arXiv subjects

G. Ruocco

Publications and source records attributed to G. Ruocco.

At least 37 records · Page 2Linked to original sources

arXiv2009

Shear thickening in molecular liquids characterized by inverse melting

We studied the rheological behavior of a molecular solution composed of $α$-cyclodextrin, water and 4-methylpyridine, a liquid known to undergo inverse melting, at different temperatures and concentrations. The system shows a marked non-Newtonian behavior, exhibiting the typical signature of shear thickening. Specifically, a transition is observed from a Newtonian to a shear thickening regime at a critical shear rate $\dotγ_c$. The value of this critical shear rate as a function of T follows an Arrhenius behavior $\dotγ_c(T)=$ B exp$(E_a/K_BT)$, with an activation energy $E_a$ close to the value of the hydrogen bond energy of the O-H group of the $α$-CD molecules. We argue that the increase of viscosity vs shear rate (shear thickening transition) is due to the formation of hydrogen bonded aggregates induced by the applied shear field. Finally, we speculate on the possible interplay between the non-Newtonian rheology and the inverse melting behavior, proposing a single mechanism to be at the origin of both phenomena.

cond-mat.soft↗

arXiv2008

Colloidal attraction induced by a temperature gradient

Colloidal crystals are of extreme importance for applied research, such as photonic crystals technology, and for fundamental studies in statistical mechanics. Long range attractive interactions, such as capillary forces, can drive the spontaneous assembly of such mesoscopic ordered structures. However long range attractive forces are very rare in the colloidal realm. Here we report a novel strong and long ranged attraction induced by a thermal gradient in the presence of a wall. Switching on and off the thermal gradient we can rapidly and reversibly form stable hexagonal 2D crystals. We show that the observed attraction is hydrodynamic in nature and arises from thermal induced slip flow on particle surfaces. We used optical tweezers to directly measure the force law and compare it to an analytic prediction based on Stokes flow driven by Marangoni forces.

cond-mat.soft↗

arXiv2008

Influence of an adsorbing polymer in the aging dynamics of Laponite clay suspensions

Clay-polymer dispersions in aqueous solutions have attracted a great interest in recent years due to their industrial applications and intriguing physical properties. Aqueous solutions of bare Laponite particles are known to age spontaneously from an ergodic state to a non ergodic state in a time varying from hours to months depending on Laponite concentration. When a polymer species like Polyethylene Oxide (PEO) is added to the solution, it weakly adsorbs on clay particle surfaces modifying the effective interaction potential between Laponite particles. A dynamic light scattering study, varying polymer concentration at fixed polymer molecular weight (Mw=200.000 g/mol), has been performed in order to understand the effect of polymer on the aging dynamics of the system. The results obtained show that arresting phenomena between clay particles are hindered if PEO is added and consequently the aging dynamics slows down with increasing PEO concentration. This process is possibly due to the progressive coverage of the clay surface by polymers that grow with increasing PEO concentration and may lead to steric stabilization.

cond-mat.soft↗

arXiv2008

Phase diagram and complexity of mode-locked lasers: from order to disorder

We investigate mode-locking processes in lasers displaying a variable degree of structural randomness, from standard optical cavities to multiple-scattering media. By employing methods mutuated from spin-glass theory, we analyze the mean-field Hamiltonian and derive a phase-diagram in terms of the pumping rate and the degree of disorder. Three phases are found: i) paramagnetic, corresponding to a noisy continuous wave emission, ii) ferromagnetic, that describes the standard passive mode-locking, and iii) the spin-glass in which the phases of the electromagnetic field are frozen in a exponentially large number of configurations. The way the mode-locking threshold is affected by the amount of disorder is quantified. The results are also relevant for other physical systems displaying a random Hamiltonian, like Bose-Einstein condensates and nonlinear optical beams.

physics.optics↗

arXiv2008

High frequency dynamics in liquid nickel: an IXS study

Owing to their large relatively thermal conductivity, peculiar, non-hydrodynamic features are expected to characterize the acoustic-like excitations observed in liquid metals. We report here an experimental study of collective modes in molten nickel, a case of exceptional geophysical interest for its relevance in Earth interior science. Our result shed light on previously reported contrasting evidences: in the explored energy-momentum region no deviation from the generalized hydrodynamic picture describing non conductive fluids are observed. Implications for high frequency transport properties in metallic fluids are discussed.

