Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userPhonons in lattices with rod-like particles
365
0
0.0
(
0
)
Authors
A. Sparavigna
Ask ChatGPT about the research
No Arabic abstract
The paper studies the modes of vibrations of a lattice with rod-like particles, in a continuum model where the sites of the lattice are the connections among strings and rigid rods. In these structures then, translational and rotational degrees of freedom are strongly coupled. We will discuss in particular two-dimensional lattices with auxetic-like behaviour. Auxetics are materials with a negative Poisson elastic parameter, meaning that they have a lateral extension, instead to shrink, when they are stretched. We assume as auxetic-like two-dimensional structures, structures which do not collapse, when stretched in one of the in-plane directions. The presence of rigid rod-like particles in the lattice prevents the shrinking of the membrane. Complete bandgaps between acoustic and optical modes are observed in analogy with the behaviour of crystalline materials.
rate research
Read More
We construct colloidal ``sticky rods from the semi-flexible filamentous fd virus and temperature-sensitive polymers poly(N-isopropylacrylamide) (PNIPAM). The phase diagram of fd-PNIPAM system becomes independent of ionic strength at high salt concentration and low temperature, i.e. the rods are sterically stabilized by the polymer. However, the network of sticky rods undergoes a sol-gel transition as the temperature is raised. The viscoelastic moduli of fd and fd-PNIPAM suspensions are compared as a function of temperature, and the effect of ionic strength on the gelling behavior of fd-PNIPAM solution is measured. For all fluidlike and solidlike samples, the frequency-dependant linear viscoelastic moduli can be scaled onto universal master curves.
The modes of vibrations in honeycomb and auxetic structures are studied, with models in which the lattice is represented by a planar network where sites are connected by strings and rigid rods. The auxetic network is obtained modifying a model proposed by Evans et al. in 1991, and used to explain the negative Poissons ratio of auxetic materials. This relevant property means that the materials have a lateral extension, instead to shrink, when they are stretched. For what concerns the acoustic properties of these structures, they absorb noise and vibrations more efficiently than non-auxetic equivalents. The acoustic and optical dispersions obtained in the case of the auxetic model are compared with the dispersions displayed by a conventional honeycomb network. It is possible to see that the phonon dispersions of the auxetic model possess a complete bandgap and that the Goldstone mode group velocity is strongly dependent on the direction of propagation. The presence of a complete bandgap can explain some experimental observations on the sound propagation properties of the auxetic materials.
Thermal convection of fluid is a more efficient way than diffusion to carry heat from hot sources to cold places. Here, we experimentally study the Rayleigh-Benard convection of aqueous glycerol solution in a cubic cell with suspensions of rod-like particles made of polydimethylsiloxane (PDMS). The particles are inertial due to their large thermal expansion coefficient and finite sizes. The thermal expansion coefficient of the particles is three times larger than that of the background fluid. This contrast makes the suspended particles lighter than the local fluid in hot regions and heavier in cold regions. The heat transport is enhanced at relatively large Rayleigh number ($Ra$) but reduced at small $Ra$. We demonstrate that the increase of Nusselt number arises from the particle-boundary layer interactions: the particles act as ``active mixers of the flow and temperature fields across the boundary layers.
Lipid rafts are hypothesized to facilitate protein interaction, tension regulation, and trafficking in biological membranes, but the mechanisms responsible for their formation and maintenance are not clear. Insights into many other condensed matter phenomena have come from colloidal systems, whose micron-scale particles mimic basic properties of atoms and molecules but permit dynamic visualization with single-particle resolution. Recently, experiments showed that bidisperse mixtures of filamentous viruses can self-assemble into colloidal monolayers with thermodynamically stable rafts exhibiting chiral structure and repulsive interactions. We quantitatively explain these observations by modeling the membrane particles as chiral liquid crystals. Chiral twist promotes the formation of finite-sized rafts and mediates a repulsion that distributes them evenly throughout the membrane. Although this system is composed of filamentous viruses whose aggregation is entropically driven by dextran depletants instead of phospholipids and cholesterol with prominent electrostatic interactions, colloidal and biological membranes share many of the same physical symmetries. Chiral twist can contribute to the behavior of both systems and may account for certain stereospecific effects observed in molecular membranes.
The kinetics of isotropic-nematic (I-N) and nematic-isotropic (N-I) phase transitions in dispersions of rod-like {it fd}-viruses are studied. Concentration quenches were applied using pressure jumps in combination with polarization microscopy, birefringence and turbidity measurements. The full biphasic region could be accessed, resulting in the construction of a first experimental analogue of the bifurcation diagram. The N-I spinodal points for dispersions of rods with varying concentrations of depletion agents (dextran) were obtained from orientation quenches, using cessation of shear flow in combination with small angle light scattering. We found that the location of the N-I spinodal point is independent of the attraction, which was confirmed by theoretical calculations. Surprisingly, the experiments showed that also the absolute induction time, the critical nucleus and the growth rate are insensitive of the attraction, when the concentration is scaled to the distance to the phase boundaries.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
