Theory of Capillary Tension and Interfacial Dynamics of Motility-Induced Phases

The statistical mechanics of equilibrium interfaces has been well-established for over a half century. In the last decade, a wealth of observations have made increasingly clear that a new perspective is required to describe interfaces arbitrarily far from equilibrium. In this Letter, beginning from microscopic particle dynamics that break time-reversal symmetry, we systematically derive the interfacial dynamics of coexisting motility-induced phases. Doing so allows us to identify the athermal energy scale that excites interfacial fluctuations and the nonequilibrium surface tension that resists these excitations. In addition to establishing an active capillary-wave theory, our theory identifies that, in contrast to equilibrium fluids, this active surface tension contains contributions arising from nonconservative forces which act to suppress interfacial fluctuations. The identified contribution bares a remarkable resemblance with that envisioned by Edwards and Wilkinson in their description of the surface of athermal granular materials, suggesting that dynamically-stabilized interfaces may be a phenomenon common to a wide class of nonequilibrium systems.Comment: Comments welcome

Shear-Induced Heterogeneity in Associating Polymer Gels: Role of Network Structure and Dilatancy

We study associating polymer gels under steady shear using Brownian dynamics simulation to explore the interplay between the network structure, dynamics, and rheology. For a wide range of flow rates, we observe the formation of shear bands with a pronounced difference in shear rate, concentration, and structure. A striking increase in the polymer pressure in the gradient direction with shear, along with the inherently large compressibility of the gels, is shown to be a crucial factor in destabilizing homogeneous flow through shear-gradient concentration coupling. We find that shear has only a modest influence on the degree of association, but induces marked spatial heterogeneity in the network connectivity. We attribute the increase in the polymer pressure (and polymer mobility) to this structural reorganization

Phase Coexistence Implications of Violating Newton's Third Law

Newton's third law, actio = reactio, is a foundational statement of classical mechanics. However, in natural and living systems, this law appears to be routinely violated for constituents interacting in a nonequilibrium environment. Here, we use computer simulations to explore the macroscopic phase behavior implications of breaking microscopic interaction reciprocity for a simple model system. We consider a binary mixture of attractive particles and introduce a parameter which is a continuous measure of the degree to which interaction reciprocity is broken. In the reciprocal limit, the species are indistinguishable and the system phase separates into domains with distinct densities and identical compositions. Increasing nonreciprocity is found to drive the system to explore a rich assortment of phases, including phases with strong composition asymmetries and three-phase coexistence. Many of the states induced by these forces, including traveling crystals and liquids, have no equilibrium analogue. By mapping the complete phase diagram for this model system and characterizing these unique phases, our findings offer a concrete path forward towards understanding how nonreciprocity shapes the structures found in living systems and how this might be leveraged in the design of synthetic materials

Direct synthesis of densely substituted pyridines and pyrimidines

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2011.Vita. Cataloged from PDF version of thesis.Includes bibliographical references.I. N-Isopropylidene-N'-2-Nitrobenzenesulfonyl Hydrazine. A Reagent for Reduction of Alcohols via the Corresponding Monoalkyl Diazenes The development of a new reagent N-isopropylidene-N'-2-nitrobenzenesulfonyl hydrazine (IPNBSH) is described. IPNBSH is used in the reduction of alcohols via the loss of dinitrogen from transiently formed monoalkyl diazene intermediates that can be accessed by sequential Mitsunobu displacement, hydrolysis, and fragmentation under mild reaction conditions. II. Stereospecific Palladium-Catalyzed Route to Monoalkyl Diazenes for Mild Allylic Reduction The first single-step stereospecific transition metal-catalyzed conversion of allylic electrophiles into monoalkyl diazenes is described. This synthesis of allylic monoalkyl diazenes offers a new strategy for asymmetric synthesis by the reduction of optically active substrates or the use of chiral catalyst systems for the reduction of racemic and prochiral substrates. Sensitive substrates are reduced in a highly selective manner. III. Direct Synthesis of Pyridine Derivatives A single-step conversion of various N-vinyl and N-aryl amides to the corresponding pyridine and quinoline derivatives, respectively, is described. The process involves amide activation with trifluoromethanesulfonic anhydride in the presence of 2-chloropyridine followed by n-nucleophile addition to the activated intermediate and annulation. IV. Synthesis of Densely Substituted Pyrimidine Derivatives The direct condensation of cyanic acid derivatives with N-vinyl and N-aryl amides to afford the corresponding C4-heteroatom substituted pyrimidines is described. The use of cyanic bromide and thiocyanatomethane in this chemistry provides versatile azaheterocycles poised for further derivatization. The synthesis of a variety of previously inaccessible C2- and C4- pyrimidine derivatives using this methodology is noteworthy.by Omar K. Ahmad.Ph.D

Dynamic Overlap Concentration Scale of Active Colloids

By introducing the notion of a dynamic overlap concentration scale, we identify universal and previously unreported features of the mechanical properties of active colloids. These features are codified by recognizing that the characteristic length scale of an active particle's trajectory, the run-length, introduces a new concentration scale ϕ∗. Large-scale simulations of repulsive active Brownian particles (ABPs) confirm that this new run-length dependent concentration, which is the trajectory-space analogue of the overlap concentration in polymer solutions, delineates distinct concentration regimes in which interparticle collisions alter particle trajectories. Using ϕ∗ and concentration scales associated with colloidal jamming, the mechanical equation-of-state for ABPs can be collapsed onto a set of principal curves that contain a number of previously overlooked features. The inclusion of these features qualitatively alters previous predictions of the behavior for active colloids as we demonstrate by computing the spinodal for a suspension of purely-repulsive ABPs. Our findings suggest that dynamic overlap concentration scales should be of great utility in unraveling the behavior of active and driven systems

Topological Forces in a Model System for Reptation Dynamics

We construct a micromechanical version of an early model for topologically constrained polymers -- a 2D chain amongst point-like uncrossable obstacles -- which allows us to explicitly elucidate the role of topological forces beyond confining the chain to a curvilinear tube-like path. Our simulations reveal that linear relaxation of the contour length \textit{along the tube} is slowed down by the presence of topological forces that can be considered as additional effective topological ``friction'' in quiescence. However, this perspective fails in predicting the strong forces that resist the imposed curvilinear motion of the chain during nonlinear startup microrheology. These entropic forces are nonlocal in nature and result from an unexpected coupling between orientational and longitudinal dynamics.Comment: Comments welcom

Swimming to Stability: Structural and Dynamical Control via Active Doping

External fields can decidedly alter the free energy landscape of soft materials and can be exploited as a powerful tool for the assembly of targeted nanostructures and colloidal materials. Here, we use computer simulations to demonstrate that nonequilibrium internal fields or forces -- forces that are generated by driven components within a system -- in the form of active particles can precisely modulate the dynamical free energy landscape of a model soft material, a colloidal gel. Embedding a small fraction of active particles within a gel can provide a unique pathway for the dynamically frustrated network to circumvent the kinetic barriers associated with reaching a lower free energy state through thermal fluctuations alone. Moreover, by carefully tuning the active particle properties (the propulsive swim force and persistence length) in comparison to those of the gel, the active particles may induce depletion-like forces between the constituent particles of the gel despite there being no geometric size asymmetry between the particles. These resulting forces can rapidly push the system toward disparate regions of phase space. Intriguingly, the state of the material can be altered by tuning macroscopic transport properties such as the solvent viscosity. Our findings highlight the potential wide-ranging structural and kinetic control facilitated by varying the dynamical properties of a remarkably small fraction of driven particles embedded in a host material.Comment: ACS Nan