cond-mat.dis-nn↗

arXiv2008

Hydrodynamic Interactions in Two Dimensions

We measure hydrodynamic interactions between colloidal particles confined in a thin sheet of fluid. The reduced dimensionality, compared to a bulk fluid, increases dramatically the range of couplings. Using optical tweezers we force a two body system along the eigenmodes of the mobility tensor and find that eigen-mobilities change logarithmically with particle separation. At a hundred radii distance, the mobilities for correlated and anti-correlated motions differ by a factor of two, whereas in bulk fluids, they would be practically indistinguishable. We derive the two dimensional counterpart of the Oseen hydrodynamic tensor which quantitatively reproduces the observed behavior. These results highlight the importance of dimensionality for transport and interactions in colloidal systems and proteins in biological membranes.

cond-mat.soft↗

arXiv2008

Phase diagram of a solution undergoing inverse melting

The phase diagram of $α$-cyclodextrin/water/4-methylpyridine solutions, a system undergoing inverse melting, has been studied by differential scanning calorimetry, rheological methods, and X-rays diffraction. Two different fluid phases separated by a solid region have been observed in the high $α$-cyclodextrin concentration range ($c$$\geq$150 mg/ml). Decreasing $c$, the temperature interval where the solid phase exists decreases and eventually disappears, and a first order phase transition is observed between the two different fluid phases.

cond-mat.soft↗

arXiv2008

Arrested state of clay-water suspensions: gel or glass?

The aging of a charged colloidal system has been studied by Small Angle X-rays Scattering, in the exchanged momentum range Q=0.03 - 5 nm-1, and by Dynamic Light Scattering, at different clay concentrations (Cw =0.6 % - 2.8 %). The static structure factor, S(Q), has been determined as a function of both aging time and concentration. This is the first direct experimental evidence of the existence and evolution with aging time of two different arrested states in a single system simply obtained only by changing its volume fraction: an inhomogeneous state is reached at low concentrations, while a homogenous one is found at high concentrations.

cond-mat.soft↗

arXiv2008

Light-matter interaction in Mie nanolasers

By deriving a three dimensional vector set of Maxwell-Bloch equations, we report on an ab-initio investigation of a spherical Mie nanolaser. Parallel numerical simulations predict a rich physical scenario, ranging from a nontrivial vectorial energy-matter interplay in the initial stage of evolution to mode competition and dynamical frequency pulling effects. Application of these effects could favor the realization of nonlinearly-controlled largely-tunable nanolaser devices.

physics.optics↗

arXiv2007

The very long range nature of capillary interactions in liquid films

Micron-sized objects confined in thin liquid films interact through forces mediated by the deformed liquid-air interface. This capillary interactions provide a powerful driving mechanism for the self-assembly of ordered structures such as photonic materials or protein crystals. Direct probing of capillary interactions requires a controlled force field to independently manipulate small objects while avoiding any physical contact with the interface. We demonstrate how optical micro-manipulation allows the direct measurement of capillary interactions between two micron sized spheres in a free standing liquid film. The force falls off as an inverse power law in particles separation. We derive and validate an explicit expression for this exponent whose magnitude is mainly governed by particles size. For micron-sized objects we found an exponent close to, but smaller than one, making capillary interactions a unique example of strong and very long ranged forces in the mesoscopic world.

cond-mat.soft↗

arXiv2007

Vibrational excitations in systems with correlated disorder

We investigate a $d$-dimensional model ($d$ = 2,3) for sound waves in a disordered environment, in which the local fluctuations of the elastic modulus are spatially correlated with a certain correlation length. The model is solved analytically by means of a field-theoretical effective-medium theory (self-consistent Born approximation) and numerically on a square lattice. As in the uncorrelated case the theory predicts an enhancement of the density of states over Debye's $ω^{d-1}$ law (``boson peak'') as a result of disorder. This anomay becomes reinforced for increasing correlation length $ξ$. The theory predicts that $ξ$ times the width of the Brillouin line should be a universal function of $ξ$ times the wavenumber. Such a scaling is found in the 2d simulation data, so that they can be represented in a universal plot. In the low-wavenumber regime, where the lattice structure is irrelevant there is excellent agreement between the simulation at small disorder. At larger disorder the continuum theory deviates from the lattice simulation data. It is argued that this is due to an instability of the model with stronger disorder.

cond-mat.dis-nn↗

arXiv2007

Phase transitions and topology in 2+k XY mean-field models

The thermodynamics and topology of mean-field models with 2+k body interaction terms (generalizing XY model) are derived. Focusing on two particular cases (2+4 and 2+6 body interaction terms), a comparison between thermodynamic (phase transition energy, thermodynamically forbidden energy regions) and topological (singularity and curvature of saddle entropy) properties is performed. We find that i) a topological change is present at the phase transition energy; however, ii) only one topological change occurs, also for those models exhibiting two phase transitions; iii) the order of a phase transition is not completely signaled by the curvature of topological quantities.

