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Register a new userElastic Instabilities in Flows through Pillared Micro channels
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Viscoelastic fluids exhibit elastic instabilities in simple shear flow and flow through curved streamlines. Surprisingly, we found in a porous medium such fluids show strikingly different hydrodynamic instabilities depicted by very large sideways excursions and presence of fast and slow moving lanes which have not been reported before. Particle image velocimetry (PIV) measurements through a pillared microchannel, provide experimental evidence of such instabilities at very low Reynolds number (< 0.01). We observe a transition from a symmetric laminar to an asymmetric flow, which finally transforms to a nonlinear aperiodic flow with strong lateral movements. The instability is characterized by a rapid increase in spatial and temporal fluctuations of velocity components and pressure at a critical Deborah number (De). Our experiments reveal the presence of a fascinating interplay between pore space and fluid rheology.
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The buckling and twisting of slender, elastic fibers is a deep and well-studied field. A slender elastic rod that is twisted with respect to a fixed end will spontaneously form a loop, or hockle, to relieve the torsional stress that builds. Further twisting results in the formation of plectonemes -- a helical excursion in the fiber that extends with additional twisting. Here we use an idealized, micron-scale experiment to investigate the energy stored, and subsequently released, by hockles and plectonemes as they are pulled apart, in analogy with force spectroscopy studies of DNA and protein folding. Hysteresis loops in the snapping and unsnapping inform the stored energy in the twisted fiber structures.
Mixtures of near-symmetric oppositely charged components with strong attractive short range interactions exhibit ordered lamellar phases at low temperatures. In the strong segregation limit the state of these systems can be described by the location of the interfaces between the components. It has previously been shown that these systems are stable against small deformations of the interfaces. We examine their stability in the presence of a uniform electric field. When the field is perpendicular to the lamellae, the system is unstable against long wavelength deformations for all non-zero values of the external field. A field parallel to the lamellae produces deformed but persistent interfaces. In a finite thickness system, onset of an external perpendicular field modifies the ground state. Flow between the old and new ground states requires the destruction of the original interfaces; this destruction proceeds through the instabilities identified in the bulk case. We examine the possibility of dynamical stabilization of structures by means of oscillating fields.
Charged colloidal dispersions make up the basis of a broad range of industrial and commercial products, from paints to coatings and additives in cosmetics. During drying, an initially liquid dispersion of such particles is slowly concentrated into a solid, displaying a range of mechanical instabilities in response to highly variable internal pressures. Here we summarise the current appreciation of this process by pairing an advection-diffusion model of particle motion with a Poisson-Boltzmann cell model of inter-particle interactions, to predict the concentration gradients around a drying colloidal film. We then test these predictions with osmotic compression experiments on colloidal silica, and small-angle x-ray scattering experiments on silica dispersions drying in Hele-Shaw cells. Finally, we use the details of the microscopic physics at play in these dispersions to explore how two macroscopic mechanical instabilities -- shear-banding and fracture -- can be controlled.
In this study, thin elastic films supported on a rigid substrate are brought into contact with a spherical glass indenter. Upon contact, adhesive fingers emerge at the periphery of the contact patch with a characteristic wavelength. Elastic films are also pre-strained along one axis before initiation of contact, causing the fingering pattern to become anisotropic and align with the axis along which the strain was applied. This transition from isotropic to anisotropic patterning is characterized quantitatively and a simple model is developed to understand the origin of the anisotropy.
We analyse here the problem of large deformation of dielectric elastomeric membranes under coupled electromechanical loading. Extremely large deformations (enclosed volume changes of 100 times and greater) of a toroidal membrane are studied by the use of a variational formulation that accounts for the total energy due to mechanical and electrical fields. A modified shooting method is adopted to solve the resulting system of coupled and highly nonlinear ordinary differential equations. We demonstrate the occurrence of limit point, wrinkling, and symmetry-breaking buckling instabilities in the solution of this problem. Onset of each of these reversible instabilities depends significantly on the ratio of the mechanical load to the electric load, thereby providing a control mechanism for state switching.
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