Colloidal glass transition: Beyond mode-coupling theory

A new theory for dynamics of concentrated colloidal suspensions and the colloidal glass transition is proposed. The starting point is the memory function representation of the density correlation function. The memory function can be expressed in terms of a time-dependent pair-density correlation function. An exact, formal equation of motion for this function is derived and a factorization approximation is applied to its evolution operator. In this way a closed set of equations for the density correlation function and the memory function is obtained. The theory predicts an ergodicity breaking transition similar to that predicted by the mode-coupling theory, but at a higher density.Comment: to be published in PR

Regulated peristalsis into the acidic region of the _Drosophila_ larval midgut is controlled by a novel component of the Autonomic Nervous System

The underlying cellular and molecular mechanisms that regulate and coordinate critical physiological processes such as peristalsis are complex, often cryptic, and involve the integration of multiple tissues and organ systems within the organism. We have identified a completely novel component of the larval autonomic nervous system in the _Drosophila_ larval midgut that is essential for the peristaltic movement of food from the anterior midgut into the acidic region of the midgut. We have named this region the Superior Cupric Autonomic Nervous System or SCANS. Located at the junction of the anterior and the acidic portions of the midgut, the SCANS is characterized by a cluster of a novel neuro-enteroendocrine cells that we call Lettuce Head Cells, a valve, and two anterior muscular tethers to the dorsal gastric caeca. Using cell ablation and ectopic activation via expression of the _Chlamydomonas reinhardtii_ blue-light activated channelrhodopsin, we demonstrate that the SCANS and in particular the Lettuce Head Cells are both necessary and sufficient for peristalsis and perhaps serve a larger role by coordinating digestion throughout the anterior midgut with development and growth

Glassy dynamics of partially pinned fluids: an alternative mode-coupling approach

We use a simple mode-coupling approach to investigate glassy dynamics of partially pinned fluid systems. Our approach is different from the mode-coupling theory developed by Krakoviack [Phys. Rev. Lett. 94, 065703 (2005), Phys. Rev. E 84, 050501(R) (2011)]. In contrast to Krakoviack's theory, our approach predicts a random pinning glass transition scenario that is qualitatively the same as the scenario obtained using a mean-field analysis of the spherical p-spin model and a mean-field version of the random first-order transition theory. We use our approach to calculate quantities which are often considered to be indicators of growing dynamic correlations and static point-to-set correlations. We find that the so-called static overlap is dominated by the simple, low pinning fraction contribution. Thus, at least for randomly pinned fluid systems, only a careful quantitative analysis of simulation results can reveal genuine, many-body point-to-set correlations

Controlling Rotationally Resolved Two-Dimensional Infrared Spectra with Polarization

Recent advancements in infrared frequency combs will enable facile recording of coherent two-dimensional infrared spectra of gas-phase molecules with rotational resolution (RR2DIR). Using time-dependent density-matrix perturbation theory and angular momentum algebra techniques, we derive new polarization conditions unique to freely rotating molecules and absent in the condensed phase. These polarization conditions can be used to suppress parts of 2DIR rovibrational response, clarifying complicated RR2DIR spectra. With the polarization control methods described here, RR2DIR spectroscopy can be a powerful tool for studying complex gas mixtures of polyatomic molecules

Theory of rotationally resolved two-dimensional infrared spectroscopy including polarization dependence and rotational coherence dynamics

Two-dimensional infrared (2DIR) spectroscopy is widely used to study molecular dynamics, but it is typically restricted to solid and liquid phase samples and modest spectral resolution. Only recently has its potential to study gas-phase dynamics begun to be realized. Moreover, the recently proposed technique of cavity-enhanced 2D spectroscopy using frequency combs and developments in multicomb spectroscopy is expected to dramatically advance capabilities for acquisition of rotationally resolved 2DIR spectra. This demonstrates the need for rigorous and quantitative treatment of rotationally resolved, polarization-dependent third-order response of gas-phase samples. In this article, we provide a rigorous and quantitative description of rotationally resolved 2DIR spectroscopy using density-matrix, time-dependent perturbation theory and angular momentum algebra techniques. We describe the band and branch structure of 2D spectra, decompose the molecular response into polarization-dependence classes, use this decomposition to derive and explain special polarization conditions, and relate the liquid-phase polarization conditions to gas-phase ones. Furthermore, we discuss the rotational coherence dynamics during the waiting time

Multiresolution Wavelet Analysis of the Dynamics of a Cracked Rotor

We examine the dynamics of a healthy rotor and a rotor with a transverse crack, which opens and closes due to its self weight. Using discrete wavelet transform, we perform a multiresolution analysis of the measured vibration signal from each of these rotors. In particular, the measured vibration signal is decomposed into eight frequency bands, and the rms amplitude values of the healthy and cracked rotors are compared in the three lowest-frequency bands. The results indicate that the rms vibration amplitudes for the cracked rotor are larger than those of the healthy rotor in each of these three frequency bands. In the case of externally applied harmonic force excitation to the rotor, the rms values of the vibration amplitude of the cracked rotor are also found to be larger than those of a healthy rotor in the three lowest-frequency bands. Furthermore, the difference in the rms values between the healthy and cracked rotors in each of the three lowest-frequency bands is more pronounced in the presence of external excitation than that with no excitation. The obtained results suggest that the present multiresolution approach can be used effectively to detect the presence of a crack in a rotor

Discussion of the Electromotive Force Terms in the Model of Parker-unstable Galactic Disks with Cosmic Rays and Shear

We analyze the electromotive force (EMF) terms and basic assumptions of the linear and nonlinear dynamo theories in our three-dimensional (3D) numerical model of the Parker instability with cosmic rays and shear in a galactic disk. We also apply the well known prescriptions of the EMF obtained by the nonlinear dynamo theory (Blackman & Field 2002 and Kleeorin et al. 2003) to check if the EMF reconstructed from their prescriptions corresponds to the EMF obtained directly from our numerical models. We show that our modeled EMF is fully nonlinear and it is not possible to apply any of the considered nonlinear dynamo approximations due to the fact that the conditions for the scale separation are not fulfilled.Comment: 15 pages, 12 figure

Spikes and diffusion waves in one-dimensional model of chemotaxis

We consider the one-dimensional initial value problem for the viscous transport equation with nonlocal velocity $u_t = u_{xx} - \left(u (K^\prime \ast u)\right)_{x}$ with a given kernel $K'\in L^1(\R)$. We show the existence of global-in-time nonnegative solutions and we study their large time asymptotics. Depending on $K'$, we obtain either linear diffusion waves ({\it i.e.}~the fundamental solution of the heat equation) or nonlinear diffusion waves (the fundamental solution of the viscous Burgers equation) in asymptotic expansions of solutions as $t\to\infty$. Moreover, for certain aggregation kernels, we show a concentration of solution on an initial time interval, which resemble a phenomenon of the spike creation, typical in chemotaxis models