cond-mat.stat-mech↗

arXiv2007

Shear banding phenomena in a Laponite suspension

Shear localization in an aqueous clay suspension of Laponite is investigated through dynamic light scattering, which provides access both to the dynamics of the system (homodyne mode) and to the local velocity profile (heterodyne mode). When the shear bands form, a relaxation of the dynamics typical of a gel phase is observed in the unsheared band soon after flow stop, suggesting that an arrested dynamics is present during the shear localization regime. Periodic oscillations of the flow behavior, typical of a stick-slip phenomenon, are also observed when shear localization occurs. Both results are discussed in the light of various theoretical models for soft glassy materials.

physics.flu-dyn↗

arXiv2007

Parametric Resonance of Optically Trapped Aerosols

The Brownian dynamics of an optically trapped water droplet are investigated across the transition from over to under-damped oscillations. The spectrum of position fluctuations evolves from a Lorentzian shape typical of over-damped systems (beads in liquid solvents), to a damped harmonic oscillator spectrum showing a resonance peak. In this later under-damped regime, we excite parametric resonance by periodically modulating the trapping power at twice the resonant frequency. The power spectra of position fluctuations are in excellent agreement with the obtained analytical solutions of a parametrically modulated Langevin equation.

cond-mat.stat-mech↗

arXiv2007

Acoustic attenuation in glasses and its relation with the boson peak

A theory for the vibrational dynamics in disordered solids [W. Schirmacher, Europhys. Lett. {\bf 73}, 892 (2006)], based on the random spatial variation of the shear modulus, has been applied to determine the wavevector ($k$) dependence of the Brillouin peak position ($Ω_k)$ and width ($Γ_k$), as well as the density of vibrational states ($g(ω)$), in disordered systems. As a result, we give a firm theoretical ground to the ubiquitous $k^2$ dependence of $Γ_k$ observed in glasses. Moreover, we derive a quantitative relation between the excess of the density of states (the boson peak) and $Γ_k$, two quantities that were not considered related before. The successful comparison of this relation with the outcome of experiments and numerical simulations gives further support to the theory.

cond-mat.mtrl-sci↗

arXiv2006

Mode-locking transitions in nano-structured weakly disordered lasers

We report on a statistical approach to mode-locking transitions of nano-structured laser cavities characterized by an enhanced density of states. We show that the equations for the interacting modes can be mapped onto a statistical model exhibiting a first order thermodynamic transition, with the average mode-energy playing the role of inverse temperature. The transition corresponds to a phase-locking of modes. Extended modes lead to a mean-field like model, while in presence of localized modes, as due to a small disorder, the model has short range interactions. We show that simple scaling arguments lead to observable differences between transitions involving extended modes and those involving localized modes. We also show that the dynamics of the light modes can be exactly solved, predicting a jump in the relaxation time of the coherence functions at the transition. Finally, we link the thermodynamic transition to a topological singularity of the phase space, as previously reported for similar models.

cond-mat.stat-mech↗

arXiv2006

Aging after shear rejuvenation in a soft glassy colloidal suspension: evidence for two different regimes

The aging dynamics after shear rejuvenation in a glassy, charged clay suspension have been investigated through dynamic light scattering (DLS). Two different aging regimes are observed: one is attained if the sample is rejuvenated before its gelation and one after the rejuvenation of the gelled sample. In the first regime, the application of shear fully rejuvenates the sample, as the system dynamics soon after shear cessation follow the same aging evolution characteristic of normal aging. In the second regime, aging proceeds very fast after shear rejuvenation, and classical DLS cannot be used. An original protocol to measure an ensemble averaged intensity correlation function is proposed and its consistency with classical DLS is verified. The fast aging dynamics of rejuvenated gelled samples exhibit a power law dependence of the slow relaxation time on the waiting time.

cond-mat.soft↗

arXiv2006

High Frequency dynamics in metallic glasses

Using Inelastic X-ray Scattering we studied the collective dynamics of the glassy alloy Ni$_{33}$Zr$_{67}$ in the first pseudo Brillouin zone, an energy-momentum region still unexplored in metallic glasses. We determine key properties such as the momentum transfer dependence of the sound velocity and of the acoustic damping, discussing the results in the general context of recently proposed pictures for acoustic dynamics in glasses. Specifically, we demonstrate the existence in this strong glass of well defined (in the Ioffe Regel sense) acoustic-like excitations well above the Boson Peak energy.

cond-mat.dis-nn↗