809 research outputs found
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 Οβ. 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 Οβ 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
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