Diffusion of self-propelled particles in complex media

The diffusion of active microscopic organisms in complex environments plays an important role in a wide range of biological phenomena from cell colony growth to single organism transport. Here, we investigate theoretically and computationally the diffusion of a self-propelled particle (the organism) embedded in a complex medium comprised of a collection of non-motile solid particles that mimic soil or other cells. Under such conditions we find that the rotational relaxation time of the swimming direction depends on the swimming velocity and is drastically reduced compared to a pure Newtonian fluid. This leads to a dramatic increase (of several orders of magnitude) in the effective rotational diffusion coefficient of the self-propelled particles, which can lead to "self-trapping" of the active particles in such complex media. An analytical model is put forward that quantitatively captures the computational results. Our work sheds light on the role that the environment plays in the behavior of active systems and can be generalized in a straightforward fashion to understand other synthetic and biological active systems in heterogenous environments

Dissipative Particle Dynamics for Directed Self-Assembly of Block Copolymers

The dissipative particle dynamics (DPD) simulation method has been shown to be a promising tool to study self-assembly of soft matter systems. In particular, it has been used to study block copolymer (BCP) self-assembly. However, previous parametrizations of this model are not able to capture most of the rich phase behaviors of block copolymers in thin films nor in directed self-assembly (chemoepitaxy or graphoepitaxy). Here we extend the applicability of the DPD method for BCPs to make it applicable to thin films and directed self-assembly. Our new reparametrization is able to reproduce the bulk phase behavior, but also manages to predict thin film structures obtained experimentally from chemoepitaxy or graphoepitaxy. A number of different complex structures, such as bilayer nanomeshes, 90{\deg} bend structures, circular cylinders/lamellae and Frank-Kasper phases directed by trenches, post arrays or chemically patterned substrate have all been reproduced in this work. This reparametrized DPD model should serves as a powerful tool to predict BCP self-assembly, especially in some complex systems where it is difficult to implement SCFT.Comment: 16 pages, 6 figure

Systems and method for viral detection

Systems and methods are provided for detecting viral particles, viral proteins, viral RNA, or viral DNA in multiple fluids. The methods consist of applying a magnetic torque to functionalized magnetic beads in a fluid solution resting on a functionalized substrate. The solution is comprised of one of the following: intact viral particles, viral proteins, RNA, or DNA. The presence and/or quantity of the aforementioned molecules or viruses is detected by measuring the translational velocity of the beads. The methods here described can detect multiple different species simultaneously using a multiplexed assay. Also, the systems here included are able to process multiple samples simultaneously

Pathway for insertion of amphiphilic nanoparticles into defect-free lipid bilayers from atomistic molecular dynamics simulations

Gold nanoparticles (NPs) have been increasingly used in biological applications that involve potential contact with cellular membranes. As a result, it is essential to gain a physical understanding of NP-membrane interactions to guide the design of next-generation bioactive nanoparticles. In previous work, we showed that charged, amphiphilic NPs can fuse with lipid bilayers after contact between protruding solvent-exposed lipid tails and the NP monolayer. Fusion was only observed at the high-curvature edges of large bilayer defects, but not in low-curvature regions where protrusions are rarely observed. Here, we use atomistic molecular dynamics simulations to show that the same NPs can also fuse with low-curvature bilayers in the absence of defects if NP-protrusion contact occurs, generalizing the results of our previous work. Insertion proceeds without applying biasing forces to the NP, driven by the hydrophobic effect, and involves the transient generation of bilayer curvature. We further find that NPs with long hydrophobic ligands can insert a single ligand into the bilayer core in a manner similar to the binding of peripheral proteins. Such anchoring may precede insertion, revealing potential methods for engineering NP monolayers to enhance NP-bilayer fusion in systems with a low likelihood of lipid tail protrusions. These results reveal new pathways for NP-bilayer fusion and provide fundamental insight into behavior at the nano-bio interface.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Award DMR-0819762)National Science Foundation (U.S.) (CAREER Award DMR-1054671

Design rules for self-assembled block copolymer patterns using tiled templates

Directed self-assembly of block copolymers has been used for fabricating various nanoscale patterns, ranging from periodic lines to simple bends. However, assemblies of dense bends, junctions and line segments in a single pattern have not been achieved by using sparse templates, because no systematic template design methods for achieving such complex patterns existed. To direct a complex pattern by using a sparse template, the template needs to encode the key information contained in the final pattern, without being a simple copy of the pattern. Here we develop a set of topographic template tiles consisting of square lattices of posts with a restricted range of geometric features. The block copolymer patterns resulting from all tile arrangements are determined. By combining tiles in different ways, it is possible to predict a relatively simple template that will direct the formation of non-trivial block copolymer patterns, providing a new template design method for a complex block copolymer pattern.Samsung Scholarship FoundationSemiconductor Research CorporationTokyo Electron LimitedTaiwan Semicondcutor Manufacturing CompanyNational Science Foundation (U.S.) (Award DMR1234169

Aggregation dynamics of active rotating particles in dense passive media

Active matter systems are able to exhibit emergent non-equilibrium behavior due to activity-induced effective interactions between the active particles. Here we study the aggregation and dynamical behavior of active rotating particles, spinners, embedded in 2D passive colloidal monolayers. Using both experiments and simulations we observe aggregation of active particles or spinners whose behavior resembles classical 2D Cahn–Hilliard coarsening. The aggregation behavior and spinner attraction depend on the mechanical properties of the passive monolayer and the activity of spinners. Spinner aggregation only occurs when the passive monolayer behaves elastically and when the spinner activity exceeds a minimum activity threshold. Interestingly, for the spinner concentrations investigated here, the spinner concentration does not seem to change the dynamics of the aggregation behavior. There is a characteristic cluster size which maximizes spinner aggregation by minimizing the drag through the passive monolayer and maximizing the stress applied on the passive medium. We also show a ternary mixture of passive particles and co-rotating and counter-rotating spinners that aggregate into clusters of co and counter-rotating spinners respectivelyThis work was supported by Department of Energy BES award #ER46919 (theoretical and simulation work) and the Chang Family (experimental work)