129 research outputs found
Micelle Formation and the Hydrophobic Effect
The tendency of amphiphilic molecules to form micelles in aqueous solution is
a consequence of the hydrophobic effect. The fundamental difference between
micelle assembly and macroscopic phase separation is the stoichiometric
constraint that frustrates the demixing of polar and hydrophobic groups. We
present a theory for micelle assembly that combines the account of this
constraint with a description of the hydrophobic driving force. The latter
arises from the length scale dependence of aqueous solvation. The theoretical
predictions for temperature dependence and surfactant chain length dependence
of critical micelle concentrations for nonionic surfactants agree favorably
with experiment.Comment: Accepted for publication in J. Phys. Chem.
Steered Transition Path Sampling
We introduce a path sampling method for obtaining statistical properties of
an arbitrary stochastic dynamics. The method works by decomposing a trajectory
in time, estimating the probability of satisfying a progress constraint,
modifying the dynamics based on that probability, and then reweighting to
calculate averages. Because the progress constraint can be formulated in terms
of occurrences of events within time intervals, the method is particularly well
suited for controlling the sampling of currents of dynamic events. We
demonstrate the method for calculating transition probabilities in barrier
crossing problems and survival probabilities in strongly diffusive systems with
absorbing states, which are difficult to treat by shooting. We discuss the
relation of the algorithm to other methods.Comment: 11 pages, 8 figure
Phase resetting reveals network dynamics underlying a bacterial cell cycle
Genomic and proteomic methods yield networks of biological regulatory
interactions but do not provide direct insight into how those interactions are
organized into functional modules, or how information flows from one module to
another. In this work we introduce an approach that provides this complementary
information and apply it to the bacterium Caulobacter crescentus, a paradigm
for cell-cycle control. Operationally, we use an inducible promoter to express
the essential transcriptional regulatory gene ctrA in a periodic, pulsed
fashion. This chemical perturbation causes the population of cells to divide
synchronously, and we use the resulting advance or delay of the division times
of single cells to construct a phase resetting curve. We find that delay is
strongly favored over advance. This finding is surprising since it does not
follow from the temporal expression profile of CtrA and, in turn, simulations
of existing network models. We propose a phenomenological model that suggests
that the cell-cycle network comprises two distinct functional modules that
oscillate autonomously and couple in a highly asymmetric fashion. These
features collectively provide a new mechanism for tight temporal control of the
cell cycle in C. crescentus. We discuss how the procedure can serve as the
basis for a general approach for probing network dynamics, which we term
chemical perturbation spectroscopy (CPS)
Signatures of odd dynamics in viscoelastic systems: from spatiotemporal pattern formation to odd rheology
Non-reciprocal interactions fueled by local energy consumption are found in
biological and synthetic active matter, where both viscosity and elasticity are
often important. Such systems can be described by "odd" viscoelasticity, which
assumes fewer material symmetries than traditional theories. In odd
viscoelastic systems there is an interplay between the energy-consuming odd
elastic elements and the traditional stabilizing elements. This leads to rich
dynamical behavior which, due to a lack of appropriate numerical methods, has
remained relatively unexplored. Furthermore, the implications associated with
the presence of such odd terms in actomyosin and other similar anisotropic
systems has not been addressed. Here, we study odd viscoelasticity analytically
and using hydrodynamic simulations based on the lattice Boltzmann algorithm. We
first outline how odd effects may naturally emerge from a theory of polymeric
elasticity which can describe anisotropic systems like actomyosin. Next, we
report on two striking features of odd viscoelastic dynamics: a pattern-forming
instability which produces an oscillating array of fluid vortices, and strong
transverse and rotational forces during a simulated rheological experiment.
These findings can guide efforts to detect or engineer odd dynamics in soft
active matter systems.Comment: 29 pages, 14 figure
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