Lattice Boltzmann study of chemically-driven self-propelled droplets

We numerically study the behavior of self-propelled liquid droplets whose motion is triggered by a Marangoni-like flow. This latter is generated by variations of surfactant concentration which affect the droplet surface tension promoting its motion. In the present paper a model for droplets with a third amphiphilic component is adopted. The dynamics is described by Navier-Stokes and convection-diffusion equations, solved by lattice Boltzmann method coupled with finite-difference schemes. We focus on two cases. First the study of self-propulsion of an isolated droplet is carried on and, then, the interaction of two self-propelled droplets is investigated. In both cases, when the surfactant migrates towards the interface, a quadrupolar vortex of the velocity field forms inside the droplet and causes the motion. A weaker dipolar field emerges instead when the surfactant is mainly diluted in the bulk. The dynamics of two interacting droplets is more complex and strongly depends on their reciprocal distance. If, in a head-on collision, droplets are close enough, the velocity field initially attracts them until a motionless steady state is achieved. If the droplets are vertically shifted, the hydrodynamic field leads to an initial reciprocal attraction followed by a scattering along opposite directions. This hydrodynamic interaction acts on a separation of some droplet radii otherwise it becomes negligible and droplets motion is only driven by Marangoni effect. Finally, if one of the droplets is passive, this latter is generally advected by the fluid flow generated by the active one.Comment: 14 pages, 9 figures. In press on EPJ

Lyapunov Central Limit Theorem: Theoretical Properties and Applications in Big-Data-Populated Smart City Settings

Central Limit Theorems have a fundamental role in statistics and in a wide range of practical applications. The most famous formulation was proposed by Lindeberg–Lévy and it requires the variables to be independent and identically distributed. In the real setting these conditions are rarely matched, though. The Lyapunov Central Limit Theorem overcomes this limitation, since it does not require the same distribution of the random variables. However, the cost of this generalization is an increased complexity, moderately limiting its effective applicability. In this paper, we resume the main results on the Lyapunov Central Limit Theorem, providing an easy-to-prove condition to put in practice, and demonstrating its uniform convergence. These theoretical results are supported by some relevant applications in the field of big data in smart city settings

Lattice Boltzmann modeling of cholesteric liquid crystal droplets under an oscillatory electric field

We numerically study the dynamics of quasi-two dimensional cholesteric liquid crystal droplets in the presence of a time-dependent electric field, rotating at constant angular velocity. A surfactant sitting at droplet interface is also introduced to prevent droplet coalescence. The dynamics is modeled following a hybrid numerical approach, where a standard lattice Boltzmann technique solves the Navier-Stokes equation and a finite difference scheme integrates the evolution equations of liquid crystal and surfactant. Our results show that, once the field is turned on, the liquid crystal rotates coherently triggering a concurrent orbital motion of both droplets around each other, an effect due to the momentum transfer to the surrounding fluid. In addition the topological defects, resulting from the conflict orientation of the liquid crystal within the drops, exhibit a chaotic-like motion in cholesterics with a high pitch, in contrast with a regular one occurring along circular trajectories observed in nematics drops. Such behavior is found to depend on magnitude and frequency of the applied field as well as on the anchoring of the liquid crystal at the droplet interface. These findings are quantitatively evaluated by measuring the angular velocity of fluid and drops for various frequencies of the applied field.Comment: 16 pages, 13 figures. Accepted on Communications in Computational Physic

Spitzer Observations of Galaxy Clusters

We present preliminary results of a project to study three rich nearby clusters of galaxies with the Spitzer space telescope. The Spitzer observations in the four IRAC and three MIPS bands cover a region up to three virial radii, approximately, and have been recently completed. On the basis of the first Spitzer images, we followed up spectroscopically the far-infrared sources with the multi-fiber spectrograph HYDRA on the WIYN telescope. 70% of the sources brighter than 0.3 mJy at 24 μm and r’ < 20.5 have been observed for a total of 1078 spectra. For 87% of them we were able to measure redshifts obtaining 50 to 100 members for the different clusters. This first study shows that the far-IR sources in these clusters are predominantly powered by star formation and clustered in regions far from the center. In the case of A1763, they seem to be situated along a filament supporting the idea of infalling galaxies experiencing bursts of star formation during their first contact with the hot intra-cluster medium

