the Wertheim theory for associating liquids with the Flory-Stockmayer approach for chemical gelation.
Central European Journal of Physics, 2012
Motivated by recent experimental findings in chemical synthesis of colloidal particles, we draw an analogy between self-assembly processes occurring in biological systems (e.g. protein folding) and a new exciting possibility in the field of material science. We consider a self-assembly process whose elementary building blocks are decorated patchy colloids of various types, that spontaneously drive the system toward a unique and predetermined targeted macroscopic structure.
Soft Matter, 2012
The Journal of Chemical Physics, 2016
The interactions of molecules and particles in solution often involve an interplay between isotropic and highly directional interactions that lead to a mutual coupling of phase separation and selfassembly. This situation arises, for example, in proteins interacting through hydrophobic and charged patch regions on their surface and in nanoparticles with grafted polymer chains, such as DNA. As a minimal model of complex fluids exhibiting this interaction coupling, we investigate spherical particles having an isotropic interaction and a constellation of five attractive patches on the particle's surface. Monte Carlo simulations and mean-field calculations of the phase boundaries of this model depend strongly on the relative strength of the isotropic and patch potentials, where we surprisingly find that analytic mean-field predictions become increasingly accurate as the directional interactions become increasingly predominant. We quantitatively account for this effect by noting that the effective interaction range increases with increasing relative directional to isotropic interaction strength. We also identify thermodynamic transition lines associated with self-assembly, extract the entropy and energy of association, and characterize the resulting cluster properties obtained from simulations using percolation scaling theory and Flory-Stockmayer mean-field theory. We find that the fractal dimension and cluster size distribution are consistent with those of lattice animals, i.e., randomly branched polymers swollen by excluded volume interactions. We also identify a universal functional form for the average molecular weight and a nearly universal functional form for a scaling parameter characterizing the cluster size distribution. Since the formation of branched clusters at equilibrium is a common phenomenon in nature, we detail how our analysis can be used in experimental characterization of such associating fluids.
The Journal of chemical physics, 2015
We study the kinetics of aggregation of a two site model of interacting spherical molecules. A given site on one molecule can interact with one or more sites on other neighboring molecules. The sites represent the result of a simple coarse graining of putative amino acid residues or two specifically designed sites on a colloidal particle. We study the kinetics and equilibrium morphology for a fixed angle between the two sites, for several angles between 30° and 150°. In the model, the sites interact via an attractive Asakura-Oosawa potential and the molecules have the usual hard sphere repulsion interaction. We find a transition from a micelle-like morphology at small angles to a rod-like morphology at intermediate angles and to a gel-like structure at values of the angle greater than about ninety degrees. However, at 150 degrees, after a long induction time during which there is no aggregation, we observe a nucleation and growth process that leads to a final spherical-like aggregat...
We investigate the structure of a dilute mixture of amphiphilic dimers and spherical particles, a model relevant to the problem of encapsulating globular " guest " molecules in a dispersion. Dimers and spheres are taken to be hard particles, with an additional attraction between spheres and the smaller monomers in a dimer. Using the Monte Carlo simulation, we document the low-temperature formation of aggregates of guests (clusters) held together by dimers, whose typical size and shape depend on the guest concentration χ. For low χ (less than 10%), most guests are isolated and coated with a layer of dimers. As χ progressively increases, clusters grow in size becoming more and more elongated and polydisperse; after reaching a shallow maximum for χ ≈ 50%, the size of clusters again reduces upon increasing χ further. In one case only (χ = 50% and moderately low temperature) the mixture relaxed to a fluid of lamellae, suggesting that in this case clusters are metastable with respect to crystal-vapor separation. On heating, clusters shrink until eventually the system becomes homogeneous on all scales. On the other hand, as the mixture is made denser and denser at low temperature, clusters get increasingly larger until a percolating network is formed. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4976704]
The Journal of Physical Chemistry B, 2019
Building structures with hierarchical order through the self-assembly of smaller blocks is not only a prerogative of nature, but also a strategy to design artificial materials with tailored functions. We explore in simulation the spontaneous assembly of colloidal particles into extended structures, using spheres and size-asymmetric dimers as solute particles, while treating the solvent implicitly. Besides rigid cores for all particles, we assume an effective short-range attraction between spheres and small monomers to promote, through elementary rules, dimer-mediated aggregation of spheres. Starting from a completely disordered configuration, we follow the evolution of the system at low temperature and density, as a function of the relative concentration of the two species. When spheres and large monomers are of same size, we observe the onset of elongated aggregates of spheres, either disconnected or cross-linked, and a crystalline bilayer. As spheres grow bigger, the self-assembling scenario changes, getting richer overall, with the addition of flexible membrane sheets with crystalline order and monolayer vesicles. With this wide assortment of structures, our model can serve as a viable template to achieve a better control of self-assembly in dilute suspensions of microsized particles. Various biomolecules, like phospholipids, peptides, and DNA filaments, as well as many synthetized colloidal particles, have the capability of assembling into mesophases, owing to their chemical and structural versatility (see, for instance, Ref. 1). The spontaneous assembly of colloidal particles into extended structures, like gels or membranes, is an emergent phenomenon of utmost importance in the design of functional materials. One motif that may serve different purposes is a colloidal sphere endowed with one or more attractive caps, so-called "patches", obtained by grafting appropriate functional groups to the sphere surface -see examples in Refs. 2-5. When assembled in a connected network characterized by a high surface-to-volume ratio, patchy particles may provide a practical morphology for