76 research outputs found
Structural transitions and arrest of domain growth in sheared binary immiscible fluids and microemulsions
We investigate spinodal decomposition and structuring effects in binary
immiscible and ternary amphiphilic fluid mixtures under shear by means of three
dimensional lattice Boltzmann simulations. We show that the growth of
individual fluid domains can be arrested by adding surfactant to the system,
thus forming a bicontinous microemulsion. We demonstrate that the maximum
domain size and the time of arrest depend linearly on the concentration of
amphiphile molecules. In addition, we find that for a well defined threshold
value of amphiphile concentration, the maximum domain size and time of complete
arrest do not change. For systems under constant and oscillatory shear we
analyze domain growth rates in directions parallel and perpendicular to the
applied shear. We find a structural transition from a sponge to a lamellar
phase by applying a constant shear and the occurrence of tubular structures
under oscillatory shear. The size of the resulting lamellae and tubes depends
strongly on the amphiphile concentration, shear rate and shear frequency.Comment: 12 pages, 11 figure
Stress response and structural transitions in sheared gyroidal and lamellar amphiphilic mesophases: lattice-Boltzmann simulations
We report on the stress response of gyroidal and lamellar amphiphilic
mesophases to steady shear simulated using a bottom-up lattice-Boltzmann model
for amphiphilic fluids and sliding periodic (Lees-Edwards) boundary conditions.
We study the gyroid per se (above the sponge-gyroid transition, of high
crystallinity) and the molten gyroid (within such a transition, of
shorter-range order). We find that both mesophases exhibit shear-thinning, more
pronounced and at lower strain rates for the molten gyroid. At late times after
the onset of shear, the skeleton of the crystalline gyroid becomes a structure
of interconnected irregular tubes and toroidal rings, mostly oriented along the
velocity ramp imposed by the shear, in contradistinction with free-energy
Langevin-diffusion studies which yield a much simpler structure of disentangled
tubes. We also compare the shear stress and deformation of lamellar mesophases
with and without amphiphile when subjected to the same shear flow applied
normal to the lamellae. We find that the presence of amphiphile allows (a) the
shear stress at late times to be higher than in the case without amphiphile,
and (b) the formation of rich patterns on the sheared interface, characterised
by alternating regions of high and low curvature.Comment: 15 pages, 10 figures, Physical Review E, in pres
Colloid Electrophoresis for Strong and Weak Ion Diffusivity
We study the electrophoretic flow of suspensions of charged colloids with a mesoscopic method that allows us to model generic experimental conditions. We show that for highly charged colloids their electrophoretic mobility increases significantly and displays a mobility maximum on increasing the colloidal charge for all salt concentrations. The electrophoretic mobility of highly charged colloids is also enhanced significantly when ion advection is dominant, leading to a strong heterogeneity in the local electrophoretic response especially at low-salt concentration, when ion diffuse layers overlap
The Ancient Varieties of Mountain Maize: The Inheritance of the Pointed Character and Its Effect on the Natural Drying Process
The introduction of mechanized agricultural practices after the Second World War and
the use of productive hybrids led to a gradual disappearance of local maize varieties. However,
13 landraces are still cultivated in North-Western Italy, in the Lombardy region; those that are
cultivated in mountainous areas (roughly up to 1200 m in altitude) are often characterized by the
pointed shape of their seeds (i.e., “Nero Spinoso”, “Rostrato Rosso di Rovetta”, “Spinato di Gandino”
and “Scagliolo di Carenno”) and the presence of pigments (i.e., “Nero Spinoso”, “Rostrato Rosso
di Rovetta”). The pointed shape of the seeds is an ancient characteristic of maize-ancestors, which
negatively affects the yield by not allowing optimal “filling” of the ear. This study reports work on
four different Italian varieties of pointed maize in order to assess the genetic bases of the “pointed
character” and to try to explain the reasons for this adaptation to the mountain environment. The
data obtained by genetic analysis, seed air-drying modeling and thermographic camera observations
demonstrated that the “pointed trait” is controlled by the same genes across the different varieties
studied and suggested that this peculiar shape has been selected in mountainous areas because it
promotes faster drying of the seed, with the presence of pigments implementing this effect
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