13 research outputs found
Photo-elastic properties of the wing imaginal disc of Drosophila
In the study of developmental biology, the physical properties and constraints of the developing tissues are of great importance. In spite of this, not much is known about the elastic properties of biologically relevant tissues that are studied in biology labs. Here, we characterize properties of the wing imaginal disc of Drosophila, which is a precursor organ intensely studied in the framework of growth control and cell polarity. In order to determine the possibility of measuring mechanical stresses inside the tissue during development, we quantify the photo-elastic properties of the tissue by direct mechanical manipulation. We obtain a photo-elastic constant of [Formula: see text]
Partial clustering prevents global crystallization in a binary 2D colloidal glass former
A mixture of two types of super-paramagnetic colloidal particles with long
range dipolar interaction is confined by gravity to a flat interface of a
hanging water droplet. The particles are observed by video microscopy and the
dipolar interaction strength is controlled via an external magnetic field. The
system is a model system to study the glass transition in 2D, and it exhibits
partial clustering of the small particles. This clustering is strongly
dependent on the relative concentration of big and small particles.
However, changing the interaction strength reveals that the clustering
does not depend on the interaction strength. The partial clustering scenario is
quantified using Minkowski functionals and partial structure factors. Evidence
that partial clustering prevents global crystallization is discussed
Arches and contact forces in a granular pile
Assemblies of granular particles mechanically stable under their own weight
contain arches. These are structural units identified as sets of mutually
stable grains. It is generally assumed that these arches shield the weight
above them and should bear most of the stress in the system. We test such
hypothesis by studying the stress born by in-arch and out-of-arch grains. We
show that, indeed, particles in arches withstand larger stresses. In
particular, the isotropic stress tends to be larger for in-arch-grains whereas
the anisotropic component is marginally distinguishable between the two types
of particles. The contact force distributions demonstrate that an exponential
tail (compatible with the maximization of entropy under no extra constraints)
is followed only by the out-of-arch contacts. In-arch contacts seem to be
compatible with a Gaussian distribution consistent with a recently introduced
approach that takes into account constraints imposed by the local force balance
on grains.Comment: 7 pages, 7 figures, major revisio
Jamming transition in granular systems.
Recent simulations have predicted that near jamming for collections of spherical particles, there will be a discontinuous increase in the mean contact number Z at a critical volume fraction ϕc. Above ϕc, Z and the pressure P are predicted to increase as power laws in ϕ-ϕc. In experiments using photoelastic disks we corroborate a rapid increase in Z at ϕc and power-law behavior above ϕc for Z and P. Specifically we find a power-law increase as a function of ϕ-ϕc for Z-Zc with an exponent β around 0.5, and for P with an exponent ψ around 1.1. These exponents are in good agreement with simulations. We also find reasonable agreement with a recent mean-field theory for frictionless particles
Goddard rattler-jamming mechanism for quantifying pressure dependence of elastic moduli of grain packs
An analysis is presented to show how it is possible for unconsolidated granular packings to obey overall non-Hertzian pressure dependence due to the imperfect and random spatial arrangements of the grains in these packs. With imperfect arrangement, some gaps that remain between grains can be closed by strains applied to the grain packing. As these gaps are closed, former rattler grains become jammed and new stress-bearing contacts are created that increase the elastic stiffness of the packing. By allowing for such a mechanism, detailed analytical expressions are obtained for increases in bulk modulus of a random packing of grains with increasing stress and strain. Only isotropic stress and strain are considered in this analysis. The model is shown to give a favorable fit to laboratory data on variations in bulk modulus due to variations in applied pressure for bead packs