Nonequilibrium relaxation of a trapped particle in a near-critical Gaussian field

We study the non-equilibrium relaxational dynamics of a probe particle linearly coupled to a thermally fluctuating scalar field and subject to a harmonic potential, which provides a cartoon for an optically trapped colloid immersed in a fluid close to its bulk critical point. The average position of the particle initially displaced from the position of mechanical equilibrium is shown to feature long-time algebraic tails as the critical point of the field is approached, the universal exponents of which are determined in arbitrary spatial dimensions. As expected, this behavior cannot be captured by adiabatic approaches which assume fast field relaxation. The predictions of the analytic, perturbative approach are qualitatively confirmed by numerical simulations.Comment: 33 pages, 11 figure

Microdroplets: fabrication of microdevices for interfacial phenomena studies

When fluids are confined on the length scales of microfluidic channels, typically in the range of tens and hundreds microns, their behavior may results significantly different with respect to the so called “bulk” proprieties. This is mainly due to the fact that the miniaturization is always characterized by a large surface to volume ratio, where the body forces can be normally neglected in favor of the surface forces. Notable example of this kind of systems is observable when two immiscible fluids are mixed to form droplets of emulsions. In the last ten years, the idea to use droplets in microfluidics has been inspired mainly because it allows to further scale down the typical size involved in these systems, bringing to a huge number of applications in chemistry, biology and physics. However, despite a large notoriety, microfluidic systems using droplets are not yet fully understood for the complexity of the interfacial phenomena that are involved. Aim of this thesis is to characterize the droplet systems commonly used in microfluidic devices. In detail, we worked with droplets in both open and closed microfluidic systems, focusing with the problem of their generation, control and manipulation with suitable microdevices, in presence of defects having different geometry and wettability. Regarding the open microfluidics, in Chapter 3,we first compared the shape of water droplets confined on posts having circular and square cross sections, observing that the pinning of the contact line is strongly influenced by the post shape. In particular, in the case of a circular profile, the contact line is pinned to the whole edge, confirming the Gibbs criteria, while on the square post, the contact line can spill along the vertical walls, because it is sustained by the corners. Then, in Chapter 4,we moved to investigate the change of morphological configuration from filament to bulge state, typical of liquid droplets confined on posts with rectangular cross section. This effect was already know in literature, but it was not quantify in term of post geometry and volume of the water droplets. Therefore, we realized rectangular posts with different aspect ratio ("l"), between length (L) and width (W). Changing the water volume on the posts, we observed that the morphological transition occurs for all the aspect ratios "l" and that, for "l">16, there is a bistability of the two states at the same volume. Furthermore, we started to investigate the dynamic of the transition, induced by oscillations, founding that, for posts with "l">16, it is possible to induce the transition by the oscillation, without change the volume. Next, in order to control the droplets motion, in Chapter 5, we studied the different behavior of sliding droplets on homogeneous and on chemically patterned surfaces. To do that we realized surfaces with hydrophilic and hydrophobic stripes by microcontact printing. On these surfaces, droplets show stick-slip motion, which causes the deformation of their shapes and introduces an extra friction imputable to the dissipation of energy at the contact line. With the aim to study generation and control of droplets in closed microfluidic channels, in Chapter 6, we focused our attention to define a reliable protocol for the production of droplets by T-junctions. Moreover, we investigated the swelling problem, which occurs using organic solvent into PDMS microchannels. We noticed that the swelling deformation is strongly connected with the geometry of the devices, being more evident when the aspect ratio (high to width) of the channel cross section is higher. Finally, in Chapter 7, we introduced a new method to change the wettability proprieties of thiolen resins, which are commonly used in microfluidics. In particular we worked with NOA, a commercial available resin, which shows a contact angle of 70°. Using chlorosilane chemistry, we changed its wettability to a more hydrophilic and to hydrophobic contact angles, showing that this technique can be used both to open and closed microfluidic device

HST absolute Proper Motions of NGC 6681 (M70) and the Sagittarius Dwarf Spheroidal Galaxy

We have measured absolute proper motions for the three populations intercepted in the direction of the Galactic globular cluster NGC 6681: the cluster itself, the Sagittarius dwarf spheroidal galaxy and the field. For this we used Hubble Space Telescope ACS/WFC and WFC3/UVIS optical imaging data separated by a temporal baseline of 5.464 years. Five background galaxies were used to determine the zero point of the absolute-motion reference frame. The resulting absolute proper motion of NGC 6681 is ($\mu_{\alpha}\cos\delta, \mu_{\delta}$)=($1.58\pm0.18, -4.57\pm0.16$) \masyr. This is the first estimate ever made for this cluster. For the Sgr dSph we obtain ($\mu_{\alpha}\cos\delta, \mu_{\delta})=(-2.54\pm0.18, -1.19\pm0.16$) \masyr, consistent with previous measurements and with the values predicted by theoretical models. The absolute proper motion of the Galaxy population in our field of view is ($\mu_{\alpha}\cos\delta, \mu_{\delta})=(-1.21\pm0.27, -4.39\pm0.26$) \masyr. In this study we also use background Sagittarius Dwarf Spheroidal stars to determine the rotation of the globular cluster in the plane of the sky and find that NGC 6681 is not rotating significantly:\ $v_{\rm rot}=0.82\pm1.02$ km$\,$s$^{-1}$ at a distance of 1 arcmin from the cluster center.Comment: 14 pages, 7 figures, accepted for publication by Ap