Detections of water ice, hydrocarbons, and 3.3um PAH in z~2 ULIRGs

We present the first detections of the 3um water ice and 3.4um amorphous hydrocarbon (HAC) absorption features in z~2 ULIRGs. These are based on deep rest-frame 2-8um Spitzer IRS spectra of 11 sources selected for their appreciable silicate absorption. The HAC-to-silicate ratio for our z~2 sources is typically higher by a factor of 2-5 than that observed in the Milky Way. This HAC `excess' suggests compact nuclei with steep temperature gradients as opposed to predominantly host obscuration. Beside the above molecular absorption features, we detect the 3.3um PAH emission feature in one of our sources with three more individual spectra showing evidence for it. Stacking analysis suggests that water ice, hydrocarbons, and PAH are likely present in the bulk of this sample even when not individually detected. The most unexpected result of our study is the lack of clear detections of the 4.67um CO gas absorption feature. Only three of the sources show tentative signs of this feature and at significantly lower levels than has been observed in local ULIRGs. Overall, we find that the closest local analogs to our sources, in terms of 3-4um color, HAC-to-silicate and ice-to-silicate ratios, as well as low PAH equivalent widths are sources dominated by deeply obscured nuclei. Such sources form only a small fraction of ULIRGs locally and are commonly believed to be dominated by buried AGN. Our sample suggests that, in absolute number, such buried AGN are at least an order of magnitude more common at z~2 than today. The presence of PAH suggests that significant levels of star-formation are present even if the obscured AGN typically dominate the power budget.Comment: 39 pages, 14 figures, accepted for publication in Ap

Spitzer observations of Abell 1763. III. The infrared luminosity function in different supercluster environments

Context. The study of galaxy luminosity functions (LFs) in different environments provides powerful constraints on the physics of galaxy evolution. The infrared (IR) LF is a particularly useful tool since it is directly related to the distribution of galaxy star-formation rates (SFRs). Aims. We aim to determine the galaxy IR LF as a function of the environment in a supercluster at redshift 0.23 to shed light on the processes driving galaxy evolution in and around clusters. Methods. We base our analysis on multi-wavelength data, which include optical, near-IR, and mid- to far-IR photometry, as well as redshifts from optical spectroscopy. We identify 467 supercluster members in a sample of 24-μm-selected galaxies, on the basis of their spectroscopic (153) and photometric (314) redshifts. IR luminosities and stellar masses are determined for supercluster members via spectral energy distribution fitting. Galaxies with active galactic nuclei are identified by a variety of methods and excluded from the sample. SFRs are obtained for the 432 remaining galaxies from their IR luminosities via the Kennicutt relation. Results. We determine the IR LF of the whole supercluster as well as the IR LFs of three different regions in the supercluster: the cluster core, a large-scale filament, and the cluster outskirts (excluding the filament). A comparison of the IR LFs of the three regions, normalized by the average number densities of r-band selected normal galaxies, shows that the filament (respectively, the core) contains the highest (respectively, the lowest) fraction of IR-emitting galaxies at all levels of IR luminosities, and the highest (respectively, the lowest) total SFR normalized by optical galaxy richness. Luminous IR galaxies (LIRGs) are almost absent in the core region. The relation between galaxy specific SFRs and stellar masses does not depend on the environment, and it indicates that most supercluster LIRGs are rather massive galaxies with relatively low specific SFRs. A comparison with previous IR LF determinations from the literature confirms that the mass-normalized total SFR in clusters increases with redshift, but more rapidly than previously suggested for redshifts ≲ 0.4. Conclusions. The IR LF shows an environmental dependence that is not simply related to the local galaxy density. The filament, an intermediate-density region in the A1763 supercluster, contains the highest fraction of IR-emitting galaxies. We interpret our findings within a possible scenario for the evolution of galaxies in and around clusters

Using storytelling to foster the teaching and learning of gravitational waves physics at high-school

Studies in Physics Education Research show that interdisciplinary approaches in education foster students’ motivation, creativity, curiosity, and interest in physics. We discuss their features and potential role in bringing contemporary physics topics to high school and how to use them to integrate formal educational programs. We make an explicit example of the use of storytelling and theatrical techniques to introduce secondary school students to black holes and gravitational wave topics. The Educational Division of the Physics Department at the University of Cagliari designed the activity. Participants were 200 high-school students (17 to 19 years old) from five schools (scientific, humanities) in Sardinia. Through a research questionnaire, we measure the efficacy of using artistic tools to communicate and teach the proposed subjects. We collected 76 answers. Results show that our methodology helps introduce students to contemporary physics themes, fostering their interest and learning of such content. Students from the humanities significantly appreciated the use of poetry and artistic tools more than their scientific peers. Finally, we discuss the potentiality of our approach in orientating students towards a STEAM (STEM and Arts) caree

Crowding effect on helix-coil transition: beyond entropic stabilization

We report circular dichroism measurements on the helix-coil transition of poly(L-glutamic acid) in solution with polyethylene glycol (PEG) as a crowding agent. Using small angle neutron scattering, PEG solutions have been characterized and found to be well described by the picture of a transient network of mesh size $\xi$, usual for semi-diluted chains in good solvent. We show that the increase of PEG concentration stabilizes the helices and increases the transition temperature. But more unexpectedly we also notice that the increase of crowding agent concentration reduces the mean helix extent at the transition, or in other words reduces its cooperative feature. This result cannot be accounted for by an entropic stabilization mechanism. Comparing the mean length of helices at the transition and the mesh size of the PEG network, our results strongly suggest two regimes: helices shorter or longer than the mesh size