Proper motions in Terzan 5: membership of the multi-iron sub-populations and first constrain to the orbit

By exploiting two sets of high-resolution images obtained with HST ACS/WFC over a baseline of ~10 years we have measured relative proper motions of ~70,000 stars in the stellar system Terzan 5. The results confirm the membership of the three sub-populations with different iron abudances discovered in the system. The orbit of the system has been derived from a first estimate of its absolute proper motion, obtained by using bulge stars as reference. The results of the integration of this orbit within an axisymmetric Galactic model exclude any external accretion origin for this cluster. Terzan 5 is known to have chemistry similar to the Galactic bulge; our findings support a kinematic link between the cluster and the bulge, further strengthening the possibility that Terzan 5 is the fossil remnant of one of the pristine clumps that originated the bulge.Comment: 25 pages, 14 figures, accepted for publication by Ap

Interaction with smart assistants using alternative and augmentative communication

Smart assistants utilize speech recognition, sensing, artificial intelligence, and networking technologies to enable improved human-machine interaction. However, there remain use cases where smart assistants are not easily usable by humans. For example, voice-activated assistants are not accessible to those that are hearing or speech impaired. Touchscreen based assistants are not usable by those who lack fine motor skills and/or reading ability. This disclosure adds to the modalities by which humans can control and communicate with smart assistants by enabling use of physical objects, facial expressions, gross motor skills, body movements, etc. to provide commands. Collectively, these techniques of control and communication are referred to as alternative and augmentative communication (AAC)

Chemical and kinematical properties of Galactic bulge stars surrounding the stellar system Terzan 5

As part of a study aimed at determining the kinematical and chemical properties of Terzan 5, we present the first characterization of the bulge stars surrounding this puzzling stellar system. We observed 615 targets located well beyond the tidal radius of Terzan 5 and we found that their radial velocity distribution is well described by a Gaussian function peaked at =+21.0\pm4.6 km/s and with dispersion sigma_v=113.0\pm2.7 km/s. This is the one of the few high-precision spectroscopic survey of radial velocities for a large sample of bulge stars in such a low and positive latitude environment (b=+1.7{\deg}). We found no evidence for the peak at \sim+200 km/s found in Nidever et al. 2012. The strong contamination of many observed spectra by TiO bands prevented us from deriving the iron abundance for the entire spectroscopic sample, introducing a selection bias. The metallicity distribution was finally derived for a sub-sample of 112 stars in a magnitude range where the effect of the selection bias is negligible. The distribution is quite broad and roughly peaked at solar metallicity ([Fe/H]\simeq+0.05 dex) with a similar number of stars in the super-solar and in the sub-solar ranges. The population number ratios in different metallicity ranges agree well with those observed in other low-latitude bulge fields suggesting (i) the possible presence of a plateau for |b|<4{\deg} for the ratio between stars in the super-solar (0<[Fe/H]<0.5 dex) and sub-solar (-0.5<[Fe/H]<0 dex) metallicity ranges; (ii) a severe drop of the metal-poor component ([Fe/H]<-0.5) as a function of Galactic latitude.Comment: 27 pages, 9 figures, accepted for publication by Ap

Ceci n'est pas a globular cluster: the metallicity distribution of the stellar system Terzan 5

We present new determinations of the iron abundance for 220 stars belonging to the stellar system Terzan 5 in the Galactic bulge. The spectra have been acquired with FLAMES at the Very Large Telescope of the European Southern Observatory and DEIMOS at the Keck II Telescope. This is by far the largest spectroscopic sample of stars ever observed in this stellar system. From this dataset, a subsample of targets with spectra unaffected by TiO bands was extracted and statistically decontaminated from field stars. Once combined with 34 additional stars previously published by our group, a total sample of 135 member stars covering the entire radial extent of the system has been used to determine the metallicity distribution function of Terzan 5. The iron distribution clearly shows three peaks: a super-solar component at [Fe/H]$\simeq0.25$ dex, accounting for 29% of the sample, a dominant sub-solar population at [Fe/H]$\simeq-0.30$ dex, corresponding to 62% of the total, and a minor (6%) metal-poor component at [Fe/H]$\simeq-0.8$ dex. Such a broad, multi-modal metallicity distribution demonstrates that Terzan 5 is not a genuine globular cluster but the remnant of a much more complex stellar system.Comment: 29 pages, 10 figures. Accepted for publication by Ap